Key to Family Cancridae--Cancer Crabs

Taken primarily from  Kozloff, 1987, 1996  p. 415 (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)

An alternate, more up-to-date key can be found in Wicksten, 2009.

See note regarding revisions to genus Cancer at the bottom of the page

An ancient Roman Poet, Oppian, wrote the poem Halieutica which speaks of many observations about marine species including molting in crabs.  His description of crab molting (for Cancer pagurus) can be found at the end of this page.
 
 

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1a	Tip of dactyl and fixed claw of the chelipeds dark, sometimes nearly black	2
1b	Tip of dactyl and fixed claw of the chelipeds not dark (photo)	5
2a	Dorsal surface of carapace smooth or slightly rough to the touch (like fine sandpaper), but not setose or obviously tuberculated; carapace at least 1.5 times as wide as long; width of carapace commonly exceeding 6 cm	3
2b	Much of the dorsal surface of the carapace with low tubercles, granular elevations, or seta; carapace less than 1.5 times as wide as long; width of carapace not exceeding 6 cm	4
3a	Dorsal surface of the carapace slightly rough to the touch (like fine sandpaper); propodus of chelipeds without obvious tubercles; ventral surface, in living or freshly collected specimens, with red spots; width of carapace up to 10 cm (not likely to be found north of Oregon) (Cancer antennarius in Kozloff key)	Romaleon antennarium
3b	Dorsal surface of carapace not rough to the touch, propodus of chelipeds usually with a few tubercles; ventral surface without red spots; width of carapace up to 18 cm	Cancer productus
4a	Dorsal surface of carapace obviously setose and with granular elevations; dorsal surface of carpus, propodus, and dactyl of chelipeds with spiny ridges, but without prominent tubercles; width of carapace up to 6 cm; subtidal (Cancer branneri in Kozloff key)	Romaleon branneri
4b	Dorsal surface of carapace with tubercles and sometimes also setose; dorsal surface of carpus, propodus, and dactyl of chelipeds without spiny ridges, but with prominent tubercles; width of carapace up to about 5 cm; common intertidal and subtidal species, generally nestling in holes, large barnacle shells, kelp holdfasts, etc. (Cancer oregonensis in Kozloff key)	Glebocarcinus oregonensis
5a	Carapace widest at the 10th in the series of teeth that begins lateral to the eye (photo); carpus, propodus, and dactyl of chelipeds with spiny ridges (photo); posterolateral margins of carapace without a distinct tooth just behind the tooth that marks the widest point of the carapace; width of the carapace up to 25 cm (Cancer magister in Kozloff key)	Metacarcinus magister
5b	Carapace widest at the 9th in the series of teeth that begins lateral to the eye; carpus, propodus, and dactyl of chelipeds without spiny ridges; posterolateral margins of carapace with a distinct tooth just behind the tooth that marks the widest point of the carapace; width of the carapace up to 12 cm (Cancer gracilis is Kozloff key)	Metacarcinus gracilis

Cancer antennarius
Cancer gracilis
Cancer magister
Cancer oregonensis
Cancer productus

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, Romaleon, 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).  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.

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 (Clicking on link will load a pdf of the long article)

Schram, Frederick R. and Peter K.L. Ng, 2012.  What is Cancer?  Journal of Crustacean Biology 32:4 pp. 665-672

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

All those whose body is set beneath a shell put off
the old shell and another springs up from the nether
flesh. The Pagurus, when they feel the violence of
the rending shell, rush everywhere in their desire
for food, that the separation of the slough may be
easier when they have sated themselves. But when
the sheath is rent and slips off, then at first they lie
idly stretched upon the sands, mindful neither of
food nor of aught else, thinking to be numbered with
the dead and to breathe warm breath no more, and
they tremble for their new-grown tender hide.
Afterwards they recover their spirits again and take
a little courage and eat of the sand ; but they are
weak and helpless of heart until a new shelter is
compacted about their limbs. Even as when a
physician tends a man who is laden with disease, in
the first days he keeps him from tasting food, blunting
the fierceness of his malady, and then he gives
him a little food for the sick, until he has cleared
away all his distress and his limb-devouring aches
and pains ; even so they retire, fearing for their
new-grown shells, to escape the ill fates of disease.