Evolution and classification

The face of a
dungeness crab. The two
eyes sit on eyestalks, with two
antennules on either side of the
rostrum (center, above the mouth).
The infraorder Brachyura contains about 93
families[4], as many as the remainder of the
Decapoda.
[5] The evolution of crabs is characterised by an increasing robustness of the body, and a reduction in the abdomen. Although other groups have also undergone similar processes of
carcinisation, it is most advanced in crabs. The
telson is no longer functional in crabs, and the
uropods are absent, having probably evolved into small devices for holding the reduced abdomen tight against the sternum.
[6]

A porcelain crab nestled in its sea pen host both waiting to capture floating food items at night. Found on the North coast of
Timor-Leste.
In most decapodes, the
gonopores (sexual openings) are found on the legs. However, since crabs use the first two pairs of
pleopods (abdominal appendages) for
sperm transfer, this arrangement has changed. As the male abdomen evolved into a narrower shape, the gonopores have moved towards the midline, away from the legs, and onto the
sternum.
[7] A similar change occurred, independently, with the female gonopores. The movement of the female gonopore to the sternum defines the
clade Eubrachyura, and the later change in the position of the male gonopore defines the
Thoracotremata. It is still a subject of debate whether those crabs where the female, but not male, gonopores are situated on the sternum form a
monophyletic group.
[5]
The earliest unambiguous crab
fossils date from the
Jurassic, although the
Carboniferous Imocaris, known only from its
carapace is thought to be a primitive crab.
[8] The
radiation of crabs in the
Cretaceous and afterwards may be linked either to the break-up of
Gondwana or to the concurrent radiation of
bony fish, the main
predators of crabs.
[9]
About 850 species
[10] of crab are freshwater or (semi-)terrestrial species; they are found throughout the world's
tropical and
semi-tropical regions. They were previously thought to be a closely related group, but are now believed to represent at least two distinct
lineages, one in the
Old World and one in the
New World.
[11]