Mytilus edulis beds on sublittoral sediment

30-11-2001
Researched byDr Harvey Tyler-Walters & Olivia Durkin 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.

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

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Additional information

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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 IrelandFound 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
Water clarity preferences
Limiting Nutrients Not relevant
Salinity
Physiographic
Biological Zone
Substratum
Tidal
Wave
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

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    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?

    Explanation

    Mytilus edulis populations stabilize the sediment surface and build up a bed of organically enriched 'mussel mud' to form a biogenic reef in this biotope (Holt et al., 1998). Therefore, the mussel bed modifies the substratum and adds habitat complexity in the form of hard substratum (its shells), the mussel matrix and the mussel mud. Mytilus edulis is therefore, regarded as the key structural species in this biotope. Predators such as Asterias rubens and Nucella lapillus influence the population structure and dynamics of the bed are therefore included as important functional species. Most other associated species live within the biotope without greatly influencing its structure or function.

    Species indicative of sensitivity

    Community ImportanceSpecies nameCommon Name
    Important functionalAsterias rubensCommon starfish
    Key structuralMytilus edulisCommon mussel
    Important functionalNucella lapillusDog whelk

    Physical Pressures

     IntoleranceRecoverabilitySensitivitySpecies RichnessEvidence/Confidence
    High High Moderate Major decline Moderate
    Removal of the substratum will remove of all the species within the biotope. Therefore, an intolerance of high has been recorded. Although a single good recruitment event may recolonize the substratum within a year, recovery may take up to 5 years, and is some circumstances significantly longer (see additional information below). Therefore, a recoverability of high has been recorded.
    Intermediate High Low Decline Moderate
    Intertidal Mytilus edulis beds have been reported to suffer moralities as a result on smothering by large scale movements of sand or sand scour (Holt et al., 1998; Daly & Mathieson, 1977). Similarly, biodeposition within a mussel bed results in suffocation or starvation of individuals that cannot re-surface. Young mussels have been shown to move up through a bed, avoiding smothering, while many others were suffocated (Dare, 1976; Holt et al., 1998). This suggests that a proportion of the population may be able to avoid smothering in subtidal conditions, and, therefore, an intolerance of intermediate has been recorded. Many infaunal species are likely to be not sensitive to smothering by the same grade of sediment, however, interstitial species and epifauna may be adversely affected. Although a single good recruitment event may recolonize the substratum within a year, recovery may take up to 5 years, and is some circumstances significantly longer (see additional information below). Therefore, a recoverability of high has been recorded.
    Low Immediate Not sensitive Minor decline Moderate
    Mytilus edulis has been reported to be relatively tolerant of suspended sediment and siltation and survived over 25 days at 440mg/l and on average 13 days at 1200mg/l (Purchon, 1937; Moore, 1977a). Mytilus edulis also has efficient pseudofaeces discharge mechanisms (Moore, 1977a; de Vooys, 1987). Asterias rubens flourishes in naturally turbid conditions and is capable of cleansing itself of adherent mud particles (Moore, 1977). Nucella lapillus is also found in turbid environments such as the Bristol Channel. Similarly, the barnacle Balanus crenatus was considered to be of low intolerance to suspended sediment. However, these species probably suffer a metabolic cost resulting from the cleansing mechanisms, mucus production and interrupted or impaired feeding. Therefore, a biotope intolerance of low, at the benchmark level, has been recorded. The majority of the organisms within the biotope are adapted to sedimentary, estuarine habitats and probably have mechanisms to deal with siltation and suspended sediment, so that recoverability of immediate has been recorded.
    Low High Not relevant Decline Low
    A decrease in suspended sediment, especially organic particulate could potentially reduce the food available to Mytilus edulis and the other suspension feeders within the biotope. A reduction in sedimentation could potential result in increased rates of erosion in sedimentary habitats. However, a large proportion of deposition within the mussel bed is due to accumulation of faeces and pseudofaeces. Therefore, a decrease in sedimentation at the benchmark level is probably not significant and an intolerance of low has been recorded.
