Biodiversity & Conservation

CR.MCR.M.ModT

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Removal of the substratum would result in the loss of the Modiolus modiolus bed and its associated community. Therefore, an intolerance of high has been recorded.
The epifaunal organisms such as anthozoans, hydroids, barnacles, ascidians and brittlestars are likely to take some time to recolonize but could potentially recover within five years. However, Modiolus modiolus beds, are likely to take considerable time the recolonize and to develop into a bed similar in size and in the diversity and species richness they support (see additional information below). Therefore, a recoverability of very low has been recorded.
Smothering
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Holt et al., (1998) point out that the deposit of spoil or solid wastes (e.g. from capital dredging) that settle as a mass will smother any habitat it lands on. MCR.ModT beds usually occur in areas of moderate to strong water flow (Holt et al., 1998) where accretion is probably reduced.
Biogenic reef formation involves the build up of faecal mud, suggesting that adults can move up through the accreting mud to maintain their relative position within the growing mound. However, no information on natural accretion rates was found. Holt et al. (1998) note that there are no studies of the accretion rates that Modiolus modiolus beds can tolerate. Therefore, smothering by 5cm of sediment for a month (the benchmark level) is likely to remove a proportion of the horse mussel population. Red algae such as Delesseria sanguinea and Phycodrys rubens are probably large enough to tolerate smothering by 5cm of sediment, and encrusting coralline algae would probably survive under sediment for one month (see benchmark). Ophiothrix fragilis and Balanus crenatus are likely to be smothered by 5cm of sediment, and are not able to crawl up through the sediment. Hydroids are likely to be intolerant of smothering and siltation (see below), e.g. Sertularia operculata were reported to have died when covered by a fine layer of silt during periods of low water movement (Gili & Hughes, 1995). Therefore, a proportion of the horse mussel population and its associated community may be lost due to smothering and an intolerance of intermediate has been recorded. Hydroids and brittle stars may be more intolerant, therefore, species richness is likely to decline.
Recruitment is sporadic, highly variable and some areas receive little or no recruitment for several years (see additional information below). Therefore, a recoverability of low has been recorded.
Increase in suspended sediment
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Modiolus modiolus is found in a variety of turbid and clear water conditions (Holt et al., 1998). Muschenheim & Milligan (1998) noted that the height of the horse mussels beds in the Bay of Fundy positioned them within the region of high quality seston while avoiding high levels of re-suspended inorganic particulates (2.5-1500mg/l) at the benthic boundary layer. Comely (1978) noted that a population in a high turbidity area (up to 14mg/l inorganic suspended particulates) showed excessive pearl formation and poor shell growth and condition, although the populations poor condition was probably partly due to old age and senility. Infaunal communities are probably exposed to high levels of suspended sediment at intervals (depending on variation in water flow and storms). Therefore, although high levels of suspended sediment may interrupt feeding, or result in the production of pseudofaeces at energetic cost, Modiolus modiolus is probably able to tolerate increases in suspended sediment for intervals equivalent to the benchmark and an intolerance of low has been recorded. Increases in organic suspended particulates may increase food availability and be beneficial. Horizontal surfaces in the subtidal tend to be algal dominated (where illumination permits) with animal dominated communities occurring on vertical or steep slopes (Hartnoll, 1983). However, the species identified as indicative of intolerance were assessed as 'low' intolerance to increase suspended sediment and siltation. Increased suspended sediment may clog or interfere with filter feeding or suspension feeding apparatus, which would require an energetic cost to clear. However, suspension feeders may benefit from an increase in organic particulates. Hydroids may be particularly intolerant e.g. Sertularia operculata were reported to have died when covered by a fine layer of silt during periods of low water movement (Gili & Hughes, 1995).
In areas of strong tidal flow where the biotope MCR.ModT is found, an increase suspended sediment may not result in a significant increase in siltation. Therefore, since the indicative species were of low intolerance to increases in suspended sediment an overall biotope intolerance of low has been recorded but a decline in species richness is likely due to loss of epifaunal hydroids. However, the biotopes SCR.ModCvar and SCR.ModHAs may be more intolerant of increased suspended sediment due to an increase in siltation in sheltered habitats.
Most suspension feeders are likely to recover rapidly, however, a recoverability of very high has been recorded to represent the time required for hydroids to recover their original abundance or extent.
Decrease in suspended sediment
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A decrease in suspended sediment may decrease the food availability for Modiolus modiolus and other suspension feeding species. However, Navarro & Thompson (1996) demonstrated that Modiolus modiolus was adapted to seasonal fluctuations in food availability, reducing feeding activity in winter and increasing feeding activity during the summer phytoplankton bloom, for which it had a high absorption efficiency. Similarly, Ophiothrix fragilis has a low respiration rate and can tolerate considerable loss of body mass during reproductive periods (Davoult et al., 1990) so that restricted feeding may be tolerated. Therefore, Modiolus modiolus is unlikely to be adversely affected by a decrease in suspended sediment for a month (see benchmark). Overall, therefore, suspension feeders within the biotope may suffer reduced growth or condition due to reduced food availability and an intolerance of low has been recorded. Red algae may benefit from reduced suspended sediment due to reduced turbidity (see below).
Desiccation
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Most of the species identified as indicative of intolerance may be of 'intermediate' or 'high' intolerance to desiccation, including Modiolus modiolus. Hydroids especially are also likely to be highly intolerant. However, this biotope (MCR.ModT) is found from the lower infralittoral and the circalittoral and is unlikely to be exposed to the air.
Increase in emergence regime
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Most of the species identified as indicative of intolerance may be of 'intermediate' or 'high' intolerance to desiccation and emergence regime, including Modiolus modiolus. Hydroids especially are also likely to be highly intolerant. However, this biotope (MCR.ModT) and those biotopes it has been used to represent, is found from the lower infralittoral and the circalittoral and in unlikely to be exposed to the air.
Decrease in emergence regime
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Decreased emersion is unlikely to adversely affect this biotope (or those it has been chosen to represent) and may allow members of the biotope to feed longer and improve condition, i.e. the biotope may benefit. The biotope could possibly extend its range, although the rates of increase in bed size are likely to be slow, probably longer than the benchmark level.
Increase in water flow rate
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MCR.ModT occurs in tide swept locations in moderately strong to strong tidal streams. An increase in water flow may interfere with feeding in Modiolus modiolus since in flume studies the inhalant siphon closed by about 20% in currents above 55 cm/sec (Wildish et al., 2000). Similarly, fouling of the horse mussels increases their intolerance to dislodgement by strong tidal streams (Witman, 1985). Comely (1978) suggested that areas exposed to strong currents required an increase in byssus production, at energetic cost, and resulted in lower growth rates. Therefore, an increase in water flow rates to very strong may result in loss of a proportion of the population, depending on the size of the beds, the level of fouling or the nature of the substratum. Horse mussel beds on coarse or hard substrata may be less intolerant than beds on mobile, fine sediments.
