Distribution data supplied by the Ocean Biodiversity Information System (OBIS). To interrogate UK data visit the NBN Atlas.Map Help
Researched by | Dr Harvey Tyler-Walters | Refereed by | This information is not refereed |
Authority | (Linnaeus, 1758) | ||
Other common names | - | Synonyms | - |
The shell is solid, swollen, approximately oblong or irregularly triangular in shape, with blunt umbones. The shell is dark blue or purple in colour, however the perisostracum gives adults a glossy yellow or dark brown appearance. In young animals the shell appears bluish and the periostracum is extended into long, smooth spines. The shell bears clear growth lines, and a sculpture of fine concentric lines and ridges. The inside of the shell is white, with a wide pallial line, a large anterior adductor muscle scar and small posterior adductor muscle scar. Adults are usually more than 10cm in length, although very large specimens may reach up to 22cm.
Distinguished from the common mussel (Mytilus edulis) by the blunter umbones, which are not quite anterior due to the extension of the shell margin in Modiolus modiolus.
- none -
Phylum | Mollusca | Snails, slugs, mussels, cockles, clams & squid |
Class | Bivalvia | Clams, cockles, mussels, oysters, and scallops |
Order | Mytilida | Mussels & crenellas |
Family | Mytilidae | |
Genus | Modiolus | |
Authority | (Linnaeus, 1758) | |
Recent Synonyms |
Typical abundance | High density | ||
Male size range | 35-200mm | ||
Male size at maturity | |||
Female size range | Medium(11-20 cm) | ||
Female size at maturity | |||
Growth form | Bivalved | ||
Growth rate | See additional text | ||
Body flexibility | None (less than 10 degrees) | ||
Mobility | |||
Characteristic feeding method | Active suspension feeder | ||
Diet/food source | |||
Typically feeds on | Bacteria, phytoplankton, detritus, and dissolved organic matter (DOM). | ||
Sociability | |||
Environmental position | Epifaunal | ||
Dependency | Independent. | ||
Supports | Substratum The boring sponge Cliona celata and the pea crab Pinnotheres spp. | ||
Is the species harmful? | No Modiolus modiolus beds have been exploited for food and as bait in Norwegian waters and on a lesser scale in Scottish waters (Comely, 1978; Holt et al., 1998). |
Typical abundance or density
Reported densities of horse mussel beds were relatively low (compared to common mussel beds) and variable, although grab samples and the interpretation of photographic images may be underestimates (Holt et al., 1998). Reported densities include:
Growth rates
Growth rates have been inferred from growth rings. Growth is rapid in the first 4-6 years, with energy being diverted to growth rather than reproduction. Rapid juvenile growth appears to be an adaptation to avoid predation. Once large size has been reached growth is very slow. Once individuals reach 45-60mm in length they become relatively immune to predation as only the very largest crabs and starfish can open horse mussels over 50mm in length (Seed & Brown, 1978; Anwar et al., 1990; Holt et al., 1998). The following growth rates have been reported:
Fast growing population of 10 year olds have been recorded on oil rigs in the North Sea (Holt et al., 1998). Intertidal populations have been reported to be slow growing (Anwar et al., 1990). Comely (1978) suggested that increased byssus production reduced growth rates in areas of loose sediment and/or strong currents. Comely (1978) also reported that a deep water population (200m) had reduced growth rates, possibly due to reduced food availability. Navarro & Thompson (1996) found that Modiolus modiolus in Newfoundland reduced its feeding activity in autumn and winter when food supply was poor but increased clearance rates and its absorption efficiency in spring in response to the spring phytoplankton bloom. Navarro & Thompson (1996) suggest that the horse mussel is adapted to survive in areas of intermittent food supply.
Maturation
Sexual maturity occurs at about 35-40mm according to Anwar et al. (1990) and coincides approximately with the size, at which individuals become less prone to predation and can divert resources to growth (Brown & Seed, 1977). Reported ages at maturation vary and include:
Predators
Predators, largely crabs and starfish, play an important role in the population structure of horse mussel beds and determine the survival of juveniles to adulthood (Brown & Seed, 1977; Anwar et al., 1990; Holt et al., 1998). Predation probably also limits the ability of Modiolus modiolus to colonize other habitats, such as hard substrata, e.g. Sebens (1985) noted that Asterias vulgaris and Buccinum undatum predation removed juvenile Mytilus spp. and Modiolus spp. from vertical rock walls in the Gulf of Maine. Nielsen (1975) noted that Modiolus sp. occurred regularly in the stomach of Buccinum undatum. However, he concluded that the whelk probably fed on weak, or dead horse mussels, since when large horse mussels were threatened they either stayed shut long enough to deter the whelk, or if attacked could close their shell valves with enough force to break the shell lip of the whelk itself. Presumably juveniles are less able to defend themselves
Parasites and diseases
Comely (1978) reported that ca 20% of older specimens, in an ageing population, were damaged or shells malformed by the boring sponge Cliona celata. 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.
