BIOTIC Species Information for Musculus discors
Researched byLizzie Tyler Data supplied byUniversity of Sheffield
Refereed byThis information is not refereed.
Scientific nameMusculus discors Common nameDiscordant mussel
MCS CodeW1721 Recent SynonymsModiolaria discors

PhylumMollusca Subphylum
Superclass ClassPelecypoda
Subclass OrderMytiloida
Suborder FamilyMytilidae
GenusMusculus Speciesdiscors

Additional InformationThe 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).
Taxonomy References Howson & Picton, 1999, Tebble, 1976, Fish & Fish, 1996, Morton, 1992, Phorson, 1996, Ockelmann, 1958, Forbes & Hanley, 1853, Jeffreys, 1863, MacGinitie, 1955,
General Biology
Growth formBivalved
Feeding methodPassive suspension feeder
Active suspension feeder
Mobility/Movement Environmental positionEpilithic
Typical food typesPhytoplankton, bacteria, organic particulates and dissolved organic matter (DOM). HabitAttached
BioturbatorNot relevant FlexibilityNone (< 10 degrees)
FragilityIntermediate SizeSmall(1-2cm)
HeightInsufficient information Growth RateSee additional information
Adult dispersal potential<10m DependencyIndependent
General Biology Additional Information

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).

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 -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 >20mm were nearly always covered by a byssal nest. However, in British Columbia Merrill & Turner (1963) noted that the smallest specimens with nests were 8.1mm in length and most specimens over 15mm had nests, although some specimens up to 18mm in length were without a nest.

Thorson (1935) reported that Musculus discors was 5-7mm in length by the first growth ring (presumably Ist year), 10 -11mm by the second and 13 -16mm by the third (presumably 3rd year) in east Greenland, however, growth rates will probably depend on environmental conditions.

Little other information on the biology of Musculus discors was found.
Biology References Fish & Fish, 1996, Baldock et al., 1998, Könnecker & Keegan, 1983, Ockelmann, 1958, Thorson, 1946, Merrill & Turner, 1963, Forbes & Hanley, 1853, Jeffreys, 1863, MacGinitie, 1955, Cartlidge & Hiscock, 1980, Lauckner, 1983,
Distribution and Habitat
Distribution in Britain & IrelandCommon around most of the British Isles from Shetland to the Channel Isles.
Global distributionA 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.
Biogeographic rangeNot researched Depth range
MigratoryNon-migratory / Resident   
Distribution Additional 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 magellanicus.

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 30-40m in east Greenland, to form a well developed community at 60-100m in the Fosse de la Hague in the English Channel (Cabioch, 1968), to occur from 0 -374m in the Barents Sea (Ockelmann, 1958), and to be common in most trawls from 130-741 ft (ca 39-225m) at Point Barrow, Alaska (MacGinitie, 1955).

Musculus discors occasionally forms dense aggregations, especially in strong tidal streams, covering rock surfaces, forming the biotope MCR.Mus.

Substratum preferencesAlgae
Large to very large boulders
Small boulders
Gravel / shingle
Physiographic preferencesOpen coast
Strait / sound
Ria / Voe
Enclosed coast / Embayment
Biological zoneLower Eulittoral
Sublittoral Fringe
Upper Infralittoral
Lower Infralittoral
Upper Circalittoral
Wave exposureExposed
Moderately Exposed
Very Sheltered
Extremely Sheltered
Tidal stream strength/Water flowStrong (3-6 kn)
Moderately Strong (1-3 kn)
Weak (<1 kn)
SalinityVariable (18-40 psu)
Full (30-40 psu)
Habitat Preferences Additional Information
Distribution References Thorson, 1935, Tebble, 1976, Fish & Fish, 1996, Baldock et al., 1998, Könnecker & Keegan, 1983, Ockelmann, 1958, Thorson, 1946, MacGinitie, 1955, Murray, E. et al., 1999, Thorpe, 1998, Könnecker, 1977, Cartlidge & Hiscock, 1980, Bruce et al., 1963, Seaward, 1990, Brazier et al., 1999,
Reproduction/Life History
Reproductive typeProtandrous hermaphrodite
Developmental mechanismDirect Development
Reproductive SeasonInsufficient information Reproductive LocationAs adult
Reproductive frequencyAnnual episodic Regeneration potential No
Life span3-5 years Age at reproductive maturity
Generation time3-5 years Fecundity
Egg/propagule size300 µm diameter Fertilization typeInsufficient information
Larval/Juvenile dispersal potentialVery limited (<1m) Larval settlement periodNot relevant
Duration of larval stage   
Reproduction Preferences Additional 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.
Reproduction References Thorson, 1935, Ockelmann, 1958, Thorson, 1946, Martel & Chia, 1991b, Merrill & Turner, 1963,
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