Saxidomus gigantea (Deshayes 1839)

How to Distinguish from Similar Species: Of shells commonly found on the same, protected beaches, the cockle Clinocardium nuttallii has radial ridges and an undulating aperture where the two valves come together. The gaper clam Tresus capax and the softshell clam Mya arenaria have a large chondrophore at the hinge, and Tresus capax also has a large gape at the posterior end. The bentnose clam Macoma nasuta has valves bent to the right on the posterior end. The littleneck clam Protothaca staminea has fine radial ribs, a row of small teeth on the inside of the valves close to their ventral margins, and may also have a mottled pattern of periostracum. Saxidomus nuttalli is very similar but has more prominent raised concentric lines on the shell, plus the shell is stained purple inside. It lives primarily farther south in California.

Sheltered sand, sandy mud, and gravel beaches

Depth Range: Low intertidal to 40 m

This common intertidal clam has an large, black external hinge ligament and well-developed concentric ridges but no radial ridges. The shell is only slightly longer than it is high, and the angle formed by the umbones is more than 110 degrees. The periostracum, if present, is not yellow or glossy. The adductor muscle scars are of nearly equal size. It has a true hinge plate with 3 cardinal teeth in each valve, but no chondrophore or byssal threads. The valves have a smooth but not glossy interior with a pallial sinus and a continuous pallial line. The shell gapes slightly at the posterior end. Length to 15 cm. The outside of the shell may be white, or may be stained by iron sulfide in anoxic sediments. The siphons are moderately long.

Biology/Natural History: This species burrows moderately deep. It has been extensively commercially harvested, especially for clam chowder. Predators include the seastars Pycnopodia helianthoides and Evasterias troschelii, the moon snail Euspira lewisii, Dungeness crabs Cancer magister, and sea otters. The species is especially vulnerable to paralytic shellfish poisoning, which accumulates especially in the dark tips of the siphons, so great caution should be used before eating them. The toxin, accumulated aftereating the dinoflagellate Alexandrium catanella, which blooms in sea temperatures over 13C, may persist for months in the tissues. Sea otters and seabirds seem to be able to detect the toxin and avoid tainted clams, though humans cannot. Pacific staghorn sculpins Leptocottus armatus nip off siphon tips, but after feeding on a tainted siphon will avoid them. Gulls regurgitate contaminated clams and avoid them after that. Pea crabs Pinnixa littoralis Pinnixa faba, and Fabia subquadrata may be found in the mantle cavity. The crystalline style contains many large spirochaete bacteria. Spawns in the summer. In British Columbia, about half the clams are large enough to spawn by their third year. Larvae settle from the plankton after 4 weeks. This species may live 20 years or more. Indians formerly used the shells of this clam for money.

Geographical Range: Aleutian Islands and SE Bering Sea, Alaska to San Francisco Bay, CA