Mytilus edulis beds on sublittoral sediment

30-11-2001
Researched byDr Heidi Tillin & Kathryn Mainwaring Refereed byThis information is not refereed.
EUNIS CodeA5.625 EUNIS NameMytilus edulis beds on sublittoral sediment

Summary

UK and Ireland classification

EUNIS 2008A5.625Mytilus edulis beds on sublittoral sediment
EUNIS 2006A5.625Mytilus edulis beds on sublittoral sediment
JNCC 2004SS.SBR.SMus.MytSSMytilus edulis beds on sublittoral sediment
1997 BiotopeSS.IMX.EstMx.MytVMytilus edulis beds in variable salinity infralittoral mixed sediment

Description

Shallow sublittoral mixed sediment, in fully marine coastal habitats or sometimes in variable salinity conditions in the outer regions of estuaries, are characterised by beds of the common mussel Mytilus edulis. Other characterising infaunal species may include the amphipod Gammarus salinus and oligochaetes of the genus Tubificoides. The polychaetes Harmothoe spp., Kefersteinia cirrata and Heteromastus filiformis are also important. Epifaunal species include the whelks Nucella lapillus and Buccinum undatum, the common starfish Asterias rubens, the spider crab Maja squinado and the anemone Urticina felina. Relatively few records are available for this biotope and it is possible that as more data is accumulated separate estuarine and fully marine sub-biotopes may be described. Further clarification may also be required with regard to the overlap between littoral and sublittoral mussel beds and with regard to mussel beds biotopes on hard substratum (Connor et al., 2004, JNCC).

Recorded distribution in Britain and Ireland

Found in a few scattered locations around the coast of Britain in sheltered bays and estuaries and recorded in Lough Foyle in Ireland.

Depth range

0-5 m, 10-20 m, 5-10 m

Additional information

-

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JNCC

Habitat review

Ecology

Ecological and functional relationships

  • Mytilus edulis is a active suspension feeder on organic particulates and dissolved organic matter.
  • The production of faeces and pseudofaeces enriches the underlying sediment providing a rich food source for infauna detritivores, deposit feeders, meiofauna and bacteria.
  • Dense beds of suspension feeding bivalves are important in nutrient cycling in estuarine and coastal ecosystems, transferring phytoplankton primary production and nutrients to benthic secondary production (pelagic-benthic coupling) (Dame, 1996).
  • Other suspension feeders include epifaunal barnacles and tube worms e.g. Pomatoceros triqueter.
  • Epifloral/faunal grazers, such as limpets and chitons may use the mussel bed as a refuge. Their grazing reduces epiflora/faunal fouling of Mytilus edulis shells, hence reducing the potential for dislodgement of the mussels due to strong water flow or storm surges (Suchanek, 1985).
  • The organic rich 'mussel mud' provides a food source for deposit feeding polychaetes (e.g. Scoloplos armiger and Capitella capitata and oligochaetes (e.g. Tubificoides spp.) and surface deposit feeders (e.g. Polydora spp. and Macoma baltica
  • Scavengers probably feed on dead mussels and other organic material within the mussel matrix, e.g. flatworms, polychaetes and amphipods (Kautsky, 1981; Tsuchiya & Nishihira, 1985,1986).
  • The interstices within the mussel matrix and mussel mud support epifaunal and infaunal predators such as scale worms (e.g. Harmothoe spp.), nereids (e.g. Nephtys spp.) and other polychaetes and nemerteans.
  • Fish, starfish, crabs and lobsters are potential predators on subtidal mussels beds (Kautsky, 1981; Paine, 1976; Seed, 1993; Seed & Suchanek, 1992).
  • Mussels were a major food source for the flounder (Platichthys flesus) in Morecambe Bay and subtidal mussel beds in the Baltic Sea (Dare, 1976; Kautsky. 1981) but probably of only minor importance for eelpout (Zoarces viviparus) and cod (Gadus morhua in the Baltic Sea (Kautsky, 1981).
  • The lower limit of Mytilus edulis beds is usually set by the intensity of predation, e.g. from Asterias rubens and Nucella lapillus in eastern England (Seed, 1969) or Liocarcinus spp., Carcinus maenas , Nucella lapillus and Marthasterias glacialis in Ireland (Kitching & Ebling, 1967; Holt et al., 1998). However, predation risk is size dependant, i.e. Carcinus maenas was unable to consume mussels of ca. 70mm in length and mussels >45mm long were probably safe from attack (Davies et al., 1980; Holt et al., 1998).
  • Periodic, and sporadic swarms of starfish have been observed to decimate mussel populations, and subtidal settlements in the Wash were destroyed by Asterias rubens annually (Dare, 1976, 1982; Seed, 1969; Holt et al., 1998).
  • Birds are major predators in intertidal beds but this biotope is probably only vulnerable during extreme low tides to most predatory wildfowl, however , eider ducks are capable divers. Eider duck consume large numbers of mussels, primarily over winter. Raffaelli et al. (1990) recorded the removal of 4500 mussels /m² (within the preferred size of 10-25mm) within 60 days by a flock of 500 eider. Eider remove mussels in clumps, which they shake to remove the target mussel. This results in additional mortality for those mussels removed from suitable substratum in the clump and leaves bare patches in the mussel beds, which may increase the risk of the loss of further mussels by water movement. Eider may, therefore, significantly affect the mussel bed (Seed & Suchanek, 1992; Holt et al., 1998).
  • Otters may prey on mussel beds.
  • Kautsky (1981) reported that the release of mussel eggs and larvae from subtidal beds in the Baltic Sea contributed an annual input of 600 tons of organic carbon/yr. to the pelagic system. The eggs and larvae were probably an important food source for herring larvae and other zooplankton.

Seasonal and longer term change

Mussels are capable of living to up to 18-24 years of age, however, the majority of mussels in biogenic reefs are probably young consisting of 2 -3 year old individuals due to predation and the dislodgement of clumps of mussels by wave action and storms (Holt et al., 1998). As mussel beds grow in size, individual mussels become more attached to other mussels than to the underlying substratum, so that large beds may be 'rolled up' and removed by wave action. Therefore, mussel beds may vary in size and extent, and show a continuum between thin patchy beds and well developed reefs (Holt et al., 1998). However, more stable reefs occur in sheltered environments. For example, in the German Wadden Sea, the distribution of mussels has been relatively constant since 1949 but the abundance of mussels varied due to irregular recruitment, storm surges, ice drift, and parasitism. In the Dutch Wadden Sea the distribution of mussel beds was relatively constant from 1949-1988 although the biomass varied 30 fold (Holt et al., 1998).

Habitat structure and complexity

Sub-tidal Mytilus edulis beds have been little studied but probably have features in common with intertidal beds or subtidal beds of other mussel species (e.g. Modiolus modiolus). Mussels beds can be divided into three distinct habitat components: the interstices within the mussel matrix; the biodeposits beneath the bed; and the substratum afforded by the mussel shells themselves (Suchanek, 1985; Seed & Suchanek, 1992).
  • The gaps between interconnected mussels form numerous interstices for a variety of organisms. In intertidal Mytilus sp. beds, the species richness and diversity increases with the age and size of the bed (Suchanek, 1985; Tsuchiya & Nishihira, 1985,1986; Seed & Suchanek, 1992). The mussel matrix may support sea cucumbers, anemones, boring clionid sponges, ascidians, crabs, nemerteans, errant polychaetes and flatworms (Suchanek, 1985; Tsuchiya & Nishihira, 1985,1986). However, the species richness of the IMX.MytV biotope is not particularly high (Connor et al., 1997a). Holt et al. (1998) noted that this biotope may form raised beds (biogenic reefs) and stabilize the substratum, perhaps resulting in a higher species diversity than in the sediments alone.
  • Mussel faeces and pseudo-faeces, together with silt, build up organic biodeposits under the beds. The biodeposits attract infauna such as sediment dwelling sipunculids, oligochaetes, and polychaetes (Suchanek, 1979; Seed & Suchanek, 1992). However, in areas of strong tidal streams, flushing may prevent the build up of a thick layer of biodeposits.
  • Epizoans may use the mussels shells themselves as substrata. However, Mytilus edulis can use its prehensile foot to clean fouling organisms from its shell (Theisen, 1972). Therefore, the epizoan flora and fauna is probably less developed or diverse than found in beds of other mussel species. Barnacles and tubeworms may be epizoic, however this biotope does not support a diverse epifauna.

Productivity

Mytilus spp. communities are highly productive secondary producers (Seed & Suchanek, 1992; Holt et al., 1998). For example, in Morecambe Bay, Dare (1976) estimated that production by two year old classes was 2.5-3 times their maximum standing, even though mussels in this area suffer high rates of mortality. In favourable areas low shore mussel can grow 3.5 -4cm in 30 weeks and 6-8 cm in length in 2 years (Orton, 1914; Seed, 1976). Rapid production and turnover are characteristic of estuarine or sheltered communities (Holt et al., 1998). Production of an intertidal bed in the Eastern Scheldt was estimated to be 156 g ash free dry weight (AFDW) / m² in one year (Craeymeersch et al., 1986). Similarly, Egerrup & Layrsen (1992; cited in Holt et al., 1998) estimated that annual predation on a Danish Wadden Sea mussel bed accounted for 17% of the biomass and 81% of the secondary production from a mussel biomass of 740 g AFDW /m². Dame (1996) suggested that dense beds of suspension feeding bivalves are important in nutrient cycling in estuarine and coastal ecosystems, transferring phytoplankton primary production and nutrients to benthic secondary production (pelagic-benthic coupling) and improving the productivity of the entire system.

Recruitment processes

  • Mytilus edulis recruitment is dependant on larval supply and settlement, together with larval and post-settlement mortality. Gametogenesis and spawning varies with geographic location, e.g. southern populations often spawn before more northern populations (Seed & Suchanek, 1992). Spawning is protracted in many populations, with a peak of spawning in spring and summer and settlement approximately one month later. Jörgensen (1981) estimated that larvae suffered a daily mortality of 13% in the Isefjord, Denmark. Lutz & Kennish (1992) suggested that larval mortality was approximately 99%. Larval mortality is probably due to adverse environmental conditions, especially temperature, inadequate food supply (fluctuations in phytoplankton populations), inhalation by suspension feeding adult mytilids, difficulty in finding suitable substrata and predation (Lutz & Kennish, 1992). Widdows (1991) suggested that any environmental factor that increased development time, or the time between fertilisation and settlement would increase larval mortality.
  • Recruitment in many Mytilus sp. populations is sporadic, with unpredictable pulses of recruitment (Seed & Suchanek, 1992). Mytilus sp. is highly gregarious and final settlement often occurs around or in between individual mussels of established populations. Occasional recruitment to circalittoral populations may occur as individuals dislodged from the intertidal. Competition with surrounding adults may suppress growth of the young mussels settling within the mussel bed, due to competition for food and space, until larger mussels are lost (Seed & Suchanek, 1992). However, young mussels tend to divert resources to rapid growth rather than reproduction. Persistent mussels beds can be maintained by relatively low levels of recruitment e.g. McGrorty et al., (1990) reported that adult populations were largely unaffected by large variations in spatfall between 1976-1983 in the Exe estuary.
  • The Mytilus edulis bed may act as a refuge for larvae or juveniles, however, the intense suspension feeding activity of the mussels is likely to consume large numbers of pelagic larvae. Commito (1987) suggested that species that reproduce with cocoons, brood their young (e.g. occasionally in Urticina felina) or disperse as juveniles will be favoured.
  • Recruitment in echinoderms is highly variable, for example, Asterias rubens is widespread, fecund, and with a pelagic larvae capable of widespread dispersal, however, recruitment in starfish is sporadic, unpredictable and poorly understood (Seed, 1993).
  • Nucella lapillus mates in gregarious aggregations and lays capsules, cemented to the substratum, in which the larvae develop until released as miniature adult crawl-aways. There is no pelagic phase, and although passive mucous rafting may occur occasionally, dispersal is limited to about 10-30cm. However, adults are relatively long-lived (about 6 years) and a female can produce up to 1030 hatchlings per year (see review).
  • Most species of polychaete encountered within the biotope are widespread and have a dispersive pelagic larvae (Fish & Fish, 1996), and can potentially disperse and recruit over a wide range, depending on the hydrographic regime. The larvae of Scoloplos armiger are benthic (Fish & Fish, 1996), however, passive transport of juveniles has been shown to be important for the recruitment of species in sedimentary habitats (Olafsson et al., 1994), and other polychaetes with purely benthic stages are capable of colonizing new habitats rapidly, e.g. Arenicola marina.

Time for community to reach maturity

The occurrence of this biotope requires the presence of dense Mytilus edulis beds. Mytilus spp. populations were considered to have a strong ability to recover from environmental disturbance (Holt et al., 1998; Seed & Suchanek, 1992). Larval supply and settlement could potentially occur annually, however, settlement is sporadic with unpredictable pulses of recruitment (Lutz & Kennish, 1992; Seed & Suchanek, 1992). Therefore, while good annual recruitment and rapid growth are possible, recovery of the mussel population may take up to 5 years. In certain circumstances and under some environmental conditions recovery may take significantly longer. The associated community is likely to colonize the substratum or mussel matrix rapidly.

Additional information

None entered.

Preferences & Distribution

Recorded distribution in Britain and Ireland

Found in a few scattered locations around the coast of Britain in sheltered bays and estuaries and recorded in Lough Foyle in Ireland.

Habitat preferences

Depth Range 0-5 m, 10-20 m, 5-10 m
Water clarity preferences
Limiting Nutrients Not relevant
Salinity Full (30-40 psu)
Physiographic
Biological Zone Circalittoral, Infralittoral
Substratum Mixed
Tidal Moderately Strong 1 to 3 knots (0.5-1.5 m/sec.)
Wave Moderately exposed, Sheltered
Other preferences

Additional Information

Mytilus edulis is found in circumpolar and temperate waters in the north and south hemispheres. Mytilus edulis can survive periodic freezing to -10°C for short periods (e.g. -16°C for 24hrs). In British waters an upper sustained thermal tolerance limit of 29°C has been reported (Holt et al., 1998).

Species composition

Species found especially in this biotope

    Rare or scarce species associated with this biotope

    -

    Additional information

    The MNCR recorded 133 species within this biotope (JNCC, 1999). Reduced salinity, estuarine environments are generally of low species richness due to the physiological rigors of the environment. While this biotope is probably of low species richness when compared to other full salinity or intertidal mussel biotopes, the biotope still represents a species rich habitat within reduced salinity habitats.

    Sensitivity reviewHow is sensitivity assessed?

    Sensitivity characteristics of the habitat and relevant characteristic species

    Although a wide range of species are associated with Mytilus edulis reef or bed biotopes these characterizing species occur in a range of other biotopes and are therefore not considered to be obligate associates.  Mytilus edulis beds are not dependent on associated species to create or modify habitat, provide food or other resources, although their loss would represent a loss of diversity.  It should be noted that for attached organisms the sensitivity of the Mytilus edulis biotope would be of primary concern as removal of the reef would also lead to removal of the attached species. The sensitivity assessments are therefore based on Mytilus edulis and only consider the sensitivity of associated species where they might augment any impact  or cause secondary impacts.  

