Pinnotherid crabs are true
crabs which do not have a rostrum
nor any teeth between the eyes (photo)
or on the lateral
margins of the carapace.
is a squared oval or nearly circular, and usually wider than
They are symbionts with worms, ascidians, bivalves, etc. Pinnixa
faba has a carapace
width at least 1.5 times the length, curved tips to the dactyls
of its walking legs (photo),
the outer orbits
of the eye sockets are rounded (photo),
and the female
has no gap between the propodus
of her claw. The merus
of the third leg in males is more than twice as long as wide (see
Males are hard-bodied and females are softer. The male has a
near the base of the propodus
of the cheliped.
Color variable, from white to orange. Male often has dark
Male carapace width to 1.75 cm, females to 2.5 cm.
Pinnixa faba (Dana, 1851)
Common name(s): Pea crab, Mantle pea
crab, Large pea crab
Pinnothera faba, Pinnotheres
|Pinnixa faba, likely a male, from
the mantle cavity of Tresus
capax. Scale in background = mm
|(Photo by: Dave Cowles,
How to Distinguish from
Similar Species: Pinnixia
littoralis, another common species, has an acute outer margin
and both sexes have a gap between the propodus
of the claw.
Prince of Wales
Island, Alaska to Newport Beach, CA. This species is much
than is P. littoralis in Puget Sound.
Range: Middle and low
Intertidal and subtidal to 30 m (depth range of Tresus
Primarily found within the
fat gaper clam Tresus
capax. Juveniles may be in other clam
Biology/Natural History: A number of crabs may be
a clam but usually only one pair is mature. The presence of
seems to hinder the maturation of the juveniles. The small
move around but the larger females remain sheltered by a fringe of
the visceral skirt, attached to the clam's visceral mass.
plankton from the clam's feeding mucus from this fringe. They
slight damage to the host. Alternate hosts include the clams Tresus
nasuta, Mya arenaria, Saxidomus
patula, and Protothaca
staminea; the mantle folds of the sea hare Aplysia
(in California), the atrial cavity of the tunicate Styela
and in the cloaca of the sea cucumbers Caudina arenata,
piperata. In Puget Sound adults seem to
only be found in
capax. In Washington, some females with
eggs can be found
year-round, and most raise two broods per year. A female
carries 7000-8000 eggs. Breeding is interrupted when the
molt, from late August to October. Larvae are planktonic for
then seek a clam host. These crabs have a terminal molt
postlarval instars for the male). Females are large enough to
eggs when they reach a carapace
width of 12.8 to 14 mm (15-16 instars) but continue to grow to 19.7 mm
and Fairbanks, 1966
and Carlton, 1975
Zmarzly, Deborah L., 1992. Taxonomic review of pea
in the genus Pinnixa (Decapoda: Brachyura:
on the California shelf, with descriptions of two species. J.
Biology 12:4 pp. 677-713
et al., 1980
and O'Clair, 1998
et al., 1985
General Notes and
Observations: Locations, abundances,
tips of the walking legs are curved.
There is no rostrum
and no teeth between the eyes, and the outer margins of the eye sockets
Here is another individual, with a bit more setae
than the one above. This one is likely a female (not the same
as below), from Tresus
capax. Photo by Dave Cowles, July 2008
|Below is a series of photos of a gravid
female from Tresus
capax. Click on any of them to enlarge
|From the top, the gravid
female looks similar to other individuals. Note, however,
first segment of her abdomen can be seen bulging out at her posterior
(at the bottom in this picture). That is because she is
so many eggs that she cannot fold her abdomen anywhere near flat
her underside and must hold it partly extended.
||This face-on photo shows the nature
of her problem.
Her abdomen is so full of eggs that she can hardly hold them.
she tries to walk around her abdomen bulges out so far below her that
has to stand on tiptoe in order to barely lift her abdomen off the
|This closeup shows the dark eggs
bulging out between her
abdomen and the underside of her thorax. Crabs, hermit crabs,
true shrimp carry their eggs on their abdomens like this, attached to
Females have featherlike pleopods
for attaching the eggs to, while male crabs have only the first few pleopods
and use them for copulation. Notice also her claw--the propodus
sides of the claw are contoured to fit together without a gap, which is
characteristic of P. faba.
||A view from the underside shows the
large, bulging abdominal
|This female has long, abundant setae
on the legs.
||Note the rounded orbits
(margin of the space around the eyes).
All photos by Dave Cowles, July 2008
Authors and Editors of Page:
Dave Cowles (2005): Created original page