Rough Pen Shell - Pinna rudis - Spiny Shell Hidden in the Reef

A heavily encrusted Rough Pen Shell nestles within the rocky reef at Cirkewwa, Malta, its large projecting shell scales still clearly visible.

The Rough Pen Shell or Spiny Fan Mussel (Pinna rudis) is a large Mediterranean bivalve with a triangular, elongated fan-shaped shell that narrows toward the end anchored in the substrate. Its shell is usually reddish-brown and is distinguished by a small number of strong radial ribs bearing large, grooved and projecting scales, especially near the open end of the shell. Unlike the larger Noble Pen Shell (Pinna nobilis), which is more commonly associated with sandy bottoms and seagrass meadows, Pinna rudis typically occurs in rocky crevices, depressions and shaded reef habitats. The pointed end is fixed into the substrate by tough byssus threads, leaving the broader end exposed for filter feeding. The species generally reaches around 30–40 centimetres in length and can occur from shallow water down to about 60 metres. This photograph shows a strongly encrusted individual at Cirkewwa Reef in Malta, where algae and other marine growth partially disguise the reddish shell while its distinctive protruding scales remain visible.