Description: This pea crab has a carapace
width less than 1.5x the length. Its dactyls
are strongly curved. The setae
on legs 3 and 4 are long and feathery, longer than those on other legs.
of juveniles and ovigerous
females is soft, while that of mature males and non-ovigerous
mature females is hardened. Mature, ovigerous
females also have a dorsal longitudinal groove extending posteriorly on
the dorsal carapace
from the orbit
of each eye (photo), as well as
groove (photo). Soft stages
are a translucent white, slightly more opaque in front. Eggs can
be seen through the integument
(photo). The chelipeds are
opaque white and may have yellow tips on the dactyls. Walking legs
are creamy white with yellow setae.
Hard stages (mature males, mature non-ovigerous
females) have an opaque white carapace,
tan in front. The cardiac and branchial areas have red reticulations.
The antennae are orange. Size: Males carapace
to 7 x 7.3 mm; females carapace
to 17 x 22 mm.
Fabia subquadrata (Dana, 1851)
Common name(s): Grooved mussel crab, mussel crab, pea crab, parasitic pea
crab, clam crab
subquadrata, Pinnotheres concharum, Cryptophyrs concharum, misspelled
as Faba subquadrata
|Fabia subquadrata, a small
gravid female, dorsal view. Scale in background is millimeters.
The brown globules are eggs, most of which are tucked under the abdomen
and create the dark stripe, but some are elsewhere and can be seen readily
through the exoskeleton. This is a soft stage.
|(Photo by: Dave Cowles, May 2009). This individual
was found in the mantle chamber of a Nuttallia
obscurata clam purchased from a local supermarket in May,
How to Distinguish from Similar Species: Pinnotheres
spp. have a soft carapace,
nearly straight dactyls,
and no dorsal longitudinal groove behind each eye. The setae
on legs 3 and 4 of Scleroplax granulata
are not longer than those on the other legs, plus it lives in the burrows
of Thalassinideans such as Neotrypaea andUpogebia.
Most other Pinnotherids have
more than 1.5x as wide as it is long. F. concharum, the smooth
mussel crab which lives in California, does not have the dorsal longitudinal
grooves in the carapace.
Geographical Range: Akutan Pass,
Alaska to Ensenada, Baja California, Mexico
Depth Range: Intertidal to 220
Habitat: Planktonic, or lives commensally
or parasitically within bivalves such as Modiolus
arenaria, Astarte compacta, Cardita ventricosa, Crenella
obscurata (not reported in the literature but that is where I have
found it several times), or in Kellia spp.
Biology/Natural History: This species
lives within bivalves. Unlike some other pea crabs, only one individual
is found within the host (e.g., not usually a male-female pair).
Juveniles settle from the plankton and enter a bivalve host where they
remain soft. After 7 or more molts they molt to adults, which have
The adults swim again through the plankton, where females are fertilized
in the early summer. Males apparently die soon afterward (a few may
re-enter a mussel), but females enter new mussel hosts, molt 5 more times
to the soft stage seen here, lay eggs, and fertilize them from her store
of sperm. Mating takes place in late May in Puget Sound. It
takes the female about 21-26 weeks from the time she re-enters a mussel
until she produces her eggs. Eggs are found in November in Puget
Sound (but note the May date above for the individual above and mid-April
for the second individual shown below).
females damage the clam's gills plus take food from the clam so they should
probably be considered parasitic rather than commensal.
The most common host for this species in the Puget Sound area is Modiolus
modiolus. Off California the crab is found in 1 to 3% of
the mussels (Ricketts et al., 1985) or up to 80% according to Hinton, 1987,
and in 18% of the Modiolus
population off Vancouver Island. Ricketts et al. state that mature
crabs are found only in mussels, which was disproven by these individuals.
Ricketts et al. also report that at San Juan Island they could not be found
in Mytilus mussels, though
Hinton states that they are the favored host in California. O'Clair
and O'Clair state that Modiolus capax is the preferred host in Alaska.
females are so much larger than males and are soft, they were originally
described as a separate species.
and Fairbanks, 1966
and Hanby, 2005
et al., 1980
and O'Clair, 1998
et al., 1985
Irvine, Alfred John, 1960. Laboratory culture methods and larval
stages of Fabia subquadrata (Dana). Master's Thesis, Walla
Walla College. 52 pp.
Pearce, J.B., 1966. The biology of the mussel
crab, Fabia subquadrata,
from the waters of San Juan Archipelago, Washington. Pacific Science
20:1 pp. 3-35
General Notes and Observations: Locations,
abundances, unusual behaviors:
are strongly curved. Setae
on legs 3 and 4, especially on the dorsal
edge of the merus,
are long and featherlike. According to Hart (1982), the legs of gravid
females have few setae
while males and non-gravid
females have many more setae
(which may be used for swimming), plus hard carapaces.
Hard individuals (mature males and non-gravid
females) have more opaque carapaces,
flatter legs, more long setae
on the legs, and red articulations on the carapace.
This individual seems intermediate between these conditions.
This closeup dorsal
view of the carapace
shows the egg clusters in the gill chamber. The dorsal
longitudinal grooves extending back from the orbits
as well as the transverse
groove can also be seen.
view shows the larger cluster of eggs carried on the abdomen, as is normal
for Brachyuran crabs (and other members of Suborder Pleocyemata).
I have not heard reports of other brachyuran crabs which have some of their
eggs up in the gill chambers, as this individual does. Also, several
references state that in gravid
females the abdomen is wider than the carapace,
but that is not true of this individual. Johnson and Snook (1955)
state that the propodus has two rows of setae
on the ventral side. The outer row extends to the base of the fixed
finger while the inner row extends to the tip. Those rows can be
This dorsal view shows the chelae. The tips of the chelae can
Here is a second live, gravid female, found in a live Nuttalia
obscurata clam from the local Walla Walla supermarket on April
17, 2017. Carapace
is 10 mm wide.
Note the easily-seen eggs through the dorsal
on both sides but medial
to most of the gill chamber. Perhaps they are in the coelom or in a greatly
Note also that the longitudinal lines extending back from each eye
and the transverse line joining them can be clearly seen. Photo by Dave
Cowles, April 2017
This closer view of the dorsal
clearly shows the eggs packed within the dorsal
side. Photo by Dave Cowles, April 2017
view shows an extremely wide abdomen
(11 mm wide, so at least as wide as the carapace),
and the eggs being carried there.
Photo by Dave Cowles, April 2017
This closeup view of the abdomen
in the opened position shows the eggs attached to the pleopods,
as is standard in most decapods. However, most of the eggs are still inside
the body, as seen above. Photo by Dave Cowles, April 2017
Authors and Editors of Page:
Dave Cowles (2007): Created original page
CSS coding for page developed by Jonathan Cowles (2007)