BIOTIC Species Information for Modiolus modiolus
Click here to view the MarLIN Key Information Review for Modiolus modiolus
Researched byLizzie Tyler Data supplied byUniversity of Sheffield
Refereed byThis information is not refereed.
Scientific nameModiolus modiolus Common nameHorse mussel
MCS CodeW1702 Recent SynonymsNone

PhylumMollusca Subphylum
Superclass ClassPelecypoda
Subclass OrderMytiloida
Suborder FamilyMytilidae
GenusModiolus Speciesmodiolus

Additional InformationDistinguished 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.
Taxonomy References Fish & Fish, 1996, Tebble, 1976, Howson & Picton, 1997, Hayward et al., 1996, Holt et al., 1998,
General Biology
Growth formBivalved
Feeding methodPassive suspension feeder
Active suspension feeder
Mobility/MovementTemporary attachment
Permanent attachment
Environmental positionEpifaunal
Typical food typesBacteria, phytoplankton, detritus, and dissolved organic matter (DOM). HabitAttached
Bioturbator FlexibilityNone (< 10 degrees)
FragilityIntermediate SizeMedium(11-20 cm)
HeightUp to ca 20 cm Growth RateSee additional information
Adult dispersal potential<10m DependencyIndependent
General Biology Additional InformationTypical 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:
  • 20-40 large individuals per m² north of the Isle of Man (Holt et al., 1998);
  • 4-158 /m² in Bay of Fundy, Nova Scotia (Wildish & Fader, 1998);
  • 14.4±11.2 individuals/ m² (Ojeda & Dearborn, 1989);
  • 1-2/m² in the intertidal and 37/m² at 100m from the west coast of Scotland (Comely, 1978).
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:
  • average lengths of 70mm or less reached at about 20 years of age in Firth of Lorne (Comely, 1978) and south east of the Isle of Man (Jasim, 1986);
  • 35-40mm in length after 4-6 years (Anwar et al., 1990);
  • many populations reach 100mm at 12-18 years of age, e.g., west Scotland, Strangford Lough, and the Isle of Man (Comely, 1978, 1981; Jasim, 1986; Seed & Brown, 1975; Holt et al., 1998).
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.

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:
  • 3-4 years of age in the Isle of Man (Jasim, 1986);
  • 5-6 years in Norwegian waters (Wiborg, 1946);
  • 7-8 years in Canadian populations (Rowell, 1967), and
  • over 4 years of age in Strangford Lough (Seed & Brown, 1978).
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.
Biology References Holt et al., 1998, Brown & Seed, 1977, Comely, 1978, Seed & Brown, 1978, Nielsen, 1975, Anwar et al., 1990, Wildish et al., 1998, Seed, 1976, Jones et al., 2000, Wildish & Fader, 1998, Roberts, 1975, Seed & Brown, 1975, Jasim, 1986, Nielsen, 1975, Sebens, 1985, Navarro & Thompson, 1996, Hayward & Ryland, 1990, Julie Bremner, unpub data,
Distribution and Habitat
Distribution in Britain & IrelandFound off all British coasts. Extensive beds are most common on northern or western coasts but absent south of the Irish Sea and Humber estuary.
Global distributionRecorded from the White sea and Norway, off the Faroes and Iceland, south to the Bay of Biscay and occasionally North Africa. Also from Labrador to North Carolina in the Atlantic and from the Bering Sea south to Japan and California in the Pacific.
Biogeographic rangeNot researched Depth range0 - 150 m
MigratoryNon-migratory / Resident   
Distribution Additional InformationModiolus modiolus was also recorded as a dominant member of the species rich benthic assemblage found throughout the gravely sediments of St. George's Channel, and the outer regions of Cardigan Bay and Caernarfon Bay (Mackie et al., 1995). Holt et al., (1998; Table 1) lists the predominant forms of horse mussel beds in British waters.

