Musculus discors beds on moderately exposed circalittoral rock

Summary

UK and Ireland classification

Description

This biotope typically occurs on the upper faces of moderately exposed, moderately tide-swept bedrock, boulders and cobbles in slightly silty conditions. The mussel Musculus discors occurs in dense mats and occasionally completely coats all available surfaces. There is also often a layer of pseudofaeces, forming a thick, silty matrix. A relatively diverse fauna of cushion and branching sponges is often present on rocky outcrops and other hard substratum that is free of mussels. These include Tethya aurantiumSycon ciliatumPachymatisma johnstoniaDysidea fragilisCliona celata and Stelligera stuposa. There may be isolated clumps of silt-tolerant bryozoans such as Flustra foliacea and Crisularia plumosa. Various species may be observed on top of the mussels, including Asterias rubensCrossaster papposus and the brittlestar Ophiura albida. Occasional Alcyonium digitatum and clumps of the hydroid Nemertesia antennina are found attached to rocky outcrops and boulders whilst the anemone Urticina felina may be seen in crevices in the rock or on gravelly patches between boulders. Colonial ascidians such as Clavelina lepadiformis and didemnids may occasionally be present. A wide range of seaweeds may be present, including Dictyota dichotomaPlocamium cartliagineumDictyopteris polypodioidesCryptopleura ramosa and Heterosiphonia plumosa. The crab Cancer pagurus may be observed in crevices. The majority of the records for this biotope are from the Lleyn Peninsula. (Information from JNCC, 2022).

Depth range

10-20 m

Additional information

Although several surveys of this biotope are available (for example Cabioch, 1968; Hiscock, 1984; Könnecker & Keegan, 1983; Baldock et al., 1998; JNCC, 1999, Connor et al., 2004), little information on the ecology of the biotope was found.

Listed By

Habitat review

Ecology

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 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 foliacea), 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.

Productivity

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

Little information concerning recruitment in Musculus discors was found.  Musculus discors is a protandrous hermaphrodite (Ockelmann, 1958). One year old individuals are functionally male. Eggs develop in thier second year, and they pass through a hermaphroditic phase before becoming functional females at the end of their third year (Ockelmann, 1958).  They lay large eggs (ca 300x200 μm) in strings within the nest of the parent. The embyos develope witihin the gelatinous egg-string without any pelagic phase. The embyonic shell was found to be ca 400 μm in length, while still within the string (Thorson, 1935, 1936 as cited in Thorson, 1946 and Ockelmann, 1958). Eggs strings are laid in the summer months in north east Greenland and Denmark (Thortson, 1946; Ockelmann, 1958). No information on spawning times was found for Britain and Ireland.

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, direct development withn the nest of the parent probably results in relatively lower levels of juvenile mortality. Therefore, recruitment within populations  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 from the biotope, so that most recruits of species with pelagic life stages come from outside the community. However, direct development in Musculus discors probably ensures a relatively good, local recruitment in the vicinty of adults.

Time for community to reach maturity

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

Additional information

No text entered.

Preferences & Distribution

Habitat preferences

Depth Range 10-20 m
Water clarity preferences
Limiting Nutrients Data deficient
Salinity preferences Full (30-40 psu)
Physiographic preferences Open coast
Biological zone preferences Upper circalittoral
Substratum/habitat preferences Bedrock, Large to very large boulders
Tidal strength preferences Moderately strong 1 to 3 knots (0.5-1.5 m/sec.)
Wave exposure preferences Moderately exposed, Sheltered
Other preferences

Additional Information

Könnecker (1977) suggested that the Musculus discors association in Kilkieran Bay, Ireland was an example of an eurythermal and eurysaline community. The MNCR biotope classification (Conner et al., 1997a) suggested that this biotope was associated with moderate wave exposure and weak to moderately strong tidal streams. However, the Musculus discors communities described by Cabioch (1968) occurred in areas subject to strong currents, and the Musculus discors communities in Kilkieran Bay were associated with currents greater than 2.5m/sec (5 knots) (Könnecker, 1977; 1983).

Species composition

Species found especially in this biotope

Rare or scarce species associated with this biotope

-

Additional information

The epifauna may be exceptionally rich (Könnecker, 1977; Könnecker & Keegan, 1983). The MNCR recorded 323 species within this biotope, although not all species were present in all records of the biotope. Dominant species were detailed in other surveys by Cabioch (1968), Baldock et al. (1998) and JNCC (1999).

Sensitivity review

Sensitivity characteristics of the habitat and relevant characteristic species

Musculus discors is the dominant space occupying species within this biotope, and may smother other species. The other species in the community are widespread and characteristic of the open coast, in which the Musculus discors beds are found. Therefore, the associated species vary with location and have little significant association with the Musculus discors bed itself. Reference has been made to Nemertesia ramosa to represent hydroids, Pentapora foliacea to represent bryozoans and Clavelina lepadiformis to represent ascidians and Urticina felina and Alcyonium digitata to represent anthozoans occurring within the biotope. However, the biotope is characterized by the Musculus discors bed. A reduction in Musculus discors density or loss of the bed would result in a significant change in the character of the community in the loss of the biotope. Therefore, the sensitivity of this biotope is dependent on the sensitivity of the Musculus discors bed. 

