Bivalves

Click on photo to enlarge. Scale line in photo equals 1cm unless otherwise specified.
* Species which are commonly encountered on the beach.

Mactromeris polynyma (Stimpson, 1860)
Arctic Surfclam *
intertidal to 110m size to 15.5cm
northern Washington to northern Alaska; circumboreal - south to Japan & Massachusetts
This is occasionally found intertidally in its southern range and commonly found to the north. It prefers a muddy location. The periostracum is dark, shiny, and rough, and tends to be worn away near the hinged end of the shell.
(previous name - Spisula polynyma)

all from Oak Bay, WA intertidal, photographed in an aquarium

Simomactra falcata (Gould, 1850)
Hooked Surfclam
low intertidal to 50m northern Mexico to northern BC size to 10cm
This is infrequently found on the beach. It could only be confused with M. polynyma. The periostracum of this species is thin, dark and silky rather than shiny. The anterior end is also proportionately longer.
(previous name - Spisula falcata)

Birch Bay, WA Totten Inlet, WA, intertidal Thetis Island, BC, intertidal

Tresus nuttallii (Conrad, 1837)
Pacific Horse Clam or Pacific Gaper *
intertidal to 80m northern Mexico to central Alaska size to 23cm
This is commonly found and one can see its exposed siphons at low tide. It is more elongated than T. capax and also lives buried up to 20 inches deep. The small projections inside the incurrent siphon tend to be dark green on the T. nuttallii and gold on the T. capax. This species never hosts pea crabs.
(previous name - Schizothaerus nuttallii)

Tresus allomyax Coan & Valentich Scott, 2000
Strange Gaper
subtidal size of largest known - 149mm
California to at least Haida Gwaii
This is very rarely seen and, as yet, live individuals have not been found in local waters. It is only known to exist off the open coast and seems to only be subtidal. It would most closely resemble T. capax.

Tresus capax (Gould, 1850)
Fat Horse Clam or Fat Gaper *
intertidal to 30m size to 28cm
southern California to Aleutian Islands, AK
This shell is very common and one frequently sees its exposed siphons at low tide. It is sometimes mistaken for a geoduck. The clam may be buried as much as 20 inches below the surface of the sand. The shell is somewhat oval in shape. Commensal pea crabs are often found inside its shell.
(previous name - Schizothaerus capax)

The siphons of the horseclams can be distinguished from that of the geoduck in that the horseclam siphons have finger-like projections on the interior rim of the siphon holes. The exterior tips of the siphons also have leathery plates. Geoduck siphons are smooth and have no projections inside the siphon. When both groups protude significantly from the sand at low tide, they can look very similar if the siphon is pinched shut and the siphons are clean of attached algae.

Family Mactridae