Mytilus edulis beds with hydroids and ascidians on tide-swept exposed to moderately wave-exposed circalittoral rock

20-06-2001
Researched byDr Harvey Tyler-Walters Refereed byThis information is not refereed.
EUNIS CodeA4.241 EUNIS NameMytilus edulis beds with hydroids and ascidians on tide-swept exposed to moderately wave-exposed circalittoral rock

Summary

UK and Ireland classification

EUNIS 2008A4.241Mytilus edulis beds with hydroids and ascidians on tide-swept exposed to moderately wave-exposed circalittoral rock
EUNIS 2006A4.241Mytilus edulis beds with hydroids and ascidians on tide-swept exposed to moderately wave-exposed circalittoral rock
JNCC 2004CR.MCR.CMus.CMytMytilus edulis beds with hydroids and ascidians on tide-swept exposed to moderately wave-exposed circalittoral rock
1997 BiotopeCR.MCR.M.MytHAsMytilus edulis beds with hydroids and ascidians on tide-swept moderately exposed circalittoral rock

Description

Dense mussel Mytilus edulis beds occur in strong tides on a variety of substrata. Species richness is not particularly high. Asterias sp. are usually common, as are crabs such as Cancer pagurus, Carcinus maenas and Necora puber. Hydroids such as Kirchenpaueria pinnata and those characteristic of strong tides and a little scour are also often present such as Sertularia argentea and Tubularia indivisa. Ascidians such as Molgula manhattensis and Polycarpa spp. and Flustra foliacea may be present, particularly in silty conditions, although not often on the mussels themselves. (Information taken from the Marine Biotope Classification for Britain and Ireland, Version 97.06: Connor et al., 1997a, b).

Recorded distribution in Britain and Ireland

The biotope has a restricted distribution around the UK. It has been described in surveys of Flamborough Head, Selsey Bill to Beachy Head, the Lizard, Skomer, the Lleyn Peninsula, the Menai Straits, Anglesey, Liverpool Bay, and Jura & Islay, west Scotland. It is also known to occur off the Gower Peninsula and in localized areas of Plymouth Sound.

Depth range

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Additional information

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Habitat review

Ecology

Ecological and functional relationships

The ecology of circalittoral Mytilus edulis beds have been poorly studied and little information was found. Mussel beds colonizing artificial substrata such as jetty piles and the legs of oil production platforms, together with data on mussel beds in general has been used to derive the following information.
  • Mytilus edulis is a active suspension feeder on organic particulates and dissolved organic matter.
  • Mytilus edulis probably competes for space with other species such as Sabellaria spinulosa and Tubularia indivisa and other mussel species (e.g. Musculus discors).
  • Epifloral/faunal grazers, such as limpets, chitons and sea urchins (e.g. Echinus esculentus), 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).
  • Fish, starfish, crabs and lobsters are potential predators on subtidal mussels beds (Kautsky, 1981; Paine, 1976; Seed, 1993; Seed & Suchanek, 1992). The common starfish Asterias rubens and the plaice Pleuronectes platessa were observed feeding on Mytilus edulis in the biotope off Flamborough Head (Brazier et al., 1998).
  • Kautsky (1981) examined subtidal mussel beds in the Baltic Sea and reported that mussels were a major food source for the flounder (Platichthys flesus) but probably of only minor importance for eelpout (Zoarces viviparus) and cod (Gadus morhua).
  • The lower limit of Mytilus edulis beds is usually set by the intensity of predation. The formation of a bed at depth suggests either a scarcity of predators or the rapid growth of the individual mussels during a lull in predator numbers to a size above the handling size of most predators. For example, 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).
  • Starfish would be expected to be significant predators in the subtidal, however, the population dynamics of starfish populations are poorly understood (Seed, 1993). 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).
  • Scavengers probably feed on dead mussels within the matrix, e.g. flatworms and polychaetes (Kautsky, 1981; Tsuchiya & Nishihira, 1985,1986). However, Kautsky (1981) demonstrated little scavenger activity in the subtidal Mytilus edulis beds in the Baltic Sea.
  • 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

Little information concerning the population dynamics of subtidal Mytilus edulis populations was found. Kautsky (1981) reported that no major fluctuations in distribution and abundance of Mytilus edulis was noted in the Baltic Sea over a ten year period, although a large proportion of biomass fluctuated with the build up and subsequent release of gametes. However, his studied population was not significantly affected by predation. It is likely that subtidal populations are periodically removed or significantly reduced by sporadic and unpredictable swarms or starfish.

