Tresus nuttallii (Conrad, 1837)

(Tresus nuttallii)
(Adult Tresus nuttallii - South Bamfield Inlet, Bamfield, BC - 175mm!)

(Baby Tresus nuttallii)
(Baby Tresus nuttallii - In anoxic mud and shell grit - Bowen Island, BC - 21mm)


Schizothaerus nuttallii
Common Name:
Pacific Horse Clam, Gaper, Summer Clam, Otter Clam.
To 225mm (9") in length!
Wide gape at posterior end of shell. Shell elongate in shape. Length is generally more than 1.5 times the height. Umbones located near the end of the anterior quarter of the shell. Lower margin of shell less rounded than in Tresus capax; more smoothly curved. Siphon has larger leathery siphonal plates than in T. capax and the siphon has a persistent wrinkled brown periostracum.
Alaska to Baja California. More common in its southern range.
Tresus nuttallii burrows in sandier substrate than T. capax, both intertidally and subtidally, tends to burrow deeper, and is found lower down in the water column (low intertidal zone to 50m).
Less common in British Columbia than Tresus capax. 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 (50 species have been found on its siphons). They are preyed upon by moon snails, Dungeness crab and seastars, as well as the juveniles being eaten by the Starry Flounder. Both species of Tresus are edible, the body meat and the siphon.

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