    Resilience and recovery rates of habitat

    Blue mussels, Mytilus edulis, are sessile, attached organisms that are unable to repair significant damage to individuals. Mussels do not reproduce asexually and therefore the only mechanism for recovery from significant impacts (where resistance is assessed as ‘None’, ‘Low’ or ‘Medium’) is larval recruitment to the bed or the area where previously a bed existed. Spawning occurs in spring and later summer allowing two periods of recruitment (Seed, 1969).  Mytilus edulis has a high fecundity producing >1,000,000 eggs per spawning event.  Larvae stay in the plankton for between 20 days to two months depending on water temperature (Bayne, 1976).  In unfavourable conditions they may delay metamorphosis for 6 months (Lane et al., 1985).  Larval dispersal depends on the currents and the length of time they spend in the plankton.  Larvae subject to ocean currents for up to six months can have a high dispersal potential. Settlement occurs in two phases, an initial attachment using their foot (the pediveliger stage) and then a second attachment by the byssus thread before which they may alter their location to a more favourable one (Bayne, 1964).  The final settlement often occurs around or between individual mussels of an established population. In areas of high water flow the mussel bed will rely on recruitment from other populations as larvae will be swept away and therefore recovery will depend on recruitment from elsewhere. 

    Larval mortality can be as high as 99% due to adverse environmental conditions, especially temperature, inadequate food supply (fluctuations in phytoplankton populations), inhalation by suspension feeding adult mytilids, difficulty in finding suitable substrata and predation (Lutz & Kennish 1992).  After settlement the larvae and juveniles are subject to high levels of predation as well as dislodgement from waves and sand abrasion depending on the area of settlement.  Height on the shore generally determines life span with mussels in the low shore only surviving between 2-3 years due to high predation levels whereas higher up on the shore a wider variety of age classes are found (Seed, 1969).  Theisen (1973) reported that specimens of Mytilus edulis could reach 18-24 years of age. 

    Mainwaring et al. (2014) reviewed the evidence for recovery of Mytilus.edulis beds from disturbance and an earlier study by  Seed & Suchanek (1992) reviewed studies on the recovery of ‘gaps’ in Mytilus spp. beds.  It was concluded that beds occurring high on the shore and on less exposed sites took longer to recover after a disturbance event than beds found low on the shore or at more exposed sites.  However, the slowest recovering sites (high shore and sheltered shores) are at the least risk of natural disturbance and often considered more ‘stable’ (Lewis, 1964) as they are less vulnerable to removal by wave action or wave driven logs. Continued disturbance will lead to a patchy distribution of mussels.

    Recruitment of Mytilus edulis is often sporadic, occurring in unpredictable pulses (Seed & Suchanek, 1992), although persistent mussel beds can be maintained by relatively low levels or episodic recruitment (McGrorty et al., 1990).  A good annual recruitment could result in rapid recovery (Holt et al., 1998).  However, the unpredictable pattern of recruitment based on environmental conditions could result in recruitment taking much longer.  In the northern Wadden Sea, strong year classes (resulting from a good recruitment episode) that lead to rejuvenation of blue mussel beds are rare, and usually follow severe winters, even though mussel spawning and settlement are extended and occur throughout the year (Diederich, 2005).  In the List tidal basin (northern Wadden Sea) a mass recruitment of mussels occurred in 1996 but had not been repeated by 2003 (the date of the study), i.e. for seven years (Diederich, 2005). 

    In some long term studies of Mytilus californianus it was observed that gaps could continue to increase in size post disturbance due to wave action and predation (Paine & Levin, 1981; Brosnan & Crumrine, 1994; Smith & Murray, 2005) potentially due to the weakening of the byssus threads leaving them more vulnerable to environmental conditions (Denny 1987).  On rocky shores barnacles and fucoids are often quick to colonise the ‘gaps’ created.  The presence of macroalgae appears to inhibit recovery whilst the presence of barnacles enhances subsequent mussel recruitment (Seed & Suchanek 1992). Brosnan & Crumrine (1994) observed little recovery of the congener Mytilus californianus in two years after trampling disturbance.  Paine & Levin (1981) estimated that recovery times of beds could be between 8-24 years while Seed & Suchaneck (1992) suggested it could take longer-time scales, suggesting that meaningful recovery is unlikely in some areas.  It has, however, been suggested that Mytilus edulis recovers quicker than other Mytilus species (Seed & Suchanek 1992), which may mean that these predicted recovery rates are too low for Mytilus edulis.

    Resilience assessment. The evidence for recovery rates of Mytilus edulis beds from different levels of impact is very limited and whether these rates are similar, or not, between biotopes is largely unclear.  Recovery rates are clearly determined by a range of factors such as degree of impact, season of impact, larval supply and local environmental factors including hydrodynamics so that confidence in the applicability of generic assessments is ‘Low’. Overall, Mytilus spp. populations are considered to have a strong ability to recover from environmental disturbance (Holt et al., 1998; Seed & Suchaneck, 1992).  A good annual recruitment may allow a bed to recovery rapidly, though this cannot always be guaranteed within a certain time-scale due to the episodic nature of Mytilus edulis recruitment (Lutz & Kennish, 1992; Seed & Suchanek, 1992) and the influence of site-specific variables.  Resilience will vary depending of larval supply and wave exposure with areas with low larval supply and high wave exposure on sandy substrata experiencing the longest recovery rates.  The sensitivity assessment have adopted the rates used by Mainwaring et al. (2014) who suggested that where resistance is ‘High’ then there is no effect to recover from and resilience should be assessed as ‘High’.  Littoral and sublittoral beds are considered to have ‘Medium’ resilience (2 -10 years) to represent the potential for recovery within a few years where a proportion of the bed remains (‘Medium’ or ‘Low’ resistance).  Resilience is assessed as ‘Low’ (over 10 years) for all biotopes where resistance is assessed as ‘None’, as recovery is dependent on recruitment from other areas and recruitment can be sporadic.  Due to the variation in recovery rates reported in the literature, while the evidence for resilience is of ‘High’ quality and ‘High’ applicability (for recovery from the same pressures or otherwise assessed as ‘Low’), the degree of concordance is ‘Medium’. 

     NB: The resilience and the ability to recover from human induced pressures is a combination of the environmental conditions of the site, the frequency (repeated disturbances versus a one off event) and the intensity of the disturbance.  Recovery of impacted populations will always be mediated by stochastic events and processes acting over different scales including, but not limited to, local habitat conditions, further impacts and processes such as larval-supply and recruitment between populations. Full recovery is defined as the return to the state of the habitat that existed prior to impact.  This does not necessarily mean that every component species has returned to its prior condition, abundance or extent but that the relevant functional components are present and the habitat is structurally and functionally recognizable as the initial habitat of interest. It should be noted that the recovery rates are only indicative of the recovery potential.  

    Hydrological Pressures

     ResistanceResilienceSensitivity
    High High No evidence (NEv)
    Q: High
    A: High
    C: Medium
    Q: High
    A: High
    C: High
    Q: High
    A: High
    C: Medium

    Local populations may be acclimated to the prevailing temperature regime and may therefore exhibit different tolerances to other populations subject to different salinity conditions and therefore caution should be used when inferring tolerances from populations in different regions. Mytilus edulis is a eurytopic species found in a wide temperature range from mild, subtropical regions to areas which frequently experience freezing conditions and are vulnerable to ice scour (Seed & Suchanek ,1992).  In recent years, Mytilus edulis has been observed to be expanding its range pole-wards and has reappeared in Svalbard, due to an increase of sea temperature in that region (Berge et al., 2005), whilst its equatorial limits are contracting due to increases in water temperature beyond the lethal limit (Jones et al., 2010).  In British waters 29°C was recorded as the upper sustained thermal tolerance limit for Mytilus edulis (Read & Cumming, 1967; Almada-Villela, et al., 1982), although it is thought that European mussels will rarely experience temperatures above 25°C (Seed & Suchanek, 1992). 

    Tsuchiya (1983) documented the mass mortality of Mytilus edulis in in Mutsu Bay, northern Japan in August 1981 due to air temperatures of 34°C that resulted in mussel tissue temperatures in excess of 40°C.  In one hour, 50% of the Mytilus edulis from the upper 75% of the shore had died.  It could not be concluded from this study whether the mortality was due to high temperatures, desiccation or a combination of the two.  Lethal water temperatures appear to vary between areas (Tsuchiya, 1983) although it appears that their tolerance at certain temperatures vary, depending on the temperature range to which the individuals are acclimatised (Kittner & Riisgaard, 2005).  After acclimation of individuals of M. edulis to 18°C, Kittner & Riisgaard (2005) observed that the filtrations rates were at their maximum between 8.3 and 20°C and below this at 6°C the mussels closed their valves.  However, after being acclimated at 11°C for five days, the mussels maintained the high filtration rates down to 4°C.  Hence, given time, mussels can acclimatise and shifting their temperature tolerance.  Filtration in Mytilus edulis was observed to continue down to -1°C, with high absorption efficiencies (53-81%) (Loo, 1992).

    At the upper range of a mussels tolerance limit, heat shock proteins are produced, indicating high stress levels (Jones et al., 2010).  After a single day at 30°C, the heat shock proteins were still present over 14 days later, although at a reduced level.  Increased temperatures can affect reproduction in Mytilus edulis (Myrand et al., 2000).  In shallow lagoons mortality began in late July at the end of a major spawning event when temperatures peaked at >20°C.  These mussels had a low energetic content post spawning and had stopped shell growth.  It is likely that the high temperatures caused mortality due to the reduced condition of the mussels post spawning (Myrand et al., 2000). Gamete production does not appear to be affected by temperature (Suchanek, 1985).

    Temperature changes may also lead to indirect effects  For example, an increase in temperature increases the mussels’ susceptibility to pathogens (Vibrio tubiashii) in the presence of relatively low concentrations of copper (Parry & Pipe, 2004).  Increased temperatures may also allow for range expansion of parasites or pathogens which will have a negative impact upon the health of the mussels if they become infected.

    Power stations have the potential to cause an increase in sea temperature of up to 15°C (Cole et al., 1999), although this impact will be localised.  However, as mussels are of the most damaging biofouling organisms on water outlets of power stations, they are clearly not adversely affected (Whitehouse et al., 1985; Thompson et al., 2000).

    Sensitivity assessment. Based on the wide range of temperature tolerance of Mytilus edulis and its limited effect on its physiology, it is concluded that the acute and chronic changes described by the benchmarks of 2-5°C would have limited effect.  Therefore the biotopes are considered to have a ‘High’ resistance to temperature change and ‘High’ resilience and are therefore considered to be 'Not Sensitive'.

    Q: High
    A: High
    C: Medium
    Q: High
    A: High
    C: High
    Q: 
    A: 
    C: 

    Local populations may be acclimated to the prevailing temperature regime and may therefore exhibit different tolerances to other populations subject to different salinity conditions and therefore caution should be used when inferring tolerances from populations in different regions. Mytilus edulis is a eurytopic species found in a wide temperature range from mild, subtropical regions to areas which frequently experience freezing conditions and are vulnerable to ice scour (Seed & Suchanek 1992). 

    The lower lethal limit of Mytilus edulis depends on the length of time exposed to a low temperature and the frequency of exposure (Bourget, 1983).  Williams (1970) observed that Mytilus edulis tolerated a tissue temperature as low as -10 °C.  In a laboratory experiment, Bourget (1983) showed that the median lethal temperature for 24 hour of exposure in Mytilus edulis was -16 °C for large mussels (>3cm) and -12.5 °C for juveniles (<1.5cm).  However, when exposed to reduced temperatures for only 16 hours, the median lethal temperature of large mussels decreased to -20 °C.  It was also reported that mussels exposed to sub lethal temperatures cyclically, e.g. -8 °C every 12.4 hours for 3-4 days, suffered significant damage likely to lead to death (Bourget, 1983), which suggested that while Mytilus edulis could tolerate occasional sharp frost events it was not likely to survive prolonged periods of very low temperatures.  During the cold winter of 1962/63, Mytilus edulis was reported to have experienced relatively few effects with only 30% mortality being recorded from the south east coast of England (Whitstable area) and only about 2% mortality was reported from Rhosilli in South Wales (Crisp, 1964).  Crisp (1964) also noted that the mortality was mainly from predation on the individuals that were weakened by the low temperatures rather than the temperature itself.  It is thought that the use of nucleating agents in the haemolymph and the maintenance of a high osmotic concentration in the mantle fluid during periods of winter isolation allows Mytilus edulis to tolerate such  low temperatures (Aunaas et al., 1988).

    Shell growth is not expected to be majorly influenced by low temperatures.  Bayne (1976) demonstrated that between 10-20 °C water temperature had little effect on scope for growth, similar to the findings of  (Page & Hubbard, 1987) who found that a temperature range of 10-18 °C did not influence growth rate.  In addition, Loo (1992) recorded growth rates of up to 0.7 % at temperatures as low as -1 °C, with an excess of seston, a rate higher than the same author recorded in mussel culture in Sweden (Loo & Rosenberg, 1983).  They concluded that food availability was more of a limiting factor to growth than temperature (Loo, 1992).

    Sensitivity assessment. Based on the wide range of temperature tolerance of Mytilus edulis and its limited effect on its physiology, it is concluded that the acute and chronic changes described by the benchmarks of 2-5°C would have limited effect.  Therefore the biotopes are considered to have a ‘High’ resistance to temperature change and ‘High’ resilience and are therefore considered to be 'Not Sensitive'.

     

    High High Not sensitive
    Q: High
    A: High
    C: Medium
    Q: High
    A: High
    C: High
    Q: 
    A: 
    C: 

    Local populations may be acclimated to the prevailing salinity regime and may therefore exhibit different tolerances to other populations subject to different salinity conditions and therefore caution should be used when inferring tolerances from populations in different regions.  Mytilus edulis is found in a wide range of salinities from variable salinity areas (18-35ppt) such as estuaries and intertidal areas, to areas of more constant salinity (30-35ppt) in the sublittoral (Connor et al., 2004).  Furthermore, mussels in rock pools are likely to experience hypersaline conditions on hot days.  Newell (1979) recorded salinities as high as 42psu in intertidal rock pools, suggesting that Mytilus edulis can tolerate high salinities.  

    Sensitivity assessment.  Increased salinity is likely to change a reduced salinity area to a fully marine area where it is known that mussels can survive in abundance.  Also, an increase in salinity from full to raised salinity (38 units), is less than that encountered in rock pools, where Mytilus edulis survives.  Therefore, Mytilus edulis is recorded as having a 'High' resistance to an increase in salinity at the pressure benchmark and a 'High' resilience and this biotope is therefore assessed as 'Not sensitive'.

    High High Not sensitive
    Q: High
    A: High
    C: Medium
    Q: High
    A: High
    C: High
    Q: High
    A: High
    C: Medium

    Local populations may be acclimated to the prevailing salinity regime and may therefore exhibit different tolerances to other populations subject to different salinity conditions and therefore caution should be used when inferring tolerances from populations in different regions. Mytilus edulis is found in a wide range of salinities from variable salinity areas (18-35ppt) such as estuaries and intertidal areas, to areas of more constant salinity (30-35ppt) in the sublittoral (Connor et al., 2004).  In addition, Mytilus edulis thrives in brackish lagoons and estuaries, although, this is probably due to the abundance of food in these environments rather than the salinity (Seed & Suchanek, 1992).Furthermore, mussels in rock pools are likely to experience hypersaline conditions on hot days.  Newell (1979) recorded salinities as high as 42psu in intertidal rock pools, suggesting that Mytilus edulis can tolerate high salinities.  Also, Mytilus edulis was recorded to grow in a dwarf form in the Baltic sea where the average salinity was 6.5psu (Riisgård et al., 2013). 

