Crabs

Scientific name : Brachyura

Crabs belong to the phylum Crustacea, class Malacostraca, order Decapoda.
Like all the animals in their phylum, they have a hard shell named exoskeleton covering and protecting the whole body. The legs are articulated , very mobile and efficient.
Like shrimps, also crab have a body divided into 2 parts: a cephalothorax (1 - head + chest), covered with a hard shell (carapace), from where the 5 main pairs of legs originate ( 4 pairs of legs and 1 pair of claws - 2), and an abdomen, small and folded under the cephalothorax.
Due to the legs position, for many crabs the easy way to move is a side walk, in any direction (Many species typically make a 180° rotation every now and then, to ehchange the pushing and pulling legs).
The head has many sense organs (3): compound eyes, spherical and stalked, that can see all around, and sensorial antennae (touch, smell).
Spines along the edge of the carapace (4) and eventual rostrum can be used for species identification. Male's abdomen has a triangular shape, in the female it is larger and rounded. The eggs (5) are incubated attached to the abdomen, visible from outside.
There are crabs in every sea environment, with many species. Often they go unnoticed, because of their small size, or because they have cryptic colour patterns, or because they live, perfectly camouflaged, in association with other animal species. To be able to observe crustaceans it is necessary to look actively for them. In each species pages (ecology and associations) we give information about where to look for them.
Most crabs have nocturnal habits. At night many of their more dangerous predators (mainly fishes) sleep, and the time is right for looking for food or for a mate.
Crustaceans have all the possible feeding habits. We can observe herbivorous species, carnivorous, plankton feeders and detritus feeders.
Reproduction is by internal fecundation. In many crab species mating is possible only at the moment of the mould. When the males identifies a female about to mould (from smell), hugs her among his claws and keep her with him, waiting for the right time.
The female carries the fecunded eggs until hatching. Small planktonic larvae come out from the eggs, and they will become small crabs only after many metamorphosis.
Crabs are distinguished by small abdomen folded under the carapace from shrimps (shelled long abdomen, ending in a tail) and from hermit crabs or Anomura (with unshelled abdomen, usually protected into an empty snail shell). Two groups of anomura, the squat lobsters and the porcelain crabs, use no shell and looks symilar to crabs. They are identified by the longer shape, antennae, and by the claws, extended forward. The position of antennae and antennulae, originating before the eyes, distinguish crabs from hermit crabs.
The crustacean are the first animal group (in evolutionary terms) to develop true legs, very effective for walking, for defense and for handling food. This can explain their evolutionary success. The reciprocal position of muscles (internal) and skeleton (external) is opposite to vertebrate, but not less efficient.
An external skeleton, on the other side, has an obvious disadvantage: being rigid, prevent the animal from growing. Growing is possible only abandoning the old shell and changing it, through the mould.
The curstacean leaves the old shell with a new one already on, still soft and extensible. Then it inflates, absorbing water, to extend the shell. At the beginning it will wear an over-size suit, that will fill gradually.

Associated organisms
Many species can establish associations with hard and soft corals, sponges, echinoderms, mollusks.