Epifauna such as hydroids may be damaged, or their feeding prevented by strong water flow (Gili & Hughes, 1995). The characterizing hydroids may be replaced by hydroid species more tolerant of strong water flow such as Tubularia indivisa. Brittlestars such as Ophiothrix fragilis may be swept away by increased water flow, e.g. above a certain water speed (25 cm/s) the feeding arms are withdrawn from the water column (Warner & Woodley, 1975; Hiscock, 1983). At water speeds above about 28 cm/s individuals or even small groups may be displaced from the substratum and they have been observed being rolled along the seabed by the current (Warner, 1971). Living in dense aggregations may reduce displacement of brittlestars by strong currents (Warner & Woodley, 1975) and living within crevices in the horse mussel beds will presumably also provide some protection. Sea urchins, such as Echinus esculentus, are known to be swept away by strong currents and, although not killed, may be removed from the community and unable to return until water flow rates return to prior conditions.
Overall, therefore a proportion of the horse mussel population may be removed, together with several members of the community and an intolerance of intermediate has been recorded.
The biotopes £SCR.ModCvar£ and £SCR.ModHAs£ may be more intolerant of dislodgement due to there muddy substratum. The associated community will probably change from species tolerant of siltation and low water flow to species tolerant of higher water flow, perhaps coming to resemble MCR.ModT.
Horse mussel recruitment is sporadic, highly variable and some areas receive little or no recruitment for several years (see additional information below). Therefore, a recoverability of low has been recorded.
Decrease in water flow rate
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Flume experiments suggested that Modiolus sp. can deplete the seston directly over dense beds when water flow is low, resulting in a reduction in the density of the mussel bed (Wildish & Kristmanson, 1984, 1985: Holt et al., 1998). Alcyonium digitatum prefers areas of high water flow, and its abundance may decline in reduced water flow. Brittlestars such as Ophiothrix fragilis are passive suspension feeders and require water flow to supply them with food particles. A reduction in water flow may reduce food availability, however Ophiothrix fragilis can survive considerable loss of body mass during reproductive periods (Davoult et al., 1990) so restricted feeding may be tolerated, and this species is found in sheltered areas of reduced water flow. Hydroids and bryozoans also require water flow to provide them with food particles but hydroid species in deeper water, with generally less water movement, have higher biomass, are larger and longer-lived than in shallower waters. Therefore, a reduction in water flow may reduce the density of the horse mussel bed, and may change the associated community favouring species that prefer low water flow. The biotope MCR.ModT may come to resemble the sheltered horse mussels beds (£SCR.ModCvar£ or £SCR.ModHAs£). In addition, in the sheltered biotopes decreased water flow will increase the risk of deoxygenated conditions (see below). Overall, therefore, an intolerance of intermediate has been recorded.
Horse mussel recruitment is sporadic, highly variable and some areas receive little or no recruitment for several years (see additional information below). Therefore, a recoverability of low has been recorded.
Increase in temperature
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Modiolus modiolus is a boreal species reaching its southern limit in British waters (Holt et al., 1998). Davenport & Kjørsvik (1982) suggested that its inability to tolerate temperature change was a factor preventing the horse mussel from colonizing the intertidal in the UK. Intertidal specimens were more common on northern Norwegian shores (Davenport & Kjørsvik, 1982). Little information on temperature tolerance in Modiolus modiolus was found, however, its upper lethal temperature is lower than that for Mytilus edulis (Bayne et al., 1976) by about 4 °C (Henderson, 1929; cited in Davenport & Kjørsvik, 1982).
Subtidal populations are protected from major, short term changes in temperature by their depth. However, Holt et al. (1998) suggested that because Modiolus modiolus reaches its southern limit in British waters it may be susceptible to long term increases in summer water temperatures.
Therefore, the absence of this species from the intertidal in the UK (with a few exceptions) suggests that it is intolerant of temperature change. The suggested susceptibility to long-term summer temperature rise could result in a reduction in the extent of the UK population and its associated community.
Lower infralittoral to circalittoral populations are exposed to a narrow range of temperatures when compared to the intertidal or even the shallow subtidal. Deep water species are therefore, likely to be intolerant of temperature change, especially short term acute change. For example, eight deep water red algae species had lower upper lethal temperatures than three shallow water red algae (Kain & Norton, 1990). Delesseria sanguinea is tolerant of 23 °C for a week (Lüning, 1984) but dies rapidly at 25 °C. North Sea and Baltic specimens grew between 0-20 °C, survived at 23 °C but died rapidly at 25 °C (Rietema, 1993). Rietema (1993) reported temperature differences in temperature tolerance between North Sea and Baltic specimens. Lüning (1990) reports optimal growth in Delesseria sanguinea between 10 - 15 °C and optimal photosynthesis at 20 °C. However, the upper limit of temperature tolerance in red algae reduced by lowered salinity (Kain & Norton, 1990).
Temperature is a critical factor in stimulating or preventing hydroid reproduction and most species exhibit an optimal range (Gili & Hughes, 1995).
Bishop (1985) noted that gametogenesis in Echinus esculentus proceeded at temperatures between 11 - 19 °C although continued exposure to 19 °C destroyed synchronicity of gametogenesis between individuals. Bishop (1985) suggested that this species cannot tolerate high temperatures for prolonged periods due to increased respiration rate and resultant metabolic stress, suggesting intolerance to acute temperature change. However, Echinus esculentus is recorded from southern and northern British Isles suggesting tolerance of the temperature range found in the UK. Short term acute changes in temperature are noted to cause a reduction in the loading of subcutaneous symbiotic bacteria in echinoderms such as Ophiothrix fragilis. Reductions in these bacteria are probably indicative of levels of stress and may lead to mortality (Newton & McKenzie, 1995). However, the distribution of Ophiothrix fragilis is large, ranging from northern Norway south to the Cape of Good Hope. Consequently this species is exposed to temperatures both above and below those found in the British Isles.
Overall, therefore, it is likely that a proportion of the horse mussel population and the associated community may be lost due to acute temperature change (see benchmark). Long term increases in temperature may reduce the populations range in the UK. Therefore, an intolerance of intermediate has been recorded. While, several members of the community are likely to recover within a few years, horse mussel recruitment is sporadic, varies with season, annually and with location and hydrographic regime and is generally low, therefore it may take many years for a population to recover from damage and a recoverability of low (10-25 years) has been recorded.
Decrease in temperature
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Modiolus modiolus is a boreal species reaching its southern limit in British waters (Holt et al., 1998). Lower infralittoral to circalittoral populations are exposed to a narrow range of temperatures when compared to the intertidal or even the shallow subtidal. Deep water species are therefore, likely to be intolerant of temperature change, especially short term acute change. Long term decreases in temperature could allow Modiolus beds and, therefore, the biotope to extend its range southwards. Other members of the community have a wide distribution in the north east Atlantic, although hydroids may be affected by decreased temperatures, especially short term acute changes. However, the biotope could potentially extend its range due to a decrease in temperature and 'not sensitive*' has been recorded. Short term acute change may remove members of the epifaunal community and a minor decline in species richness may result.