Modiolus modiolus is adapted to live semi-infaunally with an endobyssate attachment to the substratum but may also be found attached to hard substratum, epifaunally in a manner similar to the common mussel, Mytilus edulis. Modiolus modiolus beds vary in size, density, thickness and form.
  • Epifaunal horse mussels carpet steep surfaces and replace Mytilus spp. on offshore structures in cold waters, e.g., in Loch Duich, Loch Long and Loch Alsh (Holt et al., 1998).
  • Semi-infaunal reefs of large individuals about two-thirds embedded into sediment, forming an irregularly clumped layer and younger individuals between the byssal threads of larger individuals.
  • Reefs form due to the build up of a mound of faecal mud and shell debris over several years. Reefs vary in size and height, depending on the build up of faecal mud and water flow rates. In strong tidal streams the beds may not retain the faecal mud and do not form raised beds (Holt et al., 1998).
  • On coarser grounds and in strong currents the horse mussels bind together the coarse sediment forming banks and live nested infaunally within the deposit.
  • Coarse sediment banks formed by horse mussel beds form wave-like mounds or bioherms in the Bay of Fundy up to 3m high and 20m wide and 10s -100s of metres in length, which are visible on sidescan sonar (Wildish et al., 1998; Wildish & Fader, 1998). Similar banks, up to 1m high occur of the north east Isle of Man and Codling Bank, Ireland (Holt et al., 1998).
  • Horse mussel beds may occur as large and continuous banks (biogenic reefs) or as scattered clumps.
  • Holt et al., (1998) noted that extensive horse mussel beds or reefs were restricted to between 5 -50m in UK, although the large reefs recorded in the Bay of Fundy, Nova Scotia occurred at ca 80m (Wildish & Fader, 1998; Wildish et al., 1998).

Substratum preferencesBedrock
Large to very large boulders
Small boulders
Gravel / shingle
Muddy gravel
Coarse clean sand
Fine clean sand
Sandy mud
Muddy sand
Biogenic reef
Artificial (e.g. metal/wood/concrete)
Under boulders
Physiographic preferencesOpen coast
Offshore seabed
Strait / sound
Ria / Voe
Enclosed coast / Embayment
Biological zoneLower Eulittoral
Sublittoral Fringe
Upper Infralittoral
Lower Infralittoral
Upper Circalittoral
Lower Circalittoral
Wave exposureModerately Exposed
Very Sheltered
Tidal stream strength/Water flowStrong (3-6 kn)
Moderately Strong (1-3 kn)
Weak (<1 kn)
Very Weak (negligible)
SalinityFull (30-40 psu)
Habitat Preferences Additional Information
Distribution References Fish & Fish, 1996, Tebble, 1976, Hayward et al., 1996, Holt et al., 1998, Anonymous, 1999(u), Brown & Seed, 1977, Davenport & Kjørsvik, 1982, Comely, 1978, Brown, 1984, Wildish et al., 1998, Seed, 1976, Mackie et al., 1995, Jones et al., 2000, Wildish & Fader, 1998, Bayne et al., 1976b, Hayward & Ryland, 1990, Julie Bremner, unpub data,
Reproduction/Life History
Reproductive typeGonochoristic
Developmental mechanismPlanktotrophic
Reproductive SeasonSee additional information Reproductive LocationInsufficient information
Reproductive frequency Regeneration potential No
Life span51-100 years Age at reproductive maturity6-10 years
Generation time6-10 years Fecundity1000000
Egg/propagule size Fertilization typeInsufficient information
Larval/Juvenile dispersal potential>10km Larval settlement periodInsufficient information
Duration of larval stage11-30 days   
Reproduction Preferences Additional InformationLife span
Mussels over 25 years old are frequent in British populations, with occasional records of individuals of up to 35 years old. However, maximum ages are thought likely to be in excess of 50 years (Anwar et al., 1990).