Resilience and recovery rates of habitat

Life history and recruitment characteristics of the dominant species groups are presented under 'recruitment processes' above.  Direct development in eggs strings, within the adult nest, in Musculus discors, probably results in relatively low levels of juvenile mortality and good local recruitment. In addition,  direct development and the high energetic investment in relatively few offspring (compared with broadcast spawners) may allow rapid colonization of suitable habitat but restrict long range dispersal. However, Martel & Chia (1991) suggested that in species that brood their offspring or have direct development (such as Musculus discors) bysso-pelagic drifting probably contributed to rapid local dispersal and recruitment, depending on the hydrographic regime.

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 varies 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 within the adult nest would avoid the loss of juveniles from the population while allowing bysso-pelagic transport of a proportion of the juveniles, that may themselves colonize suitable habitat elsewhere.

There is no direct evidence of recovery within populations of Musculus discors or their beds. The epifaunal community described within this biotope is primarily dependent on the Musculus discors bed.

Resilience assessment. Recruitment within a population or between adjacent populations and recovery of Musculus discors is probably fairly rapid. Therefore, where some fo the population is lost or its abundance reduced (e.g. 'Medium' resistance) it is suggested that prior abundance may recover within up to two years, and resilience assessed as 'High'. However, where the bed is significantly or severely damaged (e.g. resistance in 'Low' ) and recovery is dependant on recruitment from distant populations recruitment may take longer. If a population is removed (resistance is 'None') 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 or significant damage to a population may take about up to 10 years so that resilience would be assessed as 'Medium'.  However, confidence in this assessment is 'Low'. The associated epifaunal community will probably develop within less than 5 years although slow growing sponges may take many years to develop.

Hydrological Pressures

Use [show more] / [show less] to open/close text displayed

ResistanceResilienceSensitivity
Temperature increase (local) [Show more]

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

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 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 dependent. However, loss of a few epifaunal or epifloral species will not significantly affect the biotope, and are likely to recover quickly. Therefore, a resistance of High has been recorded. Hence, resilience is High (by default) and the biotope is recorded as Not sensitive at the benchmark level.

High
Medium
Low
Medium
Help
High
High
High
High
Help
Not sensitive
Medium
Low
Medium
Help
Temperature decrease (local) [Show more]

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

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 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 dependent. However, loss of a few epifaunal or epifloral species will not significantly affect the biotope, and are likely to recover quickly. Therefore, a resistance of High has been recorded. Hence, resilience is High (by default) and the biotope is recorded as Not sensitive at the benchmark level.

High
Medium
Low
Medium
Help
High
High
High
High
Help
Not sensitive
Medium
Low
Medium
Help
Salinity increase (local) [Show more]

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Könnecker (1977) classified Musculus discors associations as 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).  However, no evidence was found on the effect of hypersaline (>40 psu) conditions.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
No evidence (NEv)
NR
NR
NR
Help
Salinity decrease (local) [Show more]

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Könnecker (1977) classified Musculus discors associations as 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 itself is probably tolerant of a reduction in salinity from full to variable or even reduced for a year.  However, Connor et al. (2004) noted that Musculus discors occurred in the lagoonal biotope R.LIR.Lag.AscSpAs at reduced salinity but at lower densities than occurred in this biotope.  Hence, a decrease in salinity from ‘full’ to ‘reduced’ for a year may result in a reduction in the abundance Musculus discors and possibly extent of the bed.  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 reduction in salinity from ‘full’ to ‘reduced’ is likely to have adverse effects, reducing the extent of the Musculus discors populations and significantly reducing the richness of the associated epifauna.  Therefore, a resistance of Low is recorded, so that resilience is probably Medium and a sensitivity of Medium is recorded.  

Low
Medium
Low
Medium
Help
Medium
Low
NR
NR
Help
Medium
Low
Low
Low
Help
Water flow (tidal current) changes (local) [Show more]

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

The Musculus discors bed biotope is recorded in areas subject to moderately strong tidal streams (0.5-1.5 m/s) (Connor et al., 2004). However, the Musculus discors communities described by Cabioch (1968) occurred in areas subject to strong currents, and the Musculus discors communities in Kilkieran Bay were associated with currents greater than 2.5 m/sec (5 knots, strong tidal streams) (Könnecker, 1977; 1983).  Water flow is probably important for this epifaunal community, in order to provide food (as particulates and plankton), oxygenate the water column and keep the habitat free of excessive silt.  Therefore, a decrease in water flow to e.g. weak would probably be detrimental to the biotope. Similarly, and increase in water flow to e.g very strong may be detrimental if the resultant water flow removed or destabilised the bed. However, no direct evidence of disturbance due to changes in water flow or storms was found.  Nevertheless, a change in water flow of 0.1-0.2 m/s (the benchmark) is within the normal range experienced by the biotope. Therefore, the biotope is considered to be Not sensitive at the benchmark level.

