The Marine Life Information Network

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.

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.

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.

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.  

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.

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.

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.

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.

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.

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. 

Radionuclide contamination

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

Evidence

No evidence was found.

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.

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.

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.

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.

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.

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’.

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.

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.

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.

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.

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.

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.

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. 

Litter

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

Evidence

Not assessed.

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

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.

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.

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.

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’.

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.

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.

Introduction or spread of invasive non-indigenous species

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

Evidence

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ‘ecosystem engineer’ that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009). Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; Mckenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024). In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Michin & Nunn, 2013; Bishop et al., 2015; Mckenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024).

Although a widespread invader, Didemnum vexillum has a limited ability for natural dispersal since the pelagic larvae remain in the water column for a short time (up to 36 hours). Therefore, it has a short dispersal phase that can allow the species to build localized populations (Herborg et al., 2009; Vercaemer et al., 2015; Holt, 2024). However, Bullard et al. (2007) suggested that Didemnum vexillum can form new colonies asexually by fragmentation. Colonies can produce long tendrils from an encrusting colony, which can fragment, disperse and settle, attaching to suitable hard substrata elsewhere (Bullard et al., 2007; Lambert, 2009; Stefaniak & Whitlatch, 2014). A fragmented colony can spread naturally for up to three weeks transported by ocean currents, attached to floating seaweed, seagrass or other floating biota, or as free-floating spherical colonies (Bullard et al., 2007; Lengyel et al., 2009; Stefaniak & Whitlatch, 2014; Holt, 2024). Fragments can reattach to suitable substrata within six hours of contact. Fragments have the potential to disperse around 20 km before reattachment (Lengyel et al., 2009). Valentine et al. (2007a) reported that colonies of Didemnum vexillum enlarged by 6 to 11 times by asexual budding after 15 days and enlarged 11 to 19 times after 30 days. Valentine et al. (2007a) concluded fragments could successfully grow, survive, and help to spread Didemnum vexillum.

While natural fragmentation of tendrils is thought to allow Didemnum vexillum to invade longer distances and increase its dispersal potential, Stefaniak & Whitlatch (2014) found that only a one tendril out of 80 reattached to the flat, bare substrata used in their study, because tendrils required an extensive (at least eight hour) period of contact to reattach. Stefaniak & Whitlatch (2014) suggested that once fragmented from a colony, the success of tendril reattachment was limited, and reattachment was not a major contributor to the invasive success of Didemnum vexillum. However, Stefaniak & Whitlatch (2014) also found that larvae-packed tendril fragments may increase natural dispersal distance, reproduction, and invasive success of Didemnum vexillum, and increase the distance larvae can travel. Not all colonies produce tendrils at all locations.

Human-meditated transport via aquaculture facilities, boat hulls, commercial fishing vessels, and ballast water is probably the most important vector that has aided the long-distance dispersal of Didemnum vexillum and explains its prevalence in harbours and marinas (Bullard et al., 2007; Dijkstra et al., 2007; Griffith et al., 2009; Herborg et al., 2009). Fragmentation of colonies during transport or human disturbance (such as trawling or dredging) could indirectly disperse the species and enable it to find suitable conditions for establishment (Herborg et al., 2009). For example, in oyster farms in British Columbia, large fragments of Didemnum sp. come off oyster strings when they are pulled out of water and other fragments can be pulled off oysters and mussels and thrown back into the water, which is likely to aid dispersal of the invasive species (Bullard et al., 2007). Dijkstra et al. (2007) hypothesised that Didemnum sp. was introduced to the Gulf of Maine with oyster aquaculture in the Damariscotta River and transported via Pacific oysters.

Didemnum vexillum was likely introduced into the UK from northern Europe or Ireland via poorly maintained or not antifouled vessels, movement of contaminated shellfish stock and aquaculture equipment, or via marine industries such as oil, gas, renewables, and dredging (Holt, 2024). Recent evidence from genetic material suggests that human-mediated dispersal, between marinas and shellfish culture sites, is the most likely pathway for connectivity of Didemnum vexillum populations throughout Ireland and Britain (Prentice et al., 2021; Holt, 2024). Didemnum vexillum can disperse away from artificial substrata, invading and colonizing natural substrata in surrounding areas (Tillin et al., 2020). Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. The current evidence of Didemnum vexillum’s ability to spread on natural habitats in this area is sparse and often conflicting, complicated by genetics and its apparent variable habitat preferences and tolerances and its variable ability to adapt to ‘new’ conditions (Holt 2024).

