Romaleon antennarium (Stimpson), Schweitzer and Feldmann, 2000

Common name(s): Pacific rock crab, Common rock crab, Spot-bellied rock crab, Rock crab, Red rock crab

Synonyms:  Cancer antennarius Male from Hole In Wall, Rialto Beach
Phylum Arthropoda 
 Subphylum Crustacea 
  Class Malacostraca 
   Subclass Eumalacostraca 
    Superorder Eucarida 
     Order Decapoda 
      Suborder Pleocyemata 
       Infraorder Brachyura
        Superfamily Cancroidea 
        Family Cancridae
A male Romaleon antennarium, approximately 10 cm carapace width, from Hole in the Wall, Rialto Beach, WA.
(Photo by: Dave Cowles July 2021)
Description:  This large Cancer crab has a dark tip to its claws (photo).  The dorsal surface of the carapace is nearly smooth or only slightly rough, and does not have rough tubercles or setae.  The carapace is at least 1.5 times as wide as long and frequently exceeds 6 cm.  The ventral surface of living individuals has red spots on a pale yellow background (photo), and the dorsal surface is purplish red. The undersurface of the carapace also has setae (photo), as does the dorsal surface of young ones (but not adults).  Carapace usually up to about 13 cm wide but may be up to 15 cm. The carapace has 11 anterolateral teeth and is widest at the 8th tooth. Some of the teeth curve forward.

How to Distinguish from Similar Species:  This is one of the two large local Cancer crabs with black tips on their chelae.  The other, Cancer productus, has a rough dorsal surface on its carapace and a few tubercles and has no ventral red spots.  Its carapace is also produced (projected) forward between the eyes.  Other Cancer crabs with black chelae, such as Glebocarcinus oregonensis, have a tuberculate carapace which is usually less than 6 cm wide and is not 1.5x or more as wide as long.

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, CancerRomaleon, 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, lobed, and separated by deep fissures plus the male has a sharply pointed telson (Schram and Ng, 2012). Romaleon can be distinguished from Cancer and Metacarcinus because it has a distinct tooth on the anterior third of the posterolateral margin of the carapace while the other two genera do not.

Geographical Range:  British Columbia to Cabo San Lucas, Baja California but rarely seen north of Coos Bay, Oregon. (But common in Barkley Sound on Vancouver Island)

Depth Range:  Intertidal to 91 m, usually less than 45 m.

Habitat:  Common in the low rocky intertidal.  Often found under rocks, sometimes partly buried in sand under the rocks.  Subtidally may be on gravel bottoms or in kelp beds.  Especially common on the exposed coast.

Biology/Natural History:  Food includes scavenged bits and animals such as Chlorostoma funebralis and hermit crabs.  It captures the hermit crabs by gradually chipping away the edges of the hermit's shell until the hermit crab has nowhere else to hide.  Sometimes harvested by humans for crab legs.  In California, become mature in about 2 years.  Berried (egg-carrying) females are most often seen November to January.  The young have vivid color patterns on the carapace similar to those of C. productus young.  Most active at night.

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Dichotomous Keys:
  Kozloff 1987, 1996
  Smith and Carlton, 1975
  Wicksten, 2009

General References:
  Brandon and Rokop, 1985
  Hinton, 1987
  Jensen, 1995
  Johnson and Snook, 1955
  Kozloff, 1993
  McConnaughey and McConnaughey, 1985
  Morris et al., 1980
  Niesen, 1994
  Niesen, 1997
  Ricketts et al., 1985

Scientific Articles:

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:
Although this species is rarely found north of Coos Bay, OR; in July 2021 we found multiple individuals, including reproducing adults, at Hole in the Wall, Rialto Beach, WA, as shown in the photos below. Notice the reddish-purple spots on the underside of the carapace that are characteristic of both males and females of this species:
This female, about 8 cm in carapace width, is heavily gravid ("berried") with eggs. She was partly buried in the sand among rocks. Photos by Dave Cowles, July 2021
Gravid female dorsal view
Gravid female
Gravid female
This male, about 10 cm carapace width, also appears to be of reproductive age. Photos by Dave Cowles, July 2021
Male dorsal
Male underside
Mating pair
This photo, by Maggie Justice and Josie Parks, shows a mating pair at Hole in the Wall, Rialto Beach. Mating takes place at the time of the female's molt, when her gonopores are soft and open. Males detect females ready to molt and pick them up, holding them underneath themselves. They protect and also mate with them during their vulnerable molting process. They continue to hold them until the female's new exoskeleton has hardened. The growths present on this female's carapace suggest that she has not yet molted.


A preserved Romaleon antennarium

The claws of Romaleon antennarium are tipped in black.  Photos of a preserved specimen from California by Dave Cowles, 2008

Authors and Editors of Page:
Dave Cowles (2007):  Created original page