Metacarcinus gracilis
The graceful rock crab or slender crab, Metacarcinus gracilis (the naming convention recognized by WoRMS) or Cancer gracilis (the naming convention recognized by ITIS), is one of only two members of the genus "Metacarcinus," recognized by WoRMS, whose chelae (claws) are white tipped, the other crab being "M. magister" (Dungeness crab). Both of these eastern Pacific crab species are recognized by ITIS as belonging to the much larger genus Cancer. "M. gracilis" has been caught from Alaska to Bahía Magdelena, Baja California. Although "M. gracilis" is only found in the Pacific Ocean, it has cousins in the Atlantic  Ocean. The genus "Cancer" ("sensu lato") apparently evolved in the Pacific Ocean and later migrated to the Atlantic Ocean. Larvae and small juveniles of this species are often seen riding jellyfish, especially "Phacellophora camtschatica". The juvenile crabs steal food from the jellyfish and also clean off parasitic amphipods. Morphology. Like the other members of the family Cancridae, the slender crab has a very broad and oval carapace with dull, tooth-like protrusions toward the front of the carapace. Female crabs can be distinguished from males by a broad tail flap on their undersides, which are used for protecting their eggs  when they are gravid. The slender crab carapace is usually olive brown, and its legs vary from yellowish brown to purple. "M. gracilis" only grows to a width of about and resembles a juvenile "M. magister". Often the two species can be confused, as "M. gracilis" looks very similar to a juvenile Dungeness crab. They can readily be distinguished from each other, though "M. gracilis" has a notch behind the widest point of its carapace that "M. magister" lacks, and the top joint of its pincers are not serrated like those of "M. magister". More obscure ways to tell are  by the appearance of hair on the posterior three legs of "M. magister" where "M. gracilis" is almost always hairless. Another telling characteristic of the slender crab is that the last segment of its tail flap is pointed, where "M. magister" is curved.