''Tresus'' is a genus of saltwater clams, marine

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

s in the family

Mactridae Mactridae, common name the trough shells or duck clams, is a family of saltwater clams, marine bivalve mollusks in the order Venerida. Description These clams have two short siphons, each with a horny sheath. The shell is shaped like a rounded- ...

. Many of them are known under the common name the horse clam or as species of gaper clam. They are similar to geoducks.

Species

Species within the genus ''Tresus'' include: * ''

Tresus allomyax ''Tresus'' is a genus of saltwater clams, marine bivalve mollusks in the family Mactridae. Many of them are known under the common name the horse clam or as species of gaper clam. They are similar to geoducks. Species Species within the genus ...

'' (Coan & Scott, 2000) – strange gaper * ''

Tresus capax ''Tresus capax'' is a species of saltwater clam, marine bivalve mollusk, common name the fat gaper, in the family Mactridae. It also shares the common name horse clam with '' Tresus nuttallii'' a species which is similar in morphology and lifes ...

'' (Gould, 1850) – fat gaper * ''

Tresus keenae ''Tresus'' is a genus of saltwater clams, marine bivalve mollusks in the family Mactridae. Many of them are known under the common name the horse clam or as species of gaper clam. They are similar to geoducks. Species Species within the genus ...

'' (Kuroda & Habe, 1950) – mirugai clam * ''

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 an ...

'' (Conrad, 1837) – Pacific gaper * ''

Tresus pajaroanus ''Tresus'' is a genus of saltwater clams, marine bivalve mollusks in the family Mactridae. Many of them are known under the common name the horse clam or as species of gaper clam. They are similar to geoducks. Species Species within the genus ...

'' (Conrad, 1857) – lost gaper

Habitat

These species' habitat is the lower intertidal zones on out to waters as deep as 50–60 feet (13–15 m). 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. Their preferred substrates are also preferred by butter and littleneck clams, so horse clams are often taken incidentally in commercial harvesting. ''Tresus'' clams often have a relationship with small commensal

pea crab The pea crab, ''Pinnotheres pisum'', is a small crab in the family Pinnotheridae that lives as a parasite in oysters, clams, mussels, and other species of bivalves. Description Pea crabs are small crustaceans about the size of a pea or dime, wit ...

s, often a mating pair, which enter through the large siphon and live within the mantle cavity of the horse clam. The crabs are easily seen and in no way affect the clam as food. The meat is good and makes excellent chowder. They tend to be ignored by sport diggers in Washington but not in Oregon.

Notes and references

{{Taxonbar, from=Q3012763 Mactridae Bivalve genera Marine molluscs of North America Molluscs of the Pacific Ocean Molluscs of the United States Western North American coastal fauna