Petrolisthes eriomerus Stimpson, 1871

Common name(s): Flattop crab, Porcelain crab

Phylum Arthropoda
 Subphylum Crustacea
  Class Malacostraca
   Subclass Eumalacostraca
    Superorder Eucarida
     Order Decapoda
      Suborder Pleocyemata
       Infraorder Anomura
        Superfamily Galatheoidea
        Family Porcellanidae
Petrolisthes eriomerus collected from Sares Head.  Carapace width 1.3 cm
(Photo by: Dave Cowles June 26, 2005)
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.  The abdomen of Petrolisthes has uropods. Petrolisthes 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.

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.

Biology/Natural History: Filter feeds (mostly diatoms) using long setae on its second and third maxillipeds, and also uses the setal tufts (photo) on its chelipeds 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 zoea larvae have extremely long, distinctive rostrums.  As with most porcelain crabs, this species will very readily autotomize its chelae if handled.  Unlike P. cinctipes, the autotomized claw of this species can continue pinching.

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Dichotomous Keys:
  Coffin, 1952
  Hart, 1982
  Kozloff 1987, 1996
  Smith and Carlton, 1975
  Wicksten, 2009

General References:
  Brusca and Brusca, 1978
  Jensen, 2014
  Johnson and Snook, 1955
  Morris et al., 1980
  O'Clair and O'Clair, 1998
  Sept, 1999

Scientific Articles:
 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, unusual behaviors:

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
This individual, found at Swirl Rocks, clearly shows the blue coloration present on the maxillipeds and the chelae.  Photos by Dave Cowles, July 2007
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.
Blue crab Kirt Onthank
The crab above was photographed at Sares Head by Kirt Onthank, June 2021
Blue crab Alexandra Tyler-1
The crab above and below was photographed by Alexandra Tyler on Orcas Island, July 2021
Blue crab Alexandra Tyler-3

Authors and Editors of Page:
Dave Cowles (2005):  Created original page