Biodiversity & Conservation

CR.MCR.CMus.Mdis

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
(View Benchmark)
Removal of the substratum whether the macroalgae to which Musculus discors was attached, or the rocky substratum itself will result in loss of the community. Therefore, an intolerance of high has been recorded.
Recoverability will depend on recruitment from adjacent or nearby populations and may take many years (see additional information below).
Smothering
(View Benchmark)
Musculus discors lives in fixed nests of byssus threads on the surface of the substratum. While the nest will protect the bivalve from the direct effects of smothering, they are unlikely to be able to burrow up through deposited spoil or other smothering agent. 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 was considered to be highly intolerant. 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 5cm of sediment. However, smaller or encrusting forms and some ascidians (e.g. Clavelina lepadiformis) may be adversely affected. Overall, however, loss of the Musculus discors population would result in loss of the biotope and a biotope intolerance of high has been recorded.
Recoverability will depend on recruitment from adjacent or nearby population and may take many years (see additional information below).
Increase in suspended sediment
(View Benchmark)
Dense beds of Musculus discors in the north of the Llyn Peninsula and Holy Island, Anglesey were reported to 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 3-4m. Other dense aggregations of Musculus discors were reported from areas of strong tidal streams and presumably low levels of suspended sediment and siltation. Therefore, Musculus discors is probably tolerant of a wide range of suspended sediment levels.

Increased suspended sediment concentrations may clog suspension feeding apparatus, lead to 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 and an overall biotope intolerance of low has been recorded.