    Low Immediate Not sensitive Minor decline Low
    This biotope occurs from 0 -10m in depth and, therefore, only populations in the shallowest parts of its distribution may be exposed to desiccation by extreme low tides. The upper limit of Mytilus edulis population is primarily controlled by the synergistic effects of temperature and desiccation (Suchanek, 1978; Seed & Suchanek, 1992; Holt et al., 1998). However, Mytilus spp. beds occur in the mid to lower intertidal, so that a change in desiccation at the benchmark level is unlikely to adversely affect the bed. Similarly, Nucella lapillus also occur in the intertidal and is unlikely to be adversely affect at the benchmark level.
    Asterias rubens has a high surface to volume ratio and is highly intolerant of desiccation. Its distribution is restricted to the sublittoral, or sublittoral fringe. Most infauna or interstitial fauna are protected from desiccation by their habitat but can tolerate intertidal conditions. Therefore an intolerance of low has been recorded at the benchmark level.
    Intermediate High Low Minor decline Low
    An increase in emergence will effectively move the upper limits of the biotope into the lower intertidal. Mytilus edulis can form extensive beds in the intertidal. Growth rates will decrease due to loss of feeding time at low tide. However, the major predators will probably change, from the starfish and crabs of the sublittoral to birds and wildfowl in the eulittoral. Dog whelk predation will probably remain about constant, while fish predation will be limited to high tides. Most of the epifauna and infaunal polychaetes and amphipods are recorded from the lower shore and likely to be little affected. However, wildfowl predation may be significant, and is likely to change to size and age distribution within the bed and disrupt the mussel bed itself, e.g. eider duck, therefore an intolerance of intermediate has been recorded. Recovery is likely to be rapid (see additional information).
    Tolerant* Not sensitive No change Low
    An increase in tidal submergence is likely to allow the biotope to extend its range further up the shore. Therefore, a rank of not sensitive* has been recorded.
    High High Moderate Decline Low
    As mussel beds increase in size and depth, individual mussels become increasingly attached to each other rather than the substratum. As a result, the bed may become destabilised and susceptible to removal by wave action or tidal scour. However, mussels at the edge of the beds are often more strongly attached than mussels within the bed (Seed & Suchanek, 1992). On sedimentary shores, mussel beds are probably intolerant of increased water flow due to removal of the sediment resulting in loss of clumps of the bed. Mussel reefs in the Wash, Morecambe Bay and the Wadden Sea are vulnerable to destruction by storms and tidal surges (Holt et al., 1998). Therefore, a change in water flow rate from weak to strong (the benchmark) would probably result in the loss of clumps or large parts of the mussel bed, Loss of the bed would result in loss of the epifaunal and predatory species associated with them, together with the interstitial fauna and a proportion of the benthic infauna. Therefore, an intolerance of high has been recorded.
    Although a single good recruitment event may recolonize the substratum within a year, recovery may take up to 5 years, and is some circumstances significantly longer (see additional information below). Therefore, a recoverability of high has been recorded.
    Low Very high Moderate Rise Low
    This biotope is found in moderately strong to weak tidal streams and further reduction in water flow may result in an increased sedimentation (see above) and risk of low oxygen conditions (see below). The mussels, and other suspension feeders, probably require water flow to supply food (suspended particulates, benthic diatoms and phytoplankton). However, overall a reduction in water flow is likely to have only limited affects and an intolerance of low and a recoverability of very high has been recorded.
    Low Very high Very Low Minor decline Low
    Sublittoral populations are unlikely to experience rapid or extreme temperature changes due to natural events and may, therefore, be expected to be intolerant of acute temperature change. An upper, sustained temperature tolerance limit of about 29 °C has been reported for Mytilus edulis in the United Kingdom (Read & Cumming, 1967; Almada-Villa et al., 1982). Seed & Suchanek (1992) noted that European populations were unlikely to experience temperatures greater than 25°C. Therefore, Mytilus edulis was consider to be of low intolerance to temperature change. Nucella lapillus may succumb to increased temperatures in summer but is otherwise relatively tolerant. Balanus crenatus and Asterias rubens, however, were assessed as highly intolerant of increased temperatures. Overall, the biotope has been assessed as of low intolerance to increased temperatures since the key species, Mytilus edulis, is unlikely to be adversely affected. Recovery is likely to be rapid (see additional information below).