    Mytilus edulis is an osmoconformer and maintains its tissue fluids iso-osmotic (equal ionic strength) with the surrounding medium by mobilisation and adjustment of the tissue fluid concentration of free amino acids (e.g. taurine, glycine and alanine) (Bayne, 1976; Newell, 1989).  But mobilizing amino acids may result in loss of protein, increased nitrogen excretion and reduced growth. However, Koehn (1983) and Koehn & Hilbish (1987) reported a genetic basis to adaptation to salinity.   Mytilus edulis exhibits a defined behavioural response to reducing salinity, initially only closing its siphons to maintain the salinity of the water in its mantle cavity, which allows some gaseous exchange and therefore maintains aerobic metabolism for longer.  If the salinity continues to fall the valves close tightly (Davenport,1979; Rankin & Davenport, 1981).  In the long term (weeks) Mytilus edulis can acclimate to lower salinities (Almada-Villela, 1984; Seed & Suchanek 1992; Holt et al.,1998).  Almada-Villela (1984) reported that the growth rate of individuals exposed to only 13 psu reduced to almost zero but had recovered to over 80% of control animals within one month.  Observed differences in growth are due to physiological and/or genetic adaptation to salinity.

    Decreased salinity has physiological effects on Mytilus edulis; decreasing the heart rate (Bahmet et al., 2005), reducing filtration rates (Riisgård et al., 2013), reducing growth rate (Gruffydd et al., 1984) and reducing the immune function (Bussell et al., 2008).  Both Bahmet et al., (2005); Riisgård et al., (2013) noted that filtration and heart rates return to normal within a number of days acclimation or a return to the original salinity.  However, Riisgard et al., (2013) did observe that mussels from an average of 17 psu found it harder to acclimate between the salinity extremes than those from an average of 6.5 psu.  This observation may mean that mussels in a variable/ lower salinity environment are more able to tolerate change than those found at fully marine salinities.  A sharp salinity change also induces a behavioural response to close the shell (Riisgård et al., 2012) to maintain the salinity within the mantle cavity. In extreme low salinities, e.g. resulting from storm runoff, large numbers of mussels may be killed (Keith Hiscock pers comm.).  However, Bailey et al., (1996) observed very few mortalities when exposing Mytilus edulis to a range of salinities as low as 0ppt for two weeks at a range of temperatures.  It was also noted that there was a fast recovery rate. 

    Sensitivity assessment, Most of the literature found on this topic considered short term (days to weeks) impacts of changes to salinity whilst the benchmark refers to a change for one year.  However, Mytilus edulis was shown to be capable of acclimation to changes in salinity.  As Mytilus edulis is found in salinities to as low as 4-5psu (Riisgård et al., 2013), it is likely to be able to acclimate to a decrease in salinity of 4-10 units for a year. Therefore, Mytilus edulis is recorded as having a ‘High’ resistance to a decrease in salinity and a ‘High’ resilience (no impact to recover from).  The blue mussel bed biotopes are therefore considered to be ‘Not Sensitive’ at the benchmark level.    

    Medium Medium Medium
    Q: High
    A: High
    C: Medium
    Q: High
    A: Low
    C: Medium
    Q: High
    A: Low
    C: Medium

    Blue mussels are active suspension feeders generating currents by beating cilia and are therefore not entirely dependent on water flow to supply food (organic particulates and phytoplankton).  Therefore, they can survive in very sheltered areas, but water flow (due to tides, currents or wave action) can enhance the supply of food, carried from outside the area or resuspended into the water column. The evidence for Mytilus edulis sensitivity to changes in water flow was reviewed by Mainwaring et al., (2014).

    The growth rate of Mytilus edulis in relation to water flow was investigated by Langan & Howell (1994) who found that the growth rate over 24 days was 0.1, 1.8, 2.0, 1.9 and 1.5mm at flow rates of 0, 0.01, 0.02, 0.04 and 0.08 m/s respectively.  The only growth rate found to be significantly different was at zero flow.  However, the pattern did follow that predicted by the “inhalant pumping speed” hypothesis that suggested maximal growth at water speeds of about 0.02 m/s and decreased growth rates at higher and lower speeds (Langan & Howell 1994). Higher current speed brings food to the bottom layers of the water column, and hence near to the mussels, at a higher rate (Frechette et al., 1989). Frechette et al., (1989) developed a model based on measurements in the St. Lawrence River estuary (Québec).  The model suggested that Mytilus edulis consumption rate depends on the flow of water.

    Widdows et al., (2002) found that there was no change in filtration rate of Mytilus edulis between 0.05 and 0.8 m/s.  They noted that their finding contradicted earlier work that found a marked decline in filtration rates from 0.05 to 0.25 m/s (Newell, 1999; cited in Widdows et al., 2002) but suggested that the difference might be caused in differences in population studied, as the earlier work was based in the USA and their study used mussels from the Exe estuary in the UK.  Widdows et al., (2002) also noted that above 0.8 m/s the filtration rate declined mainly because the mussels became detached from the substratum in the experimental flume tank.  Widdows et al., (2002) noted that their results were consistant with field observations, as mussels show preferential settlement and growth in areas of high flow, such as the mouth of estuaries and at the base of power station cooling systems (Jenner et al., 1998).  They also reported that Jenner et al.,(1998; cited in Widdows et al., 2002) observed that biofouling of cooling water systems by mussels was only reduced significantly when mean current speeds reached 1.8-2.2 m/s and was absent at >2.9 m/s.

    Increased flow rate increases the risk of mussels being detached from the bed and transported elsewhere where their chance of survival will be significantly reduced due to the risk of predation and siltation (Dare, 1976).  It is the strength of the byssal attachment that determines the mussel’s ability to withstand increases in flow rate.  Flow rate itself has been shown to influence the strength and number of byssus threads that are produced by Mytilus edulis and other Mytilus spp. with mussels in areas of higher flow rate demonstrating stronger attachment (Dolmer & Svane, 1994; Alfaro, 2006).  Dolmer & Svane (1994) estimated the potential strength of attachment for Mytilus edulis in both still water and flows of 1.94 m/sec, by counting the number of established byssus threads and measuring the strength of attachment of individual detached byssus threads.  It was found that in still water the strength of the attachment was 21% of the potential strength whilst at 19.4 cm/sec it was 81 % of the potential strength, suggesting that Mytilus edulis has the ability to adapt the strength of its attachment based on flow rate.  The mussels were then able to withstand storm surges up to 16 m/s.  Young (1985) demonstrated that byssus thread production and attachment increased with increasing water agitation.  She observed the strengthening of byssal attachments by 25% within eight hours of a storm commencing and an ability to withstand surges up to 16 m/s.  However, it was concluded that sudden surges may leave the mussels susceptible to being swept away (Young, 1985) as they need time to react to the increased velocity to increase the attachment strength.  Mytilus edulis beds could, therefore, adapt to changes in water flow at the pressure benchmark. 

    Alfaro (2006) found that when a sudden increase in flow (to 0.13 m/s) was experienced by Perna canalicuulus (another mussel species) in areas of low flow rate they were more susceptible to detachment than those that had been exposed to a higher flow rate.  It was also noted that the individuals kept at higher water flows (e.g. 10 cm/sec) produced more byssus threads.  The increased energy used for byssus production in the high flow environments may reduce the energy that is available for other biological activities (Alfaro, 2006).

    Individuals attached to solid substrata (rock) are likely to display more resistance than individuals attached to boulders, cobbles or sediment.  For example, mussel reefs in the Wash, Morecambe Bay and the Wadden Sea are vulnerable to destruction by storms and tidal surges (Holt et al., 1998).  Widdows et al., (2002) examined mussel beds in the mouth of the Exe estuary and along the coast at Exmouth.  If flume tank studies between 0.1 and 0.35 m/s, the resuspension rate of sediment in mussel beds on sandy substrata was four and five times higher for areas with 25% and 50% mussel cover compared to bare sediment due to the increased turbulence and scouring around the mussels.  However, at high densities (100% cover) the beds remained stable (up to 0.35 m/s), with resuspension being about three times lower than areas with 0% cover, due to the high number of byssal attachments between individuals (Widdows et al., 2002). Where mussel beds occurred on pebble and sand substrata (mixed substrata) sediment erosion was lower than that of the 100% cover on the sandy substrata regardless of mussel density.  Low density mussel beds formed small clumps with a lower mass ratio of mussels attached to the substratum to increase anchorage.  In low density beds, increased scour resulted in some mussel detaching from the bed and in areas with 50% cover the erosion of the bed resulted in the burial of a large proportion of the mussels.  The mussels returned to the surface afterwards and recovered in 1-2 days.  Widdows et al., (2002) also noted a linear relationship between mussel beds density and sediment stability on cohesive mud substratum, taken from Cleethorpes, and exposed to currents of 0.15 to 0.45 m/s.  Again increased mussel cover increased sediment stability.  Widdows et al., (2002) found that the mussel bed at Exmouth experienced a peak flow of 0.9m/s before and after high water, which only reduced to 0.2 m/s at slack water. 

    Water flow also affects the settlement behaviour of larvae.  Alfaro (2005) observed that larvae settling in a low water flow environment are able to first settle and then detach and reattach displaying exploratory behaviour before finally settling and strengthening their byssus threads.  However, larvae settling in high flow environments did not display this exploratory behaviour.  Pernet et al., (2003) found that at high velocities, larvae of Mytilus spp. were not able to able to exercise much settlement preference.  It was thought that when contact with suitable substratum is made the larvae probably secure a firm attachment.  Movement of larvae from low shear velocities, where they use their foot to settle, to high shear velocities where they use their byssal thread to settle was observed by Dobretsov & Wahl (2008).

    Sensitivity assessment. This blue mussel bed biotope has been recorded (Connor et al., 2004) as occurring in areas with moderately strong tidal streams (0.5-1.5 m/s).  The sensitivity of sedimentary mussel bed biotopes to increased flow is dependent on the substratum and the degree of cover, with dense beds of ca 100% cover being more stable than patchy beds, and more stable on mixed substrata with cobble and boulders than sand and mud.  Connor et al., (2004) noted that the build-up of mussel mud beneath beds could result in a change to a muddier substrata underneath the bed, and reduce attachment resulting in increased risk of removal by storms.

    A decrease in water flow is unlikely to affect adversely blue mussel beds directly.  Evidence above suggest that they can grow at water flow as low as 0.01 – 0.02 m/s and filter at 0.05 m/s; significantly less than weak tidal streams (<0.5m/s).  At very low or negligible water flow, the effects of siltation may have adverse effects (see relevant pressure). 

    An increase in water flow at the benchmark level was considered likely by Mainwaring et al., (2014) to remove parts of the bed especially where mussel mud has accumulated and/or the beds are patchy;.  Therefore, resistance to change in water flow is assessed as ‘Medium’, resilience is assessed as ‘Medium’ and the biotopes is assessed as having ‘Medium’ sensitivity.  The more precautionary assessment is presented in the table.

    Not relevant (NR) Not relevant (NR) Not relevant (NR)
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    Changes in emergence are not relevant to this biotope (group) which is restricted to subtidal habitats.  Increased emergence at the sublittoral fringe would result in reclassification of the biotope to LS.LBR.LMus.Myt.

    Low Medium Medium
    Q: Low
    A: NR
    C: NR
    Q: High
    A: Low
    C: Medium
    Q: Low
    A: Low
    C: Low

    Blue mussel beds are found in a wide range of wave exposures, from extremely exposed areas to extremely sheltered (Seed 1976; Connor et al., 2004. This biotope (SS.SBR.SMus.MytSS) occurs in moderately wave exposed to sheltered conditions.

    Mytilus edulis are able to increase the strength of their attachment to the substratum in more turbulent conditions (Price, 1982; Young, 1985).  Young (1985) demonstrated an increase in strength of the byssal attachment by 25 % within 8 hours of a storm commencing.  When comparing mussels in areas of high flow rate and low flow rate those at a higher flow rate exhibit stronger attachments than those in the areas of lower flow (Dolmer & Svane, 1994; Alfaro, 2006).  Dolmer & Svane (1994) found that in still water the strength of the attachment was 21 % of the potential strength whilst at 1.94 m/sec it was 81 % of the potential strength.  The mussels were then able to withstand storm surges up to 16 m/s.  Alfaro (2006) also noted that the individuals kept at higher water flows produce more byssal threads.  The increased energy used for byssus production in the high flow environments may reduce the energy that is available for other biological activities (Alfaro 2006).  Whilst this clearly demonstrates the ability of mussels to adapt to the various conditions to avoid dislodgement, the mussels are unlikely to adapt instantly and a sudden increase in flow is likely to result in dislodgement (Young, 1985). 

    Widdows et al., (2002) examined mussel beds in the mouth of the Exe estuary and along the coast at Exmouth.  Where the mussel beds occurred on sandy substratum the re-suspension rate  was four and five times higher for areas with 25 % and 50 % mussel cover compared to bare sediment due to the increased turbulence and scouring around the mussels.  In low density beds this increased scour resulted in some mussel detaching from the bed and in areas with 50 % cover the erosion of the bed resulted in the burial of a large proportion of the mussels.  The mussels returned to the surface after 1-2 days and recovered.  However, at high densities (100% cover) the beds remained stable, with re-suspension being about 3 times lower than areas with  0% cover, due to the high number of byssal attachments between individuals (Widdows et al., 2002). Where mussel beds occurred on pebble and sand substratum (mixed substratum) sediment erosion was lower than that of the 100 % cover on the sandy substratum regardless of density despite experiencing flows of 0.9 m/s.  The low density mussels were observed to form small clumps with a lower mass ratio of mussels attached to the substratum to increase anchorage.

    Widdows et al., (2002) suggest that 100% mussel cover on sandy substrata reduces the risk of dislodgement.  However, Harger & Landenberger (1971) suggest that growth in mussel beds results in fewer mussels being attached to the substratum and therefore strong seas can “roll up the whole mass of mud and mussels like a carpet and break it to pieces on the foreshore”.  It was also noted that on gravelly substratum, single layer mussel beds incurred less damage in storm conditions than heavier multi-layered beds (Harger & Landenberger, 1971). 

    Large scale destruction of mussel beds has been reported in many areas such as the Wash, Morecambe Bay and the Wadden Sea (Holt et al., 1998) and it appears that because of this high wave exposure and destruction, reefs found in wave exposed areas are likely to be more dynamic (Nehls & Thiel, 1993).  Furthermore, increased wave exposure leads to a higher risk of damage from drift logs (or other flotsam), which once they have destroyed a patch of mussels leave the mussels around that patch at a higher risk of erosion (Seed & Suchanek, 1992).  Mussels with high abundance of epizoic and epiphytic (e.g. barnacles and macroalgae) growing on mussels are also more susceptible to removal in areas of high exposure due to increased drag caused by these fouling organisms (Suchanek, 1985; Seed & Suchanek, 1992).  However, mussel beds are prevalent in areas of high wave exposure suggesting a high resilience despite destruction.

