Green crenella (Musculus discors)

Distribution data supplied by the Ocean Biodiversity Information System (OBIS). To interrogate UK data visit the NBN Atlas.Map Help

Summary

Description

A small bivalve, usually up to 12 mm in length. The shell is brittle, equivalved but inequilateral and rhomboidal in outline. The beaks and umbones slightly raised and a short distance from the anterior end. The anterior and posterior portions of the shell bear different numbers of radiating ribs, while the middle region lacks ribs. Where the ribs meet the shell margin, the margin is crenulated but is smooth elsewhere. The shell is yellow to brownish in colour with a pale green to olive periostracum. The inside of the shell is pearly (nacreous) with a narrow pallial line and distinct anterior and posterior adductor scars. This genus is unusual in that the byssus threads used to fix it to the substratum are woven into a nest or cage, that may incorporate macroalgae (e.g., Fucus spp. and Laminaria spp.). This species lays its eggs in mucus strings that are retained within the byssal nest.

Recorded distribution in Britain and Ireland

Common around most of the British Isles from Shetland to the Channel Isles.

Global distribution

A panartic bivalve, found from the Arctic Circle south through the Bering Sea to Japan or to the Puget Sound in the Pacific or south to New York or Madeira in the Atlantic, including the western Baltic and Mediterranean.

Habitat

Found in scattered, gregarious clumps growing epiphytically on the holdfasts of seaweeds and amongst faunal turfs from the lower intertidal to the circalittoral subtidal on most substrata. It occasionally forms extensive, dense aggregations covering upward facing rock surfaces.

Depth range

Intertidal to ca 50m deep.

Identifying features

  • Shell rhomboidal in outline usually up to 12 mm in length.
  • Shell brittle, equivalve, and inequilateral.
  • Yellow to brownish in colour with a pale green to olive periostracum.
  • Beaks short distance from anterior end.
  • External ligament deeply inset and hinge teeth simple.
  • Anterior region of shell with 9-12 radiating ribs.
  • Posterior region with 30-45 finer, radiating ribs and fine concentric lines.
  • Posterior adductor scar fat, anterior scar long and thin.
  • Pallial line narrow.
  • Margin smooth but crenulate where radiating ribs meet the margin.

Additional information

The nest completely encloses the adult so that Musculus discors is only visible when its valves are open and it is feeding. Smooth specimens, lacking ribs, were reported from Oban and Staffa, in the Hebrides (Jeffreys, 1863).

Listed by

- none -

Biology review

Taxonomy

LevelScientific nameCommon name
PhylumMollusca
ClassBivalvia
OrderMytilida
FamilyMytilidae
GenusMusculus
Authority(Linnaeus, 1767)
Recent SynonymsModiolaria discors (Linnaeus, 1767)

Biology

ParameterData
Typical abundanceHigh density
Male size range
Male size at maturity
Female size rangeSmall(1-2cm)
Female size at maturity
Growth formBivalved
Growth rateSee additional text
Body flexibilityNone (less than 10 degrees)
Mobility
Characteristic feeding methodActive suspension feeder
Diet/food sourceDetritivore, Planktotroph
Typically feeds onPhytoplankton, bacteria, organic particulates and dissolved organic matter (DOM).
SociabilityGregarious
Environmental positionEpilithic
DependencyIndependent.
SupportsHost

ciliate parasites or commensals and trematode metacercariae.

Is the species harmful?No information

Biology information

Abundance. Musculus discors usually occurs as distinct clumps but occasionally forms dense, extensive beds (Könnecker & Keegan, 1983; Cartlidge & Hiscock, 1980; Hiscock, 1984b; Baldock et al., 1998).

Habit. Adults attach to their substratum using byssus threads. They then weave a 'nest' of several thousand of fine byssus threads around their shell, so that the shell is suspended in a network of byssus threads, similar to a 'ball of twine'. The byssus threads are not attached to the shell but only emanate from the byssal aperture. The nest completely encloses the adult so that the crenella is only visible when its valves are open and it is feeding with siphons extended (MacGinitie, 1955; Merrill & Turner, 1963). An adult may produce between 200 and 500 threads / week (Merrill & Turner, 1963). Nest construction is detailed by Merrill & Turner (1963). The nest may incorporate a variety of pieces of seaweeds or detritus or may be fouled by epifauna, the exact composition depending on the location and habitat, which provide camouflage. For example, the nest may incorporate; the stolons of hydroids, bryozoans, small bivalves and annelids (MacGinitie, 1955; Merrill & Turner, 1963); fragments of Flustra foliacea (Forbes & Hanley, 1853), or fragments or blades of fucoids and laminarians (Thorson, 1935). MacGinitie (1955) noted that specimens from East Greenland >2 cm were nearly always covered by a byssal nest. However, in British Columbia Merrill & Turner (1963) noted that the smallest specimens with nests were 8.1 mm in length and most specimens over 1.5 cm had nests, although some specimens up to 1.8 cm in length were without a nest.