    Low Very high Moderate No change Low
    Sublittoral populations are unlikely to experience rapid or extreme temperature changes due to natural events and may, therefore, be expected to be intolerant of acute temperature change. However, Mytilus edulis tolerates decreases in temperature and even freezing for short periods. Mytilus edulis was relatively little affected by the severe winter of 1962/63, with 30% mortality reported from south-east coasts of England (Whitstable area) and ca. 2% from Rhosilli in south Wales (Crisp (ed.),1964). Similarly, the barnacle Balanus crenatus, were unaffected by the severe winter of 1962/63 (Crisp, 1964). Most of the polychaetes characterizing the biotope have a wide distribution and are probably tolerant of low temperatures, especially when protected from temperature change by their infaunal habit. It appears, therefore, that most of the characterizing species within the biotope are tolerant of an acute short term temperature decrease and a biotope intolerance of low has been recorded. Recovery is likely to be rapid (see additional information below).
    Tolerant Not relevant Not relevant No change Low
    This biotope is an animal dominated community, dependant on secondary production and not dependant on light. Therefore, the biotope is probably not sensitive to changes in turbidity and light attenuation.
    Tolerant Not sensitive* No change Low
    This biotope is an animal dominated community, dependant on secondary production and not dependant on light. Therefore, the biotope is probably not sensitive to changes in turbidity and light attenuation.
    High High Moderate Major decline Moderate
    The intolerance of mussel beds probably owes more to the nature of the substratum than the strength of their attachment. Individuals attached to solid substrata (rock) are likely to be more tolerant than individuals attached to boulders, cobbles or sediment. Harger & Landenberger (1971) noted that, on gravel based substratum, small, single layered mussel beds suffered far less damage from storms that heavy, multi-layered beds. As mussel beds grow in size and thickness relatively fewer mussels are directly attached to the substratum, so that heavy seas can "roll up the whole mass of mud and mussels like a carpet and break it to pieces on the foreshore" (Harger & Landenberger, 1971). Storms and tidal surges are known to destroy mussel beds, often over hundreds of hectares in the Wash, Morecambe Bay and the Wadden Sea. Mussels beds persist in sheltered areas whereas beds in exposed areas are more dynamic (Holt et al., 1998). Although, subtidal beds are protected by depth, in the shallow sublittoral occupied by this biotope wave action may still be significant. An increase in wave action from sheltered to exposed (the benchmark) is likely to remove a large proportion of the bed, the remaining mussel mud and modify the average grain size of the sediment (from fine to coarse) resulting in major changes in the benthic infauna. Therefore an intolerance of high has been recorded. Recovery may take up to 5 years or longer once prior conditions return (see additional information below) and a recoverability of high has been recorded.
    Low Very high Moderate Rise Low
    On wave sheltered sedimentary shores decreased wave exposure (i.e. sheltered to very sheltered) is likely to have little affect on mussel beds. Therefore, sheltered shore mussels beds are probably of low intolerance to decreased wave exposure, and may be less patchy and more stable (persistent). Reduced wave action will decrease water flow over the bed (see above) and may increase the risk of deoxygenation (see below).
    Tolerant Not relevant Not relevant No change High
    Mytilus edulis and most invertebrate species within the biotope are probably insensitive to noise disturbance at the levels of the benchmark.
    Tolerant Not relevant Not relevant No change High
    Mytilus edulis and most invertebrate species within the biotope are probably insensitive to visual disturbance at the levels of the benchmark.
    Intermediate High Low Decline Moderate
    Wave driven logs have been reported to influence Mytilus edulis populations, causing the removal of patches from extensive beds that subsequently open the beds to further damage by wave action (Holt et al., 1998). A similar effect could be caused by a vessel grounding. Little information on physical disturbance in subtidal Mytilus spp. beds was found. Fishing activities, e.g. scallop dredging are know to physically disturb marine communities. Modiolus modiolus beds have been reported to have declined off the Isle of Man due to scallop dredging, presumably because the scallop dredging activity had damaged the edges of denser beds over time (Jones, 1951; Holt et al., 1998). Benthic trawls, where they occur, may affect Mytilus edulis beds similarly.

