The Marine Life Information Network

Musculus discors has a wide distribution extending from the Arctic Circle to the Mediterranean in western Europe. It is, therefore, unlikely to be affected by increases in temperature in British waters. Könnecker (1977) also suggested that Musculus discors associations were eurythermal. Similarly, many epifaunal species found in the biotope have a widespread distribution and are unlikely to be adversely affected by long-term change within British waters. Short-term acute change may have adverse effects, for example, reproduction in Clavelina lepadiformis, Delesseria sanguinea and hydroids is temperature dependent. However, loss of a few epifaunal or epifloral species will not significantly affect the biotope, and are likely to recover quickly. Therefore, a resistance of High has been recorded. Hence, resilience is High (by default) and the biotope is recorded as Not sensitive at the benchmark level.

Musculus discors has a wide distribution extending from the Arctic Circle to the Mediterranean in western Europe. It is, therefore, unlikely to be affected by increases in temperature in British waters. Könnecker (1977) also suggested that Musculus discors associations were eurythermal. Similarly, many epifaunal species found in the biotope have a widespread distribution and are unlikely to be adversely affected by long-term change within British waters. Short-term acute change may have adverse effects, for example, reproduction in Clavelina lepadiformis, Delesseria sanguinea and hydroids is temperature dependent. However, loss of a few epifaunal or epifloral species will not significantly affect the biotope, and are likely to recover quickly. Therefore, a resistance of High has been recorded. Hence, resilience is High (by default) and the biotope is recorded as Not sensitive at the benchmark level.

Könnecker (1977) classified Musculus discors associations as euryhaline but without explanation. Musculus discors was recorded from fjordic waters in East Greenland that varied between 25-30 psu (Ockelmann, 1958) and from Loch Strom, Shetland that varied between 18-35psu (Thorpe, 1998).  However, no evidence was found on the effect of hypersaline (>40 psu) conditions.

Könnecker (1977) classified Musculus discors associations as euryhaline but without explanation. Musculus discors was recorded from fjordic waters in East Greenland that varied between 25-30 psu (Ockelmann, 1958) and from Loch Strom, Shetland that varied between 18-35psu (Thorpe, 1998). Intertidal populations of Musculus discors are probably exposed to freshwater runoff and rainfall. Therefore, Musculus discors itself is probably tolerant of a reduction in salinity from full to variable or even reduced for a year.  However, Connor et al. (2004) noted that Musculus discors occurred in the lagoonal biotope R.LIR.Lag.AscSpAs at reduced salinity but at lower densities than occurred in this biotope.  Hence, a decrease in salinity from ‘full’ to ‘reduced’ for a year may result in a reduction in the abundance Musculus discors and possibly extent of the bed.  Most species or hydroids, ascidians, sponges and bryozoans are stenohaline, occurring only in full salinity waters, although some species are euryhaline. Therefore, a reduction in salinity is likely to result in a decline in species richness of epifaunal species.

Overall, a reduction in salinity from ‘full’ to ‘reduced’ is likely to have adverse effects, reducing the extent of the Musculus discors populations and significantly reducing the richness of the associated epifauna.  Therefore, a resistance of Low is recorded, so that resilience is probably Medium and a sensitivity of Medium is recorded.  

The Musculus discors bed biotope is recorded in areas subject to moderately strong tidal streams (0.5-1.5 m/s) (Connor et al., 2004). However, the Musculus discors communities described by Cabioch (1968) occurred in areas subject to strong currents, and the Musculus discors communities in Kilkieran Bay were associated with currents greater than 2.5 m/sec (5 knots, strong tidal streams) (Könnecker, 1977; 1983).  Water flow is probably important for this epifaunal community, in order to provide food (as particulates and plankton), oxygenate the water column and keep the habitat free of excessive silt.  Therefore, a decrease in water flow to e.g. weak would probably be detrimental to the biotope. Similarly, and increase in water flow to e.g very strong may be detrimental if the resultant water flow removed or destabilised the bed. However, no direct evidence of disturbance due to changes in water flow or storms was found.  Nevertheless, a change in water flow of 0.1-0.2 m/s (the benchmark) is within the normal range experienced by the biotope. Therefore, the biotope is considered to be Not sensitive at the benchmark level.

Not relevant to circalittoral habitats below 5m.