High
Medium
Low
Medium
Help
High
High
High
High
Help
Not sensitive
Medium
Low
Medium
Help
Emergence regime changes [Show more]

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

Not relevant to circalittoral habitats below 5m.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Wave exposure changes (local) [Show more]

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

The Musculus discors bed biotope is recorded from moderately wave exposed and wave sheltered conditions (Connor et al., 2004), whereas Musculus discors has been reported from wave exposed to extremely wave sheltered habitats and is, therefore, probably 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 is also likely to be removed, being replaced by more wave tolerant species, e.g. Tubularia indivisa. 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 resistant of reduced wave action or water movement, e.g. the hydroid Nemertesia spp. but the biotope is likely to be little affected.  Nevertheless, a 3-5% change in significant wave height is unlikely to adversely affect the biotope, which occurs below 10 m where wave action in attenuated.  Therefore, the biotope is considered to be Not sensitive at the benchmark level.

High
Medium
Low
Medium
Help
High
High
High
High
Help
Not sensitive
Medium
Low
Medium
Help

Chemical Pressures

Use [show more] / [show less] to open/close text displayed

ResistanceResilienceSensitivity
Transition elements & organo-metal contamination [Show more]

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

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 evidence to assess resistance to heavy metals in Musculus discors, although the above evidence for hydroids suggests that they will display sublethal effects at least.

Not Assessed (NA)
NR
NR
NR
Help
Not assessed (NA)
NR
NR
NR
Help
Not assessed (NA)
NR
NR
NR
Help
Hydrocarbon & PAH contamination [Show more]

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

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 a 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.

No direct evidence on the effects of hydrocarbon contamination on Musculus discors was found. 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.

Not Assessed (NA)
NR
NR
NR
Help
Not assessed (NA)
NR
NR
NR
Help
Not assessed (NA)
NR
NR
NR
Help
Synthetic compound contamination [Show more]

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available. 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 on 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 the 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. 

Not Assessed (NA)
NR
NR
NR
Help
Not assessed (NA)
NR
NR
NR
Help
Not assessed (NA)
NR
NR
NR
Help
Radionuclide contamination [Show more]

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

No evidence was found.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
No evidence (NEv)
NR
NR
NR
Help
Introduction of other substances [Show more]

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed.

Not Assessed (NA)
NR
NR
NR
Help
Not assessed (NA)
NR
NR
NR
Help
Not assessed (NA)
NR
NR
NR
Help
De-oxygenation [Show more]

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

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. 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.  A reduction in oxygen levels below 2 mg/l for a week would probably be detrimental, but the effects would be limited in the strong to moderately strong water flow typical of this biotope. Therefore, a resistance of Medium is suggested.  Resilience is probably High so that sensitivity is recorded as Low.

Medium
Medium
Low
Medium
Help
High
Low
NR
NR
Help
Low
Low
Low
Low
Help
Nutrient enrichment [Show more]

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

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. 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 suspended sediments above) and decreased oxygen levels due to bacterial decomposition of organic material (see above). The species composition of the epifaunal community may also change as a result. However, this biotope is considered to be 'Not sensitive' at the pressure benchmark that assumes compliance with good status as defined by the WFD.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Not sensitive
NR
NR
NR
Help
Organic enrichment [Show more]

Organic enrichment

Benchmark. A deposit of 100 gC/m2/yr. Further detail

Evidence

Dense beds of Musculus discors in the north of the Llyn Peninsula and Holy Island, Anglesey were reported to be covered by a thick layer of mucous congealed fine silt and their own pseudofaeces (Hiscock, 1984; Brazier et al., 1999). The presence of pseudofaeces suggests a resistance to localised organic enrichment, although strong to moderately strong water flow would probably prevent build up of the products of decomposition (e.g. hydrogen sulphide).  In their meta-analysis, Johnston & Roberts (2009) concluded that contaminants such a sewage and nutrients resulted in a loss of species diversity.

Therefore, it is possible that an increase in organic carbon may result in a loss of species richness, and an increase in siltation and suspended solids depending on the nutrient status of the receiving waters (i.e. oligotrophic or eutrophic). However, in the absence of any direct evidence, no assessment has been made.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
No evidence (NEv)
NR
NR
NR
Help

Physical Pressures

Use [show more] / [show less] to open/close text displayed

ResistanceResilienceSensitivity
Physical loss (to land or freshwater habitat) [Show more]

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is, therefore ‘High’.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

None
High
High
High
Help
Very Low
High
High
High
Help
High
High
High
High
Help
Physical change (to another seabed type) [Show more]

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

Musculus discors requires hard substrata for attachment with byssal threads, as do the majority of the other epifauna and flora in the biotope. Therefore, a change in substratum from hard rock to sediment would result in loss of the biotope. Resistance to the pressure is considered ’None‘, and resilience ’Very low‘ or ‘None’. The sensitivity of this biotope to change from hard rock or artificial substrata to sedimentary or soft rock substrata  is assessed as ’High’.

