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 two members of the genus '' Metacarcinus,'' with white tipped

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

(claws). The second crab in the genus to have white tipped claws is ''M. magister'' (

Dungeness crab The Dungeness crab (''Metacarcinus magister'') makes up one of the most important seafood industries along the west coast of North America. Its typical range extends from Alaska's Aleutian Islands to Point Conception, near Santa Barbara, Califo ...

). 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 Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...

Bahía Magdalena Magdalena Bay () is a long bay in Comondú Municipality along the western coast of the Mexican state of Baja California Sur. It is protected from the Pacific Ocean by the unpopulated sandy barrier islands of Isla Magdalena and Isla Santa Marg ...

Baja California Sur Baja California Sur, officially the Free and Sovereign State of Baja California Sur, is a state in Mexico. It is the 31st and last state to be admitted, in 1974. It is also the second least populated Mexican state and the ninth-largest state by ...

. Although ''M. gracilis'' is only found in the Pacific Ocean, it has cousins in the Atlantic Ocean. The genus ''Cancer'' (''

sensu lato ''Sensu'' is a Latin word meaning "in the sense of". It is used in a number of fields including biology, geology, linguistics, semiotics, and law. Commonly it refers to how strictly or loosely an expression is used in describing any particular co ...

'') 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 Amphipoda () is an order (biology), order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods () range in size from and are mostly detritivores or scavengers. There are more than 10,700 amphip ...

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 *'' Glebocarc ...

, the slender crab has a very broad and oval

carapace A carapace is a 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 tortoises, the unde ...

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 cap ...

vary from yellowish brown to purple. ''M. gracilis'' only grows to a width of about and resembles a juvenile '' M. magister (Dungeness Crab)''. 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