Modiolus modiolus

(Linné, 1758)

Description (shell):
Shell thick, tumid, irregularly oval or rhomboidal in outline; umbones anterior but subterminal, with a projecting, rounded, anterior margin; dorsal margin markedly convex, ventral margin slightly concave. Umbonal ridges well-developed. Young specimens with numerous long, smooth, slender periostracal spines, frequently with attached detritus, lost as the shell grows, represented only by a roughened area at posterior end. Sculpture of fine concentric lines and ridges, growth stages often clear (M. modiolus-drawing).

Size variable, commonly 50-100 mm.

Shell bluish white to slate-blue, darkening in larger specimens; periostracum very glossy, light horn to deeper mahogany or dark brown in old shells, usually with a lighter yellow-brown strip along the umbonal ridges. Inner surface of shell white, sometimes with a faint bluish tinge towards posterior margin.

Colour dark orange; the foot is red, whitish towards the thick and wrinkled base. Both margins of the mantle are without fringe of toothing, though covered with delicate cilia.

Burrows is muddy gravel. Distributed from lower shore to about 150 m; offshore it may form immense aggregations.

In the whole North Sea, south to the Bay of Biscay (Distr. M. modiolus).