None
High
High
High
Help
Very Low
High
High
High
Help
High
High
High
High
Help
Physical change (to another sediment type) [Show more]

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

Not relevant on hard rock substrata.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Habitat structure changes - removal of substratum (extraction) [Show more]

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

Not relevant on hard rock substrata.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Abrasion / disturbance of the surface of the substratum or seabed [Show more]

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

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

Sensitivity assessment. Physical abrasion may remove or damage a proportion of the Musculus discors bed and its associated epifauna. Therefore, a resistance of Low has been recorded. Resilience is probably Medium, so that sensitivity is recorded as Medium.

Low
Low
NR
NR
Help
Medium
Low
NR
NR
Help
Medium
Low
Low
Low
Help
Penetration or disturbance of the substratum subsurface [Show more]

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Not relevant on hard rock biotopes. However, penetrative activities may also cause abrasion as above.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Changes in suspended solids (water clarity) [Show more]

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

Dense beds of Musculus discors in the north of the Llyn Peninsula and Holy Island, Anglesey were reported to be 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 3-4 m. Other dense aggregations of Musculus discors were reported from areas of strong tidal streams and presumably low levels of suspended sediment and siltation.

Increased suspended sediment concentrations may clog suspension feeding apparatus, lead to the 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.  A decrease in suspended sediment may reduce the food supply for suspension feeding epifauna but otherwise have limited effect on the biotope.

Sensitivity assessment. Musculus discors is probably tolerant of a wide range of suspended sediment levels based on the evidence above.  Therefore, a resistance of High is recorded, so that resistance is also High (by default) and the biotope is considered to be Not sensitive at the benchmark level.

High
Low
NR
NR
Help
High
High
High
High
Help
Not sensitive
Low
Low
Low
Help
Smothering and siltation rate changes (light) [Show more]

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Musculus discors lives in fixed nests of byssus threads on the surface of the substratum. The byssal mat collects silt and pseudofaeces (Hiscock, 1984; Brazier et al., 1999), however, individual byssal nests within the mat open and the siphons of Musculus discors protrude out of the surface of the mat while feeding (Merrill & Turner, 1963; Baldock et al., 1998). While the nest will protect the bivalve from the direct effects of smothering, deposited spoil will smother the surface of the mat. Individual Musculus discors are unlikely to be able to burrow up through deposited fine spoil. 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 within the bed (or mat) are unlikely to be resistant. 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 5 cm of sediment. However, smaller or encrusting forms and some ascidians (e.g. Clavelina lepadiformis) are  likely to be smothered. 

Sensitivity assessment. The effects of smothering will depend on duration. In moderately strong tidal streams or moderate wave exposure, 5 cm of fine sediment may not remain over the biotope for more than few tidal cycles. As most bivalve molluscs can respire anaerobically for short periods, it is possible that most of the population of Musculus discors would survive. Therefore, a resistance of Medium is suggested. Resilience is probably High so that a sensitivity of Low is recorded.

Medium
Low
NR
NR
Help
High
Low
NR
NR
Help
Low
Low
Low
Low
Help
Smothering and siltation rate changes (heavy) [Show more]

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Musculus discors lives in fixed nests of byssus threads on the surface of the substratum. The byssal mat collects silt and pseudofaeces (Hiscock, 1984; Brazier et al., 1999), however, individual byssal nests within the mat open and the siphons of Musculus discors protrude out of the surface of the mat while feeding (Merrill & Turner, 1963; Baldock et al., 1998). While the nest will protect the bivalve from the direct effects of smothering, deposited spoil will smother the surface of the mat.  Individual Musculus discors are unlikely to be able to burrow up through deposited fine spoil. 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 within the bed (or mat) are unlikely to be resistant. 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 5 cm of sediment. However, smaller or encrusting forms and some ascidians (e.g. Clavelina lepadiformis) are likely to be smothered. 

Sensitivity assessment. The effects of smothering will depend on duration. In moderately strong tidal streams or moderate wave exposure, 30 cm of fine sediment may remain over the biotope for several tidal cycles. It is possible that a proportion of the Musculus discors population would succumb to anoxia in this period. Therefore, a resistance of Low is suggested. Resilience is probably Medium, so that a sensitivity of Medium is recorded. 

Low
Low
NR
NR
Help
Medium
Low
NR
NR
Help
Medium
Low
Low
Low
Help
Litter [Show more]

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

Not assessed.

Not Assessed (NA)
NR
NR
NR
Help
Not assessed (NA)
NR
NR
NR
Help
Not assessed (NA)
NR
NR
NR
Help
Electromagnetic changes [Show more]

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

No evidence was found

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
No evidence (NEv)
NR
NR
NR
Help
Underwater noise changes [Show more]

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

Musculus discors is unlikely to respond to underwater noise and is sedentary.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Introduction of light or shading [Show more]

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

Circalittoral biotopes occur below the influence of light (by definition).  Therefore, artificial light could potentially increase the depth to which red algae can colonize the biotope, assuming the light sources were strong enough to penetrate the water column. Similarly, shading may reduce red algal abundance within the biotope. Otherwise the biotope is unlikely to be adversely affected.  Resistance and resilience are considered to be High and the biotope is recorded as Not sensitive to this pressure.

