Crassostrea gigas

(Thunberg, 1793)

Description (shell):
Shell thick and very rugose; shape variable, usually elongate oval. Umbones prominent and often enrolled. Left valve deeply cupped, with coarse concentric sculpture, growth stages prominent, raised and frilled, forming flat scales; few, usually about six, very bold raised ribs, imparting grossly crenulate, saw-toothed, appearance to margin. Right valve similarly sculptured, ribs corresponding to channels in opposite valve. On inner surface the external sculpture extends about one-third of its length into the shell; adductor scar distinct, deep purplish-blue or brown.

Greatest dimension dorsoventral, up to 25 cm, but often less.

Colour off-white, yellowish, or bluish grey, often with darker purplish patches. Inner surfaces of both valves pearly, white or bluish white.

Introduced to south-coast and south-west coasts, breeding only sporadically. Dead shells may occur elsewhere. Locally abundant in Dutch estuaries (Distr. C. gigas).