The Marine Life Information Network

Substratum loss

Benchmark. All of the substratum occupied by the species or biotope under consideration is removed. A single event is assumed for sensitivity assessment. Once the activity or event has stopped (or between regular events) suitable substratum remains or is deposited. Species or community recovery assumes that the substratum within the habitat preferences of the original species or community is present. Further details

Evidence

Removal of the substratum whether the macroalgae to which Musculus discors was attached, or the rocky substratum itself will result in loss of the population. Therefore, an 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).

Smothering

Benchmark. All of the population of a species or an area of a biotope is smothered by sediment to a depth of 5 cm above the substratum for one month. Impermeable materials, such as concrete, oil, or tar, are likely to have a greater effect. Further details.

Evidence

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. Individuals on raised substrata such as the stipe of kelps may escape the effects of smothering. However, overall an 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

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Dense beds of Musculus discors in the north of the Lleyn 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). The byssus nest probably provides protection from accumulating silt. 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 and an intolerance of low has been recorded.

Decrease in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Musculus discors probably occurs in a wide range of suspended sediment conditions (see above). It is partly dependant on suspended sediment for food, so that a decrease may reduce food availability and hence, growth and reproduction but is unlikely to be lethal. In areas of strong tidal streams water flow probably carries adequate food particles. Therefore, an intolerance of low has been recorded.

Desiccation

1. A normally subtidal, demersal or pelagic species including intertidal migratory or under-boulder species is continuously exposed to air and sunshine for one hour.
2. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details.

Evidence

Intertidal populations of Musculus discors are likely to be affected by desiccation. The nest and attached debris and fauna probably provide protection from desiccation by retaining moisture. Individuals occupying interstices or the holdfasts of seaweeds will gain additional protection. For example, the fronds of Corallina officinalis retain water at low tide and support a more diverse fauna than the surrounding habitat (see species review). However, an increase in desiccation at the benchmark level is likely to reduce the upper shore extent of the population, and an intolerance of intermediate has therefore been recorded.
Recovery will probably take up to 5 years (see additional information below).

Increase in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

An increase in emergence may move a normally subtidal population into the intertidal or increase the desiccation risk of a normally intertidal population. Therefore, the extent or abundance of the population may be reduced, and an intolerance of intermediate has been recorded. Recovery will probably take up to 5 years (see additional information below).

Decrease in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

A decrease in emergence may allow a formerly intertidal population to expand in extent and/or abundance. Therefore, the population may benefit.

Increase in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

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. 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

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

Musculus discors has been recorded from weak to strong tidal streams. It is, therefore, tolerant of water flow within this range. Therefore an intolerance of low has been recorded. A further decrease in water flow to negligible may result in a stagnant deoxygenated water (see deoxygenation) and increased siltation (see above).

Increase in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

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 the Musculus discors association was eurythermal. Short term acute change may have adverse effects, however, the nest and associated fauna may buffer the species against rapid change in temperature but no information was found.

Decrease in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

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 was eurythermal. Short term acute change may have adverse effects, however, the nest and associated fauna may buffer the species against rapid change in temperature but no information was found.

Increase in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

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. However, Musculus discors can utilize other substrata such as tunicate holdfasts, animal turfs or hard substrata and is unlikely to be adversely affected. Therefore, an intolerance of low has been recorded.

Decrease in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Decreased turbidity will result in increased light penetration, macroalgal growth and phytoplankton productivity, both of which may benefit Musculus discors by providing additonal substratum for colonization and food respectively.

Increase in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

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. Populations attached to seaweeds are likely to be more intolerant, where the seaweed itself is removed or damaged by increased wave action. 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. Therefore, an intolerance of intermediate has been recorded. Recovery will probably take up to 5 years (see additional information below).

Decrease in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

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 further decrease in wave action is unlikely.

Noise

1. Underwater noise levels e.g., the regular passing of a 30-metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres for one month during important feeding or breeding periods.
2. Atmospheric noise levels e.g., the regular passing of a Boeing 737 passenger jet 300 metres overhead for one month during important feeding or breeding periods. Further details

Evidence

Musculus discors can probably detect local vibrations associated with predators, resulting in closure of its valves and nest. However, it is unlikely to respond to noise at the benchmark level.