Growth. Thorson (1935) reported that Musculus discors was 5 to 7 mm in length by the first growth ring (presumably its first year), 10 to 11 mm by the second and 13 to 16 mm by the third (presumably its third year) in east Greenland.  However, growth rates will probably depend on environmental conditions. Little other information on the biology of Musculus discors was found.

Habitat preferences

ParameterData
Physiographic preferencesEnclosed coast or Embayment, Open coast, Ria or Voe, Sea loch or Sea lough, Strait or Sound
Biological zone preferencesLower eulittoral, Lower infralittoral, Sublittoral fringe, Upper circalittoral, Upper infralittoral
Substratum / habitat preferencesBedrock, Gravel / shingle, Large to very large boulders, Macroalgae, Mud, Small boulders
Tidal strength preferencesModerately strong 1 to 3 knots (0.5-1.5 m/sec.), Strong 3 to 6 knots (1.5-3 m/sec.), Weak < 1 knot (<0.5 m/sec.)
Wave exposure preferencesExposed, Extremely sheltered, Moderately exposed, Sheltered, Very sheltered
Salinity preferencesFull (30-40 psu), Variable (18-40 psu)
Depth rangeIntertidal to ca 50m deep.
Other preferences

No text entered

Migration PatternNon-migratory or resident

Habitat Information

Musculus discors forms gregarious clumps on the holdfasts of seaweeds, especially Corallina officinalis, Fucus spp. Laminaria spp. and Desmarestia sp. On laminarians, Musculus discors may cover the holdfast and bottom part of the stipe (Jeffreys, 1863; Ockelmann, 1958; Tebble, 1976). Small specimens (<20mm) were reported nestled in interstices between barnacles and the old holdfasts of tunicates (MacGinitie, 1955). Merrill & Turner (1963) found Musculus discors fouling the upper surface of the sea scallop Placopecten magellanicusMusculus discors occasionally forms dense aggregations, especially in strong tidal streams, covering rock surfaces, forming the biotope MCR.Mus.

Most records reported Musculus discors in the shallow subtidal to depths of up to 50m in the British Isles. However, it has been reported to be abundant above 30-40 m in east Greenland, to form a well-developed community at 60-100 m in the Fosse de la Hague in the English Channel (Cabioch, 1968), to occur from 0 -374 m in the Barents Sea (Ockelmann, 1958), and to be common in most trawls from 130-741 ft (ca 39-225 m) at Point Barrow, Alaska (MacGinitie, 1955).

Life history

Adult characteristics

ParameterData
Reproductive typeProtandrous hermaphrodite
Reproductive frequency Annual episodic
Fecundity (number of eggs)No information
Generation time2-5 years
Age at maturitySee additional text
SeasonInsufficient information
Life span2-5 years

Larval characteristics

ParameterData
Larval/propagule type-
Larval/juvenile development Direct development
Duration of larval stageNot relevant
Larval dispersal potential See additional information
Larval settlement periodNot relevant

Life history information


Musculus discors is a protandrous hermaphrodite, male when small then becoming female when larger and older. One year olds are functionally male. In the following year eggs begin to develop and individuals pass through a hermaphroditic phase (2-3years olds) becoming functional females by the third year of life (Thorson, 1935; Ockelmann, 1958). Some third year individuals were found to be functionally male, suggesting that reversion may occur (Thorson, 1935).

Large eggs (300 by 220 µm) are laid in 3-4 rows in mucus strings within the adult nest. Embryos of 400µm in length are found in the mucus strings. Development is direct, there is no pelagic phase, the juveniles leave the egg string as free-living crawl-aways (Thorson, 1935; Ockelmann, 1958).

Eggs are laid throughout summer, with a peak in August in east Greenland (Thorson, 1935; Ockelmann, 1958). Egg strings were found in May in the Holbaek fjord in the Øresund Sound, Denmark (Thorson, 1946). Martel & Chia (1991) reported a peak of juveniles in British Columbia during summer. However, no information on reproduction in the UK was found.