    Blue mussels display a high resistance to increases in water flow, but the oscillatory water movement that occurs on shores of higher wave exposure is likely to have a higher impact due to the ‘to and fro’ motion which is more likely to weaken the attachments.  Westerbom & Jattu (2006) found that in subtidal mussel beds, mussel densities increased with increasing wave exposure.  The highest biomass was found in areas of intermediate exposure, potentially due to the larger mussels being removed at high wave exposure levels.  It was suggested that the lower densities found in more sheltered areas were due to low recruitment, early post-recruitment mortality, increased predation or stagnant settlement on rocks.  Furthermore, it was also noted that high sedimentation which is more prevalent in sheltered areas, as there is less energy for re-suspension, prevents colonisation and result in the death of small mussels that are living close to the sediment surface by smothering and the clogging up of their feeding apparatus (Westerbom & Jattu, 2006). Therefore, colonisation of new space in sheltered areas could be slow, particularly in areas where there is low availability of adult mussels.

    An increase in wave exposure may increase density in subtidal beds (Westerbom & Jattu, 2006), unless there is a very sudden storm surge.  Mussels on sedimentary substrata are exposed to a higher risk of dislodgement (Widdows et al., 2002). A decrease in wave exposure is likely to result in increased sedimentation and reduced densities (Westerbom & Jattu, 2006) although the risk of dislodgement will be greatly reduced creating more stable beds (Nehls & Thiel, 1993). 

    The above evidence is variable as different studies have examined beds that differ in habitat, wave exposure, substratum and mussel density.  However general trends can be seen.  In rocky habitats, increased wave exposure allows mussel to dominate and form beds, especially where the rock surface has a low slope.  Where the beds are patchy or damaged (from natural or human activities) they are more susceptible to further damage as a result of wave action or storms (Seed & Suchanek, 1992; Brosnan & Crumrine, 1994).  Multi-layered mussel beds are less susceptible to damage, especially where only the surface layer is removed.  It has been noted that the build-up of mussel mud (pseudofaeces) under the bed can reduce the attachment of the bed to the underlying substratum.  But in areas of wave exposure, the flow of water through the bed will probably prevent the ‘mussel mud’ accumulating.

    On sedimentary habitats, which themselves occur in wave sheltered environments, the mussel beds stabilise the sediment surface (Widdows et al., 2002), especially at high percentage cover, although at low cover (e.g. in patchy beds) turbulent flow caused by the mussels may increase erosion of the sediment.  Coarse and mixed sediments were more stable, although Widdows et al., (2002) also noted that cohesive muds were also stabilised by mussel beds.  Nevertheless, strong wave action or storms can roll up an entire bed or section of a bed (Harger & Landenberger, 1971), and presumably remove patches of mussels, and that multi-layered bed suffered more damage.  In sedimentary, wave sheltered habitats the build-up of mussel muds may reduce attachment to the substratum and increase the susceptibility of the bed to wave action (Seed & Suchanek, 1992).  The growth of other organisms on the mussels themselves, will increase drag and hence increase the possibility of damage due to wave action.  In sheltered conditions, large macroalgae (e.g. kelps, fucoids) growing on mussels may result in removal of clumps of mussels.

    Sensitivity assessment. .  In rocky habitats, a decrease in wave exposure will favour communities associated with lower wave exposure, and although the mussel bed will probably survive, increased fucoid cover may result in the slow loss of the bed.  However, this infralittoral rock biotope is found in tide-swept basin entrances or sea lochs where water flow from currents is more important that from wave exposure. A change at the pressure benchmark is not considered to negatively impact this biotope; resistance is assessed as 'High' and resilience as 'High' (by default) so that the biotope is considered to be 'not sensitive'. 

    Chemical Pressures

     ResistanceResilienceSensitivity
    Not relevant (NR) Not relevant (NR) Not sensitive
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    Not sensitive at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

    Contamination at levels greater than the benchmark may impact this biotope. The effects of contaminants on Mytilus sp. were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992). Widdows & Donkin (1992) list tolerances of Mytilus edulis adults and larvae but note that lethal responses give a false impression of high tolerance, since the adults can close their valves and isolate themselves from the environment for days. They suggested that sublethal effects e.g. shell growth and 'scope for growth' (SFG), are more sensitive indicators of the effects of contaminants. Reported effects of heavy metals follow.

    • Adult 15 day LC50 to 50µg/l Cu (Widdows & Donkin, 1992).
    • Crompton (1997) reported that adult bivalve mortalities occurred after 4-14 day exposure to 0.1-1 µg/l Hg, 1-10 µg/l Cu and Cd, 10-100 µg/l Zn but 1-10 mg/l for Pb and Ni.
    • Widdows et al. (1995) reported 'no observed effect thresholds' on feeding or SFG in Mytilus edulistissues of 150 µg Cd/g dry wt, 25 µg Cu/ g dry wt, (lethal at 60 µg Cu/g dry wt), 12 µg Hg/g dry wt, 10 mg Pb/g dry wt, and 300 µg Zn/g dry wt. However, the tissue concentration of heavy metals at the sites studied was not high enough to reduce SFG significantly.
    • Mussels were reported to be missing from an wider area than other shore organisms on a Cumbrian shore in the vicinity of a phosphate rich effluent outfall contaminated by a number of heavy metals (Holt et al., 1998).
    • Adults are ca >10 fold more intolerant than larvae to Cu, petroleum hydrocarbons and sewage sludge (Widdows & Donkin, 1992) (see larval sensitivity).

    Overall, Mytilus edulis is probably relatively tolerant of heavy metal contamination. But the potential mortality indicated above suggest an intolerance of intermediate. 
    Recovery may occur rapidly through good annual recruitment but examination of patches in beds of Mytilus sp. revealed that they may take many years to recover (see additional information below), depending on shore height, competition and environmental conditions. Repeated loss and recruitment results in a patchy distribution of mussels on the shore (Seed & Suchanek, 1992). Therefore, a recoverability of 'high' has been reported.

    Not relevant (NR) Not relevant (NR) Not sensitive
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    Not sensitive at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

    Contamination at levels greater than the benchmark may impact this biotope. Widdows & Donkin (1992) list tolerances of Mytilus edulis adults and larvae but note that lethal responses give a false impression of high tolerance, since the adults can close their valves and isolate themselves from the environment for days. They suggested that sublethal effects e.g. shell growth and 'scope for growth' (SFG), are more sensitive indicators of the effects of contaminants.

    • Widdows et al. (1995) demonstrated that toxic hydrocarbons, primarily poly-aromatic hydrocarbons, made a large contribution the decline in SFG observed along the North Sea coast. Hydrocarbons reduce clearance rate through 'non-specific narcosis'.
    • Mussel populations in Sullom Voe experienced moderate hydrocarbon pollution and a reduced SFG but had sufficient capacity to grow, reproduce and maintain a viable population (Widdows et al., 1987).
    • Widdows et al. (1987) examined the response of Mytilus edulis to high oil (water accommodated fraction of diesel oil) (125 ± 28 µg/l) and low oil (28 ±7 µg/l) over a 8 month period, and subsequent recovery. They observed a marked reduction in SFG (due to reduced feeding rate and food absorption efficiency), and a correlation between the reduction in SFG and the hydrocarbon tissue burden (Widdows et al., 1987; Widdows & Donkin, 1992; Widdows et al., 1995). Mussels exposed to high oil conditions showed a negative SFG and weight loss. During recovery, 22 days after removal to 'clean' sea water the high oil mussels depurated (removed) hydrocarbons more rapidly than low oil mussels, and showed an increased clearance rate and growth rate associated with 'catch-up' growth. Both high and low oil mussels recovered completely within 55 days.
    • Widdows et al. (1987) also reported that high and low oil contamination of the experimental basins resulted in 100% mortality amongst mussels kept in the basins from autumn 1982 to summer 1983 and from spring 1983 to summer 1984 respectively.
    • Widdows et al. (1992) reported the following tolerances of adult Mytilus edulis to hydrocarbons; a 4 day LC50 of 1-10 mg/l of crude oil, and a 4 month LC50 to 125 µg/l of diesel.
    • A sunflower oil tanker spill off the Anglesey coast resulted in ingestion of oil droplets and subsequent mortalities after spawning (Mudge et al., 1993; Holt et al., 1998).
    • Bokn et al., (1993) demonstrated that Mytilus edulis was lost from mesocosm experiments continuously dosed with 30.1 to 129.4 µg/l of the water accommodated fraction of diesel, and was the most intolerant of the intertidal species studied.
    • Mytilus edulis dominated jetty piles immediately adjacent to an oil refinery effluent in Milford Haven, suggesting a high tolerance of hydrocarbon contamination (K. Hiscock, pers. comm.).

    Overall, hydrocarbon tissue burden results in decreased SFG and in some circumstances may result in mortalities, reduced abundance or extent of Mytilus edulis

    Not relevant (NR) Not relevant (NR) Not sensitive
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    Not sensitive at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

    Contamination at levels greater than the benchmark may impact this biotope. The effects of contaminants on Mytilus sp. were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992). Mussels are suspension feeders and, therefore, process large volumes of water together with suspended particulates and phytoplankton. Mussels absorb contaminants directly from the water, through their diet and via suspended particulate matter (Widdows & Donkin, 1992), the exact pathway being dependant on the nature of the contaminant.

    • Widdows and Donkin (1992) reported 50% mortality from a tissue burden of 20 µg/g TBT.
    • Exposure of Mytilus edulis to detergent (BP1002) in seawater resulted in 100% mortality at 10 ppm detergent, although all survived at 5 ppm detergent (Smith, 1968).
    • Liu & Lee (1975) reported a LC50 of 250 µg/l of the herbicide trifluralin in Mytilus galloprovincialis
    • Mytilus edulis has been reported to bioaccumulate the insecticide ivermecten, although no adverse effects were observed (Cole et al., 1999).
    • Biphenyl (a dye carrier) reduced the feeding rate of Mytilus edulis by 50% at 0.3 mg/l (Donkin et al., 1989).
    • PCBs accumulate in gonads, although tissue concentrations are significantly reduced after spawning, although this may affect the next generation (Hummel et al., 1989; Holt et al., 1995).
    • Significant increases in the incidence of tumours (neoplasia) were reported in the US Mussel Watch programme in the presence of higher concentrations of combustion related poly-aromatic hydrocarbons,cis-chlordane pesticides and cadmium (Hillman, 1993; Holt et al., 1998).
    • Mytilus edulis survived in a power station cooling water culvert, exposed to 0.1-0.2 mg/l hypochlorite, although their growth rates were reduced by about a third. Mussels were able to recover in hypochlorite free periods between chlorination dosing (Thompson et al., 1997). Mytilus edulis and Mytilus galloprovincialis were reported to suffer 100% mortality after 15-135 days continuous exposure to 0.2-1.0 mg/l hypochlorite (Khalanski & Borget, 1980; cited in Thompson et al., 1997).
    • Holt et al. (1995) also report that mussels may be absent from areas of high boating activity, presumably due to TBT.

    Widdows et al. (1995) compared 'scope for growth' (SFG) and chemical contaminants in tissues of mussels from 26 coastal and 9 offshore sites around the United Kingdom. They noted that polar organics (probably derived from phytoplankton) accounted for some reduction in SFG, while organo-chlorides showed a significant correlation with an unexplained component of the decline in SFG. However, TBT levels were only high enough to cause an effect (<10% reduction in SFG) at 8 study sites (Widdows et al., 1995). Mytilus edulis is probably relatively tolerant of contaminants.  Widdows & Donkin (1992) list tolerances of Mytilus edulis adults and larvae (but note that lethal responses give a false impression of high tolerance, since the adults can close their valves and isolate themselves from the environment for days. They suggest that sublethal effects (shell growth and 'scope for growth') are more sensitive indicators of the effects of contaminants. Also, adults are ca. 4 times more sensitive than larvae to TBT (Widdows & Donkin, 1992, see larval sensitivity).

    No evidence (NEv) No Evidence (NEv) No evidence (NEv)
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    The periostracum of Mytilus edulis was reported to concentrate uranium (Widdows & Donkin, 1992). Mussels have also been reported to bioaccumulate 106Ru, 95Zr, 95Nb, 137Cs and 90Sr (Cole et al., 1999). While the above data demonstrates that Mytilus edulis can accumulate radionucleides, little information concerning the effects of radionucleides on marine organisms was found. Sensitivity to this pressure is therefore not assessed based on lack of evidence. 

    Not relevant (NR) Not relevant (NR) Not sensitive
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    Not sensitive at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

    High High Not sensitive
    Q: High
    A: High
    C: High
    Q: High
    A: High
    C: High
    Q: High
    A: High
    C: High

    Mytilus edulis is regarded as euryoxic, tolerant of a wide range of oxygen concentrations including zero (Zandee et al., 1986; Wang & Widdows 1991; Gosling, 1992; Zwaan de & Mathieu 1992; Diaz & Rosenberg 1995; Gray et al., 2002).  Diaz & Rosenberg (1995) suggest it is resistant to severe hypoxia.   Adult mytilids exhibited high tolerance of anoxia in laboratory tests, e.g. Theede et al., (1969) reported LD50of 35 days for Mytilus edulis exposed to 0.21 mg/l O2 at 10 °C, which was reduced to 25 days with the addition of sulphide (50 mg/l Na2S.9H2O).  Jorgensen (1980) observed, by diving, the effects of hypoxia (0.2 -1 mg/l) on benthic macrofauna in marine areas in Sweden over a 3-4 week period.  Mussels were observed to close their shell valves in response to hypoxia and survived for 1-2 weeks before dying (Cole et al., 1999; Jorgensen, 1980). 

    All life stages show high levels of tolerance to low oxygen levels.  Mytilus edulis larvae, for example, are tolerant down to 1.0 ml/l, and although the growth of late stage larvae is depressed in hypoxic condition, the settlement behaviour does not seem to be affected (Diaz & Rosenberg, 1995).  Based on the available evidence Mytilus edulis are considered to be resistant to periods of hypoxia and anoxia although sub-lethal effects on feeding and growth may be expected.

    Sensitivity assessment Mytilus edulis is considered to be ‘Not Sensitive’ to de-oxygenation at the pressure benchmark. Resistance is therefore assessed as ‘High’ and resilience as ‘High’ (no effect to recover from), resulting in a sensitivity of 'Not sensitive'.  

    High High Not sensitive
    Q: High
    A: High
    C: High
    Q: High
    A: High
    C: High
    Q: High
    A: High
    C: High

    This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations.  The benchmark is set at compliance with WFD criteria for good status, based on nitrogen concentration (UKTAG, 2014).  

    Nutrient enrichment may impact mussel beds by altering the biomass of phytoplankton and macroalgae.  At low levels, nutrient enrichment may stimulate the growth of phytoplankton used as food - a potential beneficial effect.  In the Wadden Sea, where fishing had caused the destruction of the local population of Sabellaria spinulosaMytilus edulis was able to colonise, partly because of the increase in coastal eutrophication (Maddock, 2008).  Conversely, Dinesen et al. (2011) observed that a reduction in nutrient loading to comply with the WFD resulted in a decrease of mussel biomass in estuaries.