Physiographic preferences | Open coast, Offshore seabed, Strait / sound, Sea loch / Sea lough, Ria / Voe, Enclosed coast / Embayment |
Biological zone preferences | Lower circalittoral, Lower eulittoral, Lower infralittoral, Sublittoral fringe, Upper circalittoral, Upper infralittoral |
Substratum / habitat preferences | Artificial (man-made), Bedrock, Biogenic reef, Coarse clean sand, Fine clean sand, Gravel / shingle, Large to very large boulders, Mixed, Mud, Muddy gravel, Muddy sand, Rockpools, Sandy mud, Small boulders, Under boulders |
Tidal strength preferences | Moderately Strong 1 to 3 knots (0.5-1.5 m/sec.), Strong 3 to 6 knots (1.5-3 m/sec.), Very Weak (negligible), Weak < 1 knot (<0.5 m/sec.) |
Wave exposure preferences | Moderately exposed, Sheltered, Very sheltered |
Salinity preferences | Full (30-40 psu) |
Depth range | Lower intertidal to ca 280m. |
Other preferences | No text entered |
Migration Pattern | Non-migratory / resident |
Reproductive type | Gonochoristic (dioecious) | |
Reproductive frequency | See additional information | |
Fecundity (number of eggs) | >1,000,000 | |
Generation time | 5-10 years | |
Age at maturity | 3-8 years (see general biology) | |
Season | See additional text | |
Life span | 20-100 years |
Larval/propagule type | - |
Larval/juvenile development | Planktotrophic |
Duration of larval stage | 11-30 days |
Larval dispersal potential | Greater than 10 km |
Larval settlement period | Insufficient information |
The MarLIN sensitivity assessment approach used below has been superseded by the MarESA (Marine Evidence-based Sensitivity Assessment) approach (see menu). The MarLIN approach was used for assessments from 1999-2010. The MarESA approach reflects the recent conservation imperatives and terminology and is used for sensitivity assessments from 2014 onwards.
Intolerance | Recoverability | Sensitivity | Evidence/Confidence | |
High | Low | High | Moderate | |
Loss of substratum would cause removal of the resident population of horse mussels. Therefore, an intolerance of high has been recorded. Recovery would depend on recolonization from other populations. However, 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. | ||||
Intermediate | Low | High | Very low | |
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. 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. 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 population and an intolerance of intermediate has been recorded. 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. | ||||
Low | Immediate | Not sensitive | Low | |
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. 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. Modiolus modiolus is adapted to a benthic sediment habitat and is probably capable of rejecting excess silt or particulates, therefore a recoverability of 'immediate' has been recorded. | ||||
Low | Immediate | Not sensitive | Low | |
A decrease in suspended sediment may decrease the food availability for Modiolus modiolus. 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. Therefore, Modiolus modiolus is unlikely to be adversely affected by a decrease in suspended sediment for a month (see benchmark). | ||||
High | Low | High | High | |
On exposure to air, Modiolus modiolus closed its shell initially, but after a few minutes to a few hours the shell valves gape. As a result the mantle fluid drains away, leaving the tissue susceptible to evaporation and desiccation (Coleman, 1973). Coleman (1973) estimated that death occurred after 30-40% water loss from the tissue, which was achieved after 1 hr in moving air. Individuals lost up to 80% of tissue water after 6 hrs emersion. Coleman (1973) concluded that Modiolus modiolus may not die if exposed to still air for 2-3 days (depending on temperature), but in moving air death would be rapid. However, Kanwisher (1955; cited in Coleman, 1973) suggested that Modiolus could withstand up to 65% water loss if the water was removed as ice, i.e. at low temperatures. Therefore, Modiolus modiolus is unable to control its gape, and shows poor adaptation to aerial exposure, which probably explains its restricted distribution in the intertidal, being limited to lower shore rockpools and areas of high humidity and low air movement. Modiolus modiolus is probably very intolerant of an increase in desiccation and, therefore, an intolerance of high has been recorded. Recovery would depend on recolonization from other populations. However, 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. | ||||
High | Low | High | Moderate | |
Davenport & Kjørsvik (1982) suggested that Modiolus modiolus was poorly adapted to life in the intertidal due to its patent byssal opening, inability to control its shell gape, thin shell, lack of mobility and restricted thermal tolerance. In addition, increased emergence will increase the risk of desiccation. The majority of populations are subtidal, however intertidal populations or shallow subtidal populations occasionally exposed at extreme low water may be vulnerable to increased emergence and hence, exposure to desiccation and temperature extremes. Therefore, an intolerance of high has been recorded. Recovery would depend on recolonization from other populations. However, 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. | ||||
Tolerant* | Not relevant | Not sensitive* | Not relevant | |
