Crabs are decapod crustaceans belonging to the Brachyura infraorder, with a very short projecting “tail” (abdomen) (Greek: romanized: brachys = short, / ura = tail), which is generally concealed totally behind the thorax. They have a single pair of pincers and dwell in all of the world’s seas, freshwater, and on land. They have a robust exoskeleton and a single set of pincers. Many other species with similar names, such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs, stone crabs, and crab lice, are not real crabs, but via a process known as carcinisation, many have acquired characteristics similar to actual crabs.

Crabs have a thick exoskeleton made largely of highly mineralized chitin and are equipped with a pair of chelae (claws). Crabs range in size from the pea crab, which is only a few millimeters broad, to the Japanese spider crab, which may have a leg span of up to 4 meters (13 ft).

Environment

Crabs may be found in all of the world’s seas, as well as freshwater and land, especially in tropical areas. Freshwater crabs come in 850 different species.

Dimorphism in the sexes

Crabs have a lot of sexual dimorphism. Males have bigger claws than females, which is especially true in fiddler crabs of the genus Uca (Ocypodidae). Male fiddler crabs have one massively expanded claw that they utilize for communication, especially to attract a partner. Another noticeable variation is the shape of the pleon (abdomen), which is small and triangular in male crabs but wider and rounder in females. Because female crabs lay fertilized eggs on their pleopods, this is the case.

Crabs use chemical (pheromones), visual, auditory, and vibratory methods to attract a partner. Most completely aquatic crabs employ pheromones, but terrestrial and semiterrestrial crabs frequently use visual cues to attract females, such as fiddler crab males waving their enormous claws. Internal fertilization is common in brachyuran crabs, and they mate belly-to-belly. Mating occurs in many aquatic species shortly after the female has molted and is still soft. Females can retain sperm for a long period before fertilizing their eggs with it. When the eggs have been fertilized, they are discharged below the tail flap on the female’s belly and fastened with a sticky substance. They are protected throughout embryonic development in this place. Because the eggs resemble spherical berries, females bearing eggs are referred to as “berried.”

When the larvae have finished developing, the mother releases them into the sea, where they become part of the plankton. The tidal and light/dark diel cycles are frequently used to timing the discharge. The tiny zoea larvae may float and take advantage of water currents since they are free-swimming. They have a spine, which helps them avoid being eaten by bigger animals. Most species’ zoea must locate food, but some crabs include enough yolk in their eggs that the larval stages may survive on it.

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