In brief

• The fan mussel is found off the north coast of Northern Ireland
• It can be seen only by diving or remote cameras, half-buried in mud, sandy mud or muddy gravel
• It is listed as a UK Priority Species
• The species has only been seen living in Northern Ireland waters once since 1989.
• The main threat is thought to be damage caused by commercial fishing gear.

Species description: The shell of the fan mussel is brittle, equivalve and triangular in outline with prominent gapes, tapering to a point. It is light yellowish-brown or dark brown in colour, occasionally with black patches, and a sculpture of concentric lines and 8 to 12 radiating ribs, sometimes with fluted spines. The colour inside is similar to the outside, but glossy. It is one of largest bivalve molluscs found in Europe, reaching up to 48cm in length, with the largest individuals being 10 to 12 years old.

Life cycle: The species breeds using external fertilization, where success is affected by the proximity of other individuals, as well as a range of environmental factors. The larvae undergo a planktonic phase that lasts 5 to 10 days.

Similar species: There are no similar species in Northern Ireland waters.

How to see this species: Only known recently from off the north coast of Northern Ireland during a trawling survey for the queen scallop Aequipecten opercularis in 1971, close to and east of Rathlin Island to Magilligan, in depths to 64 m+, off Rathlin Island in 2007 in 33 m depth and at the entrance to Belfast Lough in 1989. Shells have been found on Portstewart, Downhill and Magilligan strands and on the seabed south of the Skerries, Portrush. It is normally present all year round. It lives with its pointed end buried vertically in bottoms of mud, sandy mud or muddy gravel, attached to small stones or pieces of shell by numerous fine strands of protein known as a byssus, which passes through the shell gape. Only the broad posterior portion of the valves is visible above the surface of the substrate, often fragmented or damaged. It occurs both as a solitary animal or in small groups, and feeds by filtering organic particles from the water.

Current status: This species has only apparently been recorded living in Northern Ireland once since 1989, when it was found off the north coast of Northern Ireland at Rathlin Island. In the Republic of Ireland, it is only known recently from Galway Bay (1962, 1970s) and Valentia 1975. Unlocalised animals have been seen in local public aquaria more recently than these dates. There are many old or shell records, particularly from the southern half of the island of Ireland. Whilst apparently recorded from all round Britain apart from the east coasts, there are few known recent populations. The fan mussel is distributed south from Britain on the continental shelf to the Iberian Peninsula and the Mediterranean down to 600 m.

In Northern Ireland, it is legally protected from intentional killing, injuring or taking; possession or sale under the Wildlife Order (NI), 1985.

Why is this species a priority in Northern Ireland?

• The fan mussel is a UK Priority Species
• It has experienced rapid decline, only seen living once in 2007 since 1989, and is considered to be Rare in Northern Ireland.

Threats: The species is sensitive to substratum loss; smothering; changes in temperature, water flow, wave exposure; abrasion and physical disturbance; displacement and extraction of species – with a low recoverability to all these. For example, it is unable to burrow upwards or re-burrow following a disturbance incident. The main threat is thought to be the use of trawls and dredges in fishing, both of which disturb the seabed and damage fan mussels, often removing them from the sediment. Further threats include gravel and sand extraction.

Action: There is a UK Species Action Plan which was published in 1999.

• Implementation of the Northern Ireland Habitat Action Plan for Sublittoral Sands and Gravels.

Objectives:

• Perform targeted surveys to ascertain the current status of the species in Northern Ireland
• Establish appropriate management on appropriate historical sites
• If refound, ensure that the population is maintained
• If biologically feasible, enhance the distribution and population size in Northern Ireland.

Distribution Map from NBN: Atrina fragilis at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Atrina fragilis at iNaturalist World Species Observations database.

WoRMS: Atrina fragilis at World Register of Marine Species. Accepted name: Atrina fragilis (Pennant, 1777). AphiaID: 146524.

Literature:

Brown, R.A. (1989). Bottom trawling in Strangford Lough: problems and policies. In C.C. ten Hallers and A. Bijlsma (eds), Proc. North Sea Meeting, Rotterdam pp. 117-127. Amsterdam.

Brown, R.A. and Seed, R. (1977). Modiolus modiolus (L.) - an autecological study. In B.F. Keegan, P. ÓCéidigh and P.J.S. Boaden (eds), Biology of Benthic Organisms pp. 93-100. Pergamon Press, Oxford.

Hayward, P.J. and Ryland, J.S. (eds) (1995). Handbook of the marine fauna of north-west Europe. Oxford University Press.

Seaward, D.R. (1982). Sea Area Atlas of the Marine Molluscs of Britain and Ireland, 53pp. + 746 figures. Nature Conservancy Council, Peterborough for Conchological Society of Great Britain & Ireland.

Seaward, D.R. (1990). Distribution of the marine molluscs of north-west Europe, 114pp. Nature Conservancy Council, Peterborough for Conchological Society of Great Britain & Ireland.

Tebble, N. (1966). British Bivalve seashells. The British Museum (Natural History), London.

Classification: Biota; Animalia; Mollusca; Bivalvia; Autobranchia; Pteriomorphia; Ostreida; Pinnoidea; Pinnidae; Atrina