Increase in turbidity
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Modiolus modiolus is found in turbid to clear waters (Holt et al., 1998). Increased turbidity may decrease phytoplankton primary productivity and hence the food supply for the horse mussel. However, Navarro & Thompson (1996) concluded that the horse mussel was adapted to an intermittent and often inadequate food supply.
However, other suspension feeding species may be affected by the reduced food availability, e.g. Ophiothrix fragilis, however this species can survive loss of body mass during reproductive periods and is likely to survive reduced food availability. Alcyonium digitatum will be unaffected in the factor changes during its quiescent period (late July - December) and will probably survive during the rest of the year, although is reproductive capacity may be reduced. While encrusting coralline algae are particularly tolerant of low light conditions, increased turbidity is likely to adversely affect foliose red algae. Although shade tolerant, a decrease in light intensity, comparable to the benchmark level, is likely to reduce photosynthesis, reduce growth and affect reproduction. Increased turbidity, is therefore likely to result in loss of red algae from this biotope. However, other epifauna may benefit as a result, e.g. hydroids may increase in abundance, size and diversity. Algal grazers such as gastropods and chitons may be lost from the biotope if no alternative food sources are available. Therefore, there will be losses for some species and gains for others and an intolerance of low has been recorded due to the intolerance of red algae within the biotope.
Recoverability will depend on recolonization by red algae once turbidity returns to previous or tolerable levels e.g. Delesseria sanguinea was reported to recolonize cleared blocks within 56-59 days in one experiment and 41 weeks (8 months) in another depending on depth and spore availability (Kain, 1975). Therefore a recoverability of high has been recorded.
Decrease in turbidity
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Modiolus modiolus is found in turbid to clear waters (Holt et al., 1998). Decreases in turbidity may increase phytoplankton productivity and therefore, potentially increase food availability for the horse mussels and other suspension feeding epifauna. Increased light availability will benefit red algae, promoting growth but may reduce the abundance of hydroids by interfering with settlement, or due to competition for space with red algae (Kain & Norton, 1990; Gili & Hughes, 1995). Red algae may increase in abundance. Increased growth of algae, especially kelps, may increase the horse mussel beds vulnerability to dislodgement by strong water flow, depending on the level of grazing by sea urchins in particular (Witman, 1985). Therefore, increased fouling is likely to impair feeding and hence reproduction in horse mussels and an intolerance of low has been recorded. However, in the absence of sufficient grazing, fouling by foliose algae, especially kelps may result in dislodgement of a proportion of the mussel bed (Witman, 1985). Recovery will depend on reduction in red algae and colonization by other epifauna such as bryozoans or hydroids, which likely to be rapid, depending on local conditions and the proximity of adult colonies.
Increase in wave exposure
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An increase in wave exposure may result in increased oscillatory movement at the seabed, which can be a destructive force (Hiscock, 1983). Comely (1978) suggested that in areas of strong water flow horse mussels increased byssus production. Mytilus edulis was shown to increase byssus production in response to agitation (Young, 1985) and Modiolus modiolus may respond similarly, so that increased wave action may be resisted. Populations on mobile sediment may be removed by strong wave action due to removal or changes in the substratum. No information concerning storm damage was found.
Epifauna such as hydroids may be damaged, or their feeding prevented by strong water flow (Gili & Hughes, 1995). The characterizing hydroids may be replaced by hydroid species more tolerant of strong water flow such as Tubularia indivisa. Brittlestars such as Ophiothrix fragilis may be swept away by increased water flow, e.g. above a certain water speed (25 cm/s) the feeding arms are withdrawn from the water column (Warner & Woodley, 1975; Hiscock, 1983). At water speeds above about 28 cm/s individuals or even small groups may be displaced from the substratum and they have been observed being rolled along the seabed by the current (Warner, 1971). Living in dense aggregations may reduce displacement of brittlestars by strong currents (Warner & Woodley, 1975) and living within crevices in the horse mussel beds will presumably also provide some protection. Sea urchins, such as Echinus esculentus, are known to be swept away by strong currents and, although not killed, may be removed from the community and unable to return until calmer conditions return.
Overall, therefore a proportion of the horse mussel population may be removed, together with several members of the community and an intolerance of intermediate has been recorded. The biotopes £SCR.ModCvar£ and £SCR.ModHAs£ may be more intolerant of dislodgement due to their muddy substratum. The associated community will probably change from species tolerant of siltation and low water low to species tolerant of higher water flow, perhaps coming to resemble MCR.ModT.
Horse mussel recruitment is sporadic, highly variable and some areas receive little or no recruitment for several years (see additional information below). Therefore, a recoverability of low has been recorded.
Decrease in wave exposure
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Tidal flow rather than wave action is the predominant force in feeding, so that wave action is most important in relation to the potential destruction of beds. Providing that tidal flows remains reasonably strong, horse mussel beds may benefit from a reduction in wave action and a rank of 'not sensitive*' is suggested. Decreased wave action may allow horse mussel beds to extend into shallower depths, however, the rates of increase in bed size are likely to be slow, probably much longer than the benchmark level.
Noise
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Underwater noise may deter feeding by some fish species for short periods. However, predation from decapods and whelks and grazing by sea urchins is unlikely to be affected. No other species within the biotope are likely to response to noise at the level of the benchmark.
Visual Presence
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Shading by passing boats may deter feeding by some fish species for short periods. However, it is unlikely to significantly affect predation pressure in the long term. Few other species have the visual acuity to be affected by the factor.
Abrasion & physical disturbance
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Modiolus modiolus are large and relatively tough. Holt et al. (1998) suggested that horse mussel beds were not particularly fragile, even when epifaunal, with semi-infaunal and infaunal population being less vulnerable to physical disturbance. Clumps of horse mussels of muddy substrata may be more intolerant. However, impacts from towed fishing gear (e.g. scallop dredges) are known to flatten clumps and aggregations, may break off sections of raised reefs and probably damage individual mussels (Holt et al., 1998). The shells of older specimens can be very brittle due to infestations of the boring sponge Cliona celata (Comely, 1978; Holt et al., 1998). Holt et al., (1998) suggested that scallop dredging on areas adjacent to beds in the south east of the Isle of Man had 'nibbled away at the edges' of dense beds, which had become less dense and more scattered. Extensive beds were present to the north of the Isle of Man where scallop dredging had apparently not occurred (Holt et al., (1998).