    Of the other species in the biotope, starfish, such as Asterias rubens, have been reported to be damaged by benthic dredges but have considerable regenerative capability, and, as scavengers, benefit from the presence of other damaged or killed animals (Emson & Wilkie, 1980; Gubbay & Knapman, 1999). Therefore, it is likely that abrasion or impact at the level of the benchmark (a scallop dredge) would damage or remove patches of the population and an intolerance of intermediate has been recorded. Recovery is dependant on recruitment of Mytilus edulis and a recoverability of high has been reported (see additional information below).

    Intermediate High Low Decline Low
    Mytilus edulis is capable of re-attaching itself to suitable substrata once displaced. Dislodgement may result in increased risk of predation and some individuals may be lost if swept to unsuitable substrata. Overall, however, displacement will result in loss of mussels from this biotope. Displaced starfish are unlikely to be adversely affected and could probably return. Permanently attached species within the community such as barnacles, bryozoans and tubeworms are likely to be lost as a result of displacement. Overall, a proportion of the mussel bed would probably survive displacement and an intolerance of intermediate has been recorded. However, other members of the community are probably more intolerant, resulting in a decline in species richness until they are able to recolonize. Recovery is dependant on recruitment of Mytilus edulis and a recoverability of high has been reported (see additional information below).

    Chemical Pressures

     IntoleranceRecoverabilitySensitivityRichnessEvidence/Confidence
    Intermediate High Low Decline Moderate
    The effects of contaminants on Mytilus edulis were extensively reviewed by Widdows & Donkin (1992) and Livingstone & Pipe (1992). Overall, Mytilus edulis is probably relatively tolerant of contaminants, although mortalities have been recorded (see species review for details). For example,
    • Widdows et al., (1995) noted that polar organics, and organo-chlorines reduced scope for growth in Mytilus edulis;
    • Mytilus edulis has been shown to accumulate PCBs and ivermecten (Hummel et al., 1989; Cole et al., 1999; Holt et al., 1995);
    • the presence of poly-aromatic hydrocarbons, cis-chlordane pesticides and cadmium has been associated with an increase in tumours in Mytilus edulis (Hillman, 1993; Holt et al., 1998); and
    • mussels may be absent from areas of high boating activity, presumably due to TBT (Holt et al., 1998).
    Muricid gastropods such as Buccinum undatum but especially Nucella lapillus are highly intolerant of TBT pollution resulting in significant declines in the population of the dog whelk. Barnacles, such as Balanus crenatus were considered to be highly intolerant of chemical contaminants (Holt et al., 1995). Similarly, most pesticides and herbicides were suggested to be very toxic for invertebrates, especially crustaceans (amphipods, isopods, mysids, shrimp and crabs) and fish (Cole et al., 1999). For example, Lindane was shown to be very toxic to gobies Gobius spp. (see the Pomatoschistus minutus review) (Ebere & Akintonwa, 1992) . The pesticide ivermectin is very toxic to crustaceans, and has been found to be toxic towards some benthic infauna such as Arenicola marina (Cole et al., 1999).
    Therefore, chemical contamination may cause mortalities and sub-lethal effects in the Mytilus edulis bed but affect other members of the community to varying degrees, and an overall intolerance of intermediate has been recorded.
    Most members of the community will recolonize rapidly and a recoverability of high has been reported (see additional information below).
    Heavy metal contamination
    Intermediate High Low Decline Moderate
    Lethal threshold concentrations for several heavy metals have been determined in Mytilus edulis (see species review; Widdows & Donkin (1992) and Livingstone & Pipe (1992) for reviews). Mussels were also reported to be missing from a wider area of the Cumbrian coast than other organisms in the vicinity of a phosphate rich effluent contaminated by heavy metals (Holt et al., 1998). Widdows & Donkin (1992) noted that lethal responses give a false impression of high tolerance. However, Mytilus edulis is probably relatively tolerant of heavy metal contamination. Besten et al. (1989) suggested that cadmium (Cd) pollution posed a significant threat to populations of Asterias rubens since it affected reproduction.
