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.
Tresus capax (Gould, 1850)
Common name(s): Fat gaper clam, Gaper clam, Horse clam, Alaskan
gaper, Summer clam, Otter clam
|Synonyms: Schizothaerus capax
|Tresus capax from Padilla Bay, WA. Scale is cm
|(Photo by: Dave Cowles, July 2005)
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
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.
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
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.
and Carlton, 1975
and Brusca, 1978
and McConnaughey, 1985
et al., 1980
and O'Clair, 1998
et al., 1985
Dept of Fish and Wildlife clam page
General Notes and Observations: Locations, abundances,
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
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)
There is a shelflike chondrophore
and several hinge
teeth on both hinge plates. Top = right valve, bottom = left valve.
is to the right.
The gouge on the siphon
of this individual is due to a bite by the predatory hairy triton snail
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
This photo shows a gaper clam siphon
in situ, in the upper center of the picture. It is so overgrown with
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