A brief about crab

SCIENTIFIC CLASSIFICATION
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Crustacea
Class:	Malacostraca
Order:	Decapoda
Suborder:	Pleocyemata
Infraorder:	Brachyura

Most ghost crabs have pale-colored bodies that blend in well with the sand, though they are capable of gradually changing body coloration to match their environments and the time of day. Some species are brightly colored, such as Ocypode gaudichaudii and Ocypode ryderi.

Ghost crabs have elongated and swollen eyestalks with very large corneas on the bottom half. Their carapaces are deep and box-like, squarish when viewed from the top with straight or slightly curving sides. The regions of the carapace are usually not clearly defined. The "whip" of their antennules (antennular flagella) are small or rudimentary. They fold back into the body diagonally or almost vertically. The plate between them (the interantennular septum) is broad. The third pair of mouth appendages (maxillipeds) completely cover the mouth opening. A small orifice with edges thickly fringed with hair is found between the bases of the second and third pairs of walking legs.

Ghost crab species can be most reliably identified by means of the area where they were recovered, the presence of "horns" (styles) on their eyestalks (exophthalmy), the pattern ofstridulating (sound-producing) ridges on the inside surface of the palms of their larger claws, and the shape of the gonopods in males.

Exophthalmy is exhibited by seven species in the subfamily: Ocypode brevicornis, Ocypode ceratophthalma, Ocypode gaudichaudii,Ocypode macrocera, Ocypode mortoni, Ocypode rotundata, and Ocypode saratan. All of them are found in the Indo-Pacific region with more or less limited ranges, with the exception of Ocypode ceratophthalma, which is found widely. Ocypode cursor, found in the Atlanticand the Mediterranean Sea, also possesses a tuft of bristles at the end of its eyestalks.

Stridulating ridges also differ from species to species, with some displaying rows of tubercles, others displaying rows of smaller ridges (striae), or a combination of both. These are very important in identifying species as they are displayed by both adults and juveniles (though they may be absent in newly regenerated claws). They are used by ghost crabs for communication.

Both exophthalmy and stridulating ridges, however, cannot be reliably used to determine phylogenetic relationships between different species of ghost crabs. Sakai & Türkay (2013) regarded the shape of the gonopods as more suited for this. As gonopods are important in sexual reproduction, they are less likely to evolve randomly in response to the environment. Ghost crabs of the genus Hoplocypode can be distinguished from those in Ocypode by examining their gonopods. In the former, the first gonopod has a complex, hoof-shaped tip, while in the latter they are simple and curved.