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

bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

s in the family

Mactridae Mactridae, common name the trough shells or duck clams, is a family (biology), family of saltwater clams, marine (ocean), marine bivalve mollusks in the order (biology), order Venerida. Description These clams have two short siphons, each with a ...

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

geoduck The Pacific geoduck ( ; ''Panopea generosa'') is a species of very large saltwater clam in the family Hiatellidae. The common name is derived from the Lushootseed name, . The geoduck is native to the coastal waters of the eastern North Pacific ...

Species

Species within the genus ''Tresus'' include: * '' Tresus allomyax'' (Coan & Scott, 2000) – strange gaper * ''

Tresus capax ''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 ' ...

'' (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 and ...

'' (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 Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit f ...

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. Etymology ''Pinnotheres'' is Greek for "guard of '' Pinna''" and ''pisum'' i ...

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