Species Action Plan

Atrina (= Pinna) fragilis (Pennant) is one of the largest (30-48 cm long) European bivalve molluscs. It is commonly known as a fan shell due to the thin, fragile, triangular shaped shells that are a light yellow-brown to darker brown colour, occasionally with black patches. A series (sculpture) of raised fine concentric lines run across the shell while a variable number (<20) of low, smooth, wavy ridges radiate from the umbones, sometimes with delicate, fluted spines. The interior of the shell is glossy and the margin of the shell is smooth but often fragmented. When the shell dries out, it becomes very brittle and cracks. As the thin edge of the posterior margin of most species of fan shells can cut the feet of bathers, they are also known as 'razorfish'.

Due to the scarcity of specimens in collections and the variations in the sculpture and thickness of the shell, this species has been described under numerous names in the past by British workers, including fragilis, borealis, pectinata, muricata, ingens, laevis, rudis, papyracea, rotundata and elegans.

A. fragilis occurs from just below Low Water of Spring Tides (LWST) to a depth of 400 m, in mud, sand or fine gravel. The shell is vertically embedded in the soft substrate. The pointed anterior end of the shell is attached to small stones or pieces of shell in the sediment, by numerous, long byssus threads which emerge from the ventral gape. Between a third and two-thirds of the length of the shell can be buried below the sediment surface.

Atrina and Pinna spp exist as metapopulations, where the population is patchy, composed of small groups or patches of individuals. Atrina and Pinna spp have external fertilisation and the chances of successful fertilisation are dependent on the proximity of other spawning individuals as well as other factors, including water movement. Where the population is very sparse, fertilisation failure can be significant.

Recruitment to local populations is highly variable. The combination of limited, irregular larval dispersal and variable larval survival are the likely factors that result in the known sporadic recruitment. Mortality in the larvae of these species is unknown but food supply is crucial for planktonic larvae and peaks in plankton density may determine larval survival. Occasionally, due to variations in local conditions, the food supply may be favourable for both adults and survival of the larvae and newly settled juveniles. This may lead to a good recruitment that supports the population for a long time, known as the 'storage effect'. It is likely that A. fragilis is a species that recruits when conditions are favourable for the survival of larvae. A. fragilis larvae may enter southern and western waters from areas to the south and recruitment may be very infrequent. Dispersal between different patches is variable and the nature of the dispersal between the patches is crucial to understanding the dynamics within any patch as well as that of the whole population system.

Estimates of the shell growth of A. fragilis specimens from Valentia Bay, Ireland, based on annullar growth rings, suggest that growth is relatively slow (around 3-4 cm per year) in comparison to other species of fan shell. The large size of A. fragilis, suggests that this is long-lived species.

A. fragilis is predominantly a southern and western species, whose distribution extends from north Scotland down to the Iberian Peninsula. Jeffreys (1863) reported it did not occur north of Shetland. Seaward (1982) mapped the records of A. fragilis for the British Isles according to sea areas. A. fragilis has been recorded occurring off Shetland, Orkney, the east coast of Scotland in the Moray Firth, down the west coast of Scotland, off the north and west coasts of Ireland, the Irish Sea (Dublin Bay, Liverpool Bay and the Bristol Channel), the Scilly Isles, the Channel Isles and along the south coast of England. In Scottish waters, A. fragilis appears to be most commonly encountered amongst the Western Isles, particularly around Mull. In the south-west of England, it is thought that A. fragilis has largely disappeared from inlets where it was once common.

It is a widespread species but is rarely encountered and records generally relate to single specimens, accidentally collected through benthic fishing activities. A study in 1863 reported that A. fragilis was sometimes gregarious. However, the majority of UK records relate to single specimens and it does not appear that many aggregations of A. fragilis have been encountered around the UK. Six specimens were brought up on lines off Aberdeen during the winter of 1841-42. However, aggregations have been encountered in recent years off the Atlantic coasts of Ireland, such as within Valentia Harbour, County Kerry.

Virtually all of the information on A. fragilis relates to the few records of specimens. There are no records for A. fragilis on the Marine Nature Conservation Review database. There is almost no information concerning the population status of A. fragilis within the UK. It has been suggested that the populations of A. fragilis around the UK and Ireland have declined since the turn of the century due to the impacts of demersal fishing activities and, in some areas, sand and gravel extraction. It is possible that direct removal by collectors may have contributed to the apparent decline.

The cost of rapid growth during the first few years of life and the ability to rapidly repair the shell throughout its life, might be a reduction in gamete production. The infrequency of suitable conditions for the survival and dispersal of the larvae and the survival of the juvenile stage may contribute to the sporadic recruitment of Atrina and Pinna species. Recruitment is more successful in embayments and inlets where presumably a high proportion of the larvae are entrapped, but otherwise it is poor and variable in comparison with other bivalves such as scallops, in the same location.

These factors make these species particularly vulnerable to exploitation and natural populations will not survive heavy exploitation. Consequently, human activities that shorten the adult life of Atrina and Pinna species cannot be compensated for by an immediate reproductive response of the population and increased recruitment.

A. fragilis is protected under Schedule 5 of the Wildlife and Countryside Act 1981 and the Wildlife (NI) Order 1985. A. fragilis is not listed on Annexes II, IV or V of the EC Habitats Directive.

Demersal fishing, using trawls and dredges, can remove or damage A. fragilis and cause disturbance to the seabed where A. fragilis occurs. As much as 70% of the shell length of A. fragilis can be buried below the surface and the posterior portion of the shell that projects above the surface is vulnerable to benthic fishing gear, particularly dredges. While live animals can withdraw the mantle towards the anterior of the shell and can repair considerable damage to the posterior edge of the shell, they cannot survive being uprooted from the seabed.

As A. fragilis is considered to be a long-lived species and since aggregations are now rarely encountered, this species is particularly vulnerable to damage and removal caused by demersal fishing. Around the UK and Ireland, the numbers of A. fragilis found in scallop grounds that have been dredged have declined and few specimens remain. Most recent A. fragilis specimens have been found in areas adjacent to dredged scallop beds or in areas seldom dredged. The A. fragilis population found in a Pecten maximus bed off Glengad Head was not subjected to dredging before 1975. When dredging of these beds began, many live specimens and shells of A. fragilis were found in the scallop dredges but it is thought that this population has been destroyed by the subsequent dredging.

Sand and gravel extraction may remove or damage A. fragilis and anchoring in shallow water may cause disturbance to the seabed where A. fragilis occurs.

A number of environmental changes may affect A. fragilis, including increases in turbidity, sedimentation and certain pollutants, such as TBT (tri-butyl tin). Changes in seawater temperature and current patterns may affect the recruitment pattern of A. fragilis larvae.

Oil and gas operators are required to assess the potential impacts of their actions in relation to Atrina fragilis. If the operation proposed cannot be conducted without causing significant damage then the operation may not be allowed to proceed.

New records of A. fragilis are collected and published as and when specimens are donated to museums and research institutions.

Members of the family Pinnidae have not been the subject of major ecological or physiological research programmes.

Jean-Luc Solandt, Marine Conservation Society Tel:01989 566017

The following LBAPs are working on Atrina fragilis:

Originally published in: UK Biodiversity Group Tranche 2 Action Plans - Volume V: Maritime species and habitats (October 1999, Tranche 2, Vol V, p63)

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