The Musculus discors bed biotope is recorded from moderately wave exposed and wave sheltered conditions (Connor et al., 2004), whereas Musculus discors has been reported from wave exposed to extremely wave sheltered habitats and is, therefore, probably relatively insensitive to changes in wave exposure within this range. Should the wave exposure increase from exposed to extremely exposed, Musculus discors may be removed, even in the shallow subtidal, where the oscillatory water flow generated by wave action is likely to dislodge and remove at least a proportion of the population. Similarly, a proportion of the associated epifaunal species is also likely to be removed, being replaced by more wave tolerant species, e.g. Tubularia indivisa.  A decrease in wave exposure, e.g. from moderately exposed to very sheltered is likely to increase siltation and increase the risk of deoxygenated conditions (see below). The species composition of the epifauna is likely to change, favouring species resistant of reduced wave action or water movement, e.g. the hydroid Nemertesia spp. but the biotope is likely to be little affected.  Nevertheless, a 3-5% change in significant wave height is unlikely to adversely affect the biotope, which occurs below 10 m where wave action in attenuated.  Therefore, the biotope is considered to be Not sensitive at the benchmark level.

This pressure is Not assessed but evidence is presented where available.

No information concerning the effects of heavy metals on Musculus discors was found. However,

• Bryan (1984) stated that Hg was the most toxic metal to bivalve molluscs while Cu, Cd and Zn seemed to be most problematic in the field. In bivalve molluscs Hg was reported to have the highest toxicity, decreasing from Hg > Cu and Cd > Zn > Pb and As > Cr ( in bivalve larvae, Hg and Cu > Zn > Cd, Pb, As, and Ni > to Cr). Crompton (1997) reported that adult bivalve mortalities occurred after 4-14 day exposure to 0.1-1 µg/l Hg, 1-10 µg/l Cu and Cd, 10-100 µg/l Zn but 1-10 mg/l for Pb and Ni.
• Boero (1984) noted that Cu and Cd affected a general control mechanisms, resulting in an increase in growth, while high Cu concentrations increased production of gonozooids, i.e. sexual reproduction of pelagic larvae in the hydroid Laomedea flexuosa, possibly a response to unfavourable conditions. Bryan (1984) also reported morphological abnormalities in the hydroid Eirene viridula induced by low levels of Hg, Cd PB and Zn, while Clava multicornis showed sublethal effects after 6 weeks exposure to 200ppb Cd. Tubularia sp. was reported to be resistant to pollution, including Cu (Boero, 1984).
• Bryozoans may accumulate trace metals to a certain extent and freshwater bryozoans were more intolerant of low concentrations of Cu than other freshwater organisms (Holt et al., 1995).
• Bryan (1984) suggested that the general order for heavy metal toxicity in seaweeds is: organic Hg > inorganic Hg > Cu > Ag > Zn > Cd > Pb. Cole et al. (1999) reported that Hg was very toxic to macrophytes.

Overall, there was insufficient evidence to assess resistance to heavy metals in Musculus discors, although the above evidence for hydroids suggests that they will display sublethal effects at least.

This pressure is Not assessed but evidence is presented where available.

Subtidal populations are protected from the direct effects of oil spills by their depth but are likely to be exposed to the water soluble fraction of oils and hydrocarbons, or hydrocarbons adsorbed onto particulates.

• Suchanek (1993) noted that sub-lethal levels of oil or oil fractions reduce feeding rates, reduce respiration and hence growth, and may disrupt gametogenesis in bivalve molluscs. Widdows et al. (1995) noted that the accumulation of PAHs contributed to a reduced scope for growth in Mytilus edulis.
• Musculus discors may exhibit a similar response to hydrocarbon contamination but no information was found.
• Suchanek (1993) reported that the anemones Anthopleura spp. and Actinia spp. survived in waters exposures to spills and chronic inputs of oils. Similarly, one month after the Torrey Canyon oil spill the dahlia anemone, Urticina felina, was found to be one of the most resistant animals on the shore, being commonly found alive in pools between the tide-marks which appeared to be devoid of all other animals (Smith, 1968). However, the hydroid Tubularia sp. experienced significant mortality when exposed to low concentrations of crude oil (Suchanek, 1993).
• Laboratory studies of the effects of oil and dispersants on several red algae species, including Delesseria sanguinea (Grandy 1984 cited in Holt et al. 1995) concluded that they were all sensitive to oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages. O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination.

No direct evidence on the effects of hydrocarbon contamination on Musculus discors was found. The intolerance of the epifaunal species within the community is probably variable so that some species may be lost while others survive, so that species richness is likely to be reduced.

