These two species are distributed from Alaska to California. T. capax is the more northern form and T. nuttallii is more abundant in the south. These species are similar in appearance and most people are unaware that two species of horse clams exist. They are large clams with shells up to eight inches long. The shells of both species are oval and chalky-white or yellow with patches of brown periostracum (leather-like skin) on the shell. The shells are flared around the siphon and do not completely close (hence the name gaper). The clams are unable to completely retract the siphon within the shell. Horse clams prefer sand, mud, and gravel substrates. They are normally buried 12-16 inches in the substrate and are, therefore, much easier to dig than geoducks. They are found in the lower intertidal zones on out into water as deep as 50-60 feet. They are often found with butter and littleneck clams and, for this reason, are taken incidentally with these clams in the commercial harvest. These clams often harbor small commensal pea crabs which are easily observed when opening the clams for cleaning and which in no way affect the clam as a human food source. The meat is of good flavor and makes excellent chowder. They are largely ignored by sport diggers in Washington but are an important sport clam in Oregon. They are abundant and widespread in Puget Sound. Present sport regulations encourage more use of these clams. Perhaps the reason they tend to be avoided by sport diggers is that the edible portion compared to the total weight is low and the tough skin which covers the siphon is difficult to remove. The two species differ in several aspects, but probably the easiest method to differentiate the two is that T. nuttallii normally has larger siphonal plates (horny plates found at the tip of the siphon often with algae or barnacles attached.) than T. capax, and T. nuttallii shells are longer compared to their height than T. capax.
T. nuttallii is a summer spawner and T. capax spawns during the winter.