Saxidomus gigantea Deshayes, 1839

Common name(s): Butter clam, Washington clam, Smooth Washington clam, Money shell

Synonyms:  Saxidomus giganteus
Phylum Mollusca
 Class Bivalvia
  Subclass Heterodonta
   Order Veneroida
    Family Veneridae
Saxidomus gigantea shell from Padilla Bay, WA
(Photo by: Dave Cowles, August 2005)
Description:  This common intertidal clam has an large, black external hinge ligament (photo) and well-developed concentric ridges but no radial ribs.  The shell is only slightly longer than it is high, and the angle formed by the umbones is more than 110 degrees.  The periostracum, if present at all, is not yellow or glossy.  The two adductor muscle scars are of nearly equal size (photo).  It has a true hinge plate with 3 cardinal teeth in each valve (photo), but no chondrophore or byssal threads.  The valves have a smooth but not glossy interior with a pallial sinus and a continuous pallial line (photo).  The shell gapes slightly at the posterior end.  Length to 15 cm.  The outside of the shell may be white, or may be stained by iron sulfide in anoxic sediments.  The siphons are moderately long (about 4 cm), united, and black-tipped (photo).

How to Distinguish from Similar Species:  Of shells commonly found on the same, protected beaches, the cockle Clinocardium nuttallii has radial ribs and an undulating aperture where the two valves come together.  The gaper clam Tresus capax and the softshell clam Mya arenaria have a large chondrophore at the hinge, and Tresus capax also has a large gape at the posterior end.  The bentnose clam Macoma nasuta has valves bent to the right on the posterior end.  The littleneck clam Protothaca staminea has fine radial ribs, a row of small teeth on the inside of the valves close to their ventral margins, and may also have a mottled pattern of periostracumSaxidomus nuttalli is very similar but has more prominent raised concentric lines on the shell, plus the shell is stained purple inside.  It lives primarily farther south in California.

Geographical Range:  Aleutian Islands and SE Bering Sea, Alaska to San Francisco Bay, CA (rarely seen S of Humboldt Bay)

Depth Range:  Low intertidal to 40 m

Habitat:  Sheltered sand, sandy mud, and gravel beaches

Biology/Natural History:  This species burrows moderately deep (to 35 cm), as suggested by the deep pallial sinus.  It has been extensively commercially harvested, especially for clam chowder.  Predators include the seastars Pycnopodia helianthoides and Evasterias troschelii, the moon snail Euspira lewisii, Dungeness crabs Metacarcinus magister, and sea otters.  The species is especially vulnerable to carrying paralytic shellfish poisoning (PSP), which accumulates especially in the dark tips of the siphons, so great caution should be used before eating them.  The toxin, accumulated after the clam feeds on the dinoflagellate Alexandrium catanella, which blooms in sea temperatures over 13C, may persist for months in the tissues.  Sea otters and seabirds seem to be able to detect the toxin and avoid tainted clams, though humans cannot.  Pacific staghorn sculpins Leptocottus armatus nip off siphon tips, but after feeding on a tainted siphon will avoid them.  Gulls regurgitate contaminated clams and avoid them after that.  Pea crabs Pinnixa littoralis  Pinnixa faba, or Fabia subquadrata may be found in the mantle cavity.  The crystalline style contains many large spirochaete bacteria (Cristispira sp).  Spawns in the summer.  In British Columbia, about half the clams are large enough to spawn by their third year.  Larvae settle from the plankton after 4 weeks.  This species may live 20 years or more.

Indians formerly used the shells of this clam for money.

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Dichotomous Keys:
  Fitch 1953
  Flora and Fairbanks, 1966 (as Saxidomus giganteus)
  Kozloff 1987, 1996 (as Saxidomus giganteus)
  Smith and Carlton, 1975 (as Saxidomus giganteus)

General References:
  Carefoot, 1977
  Harbo, 1997
  Harbo, 1999
  Johnson and Snook, 1955
  Kozloff, 1993
  Morris et al., 1980
  Niesen, 1997
  Ricketts et al., 1985
  Sept, 1999

Scientific Articles:
Moore, Stephanie K., Nathan J. Mantua, Barbara M. Hickey, and Vera L. Trainer, 2010.  The relative influences of El Nino-Southern Oscillation and Pacific Decadal Oscillation on paralytic shellfish toxin accumulation in Pacific Northwest shellfish.  Limnology and Oceanography 55:6 pp. 2262-2274.

Web sites:
  Washington Dept of Fish and Wildlife clam page

General Notes and Observations:  Locations, abundances, unusual behaviors:

The hinge ligament on Saxidomus gigantea is external.

Saxidomus gigantea has three cardinal hinge teeth on each valve but no chondrophore.

The two adductor muscle scars are of similar size.  There is a continuous pallial line and a well-developed pallial sinus. This is the right valve.

Left valve scars and pallial line

The above photo, of the left valve, also shows the two distinct adductor muscle scars and the the pallial line with its deep pallial sinus.

The siphons are united (attachedd to one another) and black on the tips.

Authors and Editors of Page:
Dave Cowles (2005):  Created original page