Tresus capax is a species of saltwater clam, marine bivalve mollusk, common name the fat gaper, in the family Mactridae. It also shares the common name horse clam with Tresus nuttallii a species which is similar in morphology and lifestyle. Both species are somewhat similar to the geoduck (Panopea generosa, which is in the family Hiatellidae), though smaller, with shells up to eight inches long (20 cm), weight to 3–4 lb (1.4–1.8 kg).
The two species commonly known as horse clams inhabit the Pacific coast intertidal zones: the pacific gaper, T. nuttallii, more abundant south to California; and the fat gaper, T. capax, more abundant north to Alaska. Both have oval and chalky-white or yellow shells with patches of brown periostracum (leather-like skin) on the shell. These clams are also commonly called gapers because their shells are flared around the siphon and do not completely close, rather like geoduck clams. Like geoducks, they are unable to completely retract the siphon within the shell, though less flagrantly as the siphon on Tresus species is not as large.
An easy way to tell the two species apart is that T. nuttallii usually has relatively longer, narrower shells (longer compared to height) and larger siphonal plates (horny plates found at the tip of the siphon, often with a little algae or barnacles garden). Hence T. capax is the fat gaper.
Their habitat is the lower intertidal zones on out to waters as deep as 50–60 feet (13–15 m). They prefer sand, mud, and gravel substrates, normally burying themselves 12–16 inches (30–41 cm), so they are much easier to dig than geoducks. Their preferred substrates are also preferred by butter and littleneck clams, so horse clams are often taken incidentally in commercial harvesting.
Horse clams often have a relationship with small commensal pea crabs, Pinnixa faba, often a mating pair, which enter through the large siphon and live within the mantle cavity of the horse clam. The crabs are easily seen and in no way affect the clam as food. The meat is good and makes excellent chowder. They tend to be ignored by sport diggers in Washington but not in Oregon.
Horse clams are broadcast spawners like geoducks; T. nuttalii spawns in summer and T. capax in winter.