Magorrian & Service (1998) reported that queen scallop trawling resulted in flattening of the horse mussel bed and disruption of clumps of horse mussels and removal of emergent epifauna in Strangford Lough. They suggested that the emergent epifauna such as Alcyonium digitatum were more intolerant than the horse mussels themselves and reflected early signs of damage but were able to identify different levels of impact from impacted but largely intact to heavily trawled areas with few Modiolus modiolus intact, lots of shell debris and little epifauna (Service & Magorrian, 1997; Magorrian & Service, 1998; Service 1998). Veale et al., 2000 reported that the abundance, biomass and production of epifaunal assemblages, including Modiolus modiolus and Alcyonium digitatum decreased with increasing fishing effort. Species with fragile hard tests such as echinoids are known to be intolerant of scallop dredges (see Eleftheriou & Robertson, 1992; Veale et al., 2000). Scavengers such as Asterias rubens and Buccinum undatum were reported to be fairly robust to encounters with trawls (Kaiser & Spencer, 1995) may benefit in the short term, feeding on species damaged or killed by passing dredges. However, Veale et al. (2000) did not detect any net benefit at the population level. Scallop dredging was found to damage many of the epibenthic species found in association with Modiolus beds (Hill et al., 1997; Jones et al., 2000). Holt et al. (1998) suggested that damage by whelk potting was not likely to be severe but also noted that epifaunal populations may be more intolerant.