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 the 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, Connor et al., (1997a) reported that the species richness of the MCR.MytHAs biotope was not particularly high. Similarly, Holt et al. (1998) noted that a raised beds was not present and most associated organisms were capable of growing on the substratum in the absence of Mytilus edulis.
  • Anemones such as Urticina felina and Sagartia elegans, and branching bryozoans such as Flustra foliacea are probably attached directly to the substratum surface, and penetrate the mussel matrix.
  • Mussel faeces and pseudo-faeces, together with silt, build up organic biodeposits under the beds. The biodeposits attract infauna such as sediment dwelling sipunculids, polychaetes and ophiuroids (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. Balanus crenatus and erect branching bryozoans, in particular, may be epizootic in MCR.MytHaS.

Productivity

Mytilus spp. communities are highly productive secondary producers (Seed & Suchanek, 1992; Holt et al., 1998). Kautsky (1981) estimated that the subtidal Mytilus edulis beds in the Baltic Sea contained a biomass of 10,200 tonnes dry weight in July-August, of which 1500 was meat. Kautsky (1981) also estimated that the mussel beds contributed up to 600 tons of organic carbon to the pelagic ecosystem, as eggs and larvae. However, the Baltic Sea subtidal mussel beds were subject to low levels of predation due to the reduced salinities and therefore more productive than might be expected of mussel beds in other subtidal locations.
No information concerning the productivity of circalittoral Mytilus edulis beds in the UK (MCR.MytHAs) was found. However, it is likely that they represent an important food resource for a number of predatory species, especially starfish, decapod crustaceans and fish (see ecosystem relationships).

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 1 month later. JØrgensen (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, and 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 fertilization 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 spat fall between 1976-1983 in the Exe estuary.
  • While Asterias rubens, for example, is widespread, and fecund, with a pelagic larvae capable of widespread dispersal, recruitment in starfish is sporadic, unpredictable and poorly understood (Seed, 1993).
  • Anthozoans, such as Alcyonium digitatum and Urticina felina are long lived with potentially dispersive pelagic larvae and are relatively widespread. They are not restricted to this biotope and would probably be able to recruit rapidly (refer to the Key Information reviews).
  • Balanus crenatus is an early colonizer of available space, with a dispersive, pelagic nauplius larvae and likely to recruit into the population rapidly.
  • Flustra foliacea, and other bryozoans have a short-lived, pelagic larvae, with probably poor dispersive abilities. However, bryozoans are widespread, and not restricted to this biotope and are likely to recruit from neighbouring populations fairly rapidly. Recruitment is likely to be aided by the strong tidal streams inhabited by this biotope.
  • Ascidians such as Molgula manhattensis have external fertilisation but short lived larvae, so that dispersal is probably limited. Where neighbouring populations are present recruitment may be rapid but recruitment from distant populations may take a long time.
  • 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.

Time for community to reach maturity

Holt et al. (1998) suggested that the associated species in this biotope (MCR.MytHAs) could colonize the rock surface in the absence of Mytilus edulis. Therefore, the occurrence of this biotope requires the presence of dense Mytilus edulis.
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 is 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. However, no information on recovery in subtidal Mytilus spp. populations was found. The associated community is likely to colonize the substratum or mussel matrix rapidly.

Additional information

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Preferences & Distribution

Recorded distribution in Britain and IrelandThe biotope has a restricted distribution around the UK. It has been described in surveys of Flamborough Head, Selsey Bill to Beachy Head, the Lizard, Skomer, the Lleyn Peninsula, the Menai Straits, Anglesey, Liverpool Bay, and Jura & Islay, west Scotland. It is also known to occur off the Gower Peninsula and in localized areas of Plymouth Sound.