High
Low
NR
NR
Help
High
High
High
High
Help
Not sensitive
Low
Low
Low
Help
Barrier to species movement [Show more]

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

Musculus discors beds probably exhibit good local recruitment, so that barriers to larval transport are probably not significant (see resilience and recovery rates). However, if a bed was damaged significantly, then barriers to larval transport may prolong recovery.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Death or injury by collision [Show more]

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

Not relevant to seabed habitats.  NB. Collision by grounding vessels is addressed under ‘surface abrasion’.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Visual disturbance [Show more]

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

Musculus discors probably responds to shading by closing its values but its visual acuity is probably very limited. . However, visual disturbance as defined under the pressure benchmark is unlikely to be relevant.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help

Biological Pressures

Use [show more] / [show less] to open/close text displayed

ResistanceResilienceSensitivity
Genetic modification & translocation of indigenous species [Show more]

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

Musculus discors is not subject to translocation nor genetic manipulation via breeding programmes or genetic modification. Therefore, this pressure is not relevant.

Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Introduction or spread of invasive non-indigenous species [Show more]

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

Evidence

Rapid colonizing and carpeting species such as Didemnum vexillum and Botrylloides violaceus, have been reported to smother mussels (Mytilus spp.) (GBNNSIP, 2012a&b). Didemnum vexillum may also smother epifauna and substrata in the shallow sublittoral.  Therefore, both species could potentially smother Musculus discors beds. However, they are yet to be reported growing in the vicinity of Musculus discors beds.  Therefore, in the absence of direct evidence, no assessment has been made but may be subject to reassessment as new evidence comes to light.

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887-1890. It has spread through expansion and introductions along the full extent of the English Channel and into the European mainland (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 1999, 2018; Hinz et al., 2011; Helmer et al., 2019; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015). Crepidula fornicata is reported to settle and establish amongst mussel beds (Blanchard, 1997; Thieltges, 2005; Rayment, 2007).  If Crepidula fornicata becomes established in a bed it is likely to alter the bed structure particularly if it is on coarse sand or hard substrata. Crepidula fornicata has high fecundity and can disperse its larvae over large areas making mussel beds highly vulnerable if Crepidula fornicata is introduced even large distances away.  The larvae of Crepidula fornicata can survive transport in ballast water for a number of days allowing it to travel large distances before needing to settle in the areas where the ballast water is released (Blanchard, 1997).  Crepidula can colonize a wide range of substrata. It prefers muddy gravelly, shell-rich, substrata that include gravel, or shells of other Crepidula, or other species e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020). 

Thieltges et al. (2003) reported that Crepidula fornicata was abundant on mussel beds in the intertidal to subtidal transition zone, in the northern Wadden Sea in the year 2000. Crepidula had increased in abundance since 1948 and had expanded its range from the extinct oyster beds to mussel beds where live mussels were its main substratum. Thieltges et al. (2003) also noted that storm events removed some clumps of mussels and presumably Crepidula onto tidal flats where they disappeared, which caused their abundance to fluctuate. Thieltges et al. (2003) noted that Crepidula abundance at the intertidal to subtidal transition zone (ca 21 /m2) was significantly higher than in the upper, mid, and lower intertidal (ca <3 /m2). Thieltges (2005) reported a 28-30% mortality of Mytilus edulis when Crepidula fornicata was introduced to the beds in experimental studies. He also found that mussel shell growth was reduced by 3 to 5 times in comparison to unfouled mussels and that extra energy was probably expended on byssus production.  The most significant cause of mortality was increased drag on mussels due to the growth of stacks of Crepidula fornicata on the shells of the mussels, rather than competition for food. He concluded that Crepidula fornicata is potentially an important mortality factor for Mytilus edulis (Thieltges, 2005).  Thieltges (2005) also observed mussel beds in the shallow subtidal infested with high abundances of Crepidula fornicata with almost no living mussels, along the shore of the List tidal basin, northern Wadden Sea.  

The density of Crepidula populations in northern Europe (Germany, Denmark, and Norway) is significantly lower (ca <100 /m2) than in southern waters. Thieltges et al. (2004) reported that the population of Crepidula was affected strongly by cold winters in the Wadden Sea. The winters of 2001 and 2003 resulted in ca 56-64% mortality of intertidal Crepidula and up to 97% on one mussel bed, compared to only 11-14% in southern areas without frost. Crepidula almost vanished from the Wadden Sea after the 1978/79 winter and took ten years to recover due to moderate winters which regularly affected the population. Similarly, 25% mortality was observed in Crepidula populations on the south coast of the UK after the extreme 1962/63 winter (Crisp, 1964, Bohn et al., 2012). Thieltges et al. (2003) suggested that global warming may allow Crepidula populations to become more abundant in northern Europe. Valdizan et al. (2011) noted higher water temperatures between 2000 to 2001 and 2006 to 2007 together with elevated chlorophyll-a corresponded to an increase in gametogenesis and the duration of broods in the Crepidula population in Bournerf Bay, France. They suggested that rising temperatures in northern Europe could increase its reproductive success due to favourable breeding temperatures and increased phytoplankton (Valdizan et al., 2011).  Nehls et al. (2006) noted that the decline in mussel (Mytilus edulis) beds in the Wadden Sea was due to mild winters that favoured non-native oysters (Magellana gigas) and slipper limpets, which co-existed with the mussels.

