Crabs
Crabs typically walk sideways (a behaviour which gives us the word crabwise). This is because of the articulation of the legs which makes a sidelong gait more efficient. However, some crabs prefer to walk forwards or backwards, including raninids, Libinia emarginata and Mictyris platycheles. Some crabs, notably the Portunidae and Matutidae, are also capable of swimming.
Crabs are mostly active animals with complex
behavior patterns. They can communicate by drumming or waving their pincers. Crabs tend to be aggressive towards one another and males often fight to gain access to females. On rocky
seashores, where nearly all caves and crevices are occupied, crabs may also fight over hiding holes.
Crabs are omnivores, feeding primarily on algae, and taking any other food, including molluscs, worms, other crustaceans, fungi, bacteria and detritus, depending on their availability and the crab species. For many crabs, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness.
Crabs are known to work together to provide food and protection for their family, and during mating season to find a comfortable spot for the female to release her eggs.
The infraorder Brachyura contains 6,793 species in 93 families, as many as the remainder of the Decapoda. The evolution of crabs is characterised by an increasingly robust body, and a reduction in the abdomen. Although many other groups have undergone similar processes, carcinisation 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.
In most decapods, 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. 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