This pressure is Not assessed but evidence is presented where available. No information concerning the effects of contaminants on Musculus discors was found. However,

• PAHs contributed to a reduced scope for growth in Mytilus edulis (Widdows et al., 1995) and may have a similar effect on other members of the Mytilidae family but to an unknown degree.
• Similarly, Tri butyl-tin (TBT) was reported to affect bivalve molluscs as follows: reduced spat fall in Pecten maximus, Musculus marmoratus and Limaria hians; inhibition of growth in Mytilus edulis larvae, and inhibition of growth and metamorphosis in Mercenaria mercenaria larvae (Bryan & Gibbs, 1991).
• TBT is an endocrine disrupter and may adversely affect the normal transition from male to female in the protandrous development of Musculus discors, however, no evidence to this effect was found. It is possible, therefore, that Musculus discors is likely to be adversely affected and even killed by synthetic chemical contamination.
• Bryan & Gibbs (1991) suggested that some hydroids were intolerant of TBT levels between 100-500ng/l. Some hydroids appear to tolerate noxious conditions e.g. Tubularia sp., however, hydroid species richness is reduced in polluted conditions and hydroids may be excluded in highly polluted waters (Boero, 1984; Holt et al., 1995).
• Rees et al. (2001) suggested that the intolerance of ascidian larvae to TBT may explain their recent recorded increases in abundance in the Crouch Estuary following a decline in TBT concentrations since 1988.
• Laboratory studies of the effects of oil and dispersants on several red algae species, including Delesseria sanguinea (Grandy 1984 cited in Holt et al. 1995) concluded that they were all sensitive to oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages. O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination. Cole et al. (1999) suggested that herbicides, such as simazina and atrazine were very toxic to macrophytes.
• Cole et al. (1999) suggested that herbicides, insecticides, chlorophenols and dichlorophenols were very to highly toxic to marine organisms, especially algae, crustaceans and other invertebrates.
• Smith (1968) reported that Alcyonium digitatum was killed by exposure to dispersant (BP 1002) used to clean up the Torrey Canyon oil spill, whereas Urticina felina was found to be one of the most resistant species on the shore. Similarly, Hoare & Hiscock (1974) found that Urticina felina survived fairly near to an acidified halogenated effluent discharge in a 'transition' zone where many other species were unable to survive, suggesting a tolerance to chemical contamination.

Overall, Musculus discors may be adversely affected by synthetic chemical contamination, resulting in a loss of a proportion of the population. The associated epifaunal species, especially red algae, hydroids and ascidians, are intolerant of varying degrees and may be lost, reducing species richness. 

No evidence was found.

This pressure is Not assessed.

De Zwaan & Mathieu (1992) suggested that members of the family Mytilidae were facultative anaerobes (capable of anaerobic respiration but preferring aerobic respiration) and were tolerant of a wide range of oxygen concentrations (euryoxic). The majority of evidence is derived from the study of Mytilus spp. and no information was found on Musculus spp. Hydroids inhabit mainly environments in which the oxygen concentration exceeds 5ml/l and respiration is aerobic (Gili & Hughes, 1995). Delesseria sanguinea was reported to be very intolerant of anaerobic conditions; at 15°C death occurs within 24hrs and no recovery takes place although specimens survived at 5°C. (Hammer 1972).

Overall, Musculus discors probably exhibits facultative anaerobiosis and is probably tolerant of a degree of hypoxia, whereas some members of the associated epifauna are probably highly intolerant.  A reduction in oxygen levels below 2 mg/l for a week would probably be detrimental, but the effects would be limited in the strong to moderately strong water flow typical of this biotope. Therefore, a resistance of Medium is suggested.  Resilience is probably High so that sensitivity is recorded as Low.

Moderate increases in nutrient levels may benefit Musculus discors by increasing macroalgal and phytoplankton productivity, increasing the proportion of organic particulates and hence increasing the food supply. Similarly, increased availability of organic particulates may benefit the other suspension feeding members of the community, e.g. hydroids, bryozoans, sponges and ascidians. However, Shumway (1990) reported the toxic effects of algal blooms on commercially important bivalves. This would suggest that prolonged or acute nutrient enrichment may have adverse effects on suspension feeding bivalves such as Musculus discors. Nutrient enrichment may also lead to increased turbidity (see suspended sediments above) and decreased oxygen levels due to bacterial decomposition of organic material (see above). The species composition of the epifaunal community may also change as a result. However, this biotope is considered to be 'Not sensitive' at the pressure benchmark that assumes compliance with good status as defined by the WFD.

Dense beds of Musculus discors in the north of the Llyn Peninsula and Holy Island, Anglesey were reported to be covered by a thick layer of mucous congealed fine silt and their own pseudofaeces (Hiscock, 1984; Brazier et al., 1999). The presence of pseudofaeces suggests a resistance to localised organic enrichment, although strong to moderately strong water flow would probably prevent build up of the products of decomposition (e.g. hydrogen sulphide).  In their meta-analysis, Johnston & Roberts (2009) concluded that contaminants such a sewage and nutrients resulted in a loss of species diversity.