Habitat preferences

Depth Range
Water clarity preferences
Limiting Nutrients Data deficient
Salinity Full (30-40 psu), Variable (18-40 psu)
Physiographic Open coast, Strait / sound
Biological Zone Circalittoral
Substratum Bedrock, Large to very large boulders, Small boulders, Cobbles, Mixed
Tidal Moderately Strong 1 to 3 knots (0.5-1.5 m/sec.), Strong 3 to 6 knots (1.5-3 m/sec.)
Wave Exposed, Moderately exposed, Sheltered
Other preferences

Additional Information

Mytilus edulis is found in circumpolar and temperature 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

    A total of 278 species were reported within records of this biotope in the MNCR surveys (JNCC, 1999).

    Sensitivity reviewHow is sensitivity assessed?

    Explanation

    The MCR.MytHAs biotope is characterized by the presence of Mytilus edulis. The other members of the community, and characterizing species can probably colonize the substratum in the absence of Mytilus edulis and have not been used as species indicative of biotope sensitivity. Therefore, Mytilus edulis has been included as a key structural species, since it provides secondary productivity, and substratum within the biotope. Asterias rubens is an important predator in mussel biotopes, together with other starfish, crabs and fish and is known to limit the distribution of mussel communities. Although the selected 'indicative species' are particularly important in undertaking the assessment because they have been subject to detailed research, account is taken of knowledge of the biology of all characterizing species in the biotope when undertaking an assessment of sensitivity of this biotope.

    Species indicative of sensitivity

    Community ImportanceSpecies nameCommon Name
    Important otherAlcyonium digitatumDead man's fingers
    Important functionalAsterias rubensCommon starfish
    Important otherBalanus crenatusAn acorn barnacle
    Key structuralMytilus edulisCommon mussel
    Important otherUrticina felinaDahlia anemone

