Spiny Fan Mussel

Overview

The Spiny Fan Mussel (Pinna rudis) is a large, triangular bivalve mollusk belonging to the family Pinnidae. It is a protected species known for its robust shell and distinct, scale-like protrusions, often found anchored in marine environments across the Mediterranean and parts of the eastern Atlantic.

Distribution & habitat

This species is primarily found in the eastern Atlantic, from the Azores to Saint Helena, and has established populations in the Mediterranean and Black Seas. Unlike some of its relatives that prefer soft, sandy bottoms, the Spiny Fan Mussel typically favors hard substrates, often settling in rock crevices, small depressions, or patches of sand within rocky areas. It is frequently observed in marine protected areas, which may provide essential refuge for its populations.

Appearance

The shell of the Spiny Fan Mussel is wedge-shaped and can reach lengths of up to 50 cm, though 25–40 cm is more common. Its valves are reddish-brown, symmetrical, and feature five to ten radial ribs. A defining characteristic of adults is the presence of large, gutter-like, tile-shaped scales along these ribs, which become more prominent toward the shell's opening. These surfaces are often colonized by various epibionts, such as algae, bryozoans, and ascidians.

Biology & behavior

As a filter feeder, this bivalve draws in water to extract plankton and organic matter. It remains anchored vertically in the substrate using a bundle of strong, iridescent byssus threads. The species is a hermaphrodite and releases larvae that drift with ocean currents before settling. It is often confused with the Noble Pen Shell (Pinna nobilis), but can be distinguished by its more triangular, robust shape and larger, less numerous scales.