Brooding of offspring is a common trait in boreal and arctic marine benthic invertebrates (Ockelmann, 1958; 1965). Small juvenile Musculus discors often remain within the nest, near the edge of the adult shell, feeding in the currents produced by the adult, and larger juveniles may be found in the outer fringes of the nest (Merrill & Turner, 1963). Brooding and low levels of vagility may explain the dense aggregation and gregarious clumps of individuals found in this species but suggests that dispersal is poor. However, Martel & Chia (1991) reported that juvenile Musculus discors (<1 mm) were caught in off-bottom intertidal collectors and one specimen in offshore collectors. Therefore, juvenile Musculus discors are probably capable of drifting on fine byssal threads (bysso-pelagic transport) and may be carried considerable distances, albeit in small numbers.

Sensitivity reviewHow is sensitivity assessed?

Physical pressures

Use / to open/close text displayed

 IntoleranceRecoverabilitySensitivityEvidence / Confidence
Substratum loss [Show more]

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).

High Moderate Moderate Low
Smothering [Show more]

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).

High Moderate Moderate Low
Increase in suspended sediment [Show more]

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.

Low Immediate Not sensitive Low
Decrease in suspended sediment [Show more]

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.

Low Immediate Not sensitive Low
Desiccation [Show more]

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).

Intermediate High Low Low
Increase in emergence regime [Show more]

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).

Intermediate High Low Low
Decrease in emergence regime [Show more]

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.

Tolerant* Not relevant Not sensitive*
Increase in water flow rate [Show more]

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).

Intermediate High Low Very low
Decrease in water flow rate [Show more]

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).

Low Immediate Not sensitive
Increase in temperature [Show more]

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.

Tolerant Not relevant Not sensitive Low
Decrease in temperature [Show more]

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.

Tolerant Not relevant Not sensitive Low
Increase in turbidity [Show more]

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.

Low Very high Very Low Very low
Decrease in turbidity [Show more]

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.

Tolerant* Not relevant Not sensitive*
Increase in wave exposure [Show more]

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).

Intermediate High Low Very low
Decrease in wave exposure [Show more]

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.

Not relevant Not relevant Not relevant Low
Noise [Show more]

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.

Tolerant Not relevant Not sensitive High
Visual presence [Show more]

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.

Tolerant Not relevant Not sensitive High
Abrasion & physical disturbance [Show more]

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).

Intermediate High Low Low
Displacement [Show more]

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.

Low Very high Very Low Low

Chemical pressures

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 IntoleranceRecoverabilitySensitivityEvidence / Confidence
Synthetic compound contamination [Show more]

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).

Intermediate High Low Very low
Heavy metal contamination [Show more]

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.

No information Not relevant No information Not relevant
Hydrocarbon contamination [Show more]

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.

Low Very high Very Low Very low
Radionuclide contamination [Show more]

Radionuclide contamination

Evidence

Insufficient
information

No information Not relevant No information Not relevant
Changes in nutrient levels [Show more]

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.

Tolerant* Not relevant Not sensitive* Very low
Increase in salinity [Show more]

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.

Not relevant Not relevant Not relevant Not relevant
Decrease in salinity [Show more]

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).

Intermediate High Low
Changes in oxygenation [Show more]

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).

Intermediate High Low Very low

Biological pressures

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 IntoleranceRecoverabilitySensitivityEvidence / Confidence
Introduction of microbial pathogens/parasites [Show more]

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.

Low Very high Very Low Low
Introduction of non-native species [Show more]

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.

No information Not relevant No information Not relevant
Extraction of this species [Show more]

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.

Not relevant Not relevant Not relevant Not relevant
Extraction of other species [Show more]

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).

Intermediate High Low Low

Additional information

Recoverability. No information concerning recruitment or recovery in Musculus discors was found. Brooding Musculus discors produces relatively few offspring; tens of eggs and offspring rather than hundreds of thousands of eggs in the spawning mytilids such as Mytilus edulis. However, brooding 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 and could be one factor responsible for the wide geographical distribution of many species with direct development. Therefore, local recruitment in Musculus discors may be rapid, depending on the hydrographic regime. Hence, within a population or between adjacent populations recruitment and recovery is probably fairly rapid, and it is suggested that prior abundance may recover within up to five years. However, where recovery is dependant on recruitment from distant populations recruitment may take longer. Colonization by bysso-pelagic drifting may partly explain the wide boreal distribution of this species, however, it is probably a slow, and random process, primarily dependant on the hydrographic regime. Where 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 five to 10 years.