Bohn et al. (2013a) reported that mussel shells provided a more suitable settlement substratum for Crepidula larvae than bare panels in larval settlement experiments. However, the presence of live Mytilus edulis did not increase colonization of the site by Crepidula in the Milford Harbour Waterway, e.g., no Crepidula were found on mussels at a site with 23% cover of mussels (Bohn et al., 2015). Bohn et al. (2015) suggested that its prevalence on mussels in the Wadden Sea was due to a lack of alternative substratum, together with the cold weather mortalities. 

Crepidula fornicata is likely to alter water flow over mussel beds.  They form stacks of individuals that change water flow across the sediment surface.  When these stacks occur on the shells of Mytilus edulis they increase the drag on the mussel, increase the demands on the mussel’s energy reserves for attachment (e.g. byssus formation) and, hence, affect fecundity and survival (Thieltges, 2005; Sewell et al., 2008).  The increased drag may also result in clumps of mussels being removed by water flow (Thieltges, 2005). Competition for suspended organic matter and space is also increased.  Space for the settlement of macrobenthic organisms (Blanchard, 1997) including mussels is particularly reduced.  In addition to the reduced space for settlement, larvae of macrobenthic organisms are consumed by the slipper limpet and may affect recruitment to an area. 

Sensitivity assessment. Crepidula fornicata has not been recorded from Musculus discors beds (in 2023). No evidence was found on the potential effects of Crepidula on Musculus discors beds.  However, Tillin et al. (2020) suggested that Musculus discors beds could be vulnerable due to the association of Crepidula and Mytilus edulis, as detailed above. Musculus discors form dense mats that smother hard substratum, in which the byssus is woven into nests or cages, which in turn supports other epifauna and macroalgae. Crepidula requires hard substata for settlement (e.g., rock, gravel, shell). It has been recorded to settle on mussel shells but it is unclear if it could settle on the byssus mat created by Musculus discors, although it might settle on exposed shells or bedrock exposed where the Musculus discors mat is damaged, e.g., by storms. If Crepidula was to gain a 'foothold' in the bed, it is likely to cause further damage to the bed due to increased drag, as in blue mussel beds. It is also unclear if Musculus discors could out-compete Crepidula for space, or vice-versa. In addition, the wave exposed examples of the habitat are probably unsuitable for colonization by Crepidula, which prefers wave sheltered environments. 

At present, the evidence does not allow an assessment of sensitivity to be made with any confidence and further direct evidence is required. It is unlikely that Crepidula could colonize wave exposed examples of this biotope and uncertain if it could colonize this biotope even in areas that are sheltered from wave action. However, Crepidula might cause damage to the bed if could obtain a foothold.

Insufficient evidence (IEv)
NR
NR
NR
Help
Not relevant (NR)
NR
NR
NR
Help
Insufficient evidence (IEv)
NR
NR
NR
Help
Introduction of microbial pathogens [Show more]

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

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, a resistance of High has been recorded. Hence, resilience is High and Not sensitive is recorded.

High
Low
NR
NR
Help
High
High
High
High
Help
Not sensitive
Low
Low
Low
Help
Removal of target species [Show more]

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

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, a resistance of Medium has been recorded at the benchmark level. Resilience is probably High so that a sensitivity of Low has been recorded.

Medium
Low
NR
NR
Help
High
Low
NR
NR
Help
Low
Low
Low
Low
Help
Removal of non-target species [Show more]

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

The incidental removal of Musculus discors bed (or mat) by passing bottom fishing gear is addressed under abrasion above. However, the dense Musculus discors bed provides a unique habitat, attachment for other epifauna and macroalgae (e.g. red algae), and support infauna of other species within the nests and byssal mat. Loss of the mat would result in a loss of species diversity.  Therefore, a resistance of Low is recorded. Resilience is probably Medium so that a sensitivity of Medium is recorded.

Low
Low
NR
NR
Help
Medium
Low
NR
NR
Help
Medium
Low
Low
Low
Help

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. Blanchard, M., 2009. Recent expansion of the slipper limpet population (Crepidula fornicata) in the Bay of Mont-Saint-Michel (Western Channel, France). Aquatic Living Resources, 22 (1), 11-19. DOI https://doi.org/10.1051/alr/2009004

  3. Blanchard, M., 1997. Spread of the slipper limpet Crepidula fornicata (L.1758) in Europe. Current state and consequences. Scientia Marina, 61, Supplement 9, 109-118. Available from: http://scimar.icm.csic.es/scimar/index.php/secId/6/IdArt/290/

  4. Boero, F., 1984. The ecology of marine hydroids and effects of environmental factors: a review. Marine Ecology, 5, 93-118.

  5. Bohn, K., Richardson, C. & Jenkins, S., 2012. The invasive gastropod Crepidula fornicata: reproduction and recruitment in the intertidal at its northernmost range in Wales, UK, and implications for its secondary spread. Marine Biology, 159 (9), 2091-2103. DOI https://doi.org/10.1007/s00227-012-1997-3

  6. Bohn, K., Richardson, C.A. & Jenkins, S.R., 2015. The distribution of the invasive non-native gastropod Crepidula fornicata in the Milford Haven Waterway, its northernmost population along the west coast of Britain. Helgoland Marine Research, 69 (4), 313.