Therefore, it is possible that an increase in organic carbon may result in a loss of species richness, and an increase in siltation and suspended solids depending on the nutrient status of the receiving waters (i.e. oligotrophic or eutrophic). However, in the absence of any direct evidence, no assessment has been made.

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is, therefore ‘High’.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

Musculus discors requires hard substrata for attachment with byssal threads, as do the majority of the other epifauna and flora in the biotope. Therefore, a change in substratum from hard rock to sediment would result in loss of the biotope. Resistance to the pressure is considered ’None‘, and resilience ’Very low‘ or ‘None’. The sensitivity of this biotope to change from hard rock or artificial substrata to sedimentary or soft rock substrata  is assessed as ’High’.

Not relevant on hard rock substrata.

Not relevant on hard rock substrata.

Erect epifaunal species are particularly vulnerable to physical disturbance. Veale et al. (2000) reported that the abundance, biomass and production of epifaunal assemblages decreased with increasing fishing effort. Hydroids and bryozoans are likely to be uprooted or damaged by bottom trawling or dredging and bryozoans repair damage slowly (Holt et al., 1995).  Physical abrasion would probably physically remove some Musculus discors individuals from their substratum and break the shells of some individuals, depending on their size. Disturbance of the cohesive mat of individuals may strip away tracts of the biotope or create gaps or 'edges' that may allow peeling away of the Musculus discors mat by tidal streams or wave action. Musculus discors may be affected indirectly by physical disturbance that removes macroalgae to which they are attached.

Sensitivity assessment. Physical abrasion may remove or damage a proportion of the Musculus discors bed and its associated epifauna. Therefore, a resistance of Low has been recorded. Resilience is probably Medium, so that sensitivity is recorded as Medium.

Not relevant on hard rock biotopes. However, penetrative activities may also cause abrasion as above.

Dense beds of Musculus discors in the north of the Llyn Peninsula and Holy Island, Anglesey were reported to be covered by a thick layer of mucous congealed fine silt and their own pseudofaeces (Hiscock, 1984; Brazier et al., 1999). Brazier et al. (1999) reported that the waters around Holy Island where the Musculus discors beds were found, were highly turbid, and restricted kelps to the level of chart datum and red algae to depths of only 3-4 m. Other dense aggregations of Musculus discors were reported from areas of strong tidal streams and presumably low levels of suspended sediment and siltation.

Increased suspended sediment concentrations may clog suspension feeding apparatus, lead to the smothering of epifauna and cover the leaves of foliose algae, resulting in reduced photosynthesis. Therefore, the epifaunal community, especially of hydroids, bryozoans and ascidians, is likely to change, with intolerant species replaced by sediment tolerant species. However, although the species richness will decline, the Musculus discors populations will probably be little affected.  A decrease in suspended sediment may reduce the food supply for suspension feeding epifauna but otherwise have limited effect on the biotope.

Sensitivity assessment. Musculus discors is probably tolerant of a wide range of suspended sediment levels based on the evidence above.  Therefore, a resistance of High is recorded, so that resistance is also High (by default) and the biotope is considered to be Not sensitive at the benchmark level.

Musculus discors lives in fixed nests of byssus threads on the surface of the substratum. The byssal mat collects silt and pseudofaeces (Hiscock, 1984; Brazier et al., 1999), however, individual byssal nests within the mat open and the siphons of Musculus discors protrude out of the surface of the mat while feeding (Merrill & Turner, 1963; Baldock et al., 1998). While the nest will protect the bivalve from the direct effects of smothering, deposited spoil will smother the surface of the mat. Individual Musculus discors are unlikely to be able to burrow up through deposited fine spoil. Smothered individuals will probably succumb to the effects of anoxia. Although, individuals on raised substrata such as the stipe of kelps may escape the effects of smothering, Musculus discors within the bed (or mat) are unlikely to be resistant. Large epifauna such as Alcyonium digitatum, Nemertesia antennina, large branching or globose sponges and anemones (e.g. Urticina felina) are unlikely to be adversely affected by smothering with 5 cm of sediment. However, smaller or encrusting forms and some ascidians (e.g. Clavelina lepadiformis) are  likely to be smothered. 