    High levels of enrichment may stimulate algal blooms and macroalgal growth.  The growth of macrophytes on the mussel beds may result in increased drag on the mussel bed and hence increase susceptibility to damage from wave action and/or storms (see changes in wave exposure pressure).  Algal blooms may die off suddenly, causing de-oxygenation (see de-oxygenation pressure) where the algae decompose on the seabed.  The thresholds at which these blooms occur depend on site-specific conditions and be mitigated by the degree of mixing and tidal exchange. Some algae have been shown to negatively affect Mytilus edulis when present in high concentrations.  For example, blooms of the algae Phaeocystis sp., have been observed to block the mussels gills when present in high concentrations reducing clearing rates, and at high levels they caused a complete cessation of clearance (Smaal & Twisk, 1997).  Blockage of the gills is also likely to reduce ingestion rates, prevent growth and cause reproductive failure (Holt et al., 1998).  Other species known to negatively impact Mytilus edulis are Gyrodinium aureolum (Tangen, 1977; Widdows et al., 1979b) and non-flagellated chrysophycean alga (Tracey, 1988). The accumulation of toxins from algal blooms has also been linked to out-breaks of paralytic shellfish poisoning resulting in the closure of shell fish beds (Shumway, 1990).

    Sensitivity assessmentMytilus edulis beds are considered to be not sensitive to nutrient enrichment at levels that comply with the requirements for good status for transitional and coastal water bodies (UKTAG, 2014).  Resistance is therefore assessed as ‘High’ and resilience as ‘High’ (no effect to recover from) resulting in a sensitivity of 'Not sensitive'.   

    High High Not sensitive
    Q: High
    A: High
    C: High
    Q: High
    A: High
    C: High
    Q: High
    A: High
    C: High

    Organic enrichment can result from inputs of additional organic matter.  Organic enrichment may lead to eutrophication with adverse environmental effects including deoxygenation, algal blooms and changes in community structure (see nutrient enrichment and de-oxygenation). 

    It has been shown that regardless of the concentration of organic matter Mytilus edulis will maintain its feeding rate by compensating with changes to filtration rate, clearance rates, production of pseudofaeces and absorption efficiencies (Tracey, 1988; Bayne et al., 1993; Hawkins et al., 1996).  A number of studies have highlighted the ability of Mytilus edulis to utilise the increased volume of organic material available at locations around salmon farms.  Reid et al., (2010) noted that Mytilus edulis could absorb organic waste products from a salmon farm with great efficiency.  Increased shell length, wet meat weight, and condition index were shown at locations within 200m from a farm in the Bay of Fundy allowing a reduced time to market (Lander et al., 2012).

    Mytilus edulis have also been recorded in areas around sewage outflows (Akaishi et al., 2007; Lindahl & Kollberg, 2008; Nenonen et al., 2008; Giltrap et al., 2013) suggesting that they are highly tolerant of the increase in organic material that would occur in these areas.

    It should be noted that biotopes occurring in tide swept or wave exposed areas are less likely to experience the effects of organic enrichment as the organic matter will be rapidly removed.

    Sensitivity assessment. Based on the observation of Mytilus edulis thriving in areas of increased organic matter (Lander et al., 2012, Reid et al., 2010), it was assumed that Mytilus edulis beds had a ’High’ resistance to increased organic matter at the pressure benchmark.  Resilience is therefore assessed as ‘High’ (no effect to recover from).  

    Physical Pressures

     ResistanceResilienceSensitivity
    None Very Low High
    Q: High
    A: High
    C: High
    Q: High
    A: High
    C: High
    Q: High
    A: High
    C: High

    All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is therefore ‘High’.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.  Adjacent habitats and species populations may be indirectly affected where meta-population dynamics and trophic networks are disrupted and where the flow of resources e.g. sediments, prey items, loss of nursery habitat etc. is altered.

    None Low High
    Q: Medium
    A: NR
    C: NR
    Q: High
    A: Low
    C: Medium
    Q: Medium
    A: Low
    C: Low

    Mytilus edulis can be found on a wide range of substrata including artificial substratum (e.g. metal, wood, concrete), bedrock, biogenic reef, caves, crevices / fissures, large to very large boulders, mixed, muddy gravel, muddy sand, rock pools, sandy mud, small boulders, under boulders (Connor et al., 2004). An increase in the availability of hard substratum may be beneficial in areas where sedimentary habitats were previously unsuitable for colonisation e.g. coarse, mobile sediments.  It should be noted that differences in diversity and other structural characteristics of assemblages between natural and artificial substratum have been observed suggesting that there is not a direct, compensatory effect. 

     Mussels themselves will often cause a change in substrata by the deposition of large quantities of ‘mussel-mud’ composed of faecal matter and pseudofaeces (Dare, 1976) particularly in areas of low water movement.  A change from rock to sand in an area of high water flow would increase the mussels’ vulnerability to dislodgement and scour (Widdows et al., 2002) and potentially smothering as sand smothering has been show to set the lower limit of Mytilus beds in some areas (Daly & Mathieson 1977). 

    Sensitivity assessment.  A change in substratum type would not necessarily reduce habitat quality for individual Mytilus edulis which can colonise a wide range of hard and sedimentary habitats. However a change in substratum type would alter biotope classification. Hence, resistance of the biotope is assessed as ‘None’ (loss of >75% of extent), resilience (following habitat recovery) is assessed as ‘Low’ (2-10 years).  Sensitivity, based on combined resistance and resilience is assessed as ‘High’. The more precautionary assessment for the biotope, rather than the species, is presented in the table as it is considered that any change to a reef habitat from a sedimentary habitat would alter the biotope classification and hence the more sensitive assessment is appropriate.

    High High Not sensitive
    Q: Medium
    A: NR
    C: NR
    Q: High
    A: High
    C: High
    Q: Medium
    A: Low
    C: Low

    Mytilus edulis can be found on a wide range of sediment substrata including, mixed, muddy gravel, muddy sand, rock pools, sandy mud, small boulders, under boulders (Connor et al., 2004). It should be noted that the mussels themselves will often cause a change in substrata by the deposition of large quantities of ‘mussel-mud’ composed of faecal matter and pseudofaeces (Dare, 1976) particularly in areas of low water movement.  A change from rock to sand in an area of high water flow would increase the mussels’ vulnerability to dislodgement and scour (Widdows et al., 2002) and potentially smothering as sand smothering has been show to set the lower limit of Mytilus beds in some areas (Daly & Mathieson, 1977). 

    Sensitivity assessment The pressure benchmark refers to the simplified Folk classification developed by Long (2006) and the UK Marine Habitat Classification Littoral and Sublittoral Sediment Matrices (Connor et al., 2004).   In most instances the pathway (human activity) by which the substratum is changed would remove or smother mussels.  However, these effects are addressed under the ‘hydrological change’ , ‘abrasion’, ‘penetration and disturbance’ and ‘smothering’ pressures above.  The natural modification of the sediment due to build-up of mussel-mud could result in change of sediment type from mixed and sand dominated, to anoxic mud, depending on location and hydrography. 

    In addition, a change in sediment type would also change the biotope definition.  That is, a change from mud to sand would change the biotope from LS.LBR.LMus.Myt.Mu to LS.LBR.LMus.Myt.Sa.  Technically this change could be viewed as loss of the biotope and, hence, high sensitivity.  However, this has not been taken into account in the sensitivity assessment, as long as a blue mussel bed biotope remains.  

    The biotope LS.LBR.LMus.Myt encompasses three variant biotopes found on mud, sand and mixed sediment.  These biotopes encompass the full variety of sediments ranging from mud and sand to pebble, cobbles and medium boulders; although gravel is not mentioned (Connor et al., 2004).  The higher level biotope is therefore considered to have ‘High’ resistance and subsequently ‘High recovery and ‘Not sensitive to a change in sediment type of 1 Folk class as a change between mixed sediments, mud and sandy mud and sand and muddy sand would not adversely affect the biotopes.  The rationale for that assessment is also applied to this biotope SS.SBR.SMus.MytSS, which Connor et al., (2004) describe as the sublittoral extension of the littoral biotope.

    None Low High
    Q: High
    A: High
    C: High
    Q: High
    A: Low
    C: Medium
    Q: High
    A: Low
    C: Medium

    The process of extraction will remove the entire mussel bed and the associated community; therefore a resistance of ‘None’ is recorded.  As a result, resilience is assessed as ‘Low’, and sensitivity as ‘High’.  

    Low Medium Medium
    Q: High
    A: Medium
    C: Medium
    Q: High
    A: Low
    C: Medium
    Q: High
    A: Low
    C: Medium

    Mytilus edulis lives on the surface of the seabed held by byssus threads attached to either the substratum or to other mussels in the bed.  Activities resulting in abrasion and disturbance can either directly affect the mussel by crushing them, or indirectly affect them by the weakening or breaking of their byssus threads making them vulnerable to displacement (Denny, 1987) where they are unlikely to survive (Dare, 1976).  In addition, abrasion and sub-surface damage may attract mobile scavengers and predators including fish, crabs, and starfish to feed on exposed, dead and damaged individuals and discards (Kaiser & Spencer, 1994; Ramsay et al., 1998; Groenewold & Fonds, 2000; Bergmann et al., 2002).  This effect will increase predation pressure on surviving damaged and intact Mytilus edulis.  A number of activities or events that result in abrasion and disturbance and their impacts on mussel beds are described below, based on the review by Mainwaring et al. (2014). 

    The effects of trampling have been more widely studied in the terrestrial community showing that when areas are intensively trampled bare patches are likely to result as a result of erosion or equally soil compaction may result (Liddle, 1997).  There are a number of studies which have focused on the impact of trampling on the intertidal rocky shore whereas the impact on sedimentary shores is relatively poorly studied (Tyler-Walters & Arnold, 2008).  In general, studies have found that trampling is an additional disturbance to the natural disturbances that the intertidal organisms are adapted to tolerate. Large declines of the Mytilus californianus from mussel beds due to trampling have been reported (Brosnan, 1993; Brosnan & Crumrine, 1994; Smith & Murray, 2005).  Brosnan & Crumrine (1994) recorded the loss of 54% of mussels from a single experimental plot on one day.  Mussels continued to be lost throughout the experimental period, forming empty patches larger than the experimental plots.  The empty patches continued to expand after trampling had ceased, due to wave action.  At another site, the mussel bed was composed of two layers, so that while mussels were lost, cover remained.  Brosnan (1993) also reported a 40 % loss of mussels from mussel beds after three months of trampling, and a 50 % loss within a year.  Van de Werfhorst & Pearse (2007) examined M. californianus abundance at sites with differing levels of trampling disturbance.  The highest percentage of mussel cover was found at the undisturbed site while the severely disturbed site showed low mussel cover. 

    Smith & Murray (2005) examined the effects of low level disturbance on an extensive bed of Mytilus californianus (composed of a single layer of mussels) in southern California.  Smith & Murray (2005) reported that in experimental plots exposed to trampling, mussel loss was 20-40 % greater than in untreated plots.  A decrease in mussel mass, density, cover and maximum shell length where recorded even in low intensity trampling events (429 steps/m2).  However, only 15 % of mussel loss was as a direct result of trampling, with the remaining loss occurring during intervals between treatment applications. Brosnan & Crumrine (1994) suggested that trampling destabilizes the mussel bed, making it more susceptible to wave action, especially in winter.  Smith & Murray (2005) suggested that an indirect effect of trampling was weakening of byssal threads, which increases mussel susceptibility to wave disturbance (Denny, 1987).  Brosnan & Crumrine (1994) observed recruitment within experimental plots did not occur until after trampling had ceased, and no recovery had occurred within 2 years 

    Brosnan and Crumrine (1994) noted that mussels that occupied hard substrata but did not form beds were also adversely affected.  Although only at low abundance (2.5 % cover), all mussels were removed by trampling within 4 months.  Brosnan & Crumrine (1994) noted that mussels were not common and confined to crevices in heavily trampled sites.  Similarly, the mussel bed infauna (e.g. barnacles) was adversely affected, and were crushed or lost with the mussels to which they were attached.  However, Beauchamp & Gowing (1982) did not observe any differences in mussel density between sites that differed in visitor use.

    Paine & Levine (1981) examined natural patch dynamics in a Mytilus californianus bed in the USA.  They suggested that it may take up to seven years for large barren patches to recover.  However, chronic trampling may prevent recovery altogether.  This would result in a shift from a mussel dominated habitat to one dominated by an algal turf or crust (Brosnan & Cumrine, 1994), completely changing the biotope.  However, a small period of trampling could allow communities to recover at a similar rate to that of natural disturbance as the effects are similar.  The associated epifauna and epiflora suffer the greatest amount of damage as they are the first organisms that a foot makes contact with (Brosnan & Crumrine, 1994).  The loss of epifauna and epiflora could initially be of benefit to the mussel bed, despite the obvious decrease in species diversity, as there will be a decrease in drag for the mussels reducing the risk of dislodgement (Witman & Suchanek 1984) and freeing up more energy for growth and reproduction.  However, it is likely that after continued trampling this effect will be minimal compared with the increased risk of dislodgement caused by trampling. No studies assessing the effect of trampling on mussels on intertidal muddy sand or sediments were found.  Losses to the adult mussels by crushing or by suffocation where these are forced into the sediment are expected.  There is the potential that this will open up areas for new recruitment or it may just create a similar situation to that seen on the rocky shore where wave damage and continual trampling prevent settlement and recovery.

    Collision of objects with the bed, such as wave driven logs (or similar flotsam), is known to cause removal of patches of mussels from mussel beds (Seed & Suchanek, 1992; Holt et al., 1998).  When patches occur in mussel beds a good recruitment could result in a rapid recovery or the patch may increase in size through weakening of the byssus threads of the remaining mussels leaving them vulnerable to erosion from storm damage (Denny, 1987). Damage in areas of high wave exposure is likely to result in increased erosion and a patchy distribution although recruitment may be high.  In sheltered areas damage may take a lot longer due to limited larval supply, although the frequency of destruction through wave driven logs would be less than in high wave exposure.  Similar effects could be observed through the grounding of a vessel, the dropping of an anchor or the laying of a cable, although the scale of damage clearly differs. Shifting sand is known to limit the range of Mytilus edulis through burial and abrasion (Daly & Mathieson, 1977).

    Various fishing methods also result in abrasion of the mussel beds.  Bait collection through raking will cause surface abrasion and the removal of patches of mussel resulting in the damage and recovery times described above.  Holt et al., (1998) reported that hand collection, or using simple hand tools occurs in small artisanal fisheries.  They suggested that moderate levels of collection by experienced fishermen may not adversely affect the biodiversity of the bed.  But they also noted that even artisanal hand fisheries can deplete the mussel biomass on accessible beds in the absence of adequate recruitment of mussels. Smith & Murray (2005) observed a significant decrease in mussel mass (g/m2), density (no./m2), percentage cover and mean shell length due to low-intensity simulated bait-removal treatments (2 mussels / month) for 12 months (Smith & Murray, 2005).  They also stated that the initial effects of removal were ‘overshadowed’ by loss of additional mussels during time periods between treatments, probably due to the indirect effect of weakening of byssal threads attachments between the mussel leaving them more susceptible to wave action (Smith & Murray, 2005).  The low-intensity simulated bait-removal treatments had reduced percentage cover by 57.5 % at the end of the 12 month experimental period.  Smith & Murray (2005) suggested that the losses incurred from collection and trampling are far greater than those that occur by natural causes.  This conclusion was reached due to significant results being displayed for human impact despite the experiment taking place during a time of high natural disturbance from El Niño–Southern Oscillation (ENSO).

