Atrina fragilis
Atrina fragilis, the fan mussel, is a species of large saltwater clam, a marine bivalve mollusc in the family Pinnidae, the pen shells.
The fan mussel, Atrina fragilis is one of the largest and rarest bivalve molluscs occurring in northern European waters and the only member of the family Pinnidae to inhabit UK waters. It is one of the largest European bivalve molluscs. It is one of the rarest species of marine mollusc in the United Kingdom — so rare that surveys of nearly 9,000 sites around Britain between 1987 and 1998 found none. The fan mussel has a larval stage that is very difficult to identify due to the rarity of the specimen. Atrina fragilis is greatly affected by the industrialization of the fishing industry, and it has impacted the distribution of the rare species.
Description
The shell of the fan mussel tapers to a point at the umbos, and is very brittle. It is yellowish to dark brown with blackish patches. The two valves are equal and triangular in outline, with prominent gapes. The shell surface has a sculpture of concentric lines and 8 to 12 ribs, which may have fluted spines. Shells can reach up to 48 cm in length, and the largest individuals may be 10 to 12 years old. Atrina fragilis have also been found in the Mediterranean Sea.Taxonomy and Identification
Atrina fragilis has a large, triangular, thin and fragile shell that tapers to a point. It has two valves that are equal with prominent gaps between the two shells and a hinge on the dorsal side. The shell has concentric growth and disturbance lines with 8-12 low, wavy ridges radiating out from the umbones. The shell colour is a light golden or yellow-brown to darker brown or chestnut.Habitat and Distribution
The most recent records of the rare bivalve Atrina fragilis are from Scotland in deep waters around the Shetland Isles and Orkney, and the west coast of Scotland, with scattered records from north-east Scotland, the south coast of England, the Channel Isles, Pembrokeshire in south Wales and Northern Ireland. They have also been recorded from north Scotland down to Mauritania, including the Iberian Peninsula, and into the Mediterranean. Atrina fragilis lives embedded in sublittoral fringe, subtidal muds, sandy muds or gravels. The fan mussel, like many other mussels, attaches to small stones by the secretion of strong byssal threads through their byssus gland to avoid being swept away in the ocean. Though embedded, between one and two thirds of the shell is buried, therefore leaving the fan mussel vulnerable to fishing trawlers.Biology
This species lives with the narrow half of its shell anchored in the sediment, but the large part of the fragile shell protrudes from the sea floor. Thus, unfortunately, it is vulnerable to damage by fishing trawlers. The species is also vulnerable to industrial activity.Atrina fragilis prefers to reside on the sublittoral fringe and subtidal muds in order to embed itself or attach its byssal threads to small stones underneath the softer substrate. The biology of the mussel is not well understood, with information on its reproduction and early life-history particularly sparse. Efforts are being made to identify Atrina fragilis larvae to implement effective conservation of the rare species. Because of its brittle shell, the fan mussel is sensitive to the effects of benthic fishing gears and its distribution is believed to have been impacted by fishing over the past half century.
The shells are anchored to the sand by fine gold-coloured silk produced by special glands located in the muscular foot. This silk or byssus are fine threads which historically were used to make special royal fabrics. This was called Pinna silk or sea silk, although the species more frequently used was Pinna nobilis of the Mediterranean.