Biodiversity & Conservation

Modiolus modiolus beds with hydroids and red seaweeds on tide-swept circalittoral mixed substrata



Image Anon. - Modiolus modiolus beds with hydroids and red seaweeds on tide-swept circalittoral mixed substrata. Image width ca 50 cm.
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Distribution map

CR.MCR.M.ModT recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats
  • UK_BAP

Species indicative of sensitivity

To assess the sensitivity of the biotope, the sensitivity of component species is reviewed. Those species that are considered to be particularly indicative of the sensitivity of the biotope, and for which research has been undertaken in detail are shown below (see selection criteria). The biology of other component species of the biotope is also taken into account wherever information is known to the researcher.

Species found especially in this biotope

  • Modiolus modiolus
Community Importance Species name Common Name
Key structural Modiolus modiolus Horse mussel
Important characterizing Alcyonium digitatum Dead man's fingers
Important characterizing Ophiothrix fragilis Common brittlestar
Important functional Echinus esculentus Edible sea urchin
Important other Delesseria sanguinea Sea beech


Modiolus modiolus provides substratum and habitats for a diverse assemblage of species and refuges for others. It is, therefore, the keystone species within this biotope. Suchanek (1985) suggested that a mutualistic relationship existed between sea urchins and mussel beds (see ecological relationships). Therefore, Echinus esculentus has been included as an important functional species. Alcyonium digitatum and Ophiothrix fragilis are important characterizing species and representatives of the suspension feeding part of the community. In addition, Alcyonium digitatum represents a member of the emergent epifauna, that was used to indicate sensitivity to physical disturbance by Magorrian & Service (1998) and Service (1998). Although Delesseria sanguinea occurs only rarely within this biotope, it has been included as a representative of red algae. No representative hydroid species was available, however, reference was made to the Nemertesia ramosa review. Predators are probably very important in structuring the population of horse mussels (especially juveniles and recruitment) and for the diversity and abundance of the epifauna. However, most predators are generalists, not especially associated with the biotope and are probably not indicative of the sensitivity of the biotope.
Although the selected 'indicative species' are particularly important in undertaking the assessment because they have been subject to detailed research, account is taken of knowledge of the biology of all characterizing species in the biotope when undertaking an assessment of sensitivity of this biotope.

Additional information

Modiolus modiolus beds support species rich, diverse communities (Holt et al., 1998) which represent most of the major invertebrate groups (Brown & Seed, 1977). However estimates of species richness and descriptions of the communities vary, and depend on the sampling techniques used. Descriptions of the infauna are likely to be underestimates. Species numbers vary with season (Ojeda & Dearborn, 1989) and sampling time, e.g. predatory crabs are prevalent at night and fish more prevalent during the day (Witman, 1985).
The MNCR recorded 379 species within the MCR.ModT biotope. The following numbers of species were found within horse mussel communities, although not necessarily the biotope in question:

  • 90 invertebrate taxa in horse mussel beds in Strangford Lough (Brown & Seed, 1977);
  • 270 invertebrate taxa in horse mussel reefs on the north east of the Isle of Man (Holt & Shalla, unpublished; cited in Holt et al., 1998);
  • 89 invertebrate taxa in Scottish horse mussel beds (Comely, 1981);
  • the horse mussel communities in St. George's Channel were described by Mackie et al. (1995), and
  • the horse mussel communities of Northern Ireland, including MCR.ModT were described by Erwin et al. (1990).

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 01/12/2015]. Available from: <>