    Sensitivity assessment Based on the available evidence it is concluded that all mussel biotopes are sensitive to abrasion and that resistance is ‘Low’ (loss of 25-75% of bed within direct impact footprint),  resilience is assessed as ’Medium’, resulting in a sensitivity of ‘Medium’

    Low Medium Medium
    Q: High
    A: High
    C: Medium
    Q: High
    A: Low
    C: Medium
    Q: High
    A: Low
    C: Medium

    Mytilus edulis lives on the surface of the seabed held in one place by byssus threads that either attach to the substratum or to other mussels in the bed.  Activities resulting in penetration and disturbance can either directly affect the mussel by crushing or removal, or indirectly affect them by the weakening or breaking of their byssus threads making them vulnerable to displacement (Denny, 1987) where they are unlikely to survive (Dare, 1976). Where mussels are removed the associated fauna and flora will also be removed.  In addition, abrasion and sub-surface damage attracts mobile scavengers and predators including fish, crabs, and starfish to feed on exposed, dead and damaged individuals and discards (Kaiser & Spencer, 1994; Ramsay et al., 1998; Groenewold & Fonds, 2000; Bergmann et al., 2002).  This effect could increase predation pressure on surviving damaged and intact Mytilus edulis

    Mussel dredging is the main form of activity that results in penetration around mussel beds.  Holt et al., (1998) noted that several thousand tonnes of mussels were fished in the Wash by dredgers in good years.  Dredging will remove the substratum along with the mussels and their associated flora and fauna.  Temporary re-suspension of sediment also occurs with mussel dredging (Holt et al., 1998) in volumes of 1470 g/m2 (Riemann & Hoffmann, 1991), which could potentially result in localised smothering.   Dredging is also likely to increase the vulnerability of the remaining mussels to storm damage through the weakening of byssal attachment and creating patches in the bed (Denny, 1987).

    The Scottish MPA Project Fisheries Management Guidance (JNCC, 2013a) suggests that scallop dredges and other demersal towed gear is also likely to result in the removal of a proportion of the bed along with its associated fauna and flora.  The same report suggested that potting and other demersal static gear would have a lower impact than mobile gear.  There is no evidence for the impacts of hydraulic dredging on mussels but Hall et al., (1990) observed that when using hydraulic dredging for Ensis sp. the immediate affects were a reduction in the number of target species and many macrofaunal species.  However, after 40 days the effect of the fishing gear could no longer be seen. 

    Holt et al., (1998) noted that natural ‘wild’ beds are susceptible over-exploitation, especially in some embayments and that over-exploitation can reduce recruitment.  Holt et al., (1998) also point out that the source areas for recruitment to beds is unknown and the relationship between stock and recruitment poorly understood.  This statement is consistent with the sporadic and unpredictable nature of recruitment and recovery in mussels beds (Seed & Suchanek, 1992).

    Sensitivity assessment. The activities that penetrate the seabed could result in removal of part of a bed and its associated fauna and flora.  Therefore, based on the available evidence it is concluded that all sedimentary mussel biotopes are sensitive to ‘penetration and/or disturbance of the seabed’. Therefore, resistance is assessed as ‘Low’ (loss of 25-75% of bed within direct footprint), resilience is assessed as ’Medium’, and sensitivity as ‘Medium’.  However the infralittoral rock biotope is unlikely to be affected by penetrative gear or activities, by definition, and is probably ‘Not exposed’ but is susceptible to 'abrasion'.  

    High High Not sensitive
    Q: High
    A: High
    C: Medium
    Q: High
    A: High
    C: High
    Q: High
    A: High
    C: Medium

    Mytilus edulis does not rely on light penetration for photosynthesis.  In addition visual perception is limited and the species does not rely on sight to locate food or other resources.  An indirect effect of increased turbidity and reduced light penetration may be reduced phytoplankton productivity which could reduce the food availability for Mytilus edulis.  However, as Mytilus edulis uses a variety of food sources and food is brought in from other areas with currents and tides, the effect is likely to be minimal.  This species and the biotopes it forms are therefore not sensitive to changes in water clarity that refer to light penetration.

    Mytilus edulis are often found in areas with high levels of turbidity.  For example, the average suspended particulate matter (SPM) concentration at Hastings Shingle Bank was 15 -20 mg/l in June 2005, reaching 50 mg/l in windier (force 4) conditions, although a concentration of 200 mg/l was recorded at this site during gales (Last et al., 2011).

    Winter (1972, cited by Moore, 1977) recorded 75 % mortality of Mytilus edulis in concentrations of 1.84-7.36 mg/l when food was also available.  However, a relatively small increase in SPM concentration e.g. from 10 mg/l to 90 mg/l was found to increase growth rates (Hawkins et al., 1996).  Concentrations above 250 mg/l have been shown to impair the growth of filter-feeding organisms (Essink, 1999).  But Purchon (1937) found that concentrations of particulates as high a 440 mg/l did not affect Mytilus edulis and that mortality only occurred when mud was added to the experiment bringing the concentrations up to 1220 mg/l.  The reason for some of the discrepancy between studies may be due to the volume of water used in the experiment.  Loosanoff (1962) found that in small quantities of turbid water (due to particulates) the mussel can filter out all of the particulates within a few minutes whereas in volumes >50 gallons per individual the mussel becomes exhausted before the turbidity has been significantly lowered, causing it to close its shell and die.

    It may be possible for Mytilus edulis to adapt to a permanent increase in SPM by decreasing their gill size and increasing their palp size in areas of high turbidity (Theisen, 1982; Essink, 1999).  In areas of variable SPM it is likely that the gill size would remain the same but the palp would adapt (Essink, 1999).  Whilst the ability to adapt may prevent immediate declines in health, the energetic costs of these adaptations may result in reduced fitness; the extent of which is still to be established.

    Mytilus edulis uses the circadian clock to determine the opening of the shell gape in nocturnal gape cycles (Ameyaw-Akumfi & Naylor, 1987).  Last et al., (2011) investigated the effects on increased SPM concentrations on both the gape pattern and mortality in order to establish the effect that aggregate dredging will have on Mytilus edulis and other benthic invertebrates.  Therefore they tested concentrations similar to those expected within a few hundred meters of an aggregate extraction site.  The highest concentration tested using a pVORT (paddle VOrtex Resuspension Tanks) was ~71 mg/l.  They showed that there is a significant reduction of the strength of the nocturnal gape cycle at high suspended sediment loads as well as a change in the gape period.  The effects of these changes are not fully known but as it is likely that the gape pattern is a strategy to avoid diurnal predators the change may result in an increased risk of predation.  On the other hand the increased turbidity may reduce predation from visual predators such as fish and birds (Essink, 1999).  After continued measurements of the gape cycle for 4 days post treatment, Last et al., (2011) observed that the cycle took longer than this to recover from the cycle disruption.  Further study is required to determine the length of time required for recovery of this behavioural response (Last et al., 2011).

    Based on a comprehensive literature review, Moore (1977) concluded that Mytilus edulis displayed a higher tolerance to high SPM concentrations than many other bivalves although the upper limit of this tolerance was not certain.  He also hypothesised that the ability of the mussel to clean its shell in such conditions played a vital role in its success along with its pseudofaecal expulsion.

    A reduction in SPM concentrations may be caused by the erecting of dams and hydroelectric power stations (Moore, 1977), which could leave subtidal mussel beds more vulnerable to visual predators such as birds and fish.  The recovery time from increased predation pressures would depend on the duration of the reduced turbidity.  If reduced SPM concentration is also linked with a reduction of suspended organic matter then it could be assumed that the mussel fitness would be negatively affected by a reduction in food supply.  However, as active filter feeders they are no dependent on water flow to supply food.

    Sensitivity assessment. Evidence indicates that Mytilus edulis and hence blue mussel beds can tolerate a broad range of suspended solids.  The benchmark for this pressure refers to a change in turbidity of one rank on the Water Framework Directive (WFD) scale.  Mussel beds form in relatively clear waters of open coasts and wave exposed shores and on sediments in sheltered coast (where turbulent water flow over the mussel beds could resuspend sediments locally) and in turbid bays and estuaries.  Therefore, is unlikely that a change in turbidity by of one rank (e.g. from 300 to 100 mg/l or <10 to 100 mg/l) will significantly affect the mussel bed.   Resistance to this pressure is therefore assessed as ‘High.  Recovery is assessed ‘High’ (no impact to recover from), and sensitivity is therefore 'Not sensitive'.  

    Medium Medium Medium
    Q: High
    A: High
    C: Medium
    Q: High
    A: Low
    C: Medium
    Q: High
    A: Low
    C: Medium

    The main human activity that increases sedimentation is dredging and the dumping of dredged sediments in estuarine and coastal waters.  Aggregate dredging and fishing gear can cause localised sedimentation and smothering.  However, changes in water flow can cause localised smothering within mussel beds (Widdows et al., 2002), and storms may move large volumes of sediment and smother entire mussel beds (Dare, 1976). Mytilus edulis occurs in areas of high suspended particulate matter (SPM) and therefore a level of siltation is expected from the settling of SPM.  In addition, the high rate of faecal and pseudofaecal matter production by the mussels naturally results in siltation of the seabed, often resulting in the formation of large mounds beneath the mussel bed.  For example, at Morecambe Bay an accumulation of mussel-mud (faeces, pseudofaeces and washed sand) of 0.4-0.5m between May 1968 and September 1971 resulted in the mortality of young mussels (Daly & Mathieson, 1977).  In order to survive the mussels needed to keep moving upwards to stay on the surface.  Many individuals did not make it to the surface and were smothered by the accumulation of mussel-mud (Daly & Mathieson, 1977), so that whilst Mytilus edulis does have the capacity to vertically migrate through sediment some individuals will not survive.  

    Sand burial has been shown to determine the lower limit of Mytilus edulis beds (Daly & Mathieson, 1977a).  Burial of Mytilus edulis beds by large scale movements of sand, and resultant mortalities have been reported from Morecambe Bay, the Cumbrian coast and Solway Firth (Holt et al., 1998).  Essink (1999) recorded fatal burial depths of 1-2 cm for Mytilus edulis and suggested that they had a low tolerance of sedimentation based on investigations by R.Bijkerk (cited by Essink, 1999).  Essink (1999) suggested that deposition of sediment (mud or sand) on shallow mussel beds should be avoided.  However, Widdows et al. (2002) noted that mussels buried by 6 cm of sandy sediment (caused by resuspension of sediment due to turbulent flow across the bed) were able to move to the surface within one day.  Conversely, Condie (2009) (cited by Last et al., 2011) reported that Mytilus edulis was tolerant of repeated burial events. 

    Last et al., (2011) carried out burial experiments on Mytilus edulis in pVORTs.  They used a range of burial depths and sediment fractions and temperatures.  It was found that individual mussels were able to survive burial in depths of 2, 5 and 7 cm for over 32 days although the deeper and longer the mussels were buried the higher the mortality.  Only 16 % of buried mussels died after 16 days compared to almost 50 % mortality at 32 days.  Mortality also increased sharply with a decrease in particle size and with increases in temperature from 8.0 and 14.5 to 20 °C.  The ability of a proportion of individuals to emerge from burial was again demonstrated with approximately one quarter of the individuals buried at 2cm resurfacing.  However, at depths of 5 cm and 7 cm no emergence was recorded (Last et al., 2011).  The lower mortality when buried in coarse sands may be related to the greater number of individuals who were able to emerge in these conditions and emergence was to be significant for survival. 

    It is unclear whether the same results would be recorded when mussels are joined by byssal threads or whether this would have an impact on survival (Last et al., 2011), although Daly & Mathieson (1977) recorded loose attachments between juvenile mussels during a burial event and some of these were able to surface.  It was not clear whether the same ability would be shown by adult mussels in a more densely packed bed.

    Sensitivity assessment. Overburden by 5 cm of fine material (see benchmark) in a single incident is unlikely to result in significant mortality in blue mussel beds before sediments are removed by current and wave action.  However, the inability of Mytilus edulis to emerge from sediment deeper than 2 cm (Last et al., 2011, Essink, 1999, Daly & Matthieson, 1977) and the increased mussel mortality with depth and reduced particle size observed by Last et al. (2011) suggest that there may be some mortality and resistance is assessed as 'Medium'. Survival will be higher in winter months when temperatures are lower and physiological demands are decreased.  However, mortality will depend on the duration of smothering. Mortality is likely to be more significant in wave sheltered areas, devoid of tidal streams, where the smothering sediment remains for prolonged periods.  Mortality will be more limited, and possibly avoided, where the smothering sediment is removed due to wave action or tidal streams, depending on how long the sediment remains over the individual mussels.  Resilience is assessed as 'Medium' and sensitivity is therefore 'Medium'. 

    Medium Medium Medium
    Q: High
    A: High
    C: Medium
    Q: High
    A: Low
    C: Medium
    Q: High
    A: Low
    C: Medium

    The main human activity that increases sedimentation is dredging and the dumping of dredged sediments in estuarine and coastal waters.  Aggregate dredging and fishing gear can cause localised sedimentation and smothering.  However, changes in water flow can cause localised smothering within mussel beds (Widdows et al., 2002), and storms may move large volumes of sediment and smother entire mussel beds (Dare, 1976). Mytilus edulis occurs in areas of high suspended particulate matter (SPM) and therefore a level of siltation is expected from the settling of SPM.  In addition, the high rate of faecal and pseudofaecal matter production by the mussels naturally results in siltation of the seabed, often resulting in the formation of large mounds beneath the mussel bed.  For example, at Morecambe Bay an accumulation of mussel-mud (faeces, pseudofaeces and washed sand) of 0.4-0.5m between May 1968 and September 1971 resulted in the mortality of young mussels (Daly & Mathieson, 1977).  In order to survive the mussels needed to keep moving upwards to stay on the surface.  Many individuals did not make it to the surface and were smothered by the accumulation of mussel-mud (Daly & Mathieson, 1977), so that whilst Mytilus edulis does have the capacity to vertically migrate through sediment some individuals will not survive.  

    Sand burial has been shown to determine the lower limit of Mytilus edulis beds (Daly & Mathieson, 1977a).  Burial of Mytilus edulis beds by large scale movements of sand, and resultant mortalities have been reported from Morecambe Bay, the Cumbrian coast and Solway Firth (Holt et al., 1998).  Essink (1999) recorded fatal burial depths of 1-2 cm for Mytilus edulis and suggested that they had a low tolerance of sedimentation based on investigations by R.Bijkerk (cited by Essink, 1999).  Essink (1999) suggested that deposition of sediment (mud or sand) on shallow mussel beds should be avoided.  However, Widdows et al. (2002) noted that mussels buried by 6 cm of sandy sediment (caused by resuspension of sediment due to turbulent flow across the bed) were able to move to the surface within one day.  Conversely, Condie (2009) (cited by Last et al., 2011) reported that Mytilus edulis was tolerant of repeated burial events. 

