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

Marine natural heritage importance

Listed under EC Habitats Directive
UK Biodiversity Action Plan
National importance Uncommon
Habitat Directive feature (Annex 1) Reefs
Large shallow inlets and bays

Biotope importance

  • Clumps or beds of Modiolus modiolus significantly modify the habitat, increasing habitat complexity and forming raised beds that may be visible on side-scan sonar (Wildish & Fader, 1998; Wildish et al., 1998; Holt et al., 1998; Magorrian & Service, 1998).
  • Dense growth of foliose seaweeds or branching bryozoans and hydroids may provide an important settling area for bivalve spat, e.g. Pecten maximus, Chlamys spp. and Aequipecten opercularis, adults beds of which are often abundant in the vicinity of horse mussels beds (Holt et al., 1998).
  • Horse mussel beds also made a significant contribution to secondary productivity in the Bay of Fundy and were important in transfer of energy (as organic carbon) from the eutrophic zone to the benthos, so called benthic-pelagic coupling (see productivity) (Wildish & Fader, 1998; Holt et al., 1998).
  • Witman (1985) noted that Modiolus modiolus beds in New England provided at refuge for faunal turf forming invertebrate (e.g. hydroids and bryozoans) from intense grazing by Strongylocentrotus droebachiensis. In addition, horse mussel beds, by providing a heterogeneous environment, crevices and other hiding places, acted as a refuge from fish, crab or lobster predation for the sea urchin Strongylocentrotus droebachiensis, the bivalve Hiatella arctica and the brittlestar Ophiopholis aculata and by inference other similar species (Witman, 1985; Suchanek, 1985). Horse mussel beds in British waters probably also act as refuges from predation for a variety of invertebrates.
  • The concentration of seston (organic particulates and phytoplankton) may be reduced over horse mussel beds. Wildish & Kristmanson, 1984; 1985) demonstrated that seston concentrations in were reduced directly over dense Modiolus sp. Beds when water flow was low, and could be identified in the field by reduced horse mussel density. However, this biotope (MCR.ModT) occurs in moderately strong to strong tidal streams.


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. Service (1998) suggested limits of acceptable change for horse mussel populations in Strangford Lough, but noted that further research was required to derive ecological quality standards.

Additional information icon Additional information

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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 24/11/2015]. Available from: <>