Visual presence

Benchmark. The continuous presence for one month of moving objects not naturally found in the marine environment (e.g., boats, machinery, and humans) within the visual envelope of the species or community under consideration. Further details

Evidence

Musculus discors can probably detect shading but is likely to have only limited visual acuity, if any, and is unlikely to be affected by visual disturbance.

Abrasion & physical disturbance

Benchmark. Force equivalent to a standard scallop dredge landing on or being dragged across the organism. A single event is assumed for assessment. This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism or habitat of interest. Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale. Further details.

Evidence

Musculus discors has a reasonably tough shell. The byssus nest is made of fine threads and is likely to afford little protection against abrasion or physical damage. It is likely that physical disturbance at the benchmark level would physically remove some individuals from their substratum and break the shells of some individuals, depending on their size. Musculus discors may be affected indirectly by physical disturbance that removes macroalgae to which they are attached. Therefore an intolerance of intermediate has been recorded. Recovery will probably take up to 5 years (see additional information below).

Displacement

Benchmark. Removal of the organism from the substratum and displacement from its original position onto a suitable substratum. A single event is assumed for assessment. Further details

Evidence

Once displaced, Musculus discors, can re-attach to suitable substrata using its byssal threads and then weave another nest (Merrill & Turner, 1963). Specimens produced between 200 -500 byssus threads per week and the finished nest consists of thousands of byssus threads and attached detritus (Merrill & Turner, 1963). Overall, once displaced onto suitable substratum an individual could rebuild its nest, at energetic cost, within about a month. Therefore, the intolerance has been assessed as low and recoverability as very high.

Synthetic compound contamination

Sensitivity is assessed against the available evidence for the effects of contaminants on the species (or closely related species at low confidence) or community of interest. For example:

• evidence of mass mortality of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as high sensitivity;
• evidence of reduced abundance, or extent of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as intermediate sensitivity;
• evidence of sub-lethal effects or reduced reproductive potential of a population of the species or community of interest will be assessed as low sensitivity.

The evidence used is stated in the rationale. Where the assessment can be based on a known activity then this is stated. The tolerance to contaminants of species of interest will be included in the rationale when available; together with relevant supporting material. Further details.

Evidence

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 spatfall 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. 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

Evidence

Bryan (1984) stated that Hg is 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 g/l Hg, 1-10 g/l Cu and Cd, 10-100 g/l Zn but 1-10 mg/l for Pb and Ni.
However no information on the effects of heavy metal contamination in Musculus discors was found and no assessment could be made.

Hydrocarbon contamination

Evidence

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. Intertidal populations may become smothered by oil (see smothering).
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.

Radionuclide contamination

Evidence

Insufficient
information

Changes in nutrient levels

Evidence

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. 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). However, Musculus discors would probably benefit from increased nutrients at the benchmark level.

Increase in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Musculus discors occurs in variable and full salinity conditions. Therefore, a further increase in salinity is unlikely.

Decrease in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Musculus discors occurs in variable and full salinity conditions. 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. Therefore an intolerance of intermediate has been recorded in the absence of further evidence. Recovery will probably take up to 5 years (see additional information below).

Changes in oxygenation

Benchmark.  Exposure to a dissolved oxygen concentration of 2 mg/l for one week. Further details.

Evidence

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.
However, Musculus discors probably exhibits facultative anaerobiosis and is probably tolerant of a degree of hypoxia. Therefore, an intolerance of intermediate has been recorded albeit at very low confidence. Recovery will probably take up to 5 years (see additional information below).

Introduction of microbial pathogens/parasites

Benchmark. Sensitivity can only be assessed relative to a known, named disease, likely to cause partial loss of a species population or community. Further details.

Evidence

Musculus discors was reported to host the ciliate Hypocomides musculus, which was either parasitic or commensal. The metacercariae of the trematode Gymnophallusspp. 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

Sensitivity assessed against the likely effect of the introduction of alien or non-native species in Britain or Ireland. Further details.

Evidence

No information found.

Extraction of this species

Benchmark. Extraction removes 50% of the species or community from the area under consideration. Sensitivity will be assessed as 'intermediate'. The habitat remains intact or recovers rapidly. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Musculus discors is not known to be subject to extraction.

Extraction of other species

Benchmark. A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a keystone species in a biotope is removed. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Musculus discors is often found epiphytically on fucoids, laminarians and Desmarestia sp. Fucoids and 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. 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).