Decreased emergence is unlikely to adversely affect populations and may even allow the populations to feed longer and improve condition, i.e. they may benefit from decreased emergence. | ||||
Intermediate | Low | High | Low | |
Holt et al. (1998) suggested that water movement was an important factor in the build up of the denser reefs and beds of Modiolus modiolus, since the denser reefs were found in areas of moderate to strong tidal currents. Wildish et al. (2000) examined suspension feeding in horse mussels in a flume and noted that they kept both the exhalent and inhalent siphons open over the range of flow rates studied, 12-63 cm/sec (approximates to weak to moderately strong tidal streams). However, the inhalent siphon closed by about 20% in currents above 55 cm/sec. The horse mussel reefs or bioherms observed in the Bay of Fundy were arranged parallel to flow within the Bay (Wildish & Fader, 1998; Wildish et al., 1998). Modiolus modiolus populations are found from weak to strong tidal streams and are, therefore, probably tolerant of changes in water flow within this range. However, Comely (1978) suggested that areas exposed strong currents required an increase in byssus production, at energetic cost, and resulted in lower growth rates. Populations in strong tidal streams may be more intolerant of an increase in water flow. Fouling by epifauna and algae may also increase the populations intolerance to increased water flow. Witman (1984; cited in Suchanek, 1985) found that over 11 months in New England, 84% of fouled mussels were dislodged in comparison with 0% of unfouled individuals. Exposure to very strong water flow may also result in loss of individuals due to the removal of the substratum. Therefore, intolerance to increased water flow is dependant on the nature of the substratum and the level of epifaunal or epifloral fouling and an intolerance of intermediate has been recorded. 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. | ||||
Intermediate | Low | High | Moderate | |
Muschenheim & Milligan (1998) noted that horse mussel beds in the Bay of Fundy occurred in transitional, well mixed waters, so that the beds were supplied with high quality seston (organic particulates and phytoplankton) from the euphotic zone. They also suggested that beds in areas of moderate current flow and/or a greater depths were not exposed to high concentrations of suspended inorganic particulates, which would otherwise dilute out the high quality seston (food). However, flume experiments suggest 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). Therefore, an intolerance of intermediate has been recorded. Holt et al., 1998 note that no comparable studies has been found on British Modiolus sp. populations. Populations already present in areas of weak water flow may also be exposed to increased siltation (see above) and the risk of deoxygenation (see below). 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. | ||||
Intermediate | Low | High | Very low | |
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., 1976b) 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) and its predominantly northern distribution 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. Therefore, an intolerance of intermediate has been recorded. 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. | ||||
Tolerant* | Not relevant | Not sensitive* | Low | |
Modiolus modiolus is a boreal species reaching its southern limit in British waters (Holt et al., 1998). Although no information on its lower temperature tolerance was found its northern distribution suggest that it would probably tolerate a decrease of average winter sea temperatures in the UK (Holt et al., 1998; Davenport & Kjørsvik 1982) but may increase in abundance and extent. Therefore. Modiolus modiolus has been ranked as 'tolerant*' to decreased temperatures. | ||||
Low | Very high | Very Low | Moderate | |
Modiolus modiolus is found in a wide range of 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. Therefore, an intolerance of low has been recorded. Once conditions return to normal it is likely that the population will take time to recover condition, therefore a recoverability of very high has been recorded. | ||||
Tolerant* | Not relevant | Not sensitive* | ||
Decreases in turbidity may increase phytoplankton productivity and therefore, potentially increase food availability. Therefore, horse mussel beds may benefit. | ||||
Intermediate | Low | High | Very low | |
The majority of Modiolus modiolus populations are subtidal and unlikely to be affected by wave action directly. However, increased wave action results in increased water flow in the shallow subtidal. Wave mediated water flow tends to be oscillatory, i.e. move back and forth (Hiscock, 1983), and may result in dislodgement or removal of individuals. Mytilus edulis was shown to increase byssus production in response to agitation (Young, 1985) and Modiolus modiolus may respond similarly. However, horse mussels attached to hard substrata are probably more intolerant of wave action than Mytilus edulis due to their larger size, and hence increased drag. The intolerance of semi-infaunal or infaunal populations probably owes more to the nature of the substratum rather than their attachment. 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. In addition, shallow subtidal populations are found in sheltered conditions (e.g. Strangford Lough) rather than wave exposed conditions. Therefore, it is likely that an increase in wave action will result in loss of a proportion of the population and an intolerance of intermediate has been recorded. Shallow subtidal Modiolus modiolus beds may be more intolerant of prolonged change or storms. 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. | ||||