Disruption of the clumps or beds may result in loss of some individual horse mussels suggesting an intolerance of intermediate, however, given the intolerance of epifauna suggested above an overall intolerance of high is recorded.

Horse mussel recruitment is sporadic, varies with season, annually and with location and hydrographic regime and is generally low, therefore it may take many years for a population to recover from damage and a recoverability of low (10-25 years) has been recorded.

Displacement
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Holt et al., (1998) noted the survival of clumps torn from a horse mussel bed was not known. Modiolus modiolus displaced from the beds will probably be able to re-attach to suitable substratum using their byssus threads, although no information was found concerning their ability to burrow. The ability of clumps or individuals to maintain a viable population will depend on the location and depth of the new habitat, food supply, and the local hydrographic regime. Displacement of important species such as sea urchins may increase fouling by epiflora and epifauna increasing the risk of dislodgement and further displacement in the strong currents characteristic of this biotope (MCR.ModT). Most sessile, permanently attached epifauna are likely to be highly intolerant of displacement in their own right, e.g. Alcyonium digitatum. Mobile epifauna, e.g. crabs, whelks and lobsters will probably be largely unaffected.
Overall, displacement of Modiolus modiolus is likely to result in loss in a proportion of the biotope and an intolerance of intermediate has been recorded.
Recovery of the biotope will depend on recolonization and regrowth of clumps of horse mussels or the closure of gaps in the bed, which is likely to take considerable time (see additional information below) and a recoverability of low has been recorded.