Importance review

Policy/legislation

- no data -

Status

Non-native

ParameterData
Native-
Origin-
Date ArrivedNot relevant

Importance information

The extensive, dense aggregations formed occasionally by Musculus discors were considered to be uncommon in the British Isles (see MCR.Mus; Connor et al., 1997a).

Bibliography

  1. Baldock, B.M., Mallinson, J.M. & Seaward, D.R., 1998. Observations on extensive, dense populations of the bivalve mollusc Musculus discors (L. 1758). Journal of Conchology, 36, 43-46.

  2. Brazier, D.P., Holt, R.H.F., Murray, E. & Nichols, D.M., 1999. Marine Nature Conservation Review Sector 10. Cardigan Bay and North Wales: area summaries. Peterborough: Joint Nature Conservation Committee. [Coasts and seas of the United Kingdom. MNCR Series.]

  3. Bruce, J.R., Colman, J.S. & Jones, N.S., 1963. Marine fauna of the Isle of Man. Liverpool: Liverpool University Press.

  4. Bryan, G.W. & Gibbs, P.E., 1991. Impact of low concentrations of tributyltin (TBT) on marine organisms: a review. In: Metal ecotoxicology: concepts and applications (ed. M.C. Newman & A.W. McIntosh), pp. 323-361. Boston: Lewis Publishers Inc.

  5. Bryan, G.W., 1984. Pollution due to heavy metals and their compounds. In Marine Ecology: A Comprehensive, Integrated Treatise on Life in the Oceans and Coastal Waters, vol. 5. Ocean Management, part 3, (ed. O. Kinne), pp.1289-1431. New York: John Wiley & Sons.

  6. Cartlidge, D. & Hiscock, K., 1980. South west Britain sub-littoral survey: field survey of sublittoral habitats and species in North Pembrokeshire. Nature Conservancy Council, Peterborough, CSD Report, no. 295.

  7. Connor, D.W., Dalkin, M.J., Hill, T.O., Holt, R.H.F. & Sanderson, W.G., 1997a. Marine biotope classification for Britain and Ireland. Vol. 2. Sublittoral biotopes. Joint Nature Conservation Committee, Peterborough, JNCC Report no. 230, Version 97.06., Joint Nature Conservation Committee, Peterborough, JNCC Report no. 230, Version 97.06.

  8. Crompton, T.R., 1997. Toxicants in the aqueous ecosystem. New York: John Wiley & Sons.

  9. Fish, J.D. & Fish, S., 1996. A student's guide to the seashore. Cambridge: Cambridge University Press.

  10. Forbes, E. & Hanley, S., 1853. A history of British Mollusca and their shells, vol. 1-4. London: John van Voorst.

  11. Hiscock, K., 1984b. Sublittoral surveys of Bardsey and the Lleyn peninsula. August 13th to 27th, 1983. Report prepared by the Field Studies Council, Oil Pollution Research Unit, Pembroke for the Nature Conservancy Council, CSD Report, no. 612.

  12. Howson, C.M. & Picton, B.E. ed., 1999. The species directory of the marine fauna and flora of the British Isles and surrounding seas. CD-ROM Edition. Ulster Museum and The Marine Conservation Society, Belfast and Ross-on-Wye., Belfast: Ulster Museum. [Ulster Museum publication no. 280.]

  13. Jeffreys, J.G., 1863. British conchology, or an account of the mollusca which now inhabit the British Isles and surrounding seas, vol. 1-5. London: John van Voorst.

  14. Könnecker, G., 1977. Epibenthic assemblages as indicators of environmental conditions. In Proceedings of the 11th Symposium on Marine Biology, Galway, October 1976. Biology of Benthic Organisms (ed. B.F. Keegan, P.O. Ceidigh, & P.J.S. Boaden), pp. 391-395. Oxford: Pergamon Press.

  15. Könnecker, G.F. & Keegan, B.F., 1983. Littoral and benthic investigations on the west coast of Ireland - XVII. The epibenthic animal associations of Kilkieran Bay. Proceedings of the Royal Irish Academy Section B, 83B, 309-324.

  16. Lauckner, G., 1983. Diseases of Mollusca: Bivalvia. In Diseases of marine animals. Vol. II. Introduction, Bivalvia to Scaphopoda (ed. O. Kinne), pp. 477-961. Hamburg: Biologische Anstalt Helgoland.

  17. MacGinitie, G.E., 1955. Distribution and ecology of the marine invertebrates of Point Barrow, Alaska. Smithsonian Miscellaneous Collections, 128, 1-201.