    Cole et al. (1999) suggested that Pb, Zn, Ni and As were very toxic to algae, while Cd was very toxic to Crustacea (amphipods, isopods, shrimp, mysids and crabs), and Hg, Cd, Pb, Cr, Zn, Cu, Ni, and As were very toxic to fish. Gobies were reported to be particularly intolerant of Hg (see Pomatoschistus minutus). Bryan (1984) reported sublethal effects of heavy metals in crustaceans at low (ppb) levels. Bryan (1984) suggested that polychaetes are fairly resistant to heavy metals, based on the species studied. Short term toxicity in polychaetes was highest to Hg, Cu and Ag, declined with Al, Cr, Zn and Pb whereas Cd, Ni, Co and Se were the least toxic. However, he suggested that gastropods (e.g. limpets, Nucella lapillus and Buccinum undatum) were relatively tolerant of heavy metal pollution. Therefore, given the evidence of sub-lethal and lethal effects of heavy metals in Mytilus edulis a biotope intolerance of intermediate has been reported.
    Hydrocarbon contamination
    High High Moderate Major decline Moderate
    The effects of contaminants on Mytilus edulis were extensively reviewed by Widdows & Donkin (1992) and Livingstone & Pipe (1992). Overall, Mytilus edulis is probably relatively tolerant of contaminants, although mortalities have been recorded (see species review for details). Sublittoral populations are protected from the immediate effects of oil spills by their depth. Therefore, hydrocarbon contamination in sublittoral populations is limited to exposure to lighter oil fractions and PAHs in solution, as droplets as a result of wave exposure or adsorbed onto particulates.
    • Toxic hydrocarbons and PAHs contribute to a decline on the scope for growth in Mytilus edulis (Widows & Donkin, 1992; Widdows et al., 1995).
    • The presence of poly-aromatic hydrocarbons, cis-chlordane pesticides and cadmium gas been associated with an increase in tumours in Mytilus edulis (Hillman, 1993; Holt et al., 1998).
    • Mesocosm experiments have shown high mortalities of Mytilus edulis exposed to the water accommodated fraction of diesel (Widdows et al., 1987; Bokn et al., 1993).
    • Ingestion of droplets of sunflower oil, from a tanker spill off the Anglesey coast resulted in mortalities after spawning (Mudge et al., 1993; Holt et al., 1998).
    • Asterias rubens suffered mass mortalities after the Torrey Canyon oil spill and was reported to be lost from mesocosms treated with the water accommodated fraction of diesel (Smith, 1968; Bokn et al., 1993).
    • 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, Mytilus edulis is probably relatively tolerant of chronic hydrocarbon pollution. However, due to the incidence of mortality after exposure to diesel and oils Mytilus edulis was regarded as of intermediate intolerance to hydrocarbon contamination.
    Suchanek (1993) noted that gastropods, amphipods, infaunal polychaetes and bivalves were particularly sensitive to oil spills. For example, substantial kills of Nereis, Cerastoderma, Macoma, Arenicola and Hydrobia were reported after the Sivand oil spill in the Humber (Hailey, 1995). The toxicity of oil and petrochemicals to fish ranges from moderate to high (Cole et al., 1999). The mussel bed may benefit from a reduction in starfish, dog whelk and fish predation, however, the above evidence suggests that the associated community will be adversely affected by hydrocarbon contamination. Therefore a biotope intolerance of high has been recorded.
    Recovery is probably dependant on Mytilus edulis recruitment and a recoverability of high has, therefore, been recorded (see additional information below).
    Radionuclide contamination
    No information Not relevant No information Insufficient
    information
    Not relevant
    Insufficient
    information.
    Changes in nutrient levels
    Tolerant* Not relevant Not sensitive* Minor decline Low