Chemical Factors

Synthetic compound contamination
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No information concerning the effects of synthetic contaminants on Modiolus modiolus was found. However, it is likely to have a similar metabolism to that of Mytilus edulis and hence, possibly, a similar tolerance to chemical contaminants.
Livingstone & Pipe (1992) cite Palmork & Solbakken (1981) who reported that Modiolus modiolus accumulated poly-aromatic hydrocarbons (PAHs) and examined the depuration of phenanthrene from horse mussel tissue. However, no effects on the horse mussel were documented. PAHs contribute to a reduced scope for growth in Mytilus edulis (Widdows et al., 1995) and probably have a similar effect in the horse mussel but to an unknown degree.
Tri butyl-tin (TBT) has been 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).
Therefore, it is likely that TBT may interfere with growth and settlement of Modiolus modiolus larvae. Horse mussel populations exhibit sporadic recruitment, therefore any factor that adversely affects recruitment will have an adverse effect on the population, although the effects may not be observed for some time since the species in so long lived.
O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination, possibly due to the susceptibility of phycoerythrins to destruction, and that the filamentous forms were the most sensitive. However, most evidence relates to dispersants, e.g. heavy mortality of Delesseria sanguinea occurred down to 12 m after the Torrey Canyon oil spill (probably due to a mixture of wave action and dispersant application) (Smith, 1968). 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. Smith (1968) reported dead colonies of Alcyonium digitatum and dead Echinus esculentus at a depths of up to 16m in the locality of Sennen Cove (Pedu-men-du, Cornwall) resulting from the offshore spread and toxic effect of detergents e.g. BP 1002. Cole et al. (1999) suggested that herbicides , such as simazina and atrazine were very toxic to macrophytes. Hoare & Hiscock (1974) noted that Delesseria sanguinea was excluded from Amlwch Bay, Anglesey by acidified halogenated effluent discharge. In addition Echinus esculentus populations in the vicinity of an oil terminal in A Coruna Bay, Spain, showed developmental abnormalities in the skeleton. The tissues contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) (Gomez & Miguez-Rodriguez, 1999). Loss of epifaunal grazers such as sea urchins may adversely affect the horse mussel population due to fouling.
Therefore, evidence suggests that horse mussels are of intermediate intolerance to synthetic chemicals, however, given the additional high intolerance of Echinus esculentus and red algae an overall intolerance of high has been recorded albeit at low confidence.
Horse mussel recruitment is sporadic, varies with season, annually and with location and hydrographic regime and is generally low, therefore it may take many years for a population to recover from damage and a recoverability of low (10 -25years) has been recorded.
Heavy metal contamination
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Modiolus modiolus may exhibit tolerance to heavy metals similar to that of Mytilus edulis.
The tissue distribution of Cd, Zn, Cu, Mg, Mn, Fe and Pb was examined in Modiolus modiolus by Julshamn & Andersen (1983) who reported the presence of Cd binding proteins but did not document any adverse affects. Richardson et al. (2001) examined the presence of Cu, Pb and Zn in the shells of Modiolus modiolus from a relatively un-contaminated site and from a site affected by sewage sludge dumping. The persistence of a population of horse mussels at the sewage sludge dumping site suggests that tolerance to heavy metal contamination levels at that site. Holt et al. (1998) reported that long-term changes in contaminant loads associated with spoil dumping were detectable in the shells of horse mussels in a bed off the Humber estuary. This observation showed survival of horse mussels in the vicinity of a spoil dumping ground but no information on their condition was available (Holt et al., 1998).
Little information on the effects of heavy metal contamination of other members of the community was found. However, Echinus esculentus populations in the vicinity of an oil terminal in A Coruna Bay, Spain, showed developmental abnormalities in the skeleton. The tissues contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) (Gomez & Miguez-Rodriguez, 1999). Bryan (1984) reported that early work had shown that echinoderm larvae were intolerant of heavy metals. However, it is unlikely that established sea urchins would be adversely affected and there is no evidence to suggest that mortality would occur in associated species in the biotope. Heavy metal contamination may affect the condition of species in the biotope and, therefore, an intolerance of low has been recorded. Recovery of the biotope will depend on depuration or detoxification of the heavy metals and recovery of condition, therefore a recovery of high has been reported.
Hydrocarbon contamination
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Horse mussel beds are protected from the direct effects of oil spills due to their subtidal habitat, although shallow subtidal populations will be more vulnerable. Horse mussel beds may still be affected by oil spills and associated dispersants where the water column is well mixed vertically, e.g. in areas of strong wave action. Oils may be ingested as droplets or adsorbed onto particulates. Hydrocarbons may be ingested or absorbed from particulates or in solution, especially PAHs.
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.
Holt & Shalla (unpublished; cited in Holt et al., 1998) did not observe any visible affects on a population of Modiolus modiolus within 50m of the wellhead of a oil/gas exploration rig (using water based drilling muds) in the north east of the Isle of Man.
Echinoderms tend to be very intolerant of various types of marine pollution (Newton & McKenzie, 1995). Echinus esculentus populations in the vicinity of an oil terminal in A Coruna Bay, Spain, showed developmental abnormalities in the skeleton. The tissues contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) (Gomez & Miguez-Rodriguez 1999). The sub-cuticular bacteria that are symbiotic with Ophiothrix fragilis are reduced in number following exposure to hydrocarbons. Exposure to 30,000 ppm oil reduces the bacterial load by 50 % and brittle stars begin to die (Newton & McKenzie, 1995). However, there are no field observations of mortalities caused by exposure to hydrocarbons.
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, possibly due to the susceptibility of phycoerythrins to destruction, and that the filamentous forms were the most sensitive. Therefore, is it possible that hydrocarbon contamination may reduce reproductive success and growth rates in horse mussel populations. Reduced scope for growth may be of particular importance in juveniles that are subject to intense predation pressure, resulting in fewer individuals reaching breeding age.
However, May & Pearson (1995) reported that stations in the vicinity of ballast water diffuser, probably containing fresh petrogenic hydrocarbons, showed a consistently high diversity (since surveys started in 1978) and included patches of Modiolus sp. beds. The strong currents in the area probably flushed polluting materials away from the station, and hence reduced the stress on the population (May & Pearson, 1995). The persistence of a highly diverse community suggests low intolerance to hydrocarbon contaminated effluent. However, red algae are likely to be highly sensitive to hydrocarbon contamination (see benchmark), suggesting that while overall species richness and diversity may not be reduced significantly, some characterizing species may be lost, or their abundance reduced. Therefore, an overall biotope intolerance of intermediate has been recorded.
Recovery would depend on growth of surviving epifauna, or re-colonization and would probably require up to 5 years (see additional information below).
Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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Navarro & Thompson (1996) suggested that Modiolus modiolus was adapted to an intermittent and often inadequate food supply. The persistence of a horse mussel population in the vicinity of a sewage sludge dumping site (Richardson et al., 2001) suggests that the species is tolerant of high nutrient levels. Moderate nutrient enrichment may, therefore, be beneficial by increasing phytoplankton productivity and organic particulates, and hence food availability. However, eutrophication may have indirect adverse effects, such as increased turbidity, increased suspended sediment (see above), increased risk of deoxygenation (see below) and the risk of algal blooms. Shumway (1990) reviewed the effects of algal blooms on shellfish and reported that a bloom of Gonyaulax tamarensis (Protogonyaulax) was highly toxic to Modiolus modiolus. Shumway (1990) also noted that both Mytilus spp. and Modiolus spp. accumulated paralytic shellfish poisoning (PSP) toxins faster than most other species of shellfish, e.g. horse mussels retained Gonyaulax tamarensis toxins for up to 60 days (depending on the initial level of contamination). Landsberg (1996) also suggested that there was a correlation between the incidence of neoplasia or tumours in bivalves and out-breaks of paralytic shellfish poisoning in which bivalves accumulate toxins from algal blooms, although a direct causal effect required further research. No information on the effects of nutrient enrichment on hydroids and bryozoans was found. An increase in abundance of red algae, including Delesseria sanguinea, was associated with eutrophication in the Skagerrak area, Sweden, especially in areas with the most wave exposure or water exchange (Johansson et al., 1998). However, where eutrophication resulted in high siltation rates, the delicate foliose red algae such as Delesseria sanguinea were replaced by tougher, erect red algae (Johansson et al., 1998).
Therefore, given the potential sub-lethal effects of algal blooms and potential changes in the algal community an overall intolerance of low (at the benchmark level) has been recorded. A recoverability of very high has been recorded to represent the time required for algal toxins to be depurated from horse mussels.
Increase in salinity
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This biotope (MCR.ModT) and those biotopes in has been used to represent, are found from the lower infralittoral and the circalittoral and are unlikely to be exposed to anything but full salinity.
Decrease in salinity
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Most of the species identified as indicative of intolerance may be of 'intermediate' or 'low' intolerance to a reduction in salinity. Hydroids especially are also likely to be highly intolerant. This biotope (MCR.ModT) and those biotopes in has been used to represent, is found from the lower infralittoral and the circalittoral and would only be exposed to low salinity in exceptional circumstances. Nevertheless, after a winter and spring of extremely high rainfall, populations of Modiolus modiolus at the entrance to Loch Leven (near Fort William) were found dead, almost certainly due to low salinity outflow (K. Hiscock, pers. comm.). Therefore, an intolerance of high has been recorded.
The epifaunal organisms such as anthozoans, hydroids, barnacles, ascidians and brittlestars are likely to take some time to recolonize but could potentially recover within five years. However, Modiolus modiolus beds, are likely to take considerable time the recolonize and to develop into a bed similar in size and in the diversity and species richness they support (see additional information below). Therefore, a recoverability of very low has been recorded.
Changes in oxygenation
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Theede et al. (1969) examined the relative tolerance of gill tissue from several species of bivalve to exposure to 0.21mg/l O2 with or without 6.67mg of sulphide (at 10°C and 30psu). Modiolus modiolus tissue was found to be the most resistant of the species studied, retaining some ciliary activity after 120hrs compared with 48hrs for Mytlius edulis. While it is difficult to extrapolate from tissue resistance to whole animal resistance (taking into account behavioural adaptations such as valve closure) this suggests that horse mussels are more, or at least similarly, tolerant of hypoxia and hydrogen sulphide to the common mussel. In addition, most bivalve molluscs exhibit anaerobic metabolism to some degree. Therefore, Modiolus modiolus was assessed as of low intolerance at the benchmark level.
However, Alcyonium digitatum, Ophiothrix fragilis and Delesseria sanguinea were assessed as highly intolerant of deoxygenation, while Echinus esculentus was regarded as of intermediate intolerance. Hydroids mainly inhabit environments in which the oxygen concentration usually exceeds 5 ml/l and respiration is aerobic. Assimilation of oxygen occurs simply by diffusion through the epidermis of exposed tissues and transport to tissues is facilitated by hydroplasmic flow and ciliary activity (Hickson, 1901). Ophiothrix fragilis was known to have a low respiration rate (Migné & Davoult, 1997b), particularly during colder winter temperatures, however, extreme hypoxia was reported to cause mass mortality (Stachowitsch, 1984). The effects of deoxygenation in plants has been little studied and since plants produce oxygen they may be considered relatively insensitive. However, a study of the effects of anaerobiosis (no oxygen) on some marine algae concluded that Delesseria sanguinea was very intolerant of anaerobic conditions; at 15 °C death occurred within 24hrs and no recovery took place although specimens survived at 5 °C (Hammer 1972). Under hypoxic conditions echinoderms become less mobile and stop feeding. Death of a bloom of the phytoplankton Gyrodinium aureolum in Mounts Bay, Penzance in 1978 produced a layer of brown slime on the sea bottom. This resulted in the death of fish and invertebrates, including Echinus esculentus, presumably due to anoxia caused by the decay of the dead dinoflagellates (Griffiths et al., 1979). Although the horse mussels are probably tolerant of hypoxic condition, all the species indicative of intolerance were more intolerant, suggesting that the epifauna and epiflora would decrease in abundance or diversity under hypoxic conditions. Therefore, an overall intolerance of intermediate has been recorded.
Recovery would depend on growth of surviving epifauna, or re-colonization and would probably require up to 5 years (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
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Brown & Seed (1977) reported a low level of infestation (ca 2%) with pea crabs Pinnotheres sp. in Port Erin, Isle of Man and Strangford Lough. Comely (1978) reported that ca 20% of older specimens, in an ageing population, were damaged or shells malformed by the boring sponge Cliona celata. Infestation by the boring sponge reduces the strength of the shell and may render the population more intolerant of physical disturbance (see above). However, little other information concerning the effects of parasites or disease on the condition of horse mussels was found.
Echinus esculentus is susceptible to 'Bald-sea-urchin disease', which causes lesions, loss of spines, tube feet, pedicellariae, destruction of the upper layer of skeletal tissue and death. Bald sea-urchin disease was recorded from Echinus esculentus on the Brittany coast. Although associated with mass mortalities of Strongylocentrotus franciscanus in California and Paracentrotus lividus in the French Mediterranean it is not known if the disease induces mass mortality (Bower, 1996). However, no evidence of mass mortalities of Echinus esculentus associated with disease have been recorded in Britain and Ireland. Loss of sea-urchins may be detrimental to the horse mussel bed due to fouling (see ecological relationships). Evidence of sub-lethal effects alone was found in Modiolus modiolus and an intolerance of low has been recorded.
Introduction of non-native species
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No information concerning non-native species competitors was found.
Extraction
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Holt et al. (1998) reported that, although there was no large scale horse mussel fishery in the United Kingdom, there have been small scale local fisheries in Scotland for food or bait and that horse mussels were occasionally seen on markets in Lancashire. Holt et al. (1998) suggested that any direct fishery would be very damaging. Horse mussels, Modiolus modiolus, are the key species within this biotope (MCR.ModT) and the biotopes it has been used to represent. Extraction of Modiolus modiolus would have severe consequences for the associated community.