    Last et al., (2011) carried out burial experiments on Mytilus edulis in pVORTs.  They used a range of burial depths and sediment fractions and temperatures.  It was found that individual mussels were able to survive burial in depths of 2, 5 and 7 cm for over 32 days although the deeper and longer the mussels were buried the higher the mortality.  Only 16 % of buried mussels died after 16 days compared to almost 50 % mortality at 32 days.  Mortality also increased sharply with a decrease in particle size and with increases in temperature from 8.0 and 14.5 to 20 °C.  The ability of a proportion of individuals to emerge from burial was again demonstrated with approximately one quarter of the individuals buried at 2 cm resurfacing.  However, at depths of 5 cm and 7 cm no emergence was recorded (Last et al., 2011).  The lower mortality when buried in coarse sands may be related to the greater number of individuals who were able to emerge in these conditions and emergence was to be significant for survival. 

    It is unclear whether the same results would be recorded when mussels are joined by byssal threads or whether this would have an impact on survival (Last et al., 2011), although Daly & Mathieson (1977) recorded loose attachments between juvenile mussels during a burial event and some of these were able to surface.  It was not clear whether the same ability would be shown by adult mussels in a more densely packed bed.

    Sensitivity assessment. Overburden by 30 cm of fine material (see benchmark) in a single incident could result in significant mortality in blue mussel beds due to the limited ability of Mytilus edulis to emerge from sediment deeper than 2 cm (Last et al., 2011, Essink, 1999, Daly & Matthieson, 1977) and the increased mussel mortality with depth and reduced particle size observed by Last et al. (2011).  Survival will be higher in winter months when temperatures are lower and physiological demands are decreased.  However, mortality will depend on the duration of smothering. Mortality is likely to be significant in wave sheltered areas, devoid of tidal streams, where the smothering sediment remains for prolonged periods (e.g. more than 16 days).  Therefore, resistance has been assessed as ‘Low’ (significant mortality, loss of 25-75 % of population abundance, or extent) for the littoral sediment biotopes (A2.271) and A2.212.  Mortality will be limited, and possibly avoided, where the smothering sediment is removed due to wave action or tidal streams, depending on how long the sediment remains over the individual mussels.  Therefore, a precautionary resistance has been assessed as ‘Medium’ for the biotope A5.625 and resilience is assessed as ‘Medium’, so that sensitivity is 'Medium'.  

    Not Assessed (NA) Not Assessed (NA) Not assessed (NA)
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    Not assessed.

    No evidence (NEv) No Evidence (NEv) No evidence (NEv)
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    No evidence.

    Not relevant (NR) Not relevant (NR) Not relevant (NR)
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    Not relevant.

    No evidence (NEv) No Evidence (NEv) No evidence (NEv)
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    No evidence.

    Medium High Low
    Q: Low
    A: NR
    C: NR
    Q: High
    A: High
    C: High
    Q: Low
    A: Low
    C: Low

    No direct evidence was found to assess this pressure. As the larvae of Mytilus edulis are planktonic and are transported by water movements, barriers that reduce the degree of tidal excursion may alter the supply of Mytilus edulis to suitable habitats from source populations. However the presence of barriers may enhance local population supply by preventing the loss of larvae from enclosed habitats.  This species is therefore potentially sensitive to barriers that restrict water movements, whether this will lead to beneficial or negative effects will depend on whether enclosed populations are sources of larvae or are ‘sink’ populations that depend on outside supply of larvae to sustain the local population.

    Sensitivity assessment. As this habitat is potentially sensitive to changes in tidal excursion and exchange through larval supply, resistance is assessed as ‘Medium’ and resilience as ‘High’ (as larval supply would recover when the barrier was removed), sensitivity is therefore ‘Low’.

    Not relevant (NR) Not relevant (NR) Not relevant (NR)
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    Not relevant’ to seabed habitats.  NB. Collision by grounding vessels is addressed under ‘surface abrasion’.

    Not relevant (NR) Not relevant (NR) Not relevant (NR)
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    Not relevant.

    Biological Pressures

     ResistanceResilienceSensitivity
    No evidence (NEv) No Evidence (NEv) No evidence (NEv)
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR
    Q: NR
    A: NR
    C: NR

    Commercial cultivation of Mytilus edulis involves the collection of juvenile mussel ‘seed’ or spat (newly settled juveniles ca 1-2cm in length) from wild populations, with subsequent transportation around the UK for re-laying in suitable habitats. As the seed is harvested from wild populations from various locations the gene pool will not necessarily be decreased by translocations.  Movement of mussel seed has the potential to transport pathogens and non-native species (see relevant pressure sections). This pressure assessment is based on Mainwaring et al. (2014) and considers the potential impacts on natural mussel beds of genetic flow between translocated stocks and wild mussel beds. 

    Commercial cultivation of Mytilus edulis involves the collection of juvenile mussel ‘seed’ or spat (newly settled juveniles ca 1-2cm in length) from wild populations, with subsequent transportation around the UK for re-laying in suitable habitats. As the seed is harvested from wild populations from various locations the gene pool will not necessarily be decreased by translocations.  Movement of mussel seed has the potential to transport pathogens and non-native species (see relevant pressure sections). This pressure assessment is based on Mainwaring et al. (2014) and considers the potential impacts on natural mussel beds of genetic flow between translocated stocks and wild mussel beds. 

    Two species of Mytilus occur in the UK, Mytilus edulis and Mytilus galloprovincialis.  Mytilus edulis appears to maintain genetic homogeneity throughout its range whereas Mytilus galloprovincialis can be genetically subdivided into a Mediterranean group and an Atlantic group (Beaumont et al. 2007).  Mytilus edulis and Mytilus galloprovincialis have the ability to hybridise in areas where their distribution overlaps e.g. around the Atlantic and European coast (Gardner, 1996; Daguin et al., 2001; Bierne et al., 2002; Beaumont et al., 2004).  In the UK overlaps occur on the North East coast, North East Scotland, South West England and in the North, West and South of Ireland (Beaumont et al., 2007).  It is difficult to distinguish Mytilus  edulisMytilus galloprovincialis or hybrids based on shell shape because of the extreme plasticity of shape exhibited by mussels under environmental variation, and a genetic test is required (Beaumont et al., 2007).  There is some discussion questioning the distinction between the two species as the hybrids are fertile (Beaumont et al., 2007).  Hybrids reproduce and spawn at a similar time to both Mytilus edulis and Mytilus galloprovincialis which supports genetic flow between the taxa (Doherty et al., 2009).

    There is some evidence that hybrid larvae have a faster growth rate to metamorphosis than pure individuals which may leave pure individuals more vulnerable to predation (Beaumont et al., 1993).  As the physiology of both the hybrid and pure Mytilus edulis is so similar there is likely to be very little impact on the tolerance of the bed to neither pressures nor a change in the associated fauna. 

    A review by Svåsand et al. (2007) concluded that there was a lack of evidence distinguishing between different populations to accurately assess the impacts of hybridisation and in particular how the gene flow may be affected by aquaculture.  Therefore, it cannot be confirmed whether farming will have an impact on the genetics of this species beyond a potential for increased hybridisation.

    Sensitivity assessment. No direct evidence was found regarding the potential for negative impacts of translocated mussel seed on adjacent natural beds.  While it is possible that translocation of mussel seed could lead to genetic flow between cultivated beds and local wild populations, there is currently no evidence to assess the impact (Svåsand et al., 2007).  Hybrid beds perform the same ecological functions as Mytilus edulis so that any impact relates to genetic integrity of a bed alone.  This impact is considered to apply to all mussel bed biotopes equally, as the main habitat forming species Mytilus edulis is translocated.  Also, given the uncertainty in identification of the species, habitats or biotopes described as dominated by Mytilus edulis may well be dominated by Mytilus galloprovincialis, their hybrids or a mosaic of the three. Presently, there is no evidence of impact due to genetic modification and translocation; therefore ‘No evidence’ is reported.  The range of Mytilus galloprovincialis is thought to be extending northwards (Beaumont et al., 2007) and this assessment may require updating in the future.  

    Low Very Low High
    Q: Medium
    A: Medium
    C: Medium
    Q: High
    A: Low
    C: Medium
    Q: Medium
    A: Low
    C: Medium

    Aquaculture of Mytilus edulis requires regular movement of mussel seed from one area to another providing a significant pathway for the introduction of non-indigenous species.  Sewell et al. (2008) reviewed species with the potential to be introduced to and impact mussel beds.  These included Botrylloides violaceus, Corella eumyota, Crepidula fornicata, Didemnum vexillum, Eriocheir sinensis, Rapana venosaCrassostrea gigas and Aulocomya ater

    Three of these species were considered by Mainwaring et al., (2014) to currently impact sublittoral Mytilus edulis beds; Didemnum vexillum, Crepidula fornicata and Crassostrea gigas.

    Crepidula fornicata is reported to settle and establish amongst mussel beds, and is often transported with the transfer of oysters and oyster dredging (Minchin, 1995; Blanchard, 1997; Thieltges, 2005; Rayment, 2007)  as well as by hull fouling (Rayment, 2007).  If Crepidula fornicata becomes established in a bed it is likely to alter the bed structure particularly if it is on a coarse sand or hard substrata. Crepidula fornicata has high fecundity and can disperse its larvae over large areas making mussel beds highly vulnerable if Crepidula fornicata is introduced even large distances away.  The larvae of Crepidula fornicata can survive transport in ballast water for a number of days allowing it to travel large distances before needing to settle in the areas the ballast water is released (Blanchard, 1997).  By settling in an area and increasing the amount of pseudofaeces the substratum may be altered from hard substratum to soft sediment which again will reduce substratum availability for settlement. 

    Thieltges (2005) reported a 28-30% mortality of Mytilus edulis when Crepidula fornicata was introduced to the beds in experimental studies.  He also found that mussel shell growth was reduced by 3 to 5 times in comparison to unfouled mussels and that extra energy was probably expended on byssus production.  The most significant cause of mortality was increased drag on mussel due to the growth of stacks of Crepidula fornicata on the shells of the mussel.  He concluded that Crepidula fornicata is potentially an important mortality factor for Mytilus edulis (Thieltges, 2005).  Thieltges et al., (2003) reported that Crepidula fornicata was abundant on mussel beds in the intertidal to subtidal transition zone, in the northern Wadden Sea in the year 2000.  Thieltges (2005) also observed mussel beds in the shallow subtidal infested with high abundances of Crepidula fornicata with almost no living mussels, along the shore of the List tidal basin, northern Wadden Sea.  

    Crepidula fornicata is likely to alter water flow over mussel beds.  They form stacks of individuals that alter water flow across the sediment surface.  When these stacks occur on the shells of Mytilus edulis they increase the drag on the mussel, increase the demands on the mussel’s energy reserves for attachment (e.g. byssus formation) and, hence, effect fecundity and survival (Sewell et al., 2008).  The increased drag may also result in clumps of mussels being removed by water flow.  Competition for suspended organic matter and space is also increased.  Space for the settlement of macrobenthic organisms (Blanchard, 1997) including mussels is particularly reduced.  In addition to the reduced space for settlement, larvae of macrobenthic organisms are also consumed by the slipper limpet and effect recruitment to an area. 

    Didemnum vexillum is a suspension feeding tunicate capable of forming very large colonies; hence it is called the carpet sea squirt.  It is highly competitive in colonizing new substrata, by budding, fragmentation and larval dispersal, and has a fast growth rate (Minchin & Sides 2006; Auker & Oviatt, 2007; Auker & Oviatt, 2008).  It is reported to known to grow on and over Mytilus edulis on pontoons and settlement panels (Minchin & Sides, 2006; Auker & Oviatt, 2007). When growing on the mussels themselves Didemnum vexillum restricted the opening of valves and increased mortality (Auker & Oviatt, 2007).  Auker & Oviatt (2007) also observed a decreased recruitment rate in settlement panels colonised by Didemnum vexillum but could not rule out other factors and concluded that further research was required to determine this relationship.  Currently Didemnum vexillum is found mainly in marinas around the UK and is thought to have been brought in by hull fouling, through the movement of oyster stock, and as colony fragments in ballast water (Minchin & Sides, 2006; Cohen, 2011).  Small leisure boats are a significant pathway for the spread of this species to many other areas around the UK.  Colonies cannot survive exposure to air in winter (Valentine et al., 2007a) so it is only the subtidal blue mussel bed biotopes, such as the assessed biotope,  that are at high risk from colonisation, although due to its fast growth damage to intertidal biotopes could occur over summer.  In addition, Didemnum vexillum does not tolerate low salinities (Auker & Oviatt, 2008) so that biotopes in reduced salinities are at less risk. 

    Didemnum vexillum can grow over mussel themselves.  It is also likely to reduce feeding grounds and settlement areas, and compete for food with other suspension feeders including the mussels.  Hence, it is likely to reduce growth rates of mussels, their viability, and mussel recruitment.  Didemnum vexillum has been reported to smother benthos.  In the Oosterschelde it reached high cover (>95%) in some locations resulting in a marked decrease in brittlestar and sea urchin populations, while on pebble gravel bottom of Georges Bank off Massachusetts, where it covered the majority of the seabed locally, it resulted in significant change in the sedimentary community ((Bishop, 2012c). However, no evidence of direct effects on mussel beds was found.

    Crassostrea gigas is the most widely grown bivalve in aquaculture around the world at present and an important nuisance species in marine waters (Padilla, 2010).  Adults are also long-lived so that populations can survive with infrequent recruitment.  It has a high fecundity, a long-lived pelagic larval phase and hence high dispersal potential (>1000km).  Crassostrea gigas does not spawn at water temperatures below ca 20 °C but adults grow in colder waters, so that it was thought that this species could not escape from cultivation in cold water areas.  However, it has been suggested that climate change and warmer waters have allowed Crassostrea gigas to expand into and reproduce in previously unsuitable areas.  Established feral populations have been reported to spread via larvae (Padilla, 2010).  It is found form the mid-littoral to the upper subtidal, and grows on hard substrata but also on other bivalves (e.g. blue mussels) and polychaete reefs (Padilla, 2010).

    Crassostrea gigas can outcompete Mytilus edulis (Padilla, 2010).  In the Wadden Sea and North Sea, Crassostrea gigas overgrows mussel beds in the intertidal zone (Diederich, 2005; 2006; Kochmann et al., 2008), although they did show preference for settling on conspecifics before the mussels and struggled to settle on mussels with a fucoid covering.  Diederich (2005, 2006) examined settlement, recruitment and growth of Crassostrea gigas and Mytilus edulis in the northern Wadden Sea.  Crassostrea gigas recruitment success was dependant on temperature, and in the northern Wadden Sea, only occurred in six of the 18 years since Crassostrea gigas was first introduced.  Survival of juveniles is higher in mild than cold winters.  Also survival of both juveniles and adults on mussel beds is higher than that of the mussels themselves.  However, recruitment of Crassostrea gigas was significantly higher in the intertidal than the shallow subtidal, although the survival of adult oysters or mussels in the subtidal is limited by predation.  Deiderich (2005) concluded that hot summers could favour Crassostrea gigas reproduction while cold winters could lead to high mussel recruitment the following summer.  Diederich (2005, 2006) noted that the high survival rate of Crassostrea gigas adults and juveniles in the intertidal was likely to compensate for years of poor recruitment.  Crassostrea gigas also prefer to settle on conspecifics, so that it can build massive oyster reefs, which themselves are more resistant of storms or ice scour than the mussel beds they replace; as oysters are cemented together, rather than dependent on byssus threads.  Crassostrea gigas also grows faster than Mytilus edulis in the intertidal and reach by ca 2-3 times the length of mussels within one year.  In addition, growth rates in Crassostrea gigas were independent of tidal level (emergence regime, substratum, Fucus cover and barnacle epifauna (growing on both mussels and oysters), while growth rate of Mytilus edulis was decreased by these factors.  The faster growth rate could make Crassostrea gigas more competitive than Mytilus edulis where space or food is limiting.  Diederich (2006) concluded that the massive increase in Crassostrea gigas in the northern Wadden Sea was caused by high recruitment success, itself due to anomalously warm summer temperatures, the preference for settlement on conspecifics (and hence reef formation), and high survival rates of juveniles.  Since temperature is an important factor climate change may be an important factor in the expansion of Crassostrea gigas in the Wadden Sea (Diederich, 2006).  As oyster reefs form on former mussel beds, the available habitat for Mytilus edulis could be restricted (Diederich, 2006).

