BIOTIC Species Information for Modiolus modiolus
Researched byLizzie Tyler Data supplied byUniversity of Sheffield
Refereed byThis information is not refereed.
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,
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