Biodiversity & Conservation

Musculus discors beds on moderately exposed circalittoral rock


<i>%Musculus discors% </i>beds on moderately exposed circalittoral rock
Distribution map

CR.MCR.CMus.Mdis recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats

Ecological and functional relationships

This biotope is dominated by suspension feeding species. Little information on the ecology of this biotope was found. Musculus discors is an active suspension feeder on phytoplankton, bacteria, detritus and dissolved organic matter.

In this biotope, the Musculus discors carpet excludes and may smother other epifauna (Cartlidge & Hiscock, 1980).

Other suspension feeders include the sponges, hydroids, bryozoans, ascidians and small crustaceans found within the community. When present brittlestars (e.g. Ophiura spp.) or Henricia oculata may also suspension feed.

Kelp (e.g. Laminaria hyperborea) and foliose red algae (e.g. Delesseria sanguinea or Phycodrys rubens) probably provide primary production in the form of detritus and dissolved organic matter or grazing by gastropods (e.g. Gibbula cineria or Calliostoma zizyphinum).

The faunal turf of hydroids and bryozoans are probably grazed by echinoderms such as Henricia oculata and Echinus esculentus.

Mobile predators include crabs such as Cancer pagurus and Necora puber, which probably take some Musculus discors and gastropods. The starfish Asterias rubens probably also preys on Musculus discors, although Hiscock (1984) noted that Asterias rubens was common on areas dominated by Mytilus edulis but only occasional on Musculus discors beds.

Asterias rubens, Henricia oculata and crabs probably also act as scavengers within this biotope.

Seasonal and longer term change

Where foliose algae or kelp are present the algae may be expected to show seasonal changes in growth and development of the lamina, for examples see Delesseria sanguinea and Laminaria hyperborea reviews. Strings of the eggs of Musculus discors may be visible within the nest or byssal mass of the carpet. Eggs strings are laid in the summer months in Greenland, British Columbia and Denmark but no information on spawning times was available for Britain and Ireland. No further information regarding seasonal or temporal changes was found.

Habitat structure and complexity

  • Habitat complexity is not high because of the 'blanketing' effect of Musculus discors which forms dense carpets covering upward facing hard substrata (including kelp holdfasts and stipes when present) in the infralittoral kelp zone and below. For example, in Kilkiernan Bay, Ireland Musculus discors formed a mat of up to 25mm thick over every horizontal surface over many square metres of seabed, reaching an estimated density of 22,000 individuals per square metre (Könnecker & Keegan, 1983).
  • At other sites, below the kelp zone, Musculus discors formed a carpet covered by a mucous-congealed mat of silt or pseudofaeces bound by fine byssus threads, through which its siphons protruded when feeding (Hiscock, 1984; Brazier et al., 1999).
  • The byssus nests and interstices between individual Musculus discors probably support meiofauna and small crustaceans, scavenging flatworms and polychaetes.
  • The Musculus discors carpet may also support scattered individuals of Mytilus edulis (Könnecker & Keegan, 1983).
  • The carpet is interspersed or punctuated by epifauna such as ascidians (e.g. Polycarpa pomeria, Morchelium argus, and Clavelina lepadiformis), sponges (e.g. Hemimycale columella and Polymastia boletiformis), hydroids (e.g. Nemertesia antennina and Sertularia spp.), bryozoans (e.g. Flustra foliacea and Pentapora fascialis), Anthozoa (e.g. Alcyonium digitatum, Urticina felina and Sagartia sp.), by red foliose seaweeds (e.g. Delesseria sanguinea, Phycodrys rubens and Hypoglossum hypoglossoides) and by kelps when present (Hiscock, 1984; Connor et al., 1997; Baldock et al., 1998).
  • The surrounding rocks, vertical surfaces and probably to a lesser extent the Musculus discors carpet, supports a rich epifauna of hydroids, bryozoans, ascidians and sponges (Cabioch, 1968; Könnecker, 1977; Könnecker & Keegan, 1983; Baldock et al., 1998). The species composition of the epifauna and associated species varies with depth, light availability (especially the flora), siltation and current flow, and probably reflects the epifauna and flora of the open coast in the local area rather than the presence of the Musculus discors carpet itself (Merrill & Turner, 1963; Cabioch, 1968; Cartlidge & Hiscock, 1980; Connor et al., 1997; Baldock et al., 1998). Cartlidge & Hiscock (1980) suggested that Musculus discors had a smothering effect over other epifauna.


Little information on productivity was found. However, kelps and other macroalgae probably make an important contribution to primary productivity where abundant. Dame (1996) suggested that dense beds of bivalve suspension feeders increase turnover of nutrients and organic carbon in estuarine (and presumably coastal) environments by effectively transferring pelagic phytoplanktonic primary production to secondary production in the sediments (pelagic-benthic coupling). The Musculus discors beds probably also provide secondary productivity in the form of tissue, faeces and pseudofaeces, however, probably not to the same magnitude as common or horse mussel beds.

Recruitment processes

No information concerning recruitment 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) reported that juvenile Musculus discors (<1 mm) were caught in off-bottom intertidal collectors and one specimen in offshore collectors. Juvenile Musculus discors are probably capable of drifting on fine byssal threads (bysso-pelagic transport) and may be carried considerable distances. Therefore, local recruitment in Musculus discors may be rapid, depending on the hydrographic regime. Hence, within a population or between adjacent populations recruitment is probably fairly rapid. However, recruitment from distant populations may take longer.

For many hydroids and bryozoans in the biotope, Holt et al. (1995) suggested that they 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.

Most sponge species in the biotope produce short lived, planktonic larvae so that recruitment is localized, depending on the hydrographic regime. However, some species (e.g. Polymastia robusta) produce benthic crawling larvae that probably settle close to the parent (see Fell, 1989 for review).

Ascidians in the biotope 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.

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.

Time for community to reach maturity

No information concerning population or community development in Musculus discors was found.

Additional information

No text entered.

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 26/11/2015]. Available from: <>