Didemnum vexillum has a seasonal growth cycle that is influenced by temperature (Valentine et al., 2007a). In warmer months (June and July) colonies may be large and well-developed encrusting mats. Populations experience more rapid growth from July to September sometimes continuing into December. Colonies begin to decline in health and ‘die-off’ when temperatures drop below 5°C during winter months from around October to April (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). Cold water months cause colonies to regress and reduce in size, yet they often regenerate as temperatures warm (Griffith et al., 2009; Kleeman, 2009, Mercer et al., 2009), although some populations may not survive winter at all (Dijkstra et al., 2007). The early growth phase, from May to July, is initiated by smaller colonies developing from remnants of colonies that survived the cold water (Valentine et al., 2007a). The seasonal growth cycle is also likely influenced by location. For example, the Didemnum sp. growth cycle for colonies in Sandwich tide pool (temperature range from -1 °C to 24 °C, with daily fluctuations), probably does not occur in deep offshore subtidal habitats in Georges Bank (annual temperature range from 4 °C to 15°C, and daily fluctuations are minimal) (Valentine et al., 2007a).  Larval release and recruitment typically occur between 14 to 20°C and slow or cease below 9 to 11°C as summer ends (Griffith et al., 2009; Mckenzie et al., 2017). In New Zealand, recruitment occurs from November to July, where the highest average temperatures were recorded in February (18 to 22°C) and the lowest average temperatures were recorded in July (9 to 10°C) (Fletcher et al., 2013a). In this New Zealand study, higher water temperatures were associated with a higher level of recruitment (Fletcher et al., 2013a).

Didemnum vexillum requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems because of its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock gravel, pebble, cobble, or boulders or artificial substrata such as a variety of maritime structures such as pontoons, docks, wood and metal pilings, chains, ropes and moorings, plastic and ship hulls and at aquaculture facilities (Valentine et al., 2007 a&b; Bullard et al., 2007; Griffith et al., 2009; Lambert, 2009; Tagliapietra et al., 2012; Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent, and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020).

There are few observations of Didemnum vexillum on soft bottom habitats as evidence suggests it is unable to establish or grow easily on mud, mobile sand or other unstable substrata, and it is vulnerable to smothering by fine sediment (Bullard et al., 2007; Valentine et al., 2007a; Griffith et al., 2009). The species is usually found established in areas where the colony is protected from sedimentation and wave action (Valentine et al., 2007b; Mckenzie et al., 2017; Tillin et al., 2020). For example, at Georges Bank, USA the Didemnum vexillum mats were limited to gravelly areas and unable to colonize the sand ridges that bounded the site, which have a mobile surface that is moved daily by the strong tidal currents (Valentine et al., 2007b). In addition, evidence found the species can also not survive being buried or smothered by coarse or fine grained sediment. Furthermore, in Holyhead marina, Didemnum vexillum colonies were contained in the harbour and established on artificial pontoons, and they were not present on the natural seabed under the pontoon, which is composed of silty mud or on deeper sections of mooring chains that are immersed in mud at low spring tides (Griffith et al., 2009).

However, some studies on Georges Bank, USA and Sandwich, Massachusetts observed colonies were able to survive partial covering by sand (Bullard et al., 2007; Valentine et al., 2007a). Gittenberger et al. (2015) reported that Didemnum vexillum was able to overgrow sandy bottom (cited Gittenberger, 2007). In northern Kent, Didemnum vexillum has been recorded covering London clay boulders on Whitestable Flats, West Beach, north Kent, covering tabulate sandstone boulders (0.5 to 2 m across) on the mid-shore and colonizing sediment mounds on the low shore characterized by larger areas of sand, mud and low-lying sediment at Reculver and Bishopstone, north Kent (Hitchin, 2012). It was also recorded from muddy substrata at that site. Hitchin (2012) noted that the site was exposed to enough waves and currents to cause sedimentation. However, Didemnum vexillum grew hanging from on the underside of sandstone boulders nestled on sediment, on consolidated sediment mounds and firm clays, hence burial may prevent colonization and its survival rather than sedimentation alone.