  7. Bohn, K., Richardson, C.A. & Jenkins, S.R., 2013b. The importance of larval supply, larval habitat selection and post-settlement mortality in determining intertidal adult abundance of the invasive gastropod Crepidula fornicata. Journal of Experimental Marine Biology and Ecology, 440, 132-140. DOI https://doi.org/10.1016/j.jembe.2012.12.008

  8. Bohn, K., Richardson, C.A. & Jenkins, S.R., 2013a. Larval microhabitat associations of the non-native gastropod Crepidula fornicata and effects on recruitment success in the intertidal zone. Journal of Experimental Marine Biology and Ecology, 448, 289-297. DOI https://doi.org/10.1016/j.jembe.2013.07.020

  9. 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.]

  10. 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.

  11. 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.

  12. Cabioch, L., 1968. Contribution a la connaissance des peuplements benthiques de la Manche occidentale. Cahiers de Biologie Marine, 9, 493 - 720.

  13. 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.

  14. Cole, S., Codling, I.D., Parr, W. & Zabel, T., 1999. Guidelines for managing water quality impacts within UK European Marine sites. Natura 2000 report prepared for the UK Marine SACs Project. 441 pp., Swindon: Water Research Council on behalf of EN, SNH, CCW, JNCC, SAMS and EHS. [UK Marine SACs Project.]. Available from: http://ukmpa.marinebiodiversity.org/uk_sacs/pdfs/water_quality.pdf

  15. 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.

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

  17. Davies, C.E. & Moss, D., 1998. European Union Nature Information System (EUNIS) Habitat Classification. Report to European Topic Centre on Nature Conservation from the Institute of Terrestrial Ecology, Monks Wood, Cambridgeshire. [Final draft with further revisions to marine habitats.], Brussels: European Environment Agency.

  18. De Montaudouin, X., Andemard, C. & Labourg, P-J., 1999. Does the slipper limpet (Crepidula fornicata L.) impair oyster growth and zoobenthos diversity ? A revisited hypothesis. Journal of Experimental Marine Biology and Ecology, 235, 105-124.

  19. De Montaudouin, X., Blanchet, H. & Hippert, B., 2018. Relationship between the invasive slipper limpet Crepidula fornicata and benthic megafauna structure and diversity, in Arcachon Bay. Journal of the Marine Biological Association of the United Kingdom, 98 (8), 2017-2028. DOI https://doi.org/10.1017/s0025315417001655

  20. Fell, P.E., 1989. Porifera. In Reproductive biology of invertebrates vol. IV, part A. Fertilization, development and parental care (ed. K.G & R.G. Adiyodi), pp. 1-41. Chichester: John Wiley & Sons.

  21. GBNNSIP 2012b. Botrylloides violaceus. Factsheet. [online]. York, GB Nonnative Species Secretariat. Available from: http://www.nonnativespecies.org/factsheet/factsheet.cfm?speciesId=514 [Accessed: 04/03/2016]

  22. GBNNSIP 2018. Carpet sea squirt Didemnum vexillum. Factsheet. [online]. York, GB Nonnative Species Secretariat. Available from: http://www.nonnativespecies.org/factsheet/factsheet.cfm?speciesId=1209 [Accessed: 04/03/2016]

  23. Gili, J-M. & Hughes, R.G., 1995. The ecology of marine benthic hydroids. Oceanography and Marine Biology: an Annual Review, 33, 351-426.

  24. Helmer, L., Farrell, P., Hendy, I., Harding, S., Robertson, M. & Preston, J., 2019. Active management is required to turn the tide for depleted Ostrea edulis stocks from the effects of overfishing, disease and invasive species. Peerj, 7 (2). DOI https://doi.org/10.7717/peerj.6431

  25. Hinz, H., Capasso, E., Lilley, M., Frost, M. & Jenkins, S.R., 2011. Temporal differences across a bio-geographical boundary reveal slow response of sub-littoral benthos to climate change. Marine Ecology Progress Series, 423, 69-82. DOI https://doi.org/10.3354/meps08963

  26. 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.

  27. Hoare, R. & Hiscock, K., 1974. An ecological survey of the rocky coast adjacent to the effluent of a bromine extraction plant. Estuarine and Coastal Marine Science, 2 (4), 329-348.

  28. Holt, T.J., Jones, D.R., Hawkins, S.J. & Hartnoll, R.G., 1995. The sensitivity of marine communities to man induced change - a scoping report. Countryside Council for Wales, Bangor, Contract Science Report, no. 65.

  29. JNCC (Joint Nature Conservation Committee), 2022.  The Marine Habitat Classification for Britain and Ireland Version 22.04. [Date accessed]. Available from: https://mhc.jncc.gov.uk/

  30. Johnston, E.L. & Roberts, D.A., 2009. Contaminants reduce the richness and evenness of marine communities: a review and meta-analysis. Environmental Pollution, 157 (6), 1745-1752.

