Tresus nuttallii
Tresus nuttallii, common name the Pacific gaper, is a species of saltwater clam, a marine bivalve mollusk in the family Mactridae. It also shares the common name horse clam with Tresus capax, 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). Two species commonly known as "horse clams" inhabit the intertidal zone of the West Coast of the United States: the Pacific gaper, T. nuttallii, which is more abundant south to California; and the fat gaper, T. capax, which is 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 extremely, as the siphon on Tresus species is not as large. Identification T. nuttallii can be discriminated from T. capax on the basis of the