Crab

Crabs are  which typically have a very short projecting "tail" (abdomen)  usually entirely hidden under the thorax. They live in all the world's oceans, in fresh water, and on land, are generally covered with a thick exoskeleton and have a single pair of claws. Many other animals with similar names – such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice – are not true crabs.Crabs are generally covered with a thick exoskeleton, composed primarily of calcium carbonate, and armed with a single pair of chelae (claws). Crabs are found in all of the world's oceans, while many crabs live in fresh water and on land, particularly in tropical regions. Crabs vary in size from the pea crab, a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 metres (13 ft).

About 850 species of crab are freshwater, terrestrial or semi-terrestrial species; they are found throughout the world's tropical and semi-tropical regions. They were previously thought to be a monophyletic group, but are now believed to represent at least two distinct lineages, one in the Old World and one in the New World.The underside of a male (top) and a female (bottom) individual of Pachygrapsus marmoratus, showing the difference in shape of the abdomen.Crabs often show marked sexual dimorphism. Males often have larger claws.

Crabs attract a mate through chemical (pheromones), visual, acoustic or vibratory means. Pheromones are used by most fully aquatic crabs, while terrestrial and semi-terrestrial crabs often use visual signals, such as fiddler crab males waving their large claw to attract females. The vast number of brachyuran crabs have internal fertilisation and mate belly-to-belly. For many aquatic species, mating takes place just after the female has moulted and is still soft. Females can store the sperm for a long time before using it to fertilise their eggs. When fertilisation has taken place, the eggs are released onto the female's abdomen, below the tail flap, secured with a sticky material. In this location they are protected during embryonic development. Females carrying eggs are called "berried" since the eggs resemble round berries.

When development is complete, the female releases the newly 

hatched larvae into the water, where they are part of the plankton. The release is often timed with the tides. The free-swimming tiny zoea larvae can float and take advantage of water currents. They have a spine, which probably reduces the rate of predation by larger animals. The zoea of most species must find food, but some crabs provide enough yolk in the eggs that the larval stages can continue to live off the yolk.