Scallop beds are known to be associated with or occur in the vicinity of Modiolus modiolus beds (Holt et al., 1998; Magorrian & Service, 1998). Holt et al. (1998) suggested that horse mussel beds were not particularly fragile, even when epifaunal, with semi-infaunal and infaunal population being less vulnerable to physical disturbance from fishing activity. Clumps of horse mussels of muddy substrata may be more intolerant. However, impacts from towed fishing gear (e.g. scallop dredges) are known to flatten clumps and aggregations, may break off sections of raised reefs and probably damage individual mussels (Holt et al., 1998). Holt et al. (1998) suggested that scallop dredging on areas adjacent to beds in the south east of the Isle of Man had 'nibbled away at the edges' of dense beds, which had become less dense and more scattered (Holt et al., 1998). Extensive beds were present in the north of the Isle of Man where scallop dredging has apparently not occurred (Holt et al., (1998).

Magorrian & Service (1998) reported that queen scallop trawling resulted in flattening of horse mussel beds and disruption of clumps of horse mussels and removal of emergent epifauna in Strangford Lough. They suggested that the emergent epifauna such as Alcyonium digitatum were more intolerant than the horse mussels themselves and reflected early signs of damage. They were able to identify different levels of impact from impacted but largely intact beds to heavily trawled areas with few Modiolus modiolus intact, lots of shell debris and little epifauna (Service & Magorrian, 1997; Magorrian & Service, 1998; Service 1998). Veale et al. (2000) reported that the abundance, biomass and production of epifaunal assemblages, including Modiolus modiolus and Alcyonium digitatum decreased with increasing fishing effort. Scallop dredging was found to damage many of the epibenthic species found in association with Modiolus beds (Hill et al., 1997; Jones et al., 2000).

Scavengers such as Asterias rubens and Buccinum undatum were reported to be fairly robust to encounters with trawls (Kaiser & Spencer, 1995) and may benefit in the short term, feeding on species damaged or killed by passing dredges. However, Veale et al. (2000) did not detect any net benefit at the population level. In addition, Buccinum undatum may itself be the subject of a fishery, although its removal may not adversely affect the biotope. Species with fragile hard tests such as echinoids are known to be intolerant of scallop dredges (see Eleftheriou & Robertson, 1992; Veale et al., 2000). Removal of sea urchins may have adverse effects of the horse mussel beds due to increased fouling and potential dislodgement or loss of clumps of mussels.

Recovery will depend on recruitment of horse mussels and subsequent development of the beds, which may take many years (see additional information below). Brown (1989; cited in Ramsay et al., 2000) suggested that fishing activities may render the habitat unsuitable for recolonization by species such as Modiolus modiolus. The epifaunal organisms such as anthozoans, hydroids, barnacles, ascidians and brittlestars are likely to take some time to recolonize but could potentially recover within five years. However, Modiolus modiolus beds, are likely to take considerable time the recolonize and to develop into a bed similar in size and in the diversity and species richness they support (see additional information below). Therefore, a recoverability of very low has been recorded.

Additional information icon Additional information

Recoverability
Few members of the horse mussel assemblage (except the horse mussels themselves) are restricted to the horse mussel bed and many associated species have planktonic propagules, likely to recolonize rapidly. Therefore, the recoverability of the biotope is primarily dependant on the recovery of the horse mussel bed.
Recruitment in Modiolus modiolus is sporadic and highly variable seasonally, annually or with location (geographic and depth) (Holt et al., 1998). Some areas may have received little or no recruitment for several years. Even in areas of regular recruitment, such as enclosed areas, recruitment is low in comparison with other mytilids such as Mytilus edulis. For example, in Strangford Lough small horse mussels (<10mm) represented <10% of the population, with peaks of 20-30% in good years (Brown & Seed, 1978; Figure 3). In open areas with free water movement larvae are probably swept away from the adult population, and such populations are probably not self-recruiting but dependant on recruitment from other areas, which is in turn dependant on the local hydrographic regime. In addition, surviving recruits take several to many years to reach maturity (3-8 years, see reproduction) (Holt et al., 1998).
Holt et al., (1998) point out that where impacts are severe enough to clear extensive areas of a horse mussel bed, recovery would be unlikely even in the medium term. They also noted that both the time required for small breaks in beds to close up due to growth of surrounding clumps, and the survival of clumps torn from the bed is not known. Witman (1984) cleared 115cm2 patches in a New England Modiolus modiolus bed. None of the patches were recolonized by the horse mussel after 2 years, 47% of the area being colonized by laminarian kelps instead (Witman pers. comm. cited in Suchanek, 1985). No details on longer term studies were found.
The horse mussel is long-lived and reproduction over an extended life span may compensate for poor annual recruitment. However, any factor that reduces recruitment is likely to adversely affect the population in the long-term. Any chronic environmental impact may not be detected for some time in a population of such a long -lived species.
Overall, therefore, while some populations are probably self-sustaining it is likely that a population that is reduced in extent or abundance will take many years to recover, and any population destroyed by an impact will require a very long time to re-establish and recover, especially since newly settled larvae and juveniles require the protection of adults to avoid intense predation pressure.

This review can be cited as follows:

Tyler-Walters, H. 2006. Modiolus modiolus beds with hydroids and red seaweeds on tide-swept circalittoral mixed substrata. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 23/07/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=137&code=1997>