Pseudamussium septemradiatum

(Müller O.F., 1776)

Description (shell):
Shell thin and fairly brittle, almost circular. Equilateral. Almost equivalve, right anterior ear with very small byssal notch. Sculpture of fine radiating striations and coarser concentric lines (occasionally spinous); each valve with three to ten, most frequently five to seven, evenly spaced, broad rounded ribs, increasing rapidly in width from umbone to margin. The right valve, in addition, may have a group of four or five slender ribs close to anterior margin. Ears with about six ribs each, and concentric corrugations. Growth stages clear (P. septemradiata-drawing).

Up to 50 mm long.

Left valve dull brick-red, brown, or purple, with white or cream spots and blotches; right valve pale brick-red with spots in small specimens (<15 mm), becoming paler as animal grows. In large specimens right valve is largely creamy white with reddish umbones, colour extending, and fading, down the ribs.

Attached by a byssus when small, later free and capable of swimming by the opening and rapid closing of the valves. Lives with the left valve uppermost. The muscle for closing the valves is very large and powerful. The foot is developed as a finger-like organ, occupied in the spinning of byssal threads, which pass through the ears. The mantle margin is double, the inner finely fringed and the outer edged with long tentacles and at their base gleam a row of large opalescent eyes.

Occurs offshore to about 200 m.

Distributed from Norway to the Mediterranean and north-west Africa (Distr. P. septemradiata).