Tolerant* | Not relevant | Not sensitive* | ||
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 'tolerant*' 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. | ||||
Tolerant | Not relevant | Not sensitive | High | |
Modiolus modiolus can probably detect vibrations but is relatively immobile, and unlikely to be adversely affected by noise. | ||||
Tolerant | Not relevant | Not sensitive | High | |
Modiolus modiolus can probably detect changes in light intensity, however, it is unlikely to be adversely affected by visual presence and shading at the level of the benchmark. | ||||
High | Low | High | Low | |
Modiolus modiolus are large and relatively tough. Holt et al. (1998) suggested that horse mussels beds were not particularly fragile, even when epifaunal, with semi-infaunal and infaunal population being less vulnerable to physical disturbance. Clumps of horse mussels on muddy substrata may be more intolerant. However, impacts from towed fishing gear (e.g. scallop dredges) are known to flatten clumps and aggregations, and 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). Although scallop or queen scallop dredging was probably not viable over rough ground such as horse mussel beds, it was 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 were more intolerant than the horse mussels themselves but were able to identify different levels of impact from impacted but largely intact to few Modiolus modiolus intact with lots of shell debris (Service & Magorrian, 1997; Magorrian & Service, 1998; Service 1998). Holt et al. (1998) suggested that damage by whelk potting was not likely to be severe but noted that epifaunal populations may be more intolerant Overall, an intolerance of high has been recorded. Recruitment is sporadic, varies with season, annually, 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 | ||||
Low | Very high | Very Low | Very low | |
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. Therefore, an intolerance of low has been recorded, albeit with very low evidence/confidence. 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. |
Intolerance | Recoverability | Sensitivity | Evidence/Confidence | |
Intermediate | Low | High | Very low | |
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 form 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 mollusc as follows: reduced spatfall 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 population exhibit sporadic recruitment, therefore any factor that adversely affect 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. Therefore, an intolerance of intermediate has been recorded albeit at very low confidence. 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. | ||||
Low | Very high | Very Low | Very low | |
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 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). Overall, therefore, horse mussels may show a similar tolerance to heavy metals as Mytilus edulis but in the absence of any evidence of mortalities an intolerance of low has been recorded. On return to un-contaminated conditions, removal or depuration of heavy metals may take some time and a recoverability of very high has been recorded. | ||||
Low | Very high | Very Low | Low | |
Horse mussels are protected form the direct effects of oil spills due to their subtidal habitat, although shallow subtidal and intertidal populations will be more vulnerable. Horse mussels 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 50 m of the wellhead of a oil/gas exploration rig (using water based drilling muds) in the north east of the Isle of Man. May & Pearson (1995) reviewed the effects of the oil industry on the macrobenthos of Sullom Voe. They 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 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). However, 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, the long term persistence of a diverse bed of Modiolus sp. in the vicinity of a hydrocarbon contaminated effluent suggests an intolerance of low. Recruitment is sporadic and variable (see additional information below), therefore it may take many years for a population to recover from damage and a recoverability of low (10-25years) has been recorded. | ||||
No information | Not relevant | No information | Not relevant | |
Insufficient information | ||||
Low | Very high | Very Low | Low | |
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. Therefore, a rank of 'tolerant*' has been recorded. However, eutrophication may have indirect adverse effects, such as increased turbidity, increased risk of deoxygenation (see above) 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. Therefore, an intolerance of low (at the benchmark level) has been recorded due to the potential sub-lethal effects of algal blooms. A recoverability of very high has been recorded to represent the time required for algal toxins to be depurated from horse mussels. | ||||
Not relevant | Not relevant | Not relevant | Not relevant | |
