Fan mussel  (Atrina fragilis)

Protected under Schedule 5 of the Wildlife and Countryside Act, 1981 and the Wildlife (NI) Order 1985 (3).

Growing to an impressive 48 cm in length (2), the fan mussel (Atrina fragilis) is one of the largest of the bivalve molluscs found in Europe (3). It takes its English name from the delicate shell, which is triangular in shape, and tapers to a point. The valves of the shell are pale yellowish-brown to dark brown in colour, with a glossy interior. Living specimens are partly buried vertically in the sediment, with only the top portion of the valves visible. Fan mussels anchor themselves into the sediment by attaching to small stones with numerous fine strands of protein known as 'byssus'. The byssus of this species is so like human hair that many fishermen believed that fan shells had fed upon drowned people. They therefore deemed this species 'unclean', and would return them to the sea if they were caught (2).

This scarce species is found around the coast of Britain and Ireland, with the strongest populations occurring in Scotland and off the southwestern coast of Cornwall (4). It is believed that the fan mussel has disappeared from many inlets in the southwest of England, where it was once common (3). Elsewhere its range extends as far south as the Iberian Peninsula (2).

You can view distribution information for this species at the National Biodiversity Network Gateway.

Inhabits waters from the surface down to depths of 400m, and is found in muddy, sandy and gravelly sediments (4).

At present, relatively little is known of the biology of this species. The fan mussel occurs solitarily or in small groups and feeds by filtering organic particles from the water (2). Fertilisation is external, and success is affected by the proximity of other individuals as well as a range of environmental factors (4). The larvae undergo a planktonic phase that lasts for five to ten days (3).

Studies of annual growth rings on the shells have suggested that fan mussels grow at a rate of 3-4 cm per year (3). Based on this rate of growth, it is thought that the largest individuals are 10-12 years old, and possibly even older (4). The upper, exposed parts of the shells are often damaged, leaving sharp edges, but the shell is repaired quickly (4).

The main cause of the decline of this species 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 (3).

Atrina fragilis is listed as a priority species under the UK Biodiversity Action Plan (UK BAP) and is fully protected by the Wildlife and Countryside Act. The lack of information on the biology of this species hinders efforts to conserve it. However, recent work has shown that it may be possible to restore populations where favourable habitat remains. It has been demonstrated that live specimens are able to survive being moved and 're-planted' into the sediment (4). As the species is attached to the sediment, it is also possible that protected areas could be established where good populations survive (4). A dedicated survey of this species is needed to determine its UK distribution more accurately (4).

There may be further information about this species available via the National Biodiversity Network Gateway.

View information on this species at the UNEP World Conservation Monitoring Centre.

The UK Biodiversity Action Plan for this species is available at UK BAP.

For more on this species see:

Solandt, J-L (2003). The fan shell Atrina fragilis- a species of conservation concern. British Wildlife, 14 (number 6): 423-427.
www.mcsuk.org
Tyler-Walters, H. (2003). Atrina fragilis. Fan mussel. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from:
http://www.marlin.ac.uk/species/Atrinafragilis.htm
Woodward, F. R. (1985). The fan-mussel, Pina fragilis Pennant in Scotland. Glasgow Naturalist 21: 63-69.