Tresus capax (Gould, 1850)

(Tresus capax)
(Tresus capax - Boundary Bay, BC - 171mm!)

(Tresus capax)
(Tresus capax - Tacoma Narrows, WA - in hard clay, extreme low tide zone - 93mm)


Schizothaerus capax
Common Name:
Horse Clam, Fat Gaper, Alaska Gaper, Summer Clam, Otter Clam.
To 200mm (8") in length!
Wide gape at posterior end of shell. Shell oval in shape. Length is gennerally 1.5 times the height. Umbones located near the end of the anterior third of the shell. Lower margin of shell more rounded than in Tresus nuttallii. Siphon has smaller leathery plates than in T. nuttallii and the siphon has little periostracum.
Kodiak Alaska to central California; more common in the north.
This Horse Clam burrows in fine sandy mud, gravelly mud or stiff clay, middle intertidal zone and subtidally to 30m.
More common in British Columbia than Tresus nuttallii. Most Tresus that one finds will have a pair of small crabs living in the mantle cavity. Three species can be found: Pinnixa faba, P. littoralis, and Fabia subquadrata. Many organisms can be found living on the siphons as well, although less so on Tresus capax than T. nuttallii. These clams feed on suspended diatoms, flagellates, dinoflagellates and fine detritius. They are preyed upon by moon snails, Dungeness crab and seastars. Both species of Tresus are edible, the body meat and the siphon.

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