    Moderate nutrient enrichment, especially in the form of organic particulates and dissolved organic material, is likely to increase food availability for all the suspension feeders within the biotope. Therefore, 'not sensitive*' has been recorded. However, long term or high levels of organic enrichment may result in deoxygenation and algal blooms. Mytilus edulis has been reported to suffer mortalities due to algal blooms of Gyrodinium aureolum and Phaeocystis poucheri (Holt et al., 1998). Nucella lapillus has been shown to be severely affected by toxic algal blooms (see review; Robertson, 1991; Gibbs et al., 1999). Death of toxic and non-toxic algal blooms may result in large numbers of dead algal cells collecting on the sea bottom, resulting in local de-oxygenation as the algal decompose. Although, Mytilus edulis is probably tolerant of anoxic conditions other members of the community may be more intolerant (see oxygenation below).
    Tolerant* Not relevant Not sensitive* Rise Low
    Mytilus edulis is considered to be tolerant of a wide range of salinities. Many members of the community occur in the intertidal and estuaries, exposed to fluctuating salinities. An increase from reduced to full salinity is likely to result in a change in species composition, to include more fully marine species and increased species richness, while the mussel bed itself is likely to be little affected. Since the biotope is likely to be persist and species richness increase, not sensitive* has been recorded.
    Intermediate High Low Decline Low
    However, in the longer 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 13psu reduced to almost zero but had recovered to over 80% of control animals within one month. Mytilus edulis can survive a considerably reduced salinities, growing as dwarf individuals at 4-5 psu in the Baltic. Asterias rubens is thought to be intolerant of salinity change, although local adaptation can occur (Stickle & Diehl, 1987), and a sudden inflow of river water into an inshore coastal area caused mass mortality of the conspecific species Asterias vulgaris at Prince Edward Island, Canada (Smith, 1940, in Lawrence, 1995). Asterias rubens would probably be excluded from the biotope by a further reduction in salinity.
    Crothers (1985) noted that Nucella lapillus is usually absent from estuaries and although found in the Severn Estuary it is restricted to the lower shore up-channel from Minehead where they presumably avoid reduced salinities. Many of the infaunal species are probably tolerant of estuarine conditions, while other would be replaced by species, e.g. oligochaetes, tolerant of low salinities.
    Overall, a reduction in salinity from variable to reduced is likely to reduce growth and productivity of the mussel beds but reduce predation pressure so that although species richness will be reduced, the biotope will probably remain. However, sudden acute changes in salinity from variable to low may result in loss of proportions of the mussel bed. Therefore, an intolerance of intermediate has been recorded. Recovery will probably be rapid (see additional information below).
    Intermediate High Low Decline Low
    Mytilus edulis was regarded to be tolerant of a wide range of oxygen concentrations including zero (Zwaan de & Mathieu, 1992; Diaz & Rosenberg, 1995; see species review). Intolerance to hypoxia is variable. Echinoderms such as Asterias rubens are highly intolerant of anoxic conditions. Similarly, the barnacle Balanus crenatus was considered to be highly intolerant of anoxia (see review). Crustacea are probably intolerant of hypoxia but would be able to migrate to more suitable condition. However, most polychaetes are capable of anaerobic respiration and Capitella capitata, Hediste diversicolor and Scoloplos armiger were considered to be resistant of moderate hypoxia while Nephtys hombergii and Heteromastus filiformis were thought to be resistant of severe hypoxia (Diaz & Rosenberg, 1995). Therefore, Mytilus edulis is likely to tolerate hypoxic conditions. However, hypoxia is likely to cause species specific mortality and reduce species richness, an intolerance of intermediate. Recoverability of the associated species is likely to be rapid (see additional information below).