Sensitivity assessment. The effects of smothering will depend on duration. In moderately strong tidal streams or moderate wave exposure, 5 cm of fine sediment may not remain over the biotope for more than few tidal cycles. As most bivalve molluscs can respire anaerobically for short periods, it is possible that most of the population of Musculus discors would survive. Therefore, a resistance of Medium is suggested. Resilience is probably High so that a sensitivity of Low is recorded.

Musculus discors lives in fixed nests of byssus threads on the surface of the substratum. The byssal mat collects silt and pseudofaeces (Hiscock, 1984; Brazier et al., 1999), however, individual byssal nests within the mat open and the siphons of Musculus discors protrude out of the surface of the mat while feeding (Merrill & Turner, 1963; Baldock et al., 1998). While the nest will protect the bivalve from the direct effects of smothering, deposited spoil will smother the surface of the mat.  Individual Musculus discors are unlikely to be able to burrow up through deposited fine spoil. Smothered individuals will probably succumb to the effects of anoxia. Although, individuals on raised substrata such as the stipe of kelps may escape the effects of smothering, Musculus discors within the bed (or mat) are unlikely to be resistant. Large epifauna such as Alcyonium digitatum, Nemertesia antennina, large branching or globose sponges and anemones (e.g. Urticina felina) are unlikely to be adversely affected by smothering with 5 cm of sediment. However, smaller or encrusting forms and some ascidians (e.g. Clavelina lepadiformis) are likely to be smothered. 

Sensitivity assessment. The effects of smothering will depend on duration. In moderately strong tidal streams or moderate wave exposure, 30 cm of fine sediment may remain over the biotope for several tidal cycles. It is possible that a proportion of the Musculus discors population would succumb to anoxia in this period. Therefore, a resistance of Low is suggested. Resilience is probably Medium, so that a sensitivity of Medium is recorded. 

Not assessed.

No evidence was found

Musculus discors is unlikely to respond to underwater noise and is sedentary.

Circalittoral biotopes occur below the influence of light (by definition).  Therefore, artificial light could potentially increase the depth to which red algae can colonize the biotope, assuming the light sources were strong enough to penetrate the water column. Similarly, shading may reduce red algal abundance within the biotope. Otherwise the biotope is unlikely to be adversely affected.  Resistance and resilience are considered to be High and the biotope is recorded as Not sensitive to this pressure.

Musculus discors beds probably exhibit good local recruitment, so that barriers to larval transport are probably not significant (see resilience and recovery rates). However, if a bed was damaged significantly, then barriers to larval transport may prolong recovery.

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

Musculus discors probably responds to shading by closing its values but its visual acuity is probably very limited. . However, visual disturbance as defined under the pressure benchmark is unlikely to be relevant.

Musculus discors is not subject to translocation nor genetic manipulation via breeding programmes or genetic modification. Therefore, this pressure is not relevant.

Rapid colonizing and carpeting species such as Didemnum vexillum, Botrylloides violaceus, have been reported to smother mussels (Mytilus spp.) (GBNNSIP, 2012a&b). Didemnum vexillum may also smother epifauna and substrata in the shallow sublittoral.  Therefore, both species could potentially smother Musculus discors beds. However, they are yet to be reported growing in the vicinity of Musculus discors beds.  Therefore, in the absence of direct evidence no assessment has been made, but may be subject to reassessment as new evidence comes to light.

Musculus discors was reported to host the ciliate Hypocomides musculus, which was either parasitic or commensal. The metacercariae of the trematode Gymnophallus spp. were also reported to use Musculus discors as a secondary host (Lauckner, 1983). However, no effects were given. It is likely that any parasitic infestation will result in at least sub-lethal effects, therefore, a resistance of High has been recorded. Hence, resilience is High and Not sensitive is recorded.

Musculus discors is not known to be subject to extraction or harvesting. Laminarians are subject to harvesting and aquaculture (see Laminaria hyperborea for example). Therefore, removal of the macroalgae will result in removal of substratum and attached Musculus discors when they are abundant within the biotope (see Baldock et al., 1998 for example). However, members of the population on the surrounding rocky substratum may be unaffected, and removal of macroalgae may provide new substratum for colonization. Therefore, a resistance of Medium has been recorded at the benchmark level. Resilience is probably High so that a sensitivity of Low has been recorded.

The incidental removal of Musculus discors bed (or mat) by passing bottom fishing gear is addressed under abrasion above. However, the dense Musculus discors bed provides a unique habitat, attachment for other epifauna and macroalgae (e.g. red algae), and support infauna of other species within the nests and byssal mat. Loss of the mat would result in a loss of species diversity.  Therefore, a resistance of Low is recorded. Resilience is probably Medium so that a sensitivity of Medium is recorded.