    It has been observed that mussel beds in the Wadden Sea that are adjacent to oyster farms were quickly converted to oyster beds (Kochmann et al.,  2008).  Padilla (2010) predicted that Crassostrea gigas could either displace or overgrown mussels on rocky and sedimentary habitats of low or high energy.  Kent and Essex Inshore Fisheries and Conservation Authority (IFCA) (cited in Herbert et al., 2012) reported that Crassostrea gigas had developed a significant stock on mussel beds on the Southend foreshore and that, by 2012, there were few mussels left in the affected area, but made no conclusions as to the reason for the decline in mussels (Kent and Essex IFCA per comm cited in Herbert et al.,  2012).

    Sensitivity assessment. No evidence for any effects of Aulocomya ater or Eriocheir sinensis were found. The rapa whelk (Rapana venosa) has been shown to cause declines of Mytilus galloprovincialis populations, and may have the potential to do so with blue mussel beds in the UK but at present is not established and there is no direct evidence for effects. The sea squirts Corella eumyota and Botrylloides violaceus have both been recorded growing on mussels and are likely to reduce both viability and recruitment.  But no evidence of resultant mortality was found.  Therefore, a resistance of ‘High’ is suggested for subtidal blue mussel bed biotopes, resilience is assessed as ‘High’ by default and the biotope is ‘Not sensitive.  Further monitoring of effects is required and this assessment may need updating in the future. The carpet sea squirt Didemnum vexillum has the potential to smother areas of seabed, and presumably mussel beds, and has been shown to reduce growth, recruitment and increase mortality of mussel on artificial substrata.  Therefore, a precautionary resistance of ‘Low’ has been recorded for subtidal biotopes. Resilience is likely to be ‘Very low’ as the sea squirt would need to be removed for recovery to occur.  Therefore, a sensitivity of ‘High’ is reported. Crepidula fornicata modifies the sediment and competes with blue mussels.  In the Wadden Sea, Crepidula fornicata may outcompete and replace mussel beds.  Although, no records of Crepidula fornicata replacing or dominating mussel beds were found, it has the potential to do so.  Therefore, a precautionary resistance of ‘Low’ is suggested (significant, 25-75%, mortality and effects on the physiochemical character of the habitat) for subtidal biotopes.  Resilience is likely to be ‘Very Low’ as the slipper limpet population would need to be removed for recovery to occur.  Therefore, a sensitivity of ‘High’ is reported.

    Crassostrea gigas was reported to outcompete and replace mussel beds in the intertidal and was predicted to do so, on both soft sediment and rocky habitats (Padilla, 2010).  In the upper subtidal, Crassostrea gigas may also develop reefs or grow on mussel beds but it the evidence is less clear.  Herbert et al., (2012) noted that blue mussels were found in areas dominated by Crassostrea gigas.  But small clumps or occasional individuals would not constitute a blue mussel bed, so that the component biotopes would be lost.  A precautionary resistance of ‘None’ was suggested (significant, 25-75%, mortality and effects on the physiochemical character of the habitat) by Mainwaring et al. (2014) for intertidal blue mussel beds and ‘Medium’ to represent competition for food or space and potential loss of blue mussel abundance for subtidal mussel beds.  Resilience is likely to be ‘Very low’ as the Crassostrea gigas population would need to be removed for recovery to occur.  Therefore, subtidal beds are considered to have ‘Medium’ sensitivity.

    Medium Medium Medium
    Q: Medium
    A: Medium
    C: Medium
    Q: High
    A: Low
    C: Medium
    Q: Medium
    A: Low
    C: Medium

    Evidence for the impacts of microbial pathogens on Mytilus edulis was reviewed by Mainwaring et al. (2014) with specific reference to the shellfish pathogens Marteilosis and Bonamia. Natural Mytilus edulis beds are host to a diverse array of disease organisms, parasites and commensals from many animal and plant groups including bacteria, blue green algae, green algae, protozoa, boring sponges, boring polychaetes, boring lichen, the intermediary life stages of several trematodes, copepods and decapods (Bower, 1992; Gray et al., 1999; Bower, 2010).

     Whilst Bonamia, has been shown not to infect Mytilus edulis (Culloty et al., 1999), Marteilia refringens can infect and have significant impacts on the health of Mytilus edulis.  Its distribution, impacts on the host, diagnostic techniques and control measures are reviewed by Bower (2011). There is some debate as to whether there are two species of Marteilia, one which infects oysters (Marteilia refringens) and another that infects blue mussels (Marteilia maurini) (Le Roux et al., 2001) or whether they are just two strains of the same species (Lopez-Flores et al., 2004; Balseiro et al., 2007).  Both species are present in southern parts of the United Kingdom.  The infection of Marteilia results in Marteiliosis which disrupts the digestive glands of Mytilus edulis especially at times of spore release.  Heavy infection can result in a reduced uptake of food, reduced absorption efficiency, lower carbohydrate levels in the haemolymph and inhibited gonad development particularly after the spring spawning resulting in an overall reduced condition of the individual (Robledo et al., 1995).

    Recent evidence suggests that Marteilia is transferred to and from Mytilus edulis via the copepod Paracartia grani.  This copepod is not currently prevalent in the UK waters, with only a few records in the English Channel and along the South coast.  However, it is thought to be transferred by ballast water and so localised introductions of this vector may be possible in areas of mussel seed transfer e.g. the Menai Strait.  The mussel populations here are considered to be naive (i.e. not previously exposed) and therefore could be heavily affected, although the likelihood is slim due to the dependence on the introduction of a vector that is carrying Marteilia and then it being transferred to the mussels.

    Berthe et al. (2004) concluded that Mytilus edulis is rarely significantly affected by Marteilia sp.  However, occasions have been recorded of nearly 100 % mortality when British spat have been transferred from a ‘disease free area’ to areas in France were Marteilia sp. are present.  This suggests that there is a severe potential risk if naive spat are moved around the UK from northern waters into southern waters where the disease is resident (enzootic) or if increased temperatures allow the spread of Marteilia sp. northwards towards the naive northern populations.  In addition, rising temperatures could allow increased densities of the Marteilia sp. resulting in heavier infections which can lead to mortality.

    Sensitivity assessment. Bower (2010) noted that although Marteilia was a potentially lethal pathogen of mussels, most populations were not adversely affected by marteilioisis but that in some areas mortality can be significant in mariculture (Berthe et al., 2004).  The resultant population would be more sensitive to other pressures, even where the disease only resulted in reduced condition.  Therefore, a precautionary resistance of ‘Medium’ is suggested (<25 % mortality), with a resilience of ‘Medium’ (2-10 years) resulting in a sensitivity of ‘Medium’.  

    Low Medium Medium
    Q: High
    A: High
    C: High
    Q: High
    A: Low
    C: Medium
    Q: High
    A: Low
    C: Medium

    Mytilus edulis is a commercially targeted species worldwide and has been fished for hundreds of years and managed in England and Wales for the last hundred years (Holt et al., 1998).  Mussels are collected on a commercial scale, in both the intertidal and subtidal, by dredges of various forms and by divers (Narvarte et al., 2011).  Damage caused by direct physical impacts which are assessed in under ‘abrasion’ and ‘penetration and/or disturbance of the surface of the seabed’ pressures, the sensitivity assessment for this pressure considers any biological effects resulting from the removal of target species on Mytilus edulis beds.

    Mytilus edulis is the most important characterising species defining the assessed biotopes and therefore any removal of the species will result in the removal of its associated fauna and a decline in species richness. Removal of most of the mussel biomass will also lead to loss of or reclassification of the biotope.  The sensitivity to removal can be characterised as the immediate direct impact of harvesting and subsequent indirect effects.

    Reports of dredging efficiency vary from 15% using Baird dredges on ground previously dredged for oysters (Palmer et al., 2007) to 90% using artisanal dredges (Narvarte et al., 2011). Mussels are also regularly hand collected by fisherman for bait and food from intertidal beds which can also result in significant damage to the bed (Holt et al., 1998; Smith & Murray, 2005).

    Smith & Murray (2005) examined the effects of low level disturbance and removal on an extensive bed of Mytilus californianus (composed of a single layer of mussels) in southern California.  They observed a significant decrease in mussel mass (g/m2), density (no./m2), percentage cover and mean shell length due to low-intensity simulated bait-removal treatments (2 mussels / month) for 12 months (Smith & Murray 2005).  They also stated that the initial effects of removal were ‘overshadowed’ by loss of additional mussels during time periods between treatments, probably due to the indirect effect of weakening of byssal threads attachments between the mussel leaving them more susceptible to wave action (Smith & Murray, 2005).  The low-intensity simulated bait-removal treatments had reduced percentage cover by 57.5% at the end of the 12 month experimental period.  Smith & Murray (2005) suggested that the losses occurred from collection and trampling are far greater than those that occur by natural causes.  This conclusion was reached due to significant results being displayed for human impact despite the experiment taking place during a time of high natural disturbance from El Niño–Southern Oscillation (ENSO). In addition, Holt et al., (1998) recorded an incident of the removal of an entire bed that is adjacent to a road in Anglesey due to fishermen bait collecting. 

    Commercial removal of mussels can often be responsible for the depletion of mussel stocks.  For example, a substantial reduction in the mussel stock was observed in the Wash (England) during the 1990’s due to high fishing mortality and low recruitment (Atkinson et al., 2003).  The dredging fishery for mussels in the Limfjorden, Denmark, was reported to reduce the stock size of mussels (Dolmer et al., 1999).  The total stock of mussel in the Limfjorden was estimated to be 771 kt to 616 kt in 1993-1994, while the mean exploitation rate of the fishery was 14%.  In 1993-94 the size of mussel landings was found to correlate with a reduction in the overall stock size of the area, suggesting that mussel mortality was significantly increased by the fishery.  However, in 1995 the total stock had fallen to 494kt and the mean exploitation rate was 15% but there was no significant relationship between landings and stock size (Dolmer et al., 1999).  Divers observed that dense beds are likely to be more efficiently dredged due to their byssal attachments detaining the dredge bellow the carpet of the mussels whilst mussels in low density beds cause the dredge to bounce along the seabed resulting in reduced efficiency (Dolmer et al., 1999).  A low level of exploitation may actually increase the growth rate of the mussels by reducing the intraspecific competition for food (Dolmer et al., 1999). However, Dolmer et al., (2001) observed that the mussel biomass was significantly lower in dredged areas suggesting that the lowering of the intraspecific competition does not increase the accumulation of biomass.

    Recreational fishermen will often collect moulting Carcinus maenas or whelks by hand from intertidal mussel beds for bait.  The removal of predatory crabs could actively benefit the population this effect could be beneficial to mussel populations.

    Sensitivity assessment Mytilus edulis beds have no avoidance mechanisms to escape targeted harvesting and as a result a significant proportion of the bed can be removed (Palmer et al., 2007; Narvarte et al., 2011).  Dredging occurs on both subtidal and intertidal soft sediment and results in the removal of the mussel beds which defines the biotope.  As the majority of the mussel beds that are harvested in the UK are regularly replenished with seed, the recovery rate for maintained beds should be rapid.  In natural (wild) beds, the recovery could be significantly longer due to indirect effects from wave action and the sporadic nature of recruitment (Paine & Levin 1981; Seed & Suchanek 1992).  Mussel beds on hard substrata are unlikely to be affected by dredges and are therefore only vulnerable in the intertidal areas where they may be accessed for hand picking.  However, even hand-picking for bait can result in a significant decrease in cover, especially in beds composed of a single layer of mussels (Smith & Murray 2005).  It should be noted that dense, multi-layered mussel beds may be more resistant to the gaps and bait collection, as damage to the upper layer may not effect deeper layers, so that attachment to the substratum and each other is maintained (Brosnan & Crumrine, 1994). Based on the available evidence all  Mytilus edulis bed biotopes are considered to have ‘Low’ resistance to this pressure and ‘Medium’ resilience so that sensitivity is assessed as ‘Medium’.

    Low Medium High
    Q: High
    A: High
    C: High
    Q: High
    A: Low
    C: Medium
    Q: High
    A: Low
    C: Medium

    Blue mussel beds may be removed or damaged by static or mobile gears that are targeting other species. The direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures, while this pressure considers the ecological or biological effects of by-catch. Removal of a large part of the Mytilus edulis bed unintentionally would significantly alter the biotope as the mussels are the key characterizing, structuring and functional species.

     The removal of Mytilus edulis predators including the starfish Asterias rubens and Luidia ciliaris and the crabs Cancer pagurus and Necora puber as by-catch through commercial fishing activities could potentially benefit beds of mussels but the population of starfish and crabs are highly mobile and probably attracted to damaging and dying organisms left after dredging, and therefore likely to recover before the mussels are able to recruit (Gubbay & Knapman 1999).  Fishing activities may exposure infauna, and leave dead and damaged species on the seabed, and areas where discards and by-catch have been deposited, may also attract predators and scavengers.  But this potentially heightened level predation only lasts for a few days (Dolmer et al. 2001 observed a seven day effect) and therefore is unlikely to have an impact on the bed as a whole.

    Sensitivity assessment. The biogenic structure created by Mytilus edulis is the key characterizing, structural and funxtional feature of this biotope group. Removal of individuals as by-catch would remove the biotope and hence this group is considered to have ‘Low’ resistance to this pressure and to have ‘medium’ recovery. Sensitivity is therefore ‘Medium’.

    Importance review

    Policy/Legislation

    Habitats of Principal ImportanceBlue mussel beds
    Habitats of Conservation ImportanceBlue mussel beds
    Habitats Directive Annex 1Reefs
    UK Biodiversity Action Plan PriorityBlue mussel beds
    Priority Marine Features (Scotland)Blue mussel beds

    Exploitation

    Mussels have been harvested for food and bait since early times. British mussel production is relatively small comprising only 5% of total European Community production (Edwards, 1997). Edwards (1997) notes that the commercial development of natural beds is hampered by sporadic and unpredictable recruitment. The extent to which this biotope is exploited is uncertain.

    Additional information

    -

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    Citation

    This review can be cited as:

    Tillin, H.M. & Mainwaring, K., 2016. Mytilus edulis beds on sublittoral sediment. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/habitat/detail/36

    Last Updated: 09/02/2016