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. Learn more about Pacific Gaper