(Linné, 1758)
Description (shell):
Shell thin and brittle. Inequivalve; left valve more convex than right one. Equilateral with ears more or less equal; anterior ear of right valve slightly larger than posterior one, overlapping that of left valve and with a distinct fold along the hinge line, below it a small, rounded byssal notch, finely dentate along the lower edge. Ventral margin constituting more than a semicircle. Sculpture of about 20 bold, radiating ribs, finer ridges and concentric corrugated grooves, ears similarly sculptured; growth stages unclear; margin very crenulate (A. opercularis-drawing 2).
Size:
Up to 90 mm long, frequently less, as long as deep.
Colour:
Colour very variable: light pink to brown, orange, or yellow, usually marked with bands, zigzags, rays and spots of darker or lighter shades. Right valve often paler than left one, frequently with coloured markings on a creamy ground. (A. opercularis); (A. operculais-drawing).
Animal:
The animal is as varied in colour as the shell is. It lives attached by a byssus when small, later free and capable of swimming by the opening and rapid closing of the valves. 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; at their base gleam a row of 35-40 black ocelli (eye-spots) which have a pupil-like spot of greenish grey. This species is much used for food.
Habitat:
Occurs in dense aggregations offshore to 100 m, occasionally between tidemarks; on sand and fine gravel.
Distribution:
Widespread and common in the North Sea (Distr. A. opercularis). Distributed from Norway south to the Mediterranean and Canary Isles.