Description: Porcelain crabs are Anomuran
crabs, as can be seen by the antennae being lateral to the eyes, the reduced
last leg (picture), and the fact that the abdomen
is not held tightly against the underside of the thorax.
of Petrolisthes has uropods.
eriomerus has a carpus
about 2x as long as wide and with parallel anterior and posterior margins
(photo). The outer edge of the palp
of the maxilliped
is bright blue, and there is also blue on the cheliped.
Overall color brown to gray-blue. Carapace length up to 1.9 cm, with
roughness and granulations on the anterior part. Chelae
are usually nearly equal in size and strongly flattened. Antennal
flagellum is grayish green.
Petrolisthes eriomerus Stimpson, 1871
Common name(s): Flattop crab, Porcelain crab
|Petrolisthes eriomerus collected from Sares Head. Carapace
width 1.3 cm
|(Photo by: Dave Cowles June 26, 2005)
How to Distinguish from Similar Species: Petrolisthes
cinctipes has a shorter carpus
and the margins are not parallel, plus the palp
of its maxilliped
is orange red. Pachycheles crabs such as Pachycheles
rudis have much thicker chelae
Geographical Range: Chicagof Island, Alaska
to La Jolla, CA
Depth Range: Low intertidal to 86 m; primarily
intertidal from central CA southward.
Habitat: Under rocks, on both exposed coasts
and protected water. Also on kelp holdfasts and in mussel beds.
Most common in areas with strong currents.
Filter feeds (mostly
diatoms) using long setae
on its second and third maxillipeds,
and also uses the setal
tufts (photo) on its
to sweep up material from rock surfaces. Crabs of this species
sometimes live together in groups of males, females, and young; with several
dominant males doing most of the breeding. Females often have two
broods per year. Has little resistance to desiccation. Petrolisthes
larvae have extremely long, distinctive rostrums.
As with most porcelain crabs, this species will very readily autotomize
if handled. Unlike P. cinctipes,
claw of this species can continue pinching.
and Carlton, 1975
and Brusca, 1978
and Snook, 1955
et al., 1980
and O'Clair, 1998
Russell, Robert M., Jr., 1961. Laboratory culture and developmental
stages of Petrolisthes eriomerus
Stimpson. Master's thesis, Walla Walla College. 35 pp.
General Notes and Observations: Locations, abundances,
This species is moderately common in the lower intertidal of Sares Head
Porcelain crabs, unlike true crabs, have uropods
on their abdomens.
The undersized fifth leg is visible on porcelain crabs, folded up and
over the 3rd and 4th legs.
The tufts of setae
present on the chelipeds
can be seen when the animal is underwater. Notice also
the blue on the chelipeds.
Photo by Dave Cowles, June 2005
Sometimes this crab species appears a distinct blue color. Below are some
photos of blue crabs found in summer 2021. Gregory
Jensen (2014) suggests on page 218 of his excellent reference that
this blue color may be brought out in places on the exoskeleton which are
frequently abraded on other objects such as mussels.
|This individual, found at Swirl Rocks, clearly shows the
blue coloration present on the maxillipeds
and the chelae.
Photos by Dave Cowles, July 2007
|The crab above was photographed at Sares Head by Kirt Onthank, June
|The crab above and below was photographed by Alexandra Tyler on Orcas
Island, July 2021
Authors and Editors of Page:
Dave Cowles (2005): Created original page