  31. K├Ânnecker, 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.

  32. K├Ânnecker, 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.

  33. 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.

  34. Magorrian, B.H. & Service, M., 1998. Analysis of underwater visual data to identify the impact of physical disturbance on horse mussel (Modiolus modiolus) beds. Marine Pollution Bulletin, 36 (5), 354-359. DOI https://doi.org/10.1016/s0025-326x(97)00192-6

  35. 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.

  36. McNeill, G., Nunn, J. & Minchin, D., 2010. The slipper limpet Crepidula fornicata Linnaeus, 1758 becomes established in Ireland. Aquatic Invasions, 5 (Suppl. 1), S21-S25. DOI https://doi.org/10.3391/ai.2010.5.S1.006

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

  38. O'Brien, P.J. & Dixon, P.S., 1976. Effects of oils and oil components on algae: a review. British Phycological Journal, 11, 115-142.

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

  40. Powell-Jennings, C. & Callaway, R., 2018. The invasive, non-native slipper limpet Crepidula fornicata is poorly adapted to sediment burial. Marine Pollution Bulletin, 130, 95-104. DOI https://doi.org/10.1016/j.marpolbul.2018.03.006

  41. Preston, J., Fabra, M., Helmer, L., Johnson, E., Harris-Scott, E. & Hendy, I.W., 2020. Interactions of larval dynamics and substrate preference have ecological significance for benthic biodiversity and Ostrea edulis Linnaeus, 1758 in the presence of Crepidula fornicata. Aquatic Conservation: Marine and Freshwater Ecosystems, 30 (11), 2133-2149. DOI https://doi.org/10.1002/aqc.3446

  42. Rayment W.J., 2007. Crepidula fornicata. Slipper limpet. [online]. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [On-line]. Plymouth: Marine Biological Association of the United Kingdom.  Available from: <http://www.marlin.ac.uk>

  43. Rees, H.L., Waldock, R., Matthiessen, P. & Pendle, M.A., 2001. Improvements in the epifauna of the Crouch estuary (United Kingdom) following a decline in TBT concentrations. Marine Pollution Bulletin, 42, 137-144. DOI https://doi.org/10.1016/S0025-326X(00)00119-3

  44. Sewell, J., Pearce, S., Bishop, J. & Evans, J.L., 2008. Investigations to determine the potential risk for certain non-native species to be introduced to North Wales with mussel seed dredged from wild seed beds. CCW Policy Research Report, 835, 82 pp., Countryside Council for Wales

  45. Shumway, S.E., 1990. A review of the effects of algal blooms on shellfish and aquaculture. Journal of the World Aquaculture Society, 21, 65-104.

  46. Smith, J.E. (ed.), 1968. 'Torrey Canyon'. Pollution and marine life. Cambridge: Cambridge University Press.

  47. Stiger-Pouvreau, V. & Thouzeau, G., 2015. Marine Species Introduced on the French Channel-Atlantic Coasts: A Review of Main Biological Invasions and Impacts. Open Journal of Ecology, 5, 227-257. DOI https://doi.org/10.4236/oje.2015.55019

  48. 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

  49. Thieltges, D.W., 2005. Impact of an invader: epizootic American slipper limpet Crepidula fornicata reduces survival and growth in European mussels. Marine Ecology Progress Series, 286, 13-19. DOI https://doi.org/10.3354/meps286013

  50. Thieltges, D.W., Strasser, M. &  Reise, K., 2003. The American slipper-limpet Crepidula fornicata (L.) in the Northern Wadden Sea 70 years after its introduction. Helgoland Marine Research57, 27-33

  51. 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.]

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

  53. Thorson, G., 1936. The larval development, growth and metabolism of Arctic marine bottom invertebrates etc. Meddelelser om Gronland, 100, 1-155.

  54. 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.

  55. Tillin, H.M., Kessel, C., Sewell, J., Wood, C.A. & Bishop, J.D.D., 2020. Assessing the impact of key Marine Invasive Non-Native Species on Welsh MPA habitat features, fisheries and aquaculture. NRW Evidence Report. Report No: 454. Natural Resources Wales, Bangor, 260 pp. Available from https://naturalresourceswales.gov.uk/media/696519/assessing-the-impact-of-key-marine-invasive-non-native-species-on-welsh-mpa-habitat-features-fisheries-and-aquaculture.pdf

  56. Veale, L.O., Hill, A.S., Hawkins, S.J. & Brand, A.R., 2000. Effects of long term physical disturbance by scallop fishing on subtidal epifaunal assemblages and habitats. Marine Biology, 137, 325-337.

  57. Widdows, J., Donkin, P., Brinsley, M.D., Evans, S.V., Salkeld, P.N., Franklin, A., Law, R.J. & Waldock, M.J., 1995. Scope for growth and contaminant levels in North Sea mussels Mytilus edulis. Marine Ecology Progress Series, 127, 131-148.

  58. 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]

Citation

This review can be cited as:

Tyler-Walters, H., 2023. Musculus discors beds on moderately exposed circalittoral rock. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 12-06-2024]. Available from: https://www.marlin.ac.uk/habitat/detail/90

 Download PDF version


Last Updated: 13/10/2023