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Tresus capax

(Fat Gaper)


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Interesting Facts

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Common Names

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Common Names in English:

Fat Gaper, Alaskan Gaper, Fat Gaper Clam, Gaper Clam, Horse Clam, Otter Clam, Summer Clam


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Physical Description

Species Tresus capax

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, has a true hinge plate with teeth and a socketlike chondrophore in both valves, a pallial sinus and continuous pallial line , 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. The shell is about 1.5 times as long as high, and the umbones are near the end of the anterior third of the shell. Periostracum , where it is still attached, 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.[1]

These clams have extremely long siphons which extend up from their deep location to the surface of the mud . The siphons are fised together but the internal channels are separate.[1]


Burrowing in mud and clay , to depths of up to 1 m ; in quiet bays (occasionally on the outer coast) Depth Range: Middle and low Intertidal and subtidal to 30 m[1]

Typically found in the intertidal zone at the water's edge at a mean distance from sea level of 64.65 meters (212.11 feet).[2]

Biome: Coastal.

Ecology: 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.[1]


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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. In Washington, reaches maturity in 3-4 years. Spawn in winter.[1]



Predators include the moon snail Polinices lewisii, the crab Cancer magister, and the seastars Pycnopodia helianthoides and Pisaster brevispinus. Fusitron oregonensis will attack it if it gets the chance.[1]


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Schizothaerus capax


Name Status: Accepted Name .

Last scrutiny: 17-Jan-2006

Similar Species

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Tresus nuttallii (uncommon in our region) has a shell more than 1.5x as long as high and the umbones are near the end of the anterior quarter (Ref. 109953)

Members of the genus Tresus

ZipcodeZoo has pages for 5 species and subspecies in this genus:

T. allomyax (Strange Gaper) · T. capax (Fat Gaper) · T. nuttalli (Pacific Gaper) · T. nuttallii (Pacific Gaper) · T. pajaroana (Lost Gaper)

More Info

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Further Reading

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Data Sources

Accessed through GBIF Data Portal November 18, 2007:



  1. Cowles, Dave. Key to Invertebrates Found At or Near The Rosario Beach Marine Laboratory (a campus of Walla Walla University) Fidalgo Island, Anacortes, WA May 2009. [back]
  2. Standard Deviation = 132.870 based on 34 observations. Terrestrial altitude and ocean depth information for each observation from British Oceanographic Data Centre. [back]
Last Revised: 2/1/2015