Tresus nuttallii, common name the Pacific gaper, is a species of saltwater clam, a marine bivalve mollusk in the family Mactridae.

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).

[4] Appreciated by pre-contact local Native Americans for their size, abundance, and relatively easy capture, they are less sought after today than geoducks, which have gained a marketing cachet.

For resource sustainability, the Washington State Department of Fish and Wildlife [1] sets size and bag limits for these clams.

For example, archaeological recovery from Chumash sites in San Luis Obispo County has revealed use of horse clam shells as a scoop implement.

An unusually well decorated specimen was found at the present day town of Morro Bay during archaeological excavation.