Recolonization and recovery of epifaunal species is likely to be rapid once the prior conditions return (see additional information below).
Decrease in suspended sediment
(View Benchmark)
Musculus discors is probably tolerant of a wide range of suspended sediment levels (see above). The species composition of associated epifaunal species is likely to vary with suspended sediment concentration, with sediment tolerant species being out-competed by fast growing, but less sediment tolerant species as the suspended sediment concentration decreases. Overall, although the associated epifaunal species may change, and species richness decline temporarily, the Musculus discors carpet is unlikely to be adversely affected. Therefore, an intolerance of low has been recorded.
Desiccation
(View Benchmark)
Musculus discors was assessed as highly intolerant of desiccation (see review). Similarly, subtidal epifaunal bryozoans, hydroids, ascidians and sponges are likely to be highly intolerant of desiccation. However, this biotope is subtidal and circalittoral and is unlikely to be exposed to the air. The shallower water extent of this biotope may be exposed for short periods during on extreme low water tides, however, overall this factor is not relevant.
Increase in emergence regime
(View Benchmark)
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).
Decrease in emergence regime
(View Benchmark)
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).
Increase in water flow rate
(View Benchmark)
Musculus discors has been recorded from weak to strong tidal streams. It is, therefore, tolerant of water flow within this range. An increase to very strong tidal streams may result in loss of a proportion of the population physically removed by water flow, either due to removal of the animal itself or removal of the algae to which it was attached. Similarly, the associated epifaunal species will vary with water flow, resulting in an increase in species tolerant of increased water flow. Therefore, an intolerance of intermediate has been recorded. Recovery will probably take up to 5 years (see additional information below).
Decrease in water flow rate
(View Benchmark)
Musculus discors has been recorded from weak to strong tidal streams. It is, therefore, tolerant of water flow within this range. Decreases water flow will favour epifaunal species tolerant of reduced water flow over species that prefer high water flow rates, so that the composition of the epifaunal species will change. A decrease in water flow to negligible may result in a stagnant deoxygenated water (see deoxygenation) and increased siltation (see above). Overall, although species composition may change the biotope will be not be adversely affected and an intolerance of low and a high recoverability has been recorded (see additional information below).
Increase in temperature
(View Benchmark)
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 dependant. However, loss of a few intolerant epifaunal or epifloral species will not significantly affect the biotope, and are likely to recover quickly. Therefore an intolerance of low has been recorded, with a recoverability of high (see additional information below).
Decrease in temperature
(View Benchmark)
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 decreases in temperatures or winter temperatures in British waters. Könnecker (1977) also suggested that Musculus discors associations were eurythermal. Many associated epifaunal species have a wide geographical distribution and are unlikely to be adversely affected by decrease in temperature within British waters. A few species may be more intolerant, e.g. Clavelina lepadiformis, Delesseria sanguinea, and Pentapora fascialis where they occur. However, loss or reduction of a few intolerant epifaunal species is unlikely to adversely affect the Musculus discors beds or the biotope as a whole. Therefore, an intolerance of low, with a high recoverability, has been recorded (see additional information below).
Increase in turbidity
(View Benchmark)
Increased turbidity will reduce phytoplankton productivity and may reduce food availability for Musculus discors, however, it is probably capable of utilizing other organic particulates so that the effects would probably be sub-lethal. Increased turbidity will also decrease the depth to which kelps and other macroalgae can grow, reducing their availability as substratum for Musculus discors. 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 3-4m. However, Musculus discors can utilize other substrata such as tunicates, animal turfs or hard substrata and is unlikely to be adversely affected. Increased turbidity is likely to decrease macroalgal cover, hence increasing potential space for Musculus discors and epifaunal species. Therefore, an intolerance of low has been recorded. Recovery will depend on recolonization of available space by macroalgae and may be rapid in the case of red algae or take many years in the case of kelps (see Laminaria hyperborea for example). Therefore a recoverability of high has been recorded.
Decrease in turbidity
(View Benchmark)
Decreased turbidity will result in increased light penetration, macroalgal growth and phytoplankton productivity, both of which may benefit Musculus discors by providing additional substratum for colonization and food respectively. Increased macroalgal growth, especially red algae, may compete for space with epifaunal hydroids and bryozoans, resulting in a change in epifaunal species composition and increased abundance of algae. However, overall , the biotope would be little affected and an intolerance of low has been recorded. Recoverability is likely to be very high.
Increase in wave exposure
(View Benchmark)
This biotope has been reported from areas of moderate wave exposure, whereas Musculus discors has been reported from wave exposed to extremely wave sheltered habitats and is therefore 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 are also likely to be removed, being replaced by more wave tolerant species, e.g. Tubularia indivisa. Therefore, an intolerance of intermediate has been recorded. Recovery will probably take up to 5 years (see additional information below).
Decrease in wave exposure
(View Benchmark)
This biotope has been reported from areas of moderate wave exposure, whereas Musculus discors has been reported from wave exposed to extremely wave sheltered habitats and is therefore relatively insensitive to changes in wave exposure within this range. 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 tolerant of reduced wave action or water movement, e.g. the hydroid Nemertesia spp. Overall, however, the biotope is likely to be little affected and an intolerance of low has been recorded. Recoverability has been recorded as high, to represent the time taken for the epifauna to recover a similar species composition.
Noise
(View Benchmark)
Few species within the biotope are likely to respond to noise or vibration at the benchmark level.
Visual Presence
(View Benchmark)
Few, if any, species within the biotope have a significant visual acuity, and are unlikely to respond to visual disturbance at the benchmark level.
Abrasion & physical disturbance
(View Benchmark)
Physical disturbance at the benchmark level 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.

Erect epifaunal species are particularly vulnerable to physical disturbance. Hydroids and bryozoans are likely to be uprooted or damaged by bottom trawling or dredging and bryozoans repair damage slowly (Holt et al., 1995). Veale et al. (2000) reported that the abundance, biomass and production of epifaunal assemblages decreased with increasing fishing effort.

Overall, physical disturbance at the benchmark level may remove or damage a proportion of the Musculus discors bed and its associated epifauna. Therefore, an intolerance of intermediate has been recorded. Recovery will probably take up to 5 years (see additional information below). However, large scale physical disturbance effects (e.g. from mobile fishing gear) may be more akin to substratum removal (see above).
Displacement
(View Benchmark)
Once displaced, Musculus discors, can re-attach to suitable substrata using its byssal threads and then weave another nest. Overall, once displaced onto suitable substratum an individual could rebuild its nest, at energetic cost, within about a month (see Merrill & Turner, 1963). Most epifaunal species, e.g. macroalgae, hydroids, sponges, bryozoans and ascidians are permanently attached and would be lost if displaced. Overall, Musculus discors would be able to re-attach, but a proportion of the population may be lost, together with its epifauna, therefore, the intolerance has been assessed as intermediate. Recoverability has been recorded as high (see additional information below).

