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 ''Metacarcinus'' is a genus of crabs formerly included in the genus ''Cancer''. It includes nine exclusively fossil species and five extant species, of which four are also known from the fossil record. A molecular study using the cytochrome oxi ...

,'' recognized by WoRMS, whose

chelae A chela ()also called a claw, nipper, or pinceris a pincer (biology), pincer-like organ at the end of certain limbs of some arthropods. The name comes from Ancient Greek , through New Latin '. The plural form is chelae. Legs bearing a chela are ...

(claws) are white tipped, the other crab being ''M. magister'' (

Dungeness crab The Dungeness crab (''Metacarcinus magister'') is a species of crab inhabiting eelgrass beds and water bottoms along the west coast of North America. It typically grows to across the carapace and is a popular seafood. Its common name comes from ...

). 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 ''Phacellophora camtschatica'', commonly known as the fried egg jellyfish or egg-yolk jellyfish, is a very large jellyfish in the family Phacellophoridae. This species can be easily identified by the yellow coloration in the center of its body wh ...

''. The juvenile crabs steal food from the jellyfish and also clean off parasitic amphipods.

Morphology

Like the other members of the family

Cancridae Cancridae is a family of crabs. It comprises six extant genera, and ten exclusively fossil genera, in two subfamilies: Extant Genera Cancrinae Latreille, 1802 *'' Anatolikos'' Schweitzer & Feldmann, 2000 * ''Cancer'' Linnaeus, 1758 *''Glebocarci ...

, the slender crab has a very broad and oval

carapace A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...

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 A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element ...

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.

References

{{Taxonbar, from=Q5234072 Cancroidea Crustaceans of the eastern Pacific Ocean Crustaceans described in 1852 Taxa named by James Dwight Dana