    Physical Pressures

     IntoleranceRecoverabilitySensitivitySpecies RichnessEvidence/Confidence
    High High Moderate Major decline High
    Removal of the substratum will include the removal 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 Minor decline Moderate
    Intertidal Mytilus edulis beds have been reported to suffer mortalities 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. Although, Mytilus edulis is highly fecund, larval mortality is high. Larval development occurs within the plankton over ca 1 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 dependant on recruitment from outside the biotope and a recoverability of high has been reported (see additional information below).
    Low Immediate Not sensitive No change 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, 1977). Mytilus edulis also has efficient pseudofaeces discharge mechanisms (Moore, 1977; de Vooys, 1987). Similarly Asterias rubens flourishes in naturally turbid conditions and is capable of cleansing itself of adherent mud particles (Moore, 1977). However, both species probably suffer a metabolic cost resulting from the cleansing mechanisms, mucus production and interrupted or impaired feeding. Similarly, Urticina felina, Alcyonium digitatum and Balanus crenatus were considered to be of low intolerance to suspended sediment. In addition, the strong tidal streams characteristic of the biotope probably prevent suspended sediment settling out and hence reduces siltation. Therefore, a biotope intolerance of low, at the benchmark level, has been recorded. Hydroids, such as Sertularia spp. and Kirchenpaueria pinnata are likely to be more intolerant of siltation (Hiscock, 1983). However, greater increases in siltation may reduce the abundance of hydroids, bryozoans and anthozoans within the biotope especially on upward facing surfaces. The majority of the organisms within the biotope probably have mechanisms to deal with siltation and suspended sediment, so that recoverability of immediate has been recorded.
    Low High Not relevant Decline Moderate
    A decrease in suspended sediment, especially organic particulates could potentially reduce the food available to Mytilus edulis and the other suspension feeders within the biotope. Therefore, an intolerance of low has been recorded.
    Not relevant Not relevant Not relevant Not relevant High
    Circalittoral populations are unlikely to be exposed to desiccation.
    Not relevant Not relevant Not relevant Not relevant High
    An increase or decrease in tidal emergence is unlikely to affect circalittoral habitats.
    Not sensitive* High
    An increase or decrease in tidal emergence is unlikely to affect circalittoral habitats, except that the influence of wave action and tidal streams may be increased (see water flow rate below).
    Low Immediate Not sensitive No change Low
    The strong tidal streams characteristic of this biotope probably supply the community with adequate food in the form of particulates. This is a particular importance for passive suspension feeders such as hydroids and bryozoans. An increase in water flow may dislodge a proportion of the Mytilus edulis bed and increase competition for space from species adapted to very strong water flow rates such as Tubularia indivisa. Mytilus edulis populations are found from weak to strong tidal streams, suggesting a low intolerance to water flow rates. Similarly, Young (1985) reported that Mytilus edulis increased byssus thread production in response to increased agitation and water flow rates, and that Mytilus edulis was able to withstand surges of up to 16 m/s. However, Young (1985) also noted that mussels would be susceptible to sudden squalls and surges. Predation by Asterias rubens may also be decreased by increased water flow rates or wave exposure (Hiscock, 1983). Urticina felina and Alcyonium digitatum prefer areas of strong water flow, and Balanus crenatus is found in a wide range of water flow regimes. Species such as Molgula manhattensis and Flustra foliacea thrive in strong water flow but are found at low abundance in very wave exposed and very strong tidal streams (Hiscock, 1983). However, overall an intolerance of low has been recorded. Recoverability is likely to be rapid.
    Intermediate High Low Minor decline Very low
    The strong tidal streams characteristic of this biotope probably supply the community with adequate food in the form of particulates. This is a particular importance for passive suspension feeders such as hydroids and bryozoans. Mytilus edulis tolerates a wide range of water flow rates. However, decreases in water flow rates are likely to increase siltation (see above) and increase predation pressure from crabs, lobsters and starfish such as Asterias rubens. The biotope is likely to suffer from competition from species adapted to more sheltered conditions. Therefore, an intolerance of intermediate has been recorded. Although, Mytilus edulis is highly fecund, larval mortality is high. Larval development occurs within the plankton over ca 1 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 dependant on recruitment from outside the biotope and a recoverability of high has been reported (see additional information below).
    Low Very high Very Low No change Low
    Circalittoral 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, 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. Similarly, Urticina felina and Alcyonium digitatum were considered to be of low intolerance to temperature change. 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
    Circalittoral 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 to in temperature and even freezing for short periods. Similarly, Balanus crenatus, Alcyonium digitatum, Asterias rubens and Urticina felina were unaffected by the severe winter of 1962/63 (Crisp, 1964). 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. No information regarding the temperature tolerance of hydroids or bryozoans was found, and these groups may be more intolerant. Recovery is likely to be rapid (see additional information below).
    Tolerant Not relevant Not relevant No change Moderate
    Foliose algae have been reported in some records of this biotope (Hiscock, 1984). However, this biotope is primarily 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 Moderate
    Foliose algae have been reported in some records of this biotope (Hiscock, 1984). However, this biotope is primarily 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.
    Intermediate High Low Minor decline Low
    Wave exposure causes oscillatory flow on the sea bed, the magnitude of which is attenuated with depth. Therefore, increases in wave exposure are likely to increase water flow rates in the circalittoral (see increases in water flow above). However, oscillatory water movement is potentially far more destructive than tidal streams due to the 'to and fro' motion is more likely to loosen mussels. Therefore, an intolerance of intermediate has been recorded.
    Although, Mytilus edulis is highly fecund, larval mortality is high. Larval development occurs within the plankton over ca 1 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 dependant on recruitment from outside the biotope and a recoverability of high has been reported (see additional information below).
    Intermediate High Low Minor decline Very low
    Wave exposure causes oscillatory flow on the sea bed, the magnitude of which is attenuated with depth. Therefore, decreases in wave exposure are likely to decrease water flow rates in the circalittoral, depending on the prevalent tidal streams. See increases in water flow rates above.
    Tolerant Not relevant Not relevant No change High
    Although, some fish species may be scared off or deterred from feeding by underwater noise, the majority of the species within the biotope are unlikely to be adversely affected by, or detect underwater noise.
    Tolerant Not relevant Not relevant No change High
    None of the species within the biotope are likely to be adversely affected or detect changes in visual presence at the benchmark level.
    Intermediate High Low Minor decline Low
    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. However, benthic trawls tend to avoid rough ground, such as reefs and rocky areas. 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. Scallop dredging and otter trawls has also been reported to damage Alcyonium digitatum (Hartnoll, 1998; Holt et al., 1998). 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 passing 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 from outside the biotope 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 hydroids, bryozoans and Alcyonium digitatum as 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 edulisfrom outside the biotope and a recoverability of high has been reported (see additional information below).

