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

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

geoduck The Pacific geoduck ("gooey-duck"; ; ''Panopea generosa'') is a species of very large saltwater clam in the family Hiatellidae. The common name is derived from the Lushootseed ( Nisqually) word . The geoduck is native to the coastal waters of t ...

Species

Species within the genus ''Tresus'' include: * '' Tresus allomyax'' (Coan & Scott, 2000) – strange gaper * '' Tresus capax'' (Gould, 1850) – fat gaper * '' Tresus keenae'' (Kuroda & Habe, 1950) – mirugai clam * '' Tresus nuttallii'' (Conrad, 1837) – Pacific gaper * '' Tresus pajaroanus'' (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 fro ...

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