MBDP-NI: Priority habitat

Horse Mussel Beds

Horse Mussel Beds

Correspondence with existing habitats

OSPAR habitat: Modiolus modiolus beds

Habitats Directive Annex I: Large shallow inlets and bays

Habitats Directive Annex I: Reefs

Description: Historically, the horse mussel Modiolus modiolus formed dense beds at depths of more than 5 m in fully saline, often moderately tide-swept areas throughout the mid and northern parts of Strangford Lough. Modiolus clumps provide a suitable substratum for the attachment of a diverse epifauna including sponges, anemones, hydroids, oysters, brittlestars, sea squirts and bryozoans. They also provide protection for mobile fauna such as crabs, prawns, squat lobsters and small fish and infaunal species such as sea cucumbers and polychaete worms. The horse mussel reefs within Strangford Lough were heavily impacted by dredging and trawling, especially in 1987.

The JNCC Marine Nature Conservation Review (MNCR) has identified four major biotopes dominated by dense M. modiolus.

M. modiolus is a long-lived species and individuals within beds are frequently 25 years old or more. Juvenile M. modiolus are heavily preyed upon, especially by crabs and starfish, until they are about 3–6 years old, but predation is low thereafter. Recruitment is slow and may be very sporadic; there may be poor recruitment over a number of years in some populations.

The byssus threads secreted by M. modiolus have an important stabilising effect on the seabed, binding together living M. modiolus, dead shell, and sediments. As M. modiolus is a filter feeder, the accumulation of faeces and pseudofaeces probably represents an important flux of organic material from the plankton to the benthos. This rich food source, together with the varied habitat, means that extremely rich associated faunas, sometimes with hundreds of species, may occur on dense beds.

The composition of the biotopes is variable, and is influenced by the depth, degree of water movement, substrate, and density of M. modiolus. Sponges, ascidians, soft corals, anemones, hydroids, bryozoans, tubeworms, brittlestars, urchins, starfish, barnacles, crabs, spider crabs and other decapods, whelks and other gastropods, scallops and fish all tend to be abundant as epifauna, while there may also be coralline algae and other red seaweeds in shallower areas. Infauna often includes the sea cucumbers Thyone roscovita and Thyonidium drummondi, polychaete worms and numerous bivalves. The possible role of M. modiolus beds as nursery areas for other species has not been investigated.

Healthy horse mussel beds were previously recorded at various sites in Strangford Lough for example NE of Janes Rock and E of Long Sheelah. There have been no studies of the recovery of damaged beds but full recovery after severe damage would undoubtedly take many years at best and may not occur at all. Some beds may be self-maintaining relict features.

Associated biotopes:

Priority species: Modiolus modiolus, Thyonidium drummondi, Aequipecten opercularis, Ostrea edulis, Pecten maximus

Threats:

Fishing: Particularly using trawls and dredges for scallops and queen scallops, is known to have caused widespread and long-lasting damage to beds in Strangford Lough and off the south-east of the Isle of Man. Effects include flattening clumps of M. modiolus causing fatalities, and loss of much of the associated epifauna. Fishing impacts are likely to be occurring on M. modiolus beds elsewhere.
Sedimentation: Increase in sedimentation a s a consequence of changes in land use, ploughing, deforestation, marine aggregate extraction, dredging, drilling and mining and the development of renewable energy can result in substantial increases in sediment deposition. Increases in sediment deposition can cause mortality in Modiolus (Hutchison et al., 2016).
Physical impacts: Modiolus beds are likely to be badly damaged by any other physical impacts, such as aggregate extraction, trenching and pipe/cable-laying, dumping of spoil/cuttings, or use of jack-up drilling rigs.
Contaminants: M. modiolus is known to accumulate contaminants such as heavy metals in spoil disposal areas but the effects on condition, reproduction and mortality rates are unknown.
Natural fluctuations: In spawning, settlement and recruitment into adult sizes occur in some beds, with predation of young mussels probably being very influential. These must affect the population structure of M. modiolus beds over periods of a few years, but in the long term they seem to be stable features.
Climate change: Modiolus modiolus is a northern species in the United Kingdom and therefore is vulnerable to an increase in sea temperatures.

References:

Anwar, N.A., Richardson, C.A. & Seed, R. (1990) Age determination, growth rate and population structure of the horse mussel Modiolus modiolus. Journal of the Marine Biological Association of the UK, 70, 441–457.
Brown, R.A. (1984) Geographical variations in the reproduction of the horse mussel, Modiolus modiolus (Mollusca: bivalvia). Journal of the Marine Biological Association of the UK, 64, 751–770.
Brown, R.A. & Seed, R. (1977) Modiolus modiolus (L.). An autoecological study. In: B.K. Keegan, P.O. Cleidigh & P.J.S. Boddin, eds. Biology of Benthic Organisms. Oxford: Pergamon Press.
Comely, C.A. (1978) Modiolus modiolus (L.) from the Scottish west coast. Ophelia, 17, 167193.
Comely, C.A. (1981) The physical and biochemical condition of Modiolus modiolus (L.) in selected Shetland Voes. Proceedings of the Royal Society of Edinburgh, 80b, 299–321.
Holt, T.J., Rees, E.I., Hawkins, S.J. & Seed, R. (1998) Biogenic Reefs: An overview of dynamic sensitivity characteristics for conservation management of marine SACs. Scottish Association of Marine Science/UK Marine SACs Project, Oban, Scotland.
Jones, N.S. (1951) The bottom fauna off the south of the Isle of Man. Journal of Animal Ecology, 20, 132–144.
Magorrian, B.H., Service, M. & Clarke, W. (1995) An acoustic bottom classification survey of Strangford Lough, Northern Ireland. Journal of the Marine Biological Association of the UK, 75, 987–992.
Seed, R. & Brown, R.A. (1978) Growth as a strategy for survival in two marine bivalves, Cerastoderma edule and Modiolus modiolus. Journal of Animal Ecology, 47, 283–292.
https://webarchive.nationalarchives.gov.uk/20110303150007/
http://www.ukbap.org.uk/UKPlans.aspx?ID=37 Edited by Nikki Chapman, JNCC.
Hutchison, Z.L., Hendrick, V.J., Burrows, M.T., Wilson, B. & Last, K.S. (2016) Buried Alive: The Behavioural Response of the Mussels, Modiolus modiolus and Mytilus edulis to Sudden Burial by Sediment. PLOS ONE 11(3): e0151471. https://doi.org/10.1371/journal.pone.0151471

Marine Biodiversity Data Portal - NI

Horse Mussel Beds