    Chemical Pressures

     IntoleranceRecoverabilitySensitivityRichnessEvidence/Confidence
    Intermediate High Low Minor 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 gas 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).
    Mortality in Alcyonium digitatum was reported after exposure to the dispersant BP 1002 (Smith, 1968) whereas Smith (1968) found Urticina felina to be one of the more resistant species on the shore after the Torrey Canyon oil spill and Hoare & Hiscock (1974) reported it relatively close of a halogenated effluent discharge in Amlwch where other organisms were unable to survive. PCB exposure resulted in defective larvae in Asterias rubens (Besten et al., 1989). Barnacles, such as Balanus crenatus were considered to be highly intolerant of chemical contaminants (Holt et al., 1995). No information was found concerning the effect of contaminants hydroids, bryozoans or ascidians.
    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.
    Recovery of the mussel beds will probably require recruitment from other areas, while most other 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. Little information concerning heavy metal toxicity was found for hydroids, bryozoans and ascidians.
    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
    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). Circalittoral populations are protected from the immediate effects of oil spills by their depth. Therefore, hydrocarbon contamination in the circalittoral 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.
    Little information was found concerning the effects of hydrocarbon pollution on hydroids, bryozoans, or ascidians. Although, Asterias rubens has been assessed as highly intolerant, the mussel bed may benefit from a reduction in starfish predation, and an overall biotope intolerance of intermediate 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* No change 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, 'tolerant*' 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). Circalittoral populations may be too deep to be affected by feeding on toxic algae. However, 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).
    Not relevant Not relevant Not relevant Not relevant Not relevant
    The biotope is sublittoral and present on the open coast in full salinity conditions. Increase in salinity is therefore considered not relevant.
    Intermediate High Low Decline Moderate
    Mytilus edulis thrives in brackish lagoons and estuaries, however, this is probably due to the abundance of food in these environments rather than the salinity (Seed & Suchanek, 1992). Overall, Mytilus edulis can acclimate to a wide range of salinities and a change of salinity at the benchmark level is unlikely to adversely affect this species. However, many of the associated species in this biotope are restricted to open coast situations and are most likely intolerant of change in salinity. Therefore, an intolerance of intermediate is indicated. Once conditions return to normal, most of the associated species will recolonize from planktonic stages. A recoverability of high is given.
    Intermediate Very high Low Minor decline Moderate
    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). However, echinoderms such as Asterias rubens are highly intolerant of anoxic conditions. Similarly, Alcyonium digitatum and Balanus crenatus were considered to be highly intolerant of anoxia. Little information regarding the tolerance of ascidians and hydroids to hypoxia was found. Although Mytilus edulis is likely to tolerate hypoxic conditions, an intolerance of intermediate has been recorded due to the intolerance of the other members of the community. It should be noted that in the presence of strong to moderate tidal streams, anoxic conditions are unlikely to occur unless combined with reduced water flow rates. Recoverability of the associated species is likely to be rapid (see additional information below).

    Biological Pressures

     IntoleranceRecoverabilitySensitivityRichnessEvidence/Confidence
    Intermediate High Low Minor 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).
    Therefore, an intolerance of intermediate has been recorded. Recovery of the mussel beds will be dependant on recruitment from other populations and a recoverability of high has been recorded (see additional information below).
    Intermediate High Low No change Very low
    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 (Holt et al., 1998; Eno et al., 1997; McKay, 1994). 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).
    Intermediate High Low Major 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 for the past hundred years (Holt et al., 1998). Some shallow subtidal populations are found in turbid areas and are essentially circalittoral, and represented by this biotope.
    Subtidal mussel beds may be exploited 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 balthica.
    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 Not relevant

    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 1 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).
    The other characterizing species are likely to recolonize the substratum rapidly (see recruitment processes).

    Importance review

    Policy/Legislation

    Habitats Directive Annex 1Reefs

    Exploitation

    Little information concerning the exploitation of Mytilus edulis or other members of this community was found. For information concerning the exploitation of Mytilus edulis in general please refer to the species review.

    Additional information

    This biotope is included under the Habitats Directive Annex I features 'Reefs' and 'Estuaries'.

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    Citation

    This review can be cited as:

    Tyler-Walters, H., 2001. Mytilus edulis beds with hydroids and ascidians on tide-swept exposed to moderately wave-exposed circalittoral rock. 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/208

    Last Updated: 20/06/2001