The Fan Mussel (Atrina fragilis), is a large saltwater clam or marine bivalve mollusc in the family Pinnidae, the pen shells.

The fan mussel is one of the largest (30 to 48 mm long) European bivalve molluscs. It is one of the rarest species of marine mollusc in the United Kingdom (UK).

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 48 cm in length, and the largest individuals may be 10 to 12 years old.

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 fishing. The species is also vulnerable to industrial activity. They have been massacred by ships and anchors as they sit, half submerged in the substrate with their syphons exposed for feeding.

The shells are anchored to the sand by fine gold-coloured silk produced by special glands. This silk or byssus are fine threads which historically was 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.

The fan mussel is a UK Priority Species for conservation. This species is protected in the United Kingdom under the Wildlife and Countryside Act 1981 from killing and injuring, possession and sale since 1998.

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