In contrast, Didemnum vexillum’s preference for sheltered conditions, established colonies observed in Georges Bank and Long Island Sound were exposed to moderately strong tidal currents (1 to 2 knots; ca 0.5 to 1 m/s recorded at both sites) that may mobilise sediment (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020). However, Valentine et al. (2007b) describe the substratum as immobile, presumably consolidated, gravel, cobbles, and pebbles. Kleeman (2009), stated that the presence of a consistent mild wave action or ‘swash zone’ appears to favour Didemnum sp. establishment in the intertidal. Although some evidence suggests that waves and currents can facilitate the fragmentation and spread of Didemnum vexillum (Mckenzie et al., 2017), the tidal current velocities at some sites where Didemnum vexillum has been reported (for example, New England, where current velocities reach up to around 3 m/s) is lower than the current velocity required for the dislodgement of Didemnum vexillum fragments (around 7.6 m/s) (Reinhardt et al., 2012). This suggests that not all tidal currents are likely to dislodge Didemnum vexillum fragments. When on boat hulls the species can experience higher current velocities which is enough to cause dislodgement (Reinhardt et al., 2012).  

Didemnum vexillum can overgrow bivalve species, such as oysters, scallops, and mussels, as the hard shells can provide suitable hard substrata for settlement. It has been described as a ‘shellfish pest’ by the aquaculture industry because it is likely to completely encapsulate bivalves and smother them resulting in death or partially encapsulate and partially smother them resulting in reduced bivalve growth (Auker, 2010; Bullard et al., 2007; Coutts & Forrest, 2007, Valentine et al., 2007a; Carman et al., 2009; Kleeman, 2009; Fletcher et al., 2013b; Tillin et al., 2020). Didemnum vexillum has been recorded overgrowing mussels in Strangford Lough, Northern Ireland (Minchin & Nunn, 2013) and recorded forming large mats over Blue Mussel beds in the Gulf of Maine, completely covering individuals (Auker et al., 2014). Didemnum vexillum fouling on aquaculture equipment and bivalve species causes great economic impacts, as Didemnum vexillum removal methods are expensive, labour-intensive, and not always effective (Coutts & Forrest, 2007; Carman et al., 2009; Kleeman, 2009; Fletcher et al., 2013b; Tillin et al., 2020; Holt, 2024). The fouling on aquaculture nets and bags can restrict water flow and food availability for shellfish and smothering on mussel farms may result in crop losses (Coutts & Forrest, 2007; Carver et al., 2003 cited by Carman et al., 2009; Fletcher et al., 2013b; Holt, 2024). Effects on mussels are likely to become more prominent as Didemnum vexillum becomes more abundant (Auker, 2010).

The epibiotic relationship between Didemnum vexillum and Mytilus edulis negatively impacts mussel growth (Auker, 2010). Clean control mussels with no Didemnum vexillum overgrowth had thicker shells, a significantly thicker lip, and a greater tissue index, compared to mussels overgrown by Didemnum vexillum (Auker, 2010). Mortality of both control and overgrown mussels was relatively low over the one-year study period, but higher mortality was seen in overgrown mussels (6.7% died) compared to the clean control mussels (1.1% died) (Auker, 2010). Auker (2010) also found that Didemnum vexillum affected reproduction and recruitment of Mytilus edulis as the invasive species grew over gamete release point (siphons) or inhibited settlement of recruits, but this varied seasonally. Fletcher et al. (2013b) also noted that Didemnum vexillum clogged cages and mesh used to house shellfish (e.g. mussels and oysters), which could reduce shellfish growth rates. In contrast, the overgrowth of mussels by Didemnum vexillum has reduced the predation risk on mussels, as Didemnum vexillum mats act as refuges for blue mussels (Auker, 2010; Auker et al., 2014, Lyu et al., 2020).

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.

Didemnum vexillum has not been recorded from Musculus discors beds (in 2024). No evidence was found on the potential effects of Didemnum sp. on Musculus discors beds. However, Tillin et al. (2020) suggested that Musculus discors beds could be vulnerable as they could provide suitable attachment opportunities for Didemnum vexillum. Didemnum vexillum requires hard substrata (such as rock, gravel, and shell) or consolidated sediment for settlement (Gittenberger, 2007; Valentine et al., 2007a; Hitchin, 2012). 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. It is also unclear if Musculus discors could out-compete Didemnum sp. for space, or vice-versa. If Didemnum sp. was able to gain a 'foothold' it could overgrow, smother or cause mortality to the Musculus discors beds. In addition, Didemnum vexillum has also been recorded in moderately strong currents (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020) and predicted to survive stronger currents, as current velocity which will dislodge Didemnum vexillum is around 7.6 m/s (Reinhardt et al., 2012). Therefore, Didemnum vexillum could colonize examples of these biotopes that occur in tide-swept conditions. Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. Therefore, a resistance of 'Medium' (some mortality, <25%) is suggested as a precaution. Resilience is likely to be 'Very low' as Didemnum vexillum would need to be physically removed for recovery to occur. Hence, sensitivity to invasion by Didemnum is assessed as 'Medium'. 

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.

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.

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.