Shells of Mya truncata can reach a size of about 2.5–7.5 centimetres (0.98–2.95 in). These bivalves are similar to the soft-shell clams (Mya arenaria), but usually they are smaller. Moreover, their shells are less elongated.
Valves are rounded in the anterior end and truncated in the posterior end, with a large gape allowing the passage of an extensible siphon that can reach four times the length of the shell. The siphon can be retracted completely into the shell. The two valves are triangular and convex, but the right valve is more convex than the left one. The siphon is protected by a horny sheath and it is provided with small tentacles at its end.
The outer surface of the valves is covered with concentric growth lines. It is whitish, while the outermost layer of the valves (periostracum) may be yellowish, light olive or dark brown. The hinge of the left valve shows a spoon-shaped projection, to which the ligaments are attached. The pallial sinus is deep and broad.
Reproduction is sexual, the gametes of both sexes being expelled into the water, then the free larvae attach themselves to rocks. These clams filter the sea water in order to feed on bacteria, diatoms and larvae of invertebrates. In some years, the reproductive cycle fails completely.
This species is widespread over the Arctic seas and extends to the Bay of Biscay, to the West Atlantic coast and to Sachalin on the Pacific coast. It has been reported also in the Japan seas. Mya truncata lives from the lower shore up to depths of 70 m, burrowed in sand or sandy mud.
Shell of Mya truncata from North Wales
Shells of Mya truncata in subrecent sediments on the coast of Spitsbergen.