Metacarcinus gracilis (Dana, 1852) Schweitzer and Feldmann, 2000

Common name(s): Graceful rock crab, Graceful crab, Slender crab

Synonyms:  Cancer gracilis
Phylum Arthropoda 
 Subphylum Crustacea 
  Class Malacostraca 
   Subclass Eumalacostraca 
    Superorder Eucarida 
     Order Decapoda 
      Suborder Pleocyemata 
       Infraorder Brachyura
        Superfamily Cancroidea 
        Family Cancridae
Metacarcinus gracilis, about 5 cm carapace width, from 100 m depth, San Juan Channel
(Photo by: Dave Cowles, July 2006)
Description:  As with all Cancer crabs, this species has 5 teeth between the eyes.  Its carapace is widest at the 9th tooth lateral to the eyes, and behind this point there is a distinct 10th tooth.  The tips of the chelae are white (photo), and there are no spiny ridges on the carpus, propodus, and dactyl of the chelae (although there are two blunt teeth on the dorsal ridge of the propodus) (photo).  The carapace is slightly convex dorsally.  The dactyls of the pereopods (walking legs) are cylindrical.  The dorsal surface of the upper part of the legs is purple in many individuals.  Carapace width to 11.5 cm (males) or 10.6 cm (females) but usually not over 6 cm.

How to Distinguish from Similar Species:Metacarcinus magister has a carapace widest at the 10th tooth, with no teeth behind that point, and also has spiny ridges on the carpus, propodus, and dactyl of the chela.  It also has more flattened dactyls on the pereopods and the dorsal surface of the upper parts of the legs is not purple.

Note:  Species formerly in genus Cancer have been recently subdivided into several genera (Ng et al., 2008; Schweitzer and Feldmann, 2010).  Of our local genera, Cancer and Metacarcinus have a carapace wider than long plus only scattered setae on the carapace margins and legs while Glebocarcinus has a carapace of approximately equal length and width, often with granular regions and with setae along the edges; and setae on the outer surface of the chela as well as on the legs. Metacarcinus can be distinguished from Cancer because Metacarcinus has anterolateral carapace teeth which are distinct and sharp plus the male has a rounded tip to the telson, while Cancer has anterolateral carapace teeth which are low and lobed, separated by deep fissures plus the male has a sharply pointed telson (Schram and Ng, 2012).

Geographical Range:  Prince William Sound, Alaska to Baja California, Mexico

Depth Range:  Intertidal to 174 m

Habitat:  Mainly subtidal on sand and mud, sometimes near eelgrass beds.  May be on pilings.

Biology/Natural History:  This species is a scavenger, or eats small invertebrates.  Predators include staghorn sculpin, starry flounder, the seastar Astropecten verrelli, and the giant octopus Enteroctopus dofleini.  Females usually are found buried in the mud.  Seasonally found in bays but cannot osmoregulate and does not tolerate brackish conditions.  In Puget Sound eggs were borne from December to April.  Males protect females after mating.  Megalopae and juveniles often cling to large jellyfish such as Pelagia colorata.

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Dichotomous Keys:
  Coffin, 1952
  Flora and Fairbanks, 1966
  Hart, 1982
  Kozloff 1987, 1996
  Smith and Carlton, 1975
  Wicksten, 2009

General References:
  Gotshall, 1994
  Harbo, 1999
  Jensen, 1995
  Johnson and Snook, 1955
  Kozloff, 1993
  Morris et al., 1980
  Niesen, 1997
  O'Clair and O'Clair, 1998

Scientific Articles:
 Ng, P.K.L., D. Guinot, and P.J.F. Davie, 2008.  Systema Brachyurorum: part I.  An annotated checklist of extant brachyuran crabs of the world.  Raffles Bulletin of Zoology, Supplement 17 pp. 1-286

Schweitzer, C.F. and R.M. Feldmann, 2000.  Re-evaluation of the Cancridae Latereille, 1802 (Decapoda: Brachyura) including three new genera and three new species.  Contributions to Zoology 69:4 pp. 223-250

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:

The tips of the chelae are white.  There are no spiny ridges on the carpus, propodus, or dactyl of the chelae, though there are two spines or tubercles on the dorsal ridge of the dactyl.
The appendages seen covering the mouth are the maxillipeds, which are found in all crabs.  The white objects between the maxillipeds are the hardened tips of the mandibles.
This female C. gracilis, 8.5 cm carapace width, was found at March Point.  She appears to be gravid (carrying eggs).  The eggs, however, are much smaller and dirtier-looking than crab eggs usually are, leading me to suspect that she may be parasitized by a rhizocephalan sacculinid barnacle.  However,  the "eggs" did not form a sac-like structure as usually seen in rhizocephalans. Raising the abdomen shows the dark gray and black mass of eggs.  The female was docile and made little attempt to escape or to pinch. A view through the microscope shows the mass is composed of a large number of tiny, slightly flattened, egg-like objects attached to her pleopods.  Note the tip of her chela at the top right for size.  I will have to watch for more gravid females to see whether these tiny, dirty-looking eggs are the norm for Cancer gracilis.  Photos by Dave Cowles July 2007

This view of a Padilla Bay crab shows the purple legs seen in many individuals.  Photo by Dave Cowles July 2008

From Seawater Tank 2016
This squeak-clean individual I found in the Rosario Seawater System reservoir tank before starting up the seawater system in June 2016.  To get there, the crab would have had to pass through the intake filter (maximum clearance 1 mm) the previous summer, be pumped up the hill to the reservoit, remain in the tank 5-10 cm of water in the bottom of the tank while the rest of the water drained out at the end of the summer, and then survive and grow to this size in the perhaps 1-2 cm deep coating of silt on the bottom of the tank in total darkness  for over 9 months until we opened the seawater system again.  No doubt the crab is so clean because it has never had to deal with algal overgrowth in the darkness. Carapace width is 4 cm.

Authors and Editors of Page:
Dave Cowles (2006):  Created original page