  18. Martel, A. & Chia, F.S., 1991b. Drifting and dispersal of small bivalves and gastropods with direct development. Journal of Experimental Marine Biology and Ecology, 150, 131-147.

  19. Merrill, A.S. & Turner, R.D., 1963. Nest building in the bivalve genera Musculus and Lima. Veliger, 6, 55-59.

  20. Morton, B., 1992. The evolution and success of the heteromyarian form in the Mytiloida. In The mussel Mytilus: Ecology, physiology, genetics and culture, (ed. E.M. Gosling), pp. 21-52. Amsterdam: Elsevier Science Publ. [Developments in Aquaculture and Fisheries Science, 25]

  21. Murray, E., Dalkin, M.J., Fortune, F. & Begg, K., 1999. Marine Nature Conservation Review Sector 2. Orkney: area summaries. Peterborough: Joint Nature Conservation Committee. [Coasts and sea of the United Kingdom. MNCR Series.]

  22. Ockelmann, W.K., 1958. The zoology of east Greenland. Marine Lamellibranchiata. Meddelelser om Grønland, 122, 1-256.

  23. Phorson, J.E., 1996. Observations on the development of dentition in small juveniles of certain species of Mytilidae. The Conchologists Newsletter, 136, 603-623.

  24. Seaward, D.R., 1990. Distribution of marine molluscs of north west Europe. Peterborough: Nature Conservancy Council.

  25. Suchanek, T.H., 1993. Oil impacts on marine invertebrate populations and communities. American Zoologist, 33, 510-523. DOI https://doi.org/10.1093/icb/33.6.510

  26. Tebble, N., 1976. British Bivalve Seashells. A Handbook for Identification, 2nd ed. Edinburgh: British Museum (Natural History), Her Majesty's Stationary Office.

  27. Thorpe, K., 1998. Marine Nature Conservation Review, Sectors 1 and 2. Lagoons in Shetland and Orkney. Peterborough: Joint Nature Conservation Committee. [Coasts and seas of the United Kingdom. MNCR Series.]

  28. Thorson, G. von, 1935. Biologische Studien über die Lamellibranchier Modiolaria discors L. und Modiolaria nigra Gray in Ostgrönland. Zoologischer Anzeiger, 111, 297-304.

  29. Thorson, G., 1946. Reproduction and larval development of Danish marine bottom invertebrates, with special reference to the planktonic larvae in the Sound (Øresund). Meddelelser fra Kommissionen for Danmarks Fiskeri- Og Havundersögelser, Serie: Plankton, 4, 1-523.

  30. Zwaan de, A. & Mathieu, M., 1992. Cellular biochemistry and endocrinology. In The mussel Mytilus: ecology, physiology, genetics and culture, (ed. E.M. Gosling), pp. 223-307. Amsterdam: Elsevier Science Publ. [Developments in Aquaculture and Fisheries Science, no. 25]

Datasets

  1. Centre for Environmental Data and Recording, 2018. Ulster Museum Marine Surveys of Northern Ireland Coastal Waters. Occurrence dataset https://www.nmni.com/CEDaR/CEDaR-Centre-for-Environmental-Data-and-Recording.aspx accessed via NBNAtlas.org on 2018-09-25.

  2. Conchological Society of Great Britain & Ireland, 2018. Mollusc (marine) data for Great Britain and Ireland - restricted access. Occurrence dataset: https://doi.org/10.15468/4bsawx accessed via GBIF.org on 2018-09-25.

  3. Conchological Society of Great Britain & Ireland, 2018. Mollusc (marine) records for Great Britain and Ireland. Occurrence dataset: https://doi.org/10.15468/aurwcz accessed via GBIF.org on 2018-09-25.

  4. Environmental Records Information Centre North East, 2018. ERIC NE Combined dataset to 2017. Occurrence dataset: http://www.ericnortheast.org.ukl accessed via NBNAtlas.org on 2018-09-38

  5. Manx Biological Recording Partnership, 2018. Isle of Man historical wildlife records 1990 to 1994. Occurrence dataset:https://doi.org/10.15468/aru16v accessed via GBIF.org on 2018-10-01.

  6. NBN (National Biodiversity Network) Atlas. Available from: https://www.nbnatlas.org.

  7. OBIS (Ocean Biodiversity Information System),  2024. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. www.iobis.org. Accessed: 2024-03-19

Citation

This review can be cited as:

Tyler-Walters, H., 2001. Musculus discors Green crenella. In Tyler-Walters H. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 19-03-2024]. Available from: https://www.marlin.ac.uk/species/detail/1645

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Last Updated: 20/12/2001