Tresus capax is a species of saltwater clam, marine bivalve mollusk, common name the fat gaper, 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).

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.

[4] Appreciated by pre-contact local Native Americans for their size, abundance, and relatively easy capture, they are less sought 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.