Tresus capax (Gould, 1850)

Common name(s): Fat gaper clam, Gaper clam, Horse clam, Alaskan gaper, Summer clam, Otter clam

Synonyms:  Schizothaerus capax
Phylum Mollusca
 Class Bivalvia
  Subclass Heterodonta
   Order Veneroida
    Family Mactridae
Tresus capax from Padilla Bay, WA.   Scale is cm
(Photo by: Dave Cowles, July 2005)
Description:  As with other members of family Mactridae, this species has 2 valves similar to one another and 2 adductor muscle scars that are similar in size (photo), has a true hinge plate with teeth and a socketlike chondrophore in both valves (photo), a pallial sinus and continuous pallial line (photo), and no radial ribs. Tresus capax has a shell often over 10 cm long (up to 20 cm) and with a wide gape (over 1/4 the width of the shell) at the posterior end (photo).  The shell is about 1.5 times as long as high (photo), and the umbones are near the end of the anterior third of the shell (photo).  Periostracum, where it is still attached, is a dark brown or black.  Shell chalky white or yellow.  This is the largest intertidal clam in the northern part of its range (Alaska), though in Puget Sound the geoduck clam is larger.

How to Distinguish from Similar Species:  Tresus nuttallii (uncommon in our region of the Salish Sea but common elsewhere) has a shell more than 1.5x as long as high and the umbones are near the end of the anterior quarter

Geographical Range:  Kodiak Island, Alaska to central California.  Uncommon in California.

Depth Range:  Middle and low Intertidal and subtidal to 30 m

Habitat:  Burrowing in mud and clay, to depths of up to 1 m; in quiet bays (occasionally on the outer coast)

Biology/Natural History:  These clams have extremely long siphons which extend up from their deep location to the surface of the mud.  The incurrent and excurrent siphons are fused together but the internal channels are separate.  Predators include the moon snail Euspira lewisii, the crab Cancer magister, and the seastars Pycnopodia helianthoides and  Pisaster brevispinusFusitron oregonensis will attack it if it gets the chance.  Commensals include the crabs Pinnixa faba and Pinnixa littoralis, in the mantle cavity.  The small male crabs move around but the larger females remain sheltered by a fringe of tissue, the visceral skirt, attached to the clam's visceral mass.  They scrape plankton from this fringe.  During the winter this clam depletes its glycogen stores from the gonads.  Fat stores (in the digestive diverticula) are only used if the glycogen has been exhausted.  This clam is often eaten by man, especially by native Americans.   When dug up, this clam may not be capable of reburying itself again.  In Washington, reaches maturity in 3-4 years.  Spawn in winter.

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Dichotomous Keys:
  Kozloff 1987, 1996
  Smith and Carlton, 1975

General References:
  Brusca and Brusca, 1978
  Harbo, 1997
  Kozloff, 1993
  McConnaughey and McConnaughey, 1985
  Morris et al., 1980
  O'Clair and O'Clair, 1998
  Ricketts et al., 1985

Scientific Articles:

Web sites:
Washington Dept of Fish and Wildlife clam page

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

The shell of Tresus capax has a large gape at the posterior end to accomodate the large siphons

Because of the large siphons, Tresus capax has a very deep pallial sinus.  It has a continuous pallial line.  Note also the almost equal-sized anterior and posterior adductor muscle scars.
The posterior end is to the left.

The shell is less than 1.5 times as long as it is wide.

The umbo is about 1/3 the way from the anterior end of the shell (to the left in this photo).  The hinge (to the right of the umbo) is thin

There is a shelflike chondrophore and several hinge teeth on both hinge plates.  Top = right valve, bottom = left valve.  Anterior is to the right.

The gouge on the siphon of this individual is due to a bite by the predatory hairy triton snail Fusitron oregonensis.

This individual from March Point has a shell length of 19 cm and shell height of 11 cm.  It has retracted its siphon about as far as it can but before retraction the siphon was 18 cm long, or about as long as the shell.

On some mud flats the tips of the large siphons, which are exposed at the surface when feeding, become overgrown with red algae such as Polysiphonia, as seen here. 
Photo by Dave Cowles 2007 

Algae on siphon
This photo shows a gaper clam siphon in situ, in the upper center of the picture.  It is so overgrown with a tuft of red algae (about 8 cm across) that it is well camouflaged.
Photo by Dave Cowles, July 2016

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