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).