    Biological Pressures

     IntoleranceRecoverabilitySensitivityRichnessEvidence/Confidence
    Intermediate Very high Low Decline Low
    The diseases and parasites of Mytilus edulis were reviewed by Bower (1992) and Bower & McGladdery (1996) (see the species review).
    The boring sponge Cliona spp. has been reported from Modiolus modiolus beds and may affect subtidal Mytilus edulis beds. Similarly, subtidal beds may be affected by the boring polychaete Polydora ciliata. Both of the above boring species weaken the shell of the victim and makes them more vulnerable to predation. Polydora ciliata also causes blisters, atrophy of muscle tissue and interferes with gamete production and has resulted in substantial mortalities in European mussel populations. Asterias rubens may be parasitised by the ciliate Orchitophyra stellarum (Vevers, 1951; Bouland & Clareboudt, 1994) resulting in castration of males, and subsequent reduction in population size (Vevers, 1951). Nucella lapillus may also suffer form castration due to infestation with the larval stages of seabird trematode parasites.
    None of the above were reported to cause high mortalities so that the biotope would probably persist. Therefore, an intolerance of low and a recoverability of very high has been recorded (see additional information below).
    No information Not relevant No information Not relevant Not relevant
    Mytilus edulis is an effective space occupier and few other species are able to out-compete it for space. However, the South American mytilid Aulocomya ater has been reported recently in the Moray Firth, Scotland in 1994 and again in 1997 (McKay, 1994; Holt et al., 1998; Eno et al., 2000). Aulocomya ater is thought to have a stronger byssal attachment than Mytilus edulis and may replace Mytilus edulis in more exposed areas if it reproduces successfully (Holt et al., 1998). However, its potential effects in sheltered sedimentary habitats are unknown.
    Intermediate High Low Decline Moderate
    Large mussel beds in the intertidal and subtidal have been routinely fished for hundreds of years, and managed by local Sea Fishery Committees in England and Wales for the past hundred years (Holt et al., 1998).
    Subtidal mussel beds may be exploited by dredging. Holt et al., (1998) suggest that, in particular embayments, over-exploitation may reduce subsequent recruitment leading to long term reduction in the population or stock. The relationship between stock and recruitment is poorly understood. Loss of stock may have significant effects on other species, e.g. in the Dutch Wadden Sea in 1990 the mussel stocks fell to unprecedented low levels resulting in death or migration of eiders, and oystercatchers seeking alternative prey such as Cerastoderma edule, Mya arenaria, and Macoma baltica.
    Extraction of Mytilus edulis is likely to remove much of the epifaunal and infaunal community, resulting in a decline in species richness. Overall, an intolerance of intermediate has been recorded at the benchmark level of extraction. However, recovery is likely to occur within 5 years and a recoverability of high has been recorded (see additional information below).
    Not relevant Not relevant Not relevant Not relevant Low

    Additional information

    Recoverability
    Larval supply and settlement could potentially occur annually, however, settlement is sporadic with unpredictable pulses of recruitment (Lutz & Kennish, 1992; Seed & Suchanek, 1992).
    Mytilus edulis is highly fecund but larval mortality is high. Larval development occurs within the plankton over ca one month (or more), therefore, whilst recruitment within the population is possible, it is likely that larval produced within the biotope are swept away from the biotope to settle elsewhere. Therefore, recovery is probably dependant on recruitment from outside the biotope.
    While good annual recruitment is possible, recovery may take at least 5 years. However, it should be noted that in certain circumstances and under some environmental conditions recovery may take significantly longer. Overall, Mytilus spp. populations were considered to have a strong ability to recover from environmental disturbance (Holt et al., 1998; Seed & Suchanek, 1992).
    Polychaetes, oligochaetes and other interstitial fauna will probably recolonize rapidly (see recruitment processes), for example Boström & Bonsdorff (2000) reported that large numbers of nematodes, oligochaetes, chironomids, copepods, and the polychaete Pygospio elegans colonized artificial seagrass beds within 33-43 days. Recruitment in Asterias rubens is sporadic and larval production by one population may influence settlement some considerable distance away (Morgan, 1995), while not affecting the original population, so consequently it may take more than one or two generations for a population to return to a pre-impact state. However, recolonization is also likely to occur due to migration of adults or juveniles. Populations of Nucella lapillus appear to be capable of recovering with about 2-5 years if survivors are present either intertidally on below low water (see review) However, should a population need to recruit from surrounding area recovery may take significantly longer.

    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:

    Tyler-Walters, H. & Durkin, O.C., 2001. 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: 30/11/2001