Modiolus modiolus was considered to be poorly adapted to fluctuating salinities, due to its patent byssal aperture and restricted salinity tolerance (Bayne et al., 1976b). Pierce (1970) exposed Modiolus sp. to range of salinities between 1.5 and 54psu and reported that Modiolus modiolus survived for 21 days (the duration of the experiment) between 27 and 41psu. However, the majority of Modiolus modiolus populations are subtidal and therefore, unlikely to be exposed to salinities above that of full seawater. Only intertidal populations in rock pools or populations exposed to hypersaline effluents are likely to experience increased salinity. | ||||
High | Low | High | Moderate | |
Modiolus modiolus was considered to be poorly adapted to fluctuating salinities, due to its patent byssal aperture and restricted salinity tolerance (Bayne et al., 1976b). Pierce (1970) exposed Modiolus sp. to range of salinities between 1.5 and 54 psu and reported that Modiolus modiolus survived for 21 days (the duration of the experiment) between 27 and 41 psu. Davenport & Kjørsvik (1982) reported the presence of large horse mussels in rock pools at 16 psu in Norway, subject to freshwater inflow, and noted that they were probably exposed to lower salinities. Davenport & Kjørsvik (1982) also reported that Shumway (1977) had found the horse mussels tolerated simulated tidal fluctuations between full seawater and fresh water. Shumway (1977) reported that Modiolus modiolus survived for 10 days exposed to either gradual or sudden cyclic changes in salinity between 50 and 100% seawater. In addition, Shumway (1977) reported that the salinity of the mantle fluid changed only gradually and Piece (1970) noted that the body fluid remained hyperosmotic to the environment, suggesting that diffusion through the byssal aperture was slow (Shumway, 1977; Davenport & Kjørsvik 1982). Holt et al. (1998) note that young Modiolus occasionally occur subtidally in estuaries. Therefore, the horse mussel will probably survive short term exposure to reduced salinity conditions. However, 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. 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. | ||||
Low | Very high | Very Low | Moderate | |
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 than the common mussel. In addition, most bivalve molluscs exhibit anaerobic metabolism to some degree. Therefore, an intolerance of low has been recorded at the benchmark level. Anaerobic metabolism often results in an oxygen debt and individuals may have to make up for energy reserves depleted during hypoxia, therefore a recoverability of very high has been recorded. |
Intolerance | Recoverability | Sensitivity | Evidence/Confidence | |
Low | Very high | Very Low | Low | |
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. Therefore, an intolerance of low was recorded. | ||||
No information | Not relevant | No information | Not relevant | |
No information concerning the effects of competition with non-native or alien species was found. | ||||
High | Low | High | Moderate | |
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. suggest that any direct fishery would be very damaging. Therefore, extraction of 50% of the population (the benchmark level) would result in an intolerance of intermediate. However, extraction of 50% of a horse mussel bed may result in further dislodgement of clumps of mussels, and adversely affect subsequent juvenile survival and hence recovery, since juveniles require the presence of adults to shelter from predators. Therefore, an intolerance of high has been recorded. Recruitment is sporadic, variable 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. | ||||
Intermediate | Low | High | High | |
Impacts from towed fishing gear (e.g. scallop dredges) are known to flatten clumps and aggregations and 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). It was 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 denser beds, which had become less dense and more scattered (Holt et al., 1998). Veale et al. (2000) reported that the total abundance, biomass and production of the major taxa studied, including Modiolus modiolus, decreased with increasing scallop fishing effort in the south of the Isle of Man.
Magorrian & Service (1998) reported that queen scallop trawling resulted in flattening of the horse mussel bed, disruption of clumps of horse mussels and removal of emergent epifauna in Strangford Lough and were able to identify different levels of impact to the horse mussel bed from impacted but largely intact to few Modiolus modiolus intact with lots of shell debris (Service & Magorrian, 1997; Magorrian & Service, 1998; Service 1998). Overall, dredging for scallops is likely to result in disruption of the horse mussel beds and loss of a proportion of the population in the short term. However, the horse mussel beds may be more intolerant in the long-term, depending on the level of fishing effort. Therefore an intolerance of high has been recorded. Recruitment is sporadic, variable 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. |
National (GB) importance | Not rare/scarce | Global red list (IUCN) category | - |
Native | - | ||
Origin | - | Date Arrived | Not relevant |
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This review can be cited as:
Last Updated: 05/06/2007
Tags: bivalve mollusc reefs biogenic beds horse