The spawning season is variable or unclear and varies with depth and geographic location, probably related to temperature (de Schwienitz & Lutz, 1976; reviewed by Brown, 1984; Holt et al., 1998). For example:
  • in Strangford Lough, Ireland the population exhibits a slow, continuous release of gametes (Seed & Brown, 1977; Brown & Seed, 1977);
  • populations off south east of the Isle of Man show an annual gametogenesis and spawning cycle, with continuous release of gametes and a peak in spring and summer (Jasim & Brand; 1989);
  • Scottish populations showed a slow release of gametes throughout the year with peaks of spawning in spring and summer in some areas (Comely, 1978);
  • Swedish and northern Norwegian populations showed a distinct spawning in June -July respectively (Brown, 1984), and
  • Wiborg (1946) reported that spawning occurring only every 2nd to 3rd year in Norwegian waters.
Brown (1984) suggested that Modiolus modiolus commenced spawning over a narrow range of temperatures (7 -10°C), timed with suitable conditions for larval development. Brown (1984) also suggested that the suitable spawning temperature may limit this species' northern distribution.

Recruitment is sporadic and highly variable seasonally, annually or with location (geographic and depth) (Holt et al., 1998). For example:
  • settlement in Bristol Channel populations is dense but subsequent recruitment is low (Holt et al., 1998);
  • regular recruitment occurs in populations in Strangford Lough and in two areas south east of the Isle of Man (Seed & Brown, 1978; Jasim & Brand, 1986).
  • very irregular recruitment, with gaps of many years was reported for Norwegian (Wiborg, 1946) and Canadian populations (Rowell, 1967).
  • Scottish populations varied, with 'normal' recruitment occurring in areas of strong currents, resulting in a relatively young population, while recruitment was negligible in areas of quiet water resulting in an ageing population, and in a deep water population no recruitment had occurred for a number of years and the population was old, possibly senile and dying out (Comely, 1978).
Comely (1978) suggested that recruitment was dependant on larvae from outside the area in areas of free water movement. In open coast areas, e.g. the Llyn Peninsula and Sarnau, released larvae are probably swept away from the adult population (Comely, 1978; Holt et al., 1998). Holt et al., (1998) cite unpublished preliminary genetic data that suggest that beds off the south east of the Isle of Man receive recruits from other areas, albeit in a sporadic manner. Holt et al., (1998) suggested that enclosed areas such as Strangford Lough and the Scottish sea lochs would be relatively self sustaining. Therefore, recruitment is probably strongly affected by the local hydrographic regime.

Post-settlement mortality
Widdows (1991) noted that in Mytilus edulis larvae any environmental factor that increased the larval development time (e.g. temperature or food availability) increased larval mortality. This is probably true for other mytilid larvae, such as Modiolus modiolus.
Pre- and post-settlement mortality is high due to predation. Settling larvae prefer the byssus threads and aggregations or clumps of adults which provide a refuge from predators. In infaunal populations, however, the byssal threads and clumps of adults will be less accessible, and predation risk higher as a result (Holt et al., 1998). Most populations exhibit a bimodal size distribution of large, older specimens and small, younger specimens. Newly-settled horse mussels exhibit rapid growth prior to reaching maturity (see general biology), investing energy in growth rather than reproduction. Selection favours rapid growth to a size that is relatively immune to predation. Only the largest starfish and crabs can open mussels greater than 45-60mm and large horse mussels are thought to be largely predator free (Roberts, 1975; Seed & Brown, 1978; Holt et al., 1998). Comely (1978) noted that Modiolus modiolus <40mm were rarely found away from large horse mussels.
Reproduction References Holt et al., 1998, Brown & Seed, 1977, Comely, 1978, Schweinitz de & Lutz, 1976, Brown, 1984, Seed & Brown, 1978, Anwar et al., 1990, Seed, 1976, Jones et al., 2000, Seed & Brown, 1977, Jasim & Brand, 1989, Wiborg, 1946, Roberts, 1975, Seed & Brown, 1975, Jasim, 1986, Rowell, 1967, Julie Bremner, unpub data, Brown & Seed, 1977,
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