Chemical Factors

Synthetic compound contamination
(View Benchmark)
  • 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 in 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 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. Therefore, an intolerance of intermediate has been recorded. Recovery has been recorded as high (see additional information below). an intolerance of intermediate has been suggested but at very low confidence. Recovery will probably take up to 5 years (see additional information below).
Heavy metal contamination
(View Benchmark)
  • 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 information to assess intolerance to heavy metals in Musculus discors, although the above evidence for hydroids suggests that they will display sublethal effects at least.
Hydrocarbon contamination
(View Benchmark)
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 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.
In the absence of specific information on Musculus discors but with evidence from other bivalve molluscs above, an intolerance of low has been suggested, albeit at very low confidence. 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. Recoverability has been assessed as very high (see additional information below).
Radionuclide contamination
(View Benchmark)
Insufficient information
Changes in nutrient levels
(View Benchmark)
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 above) and decreased oxygen levels due to bacterial decomposition of organic material (see below). The species composition of the epifaunal community may also change as a result. However, Musculus discors and other suspension feeders would probably benefit from increased nutrients at the benchmark level.
Increase in salinity
(View Benchmark)
This biotope occurs in full salinity and is unlikely to encounter increases in salinity.
Decrease in salinity
(View Benchmark)
Könnecker (1977) suggested that Musculus discors associations were 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 is probably tolerant of a reduction in salinity from full to variable, and a short term decrease to reduced but would probably be adversely affected by a long term reduction in salinity. 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 long term reduction in salinity is likely to have adverse effects, reducing the extent of the Musculus discors populations and significantly reducing the richness of the associated epifauna. Therefore, an intolerance of intermediate has been recorded with a recoverability of high (see additional information below).
Changes in oxygenation
(View Benchmark)
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. Therefore, a proportion of the Musculus discors bed may be lost together with members of its epifauna, and an intolerance of intermediate has been recorded albeit at very low confidence. Recovery will probably take up to 5 years (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
(View Benchmark)
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 an intolerance of low has been recorded.
Introduction of non-native species
(View Benchmark)
No information found.
Extraction
(View Benchmark)
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, an intolerance of intermediate has been recorded at the benchmark level. Recovery will probably take up to 5 years (see additional information below).

Additional information icon Additional information

Recoverability
No information concerning recruitment or recovery in Musculus discors was found. Brooding in Musculus discors probably results in relatively lower levels of juvenile mortality. Therefore, within populations recruitment is likely to be good.

Martel & Chia (1991) suggested that in species that brood their offspring (such as Musculus discors) bysso-pelagic drifting probably contributed to rapid local dispersal and recruitment, depending on the hydrographic regime. Hence, within a population or between adjacent populations recruitment and recovery of Musculus discors is probably fairly rapid, and it is suggested that prior abundance may recover within up to 5 years. However, where recovery is dependant on recruitment from distant populations recruitment may take longer. If a population is removed, recovery will depend on recruitment from nearby populations by drifting, followed by subsequent expansion of the population. The species is widespread so that a ready supply of juveniles will probably be present, albeit in small numbers. Therefore, it is suggested that recovery after removal of a population may take about 5 to 10 years.

Holt et al. (1995) suggested that many hydroids and bryozoans were rapid colonizers, able to settle rapidly, mature and reproduce quickly. Many species have a short lived planktonic phase, resulting in relatively local recruitment, however, fecundity is high and most species are widespread, so that recruitment is likely to be rapid from surrounding populations.

Ascidians 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.

Most sponge species produce short lived, planktonic larvae so that recruitment is localized, depending on the hydrographic regime. Some species (e.g. Polymastia robusta) produce benthic crawling larvae that probably settle close to the parent (see Fell, 1989 for review). Growth rate vary between and within species, so that time to reach maturity is also variable and large colonies may take several years to develop. However, little information was found.

In strong water flow associated with this biotope, most pelagic larvae are probably transported away form the biotope, so that most recruits of species with pelagic life stages come from outside the community. However, direct development and brooding in Musculus discors probably ensures a relatively good, local recruitment.

Overall, the community is primarily dependent on Musculus discors, which may regain abundance within 5 years, or recover from removal within 5-10 years. The associated epifaunal community will probably develop within less than 5 years, although slow growing sponges may take many years to develop.


This review can be cited as follows:

Tyler-Walters, H. 2002. Musculus discors beds on moderately exposed circalittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 01/08/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=90&code=2004>