Crab general biology

[Crab Biology]

Anatomy
Although crabs come in a variety of shapes and sizes they all have the same general body plan. All crabs have one pair of chelipeds and four pairs of walking legs. Also referred to as claws, nippers or pincers, the chelipeds are the first pair of legs on a crab and their most distinguishing structure. Chelipeds are used for holding and carrying food, digging, cracking open shells and warning off would be attackers. The carapace is the hard cover or exoskeleton which protects the internal organs of the head, thorax and gills.
[crab drawing 2] Visible on the underside of a crab are the mouthparts and the abdomen. The gills through which the animal obtains oxygen cannot be seen. They are soft structures under the side of the carapace. The eyes which protrude from the front of the carapace are on the ends of short stalks. The mouthparts are a series of pairs of short legs, specialised to manipulate and chew food.
[crab drawing 3] [crab drawing 4]
The abdomen is small and tightly held against the underside of the body. Like all crustaceans the sexes are separate and the size of the abdomen distinguishes them: in males it is triangular and inset into the underside. In females it is broad and round and most obvious when the eggs are being carried.

Reproduction
Female crabs usually lay their eggs shortly after copulating but can also store sperm for many months. The eggs are fertilized as they are laid by passing through the chamber holding the sperm. Eggs are brooded in a mass attached to hair on the female's abdomen. The number of eggs carried can be very large but depends on the size of the crab. Some species may carry tens of thousands of eggs when fully grown. Once developed the egg hatches into a tiny larva called a zoea. Release of the zoea is aided by the female wafting her abdomen to and fro. The crab larvae spends its life swimming in the plankton moulting several times until it reaches a stage ready to settle on the sea floor again.

Sense Organs
Crabs have compound eyes consisting of several thousand optical units. The eyes are on stalks which can be lowered for protection into sockets on the carapace. Crabs appear to see very well with some species detecting movement 20 or 30 metres away. Crabs can also hear and produce a variety of sounds. In courtship some species attract the females attention by banging their cheliped on the ground or vibrating their walking legs. Each species has its own unique sound that can attract a female or intimidate a competing male. Crabs have bristles and hairs which act as touch receptors. The bristles occur all over the body but are most frequently found in clumps on the walking legs. These bristles signal contact with a hard surface simply by bending, whilst other shorter hairs are sensitive to water currents. Crabs can find food using chemical stimuli. The antennae have "smell detectors" which detect chemicals that stimulate a search for food. When similar detectors on the legs contact food the cheliped quickly grasps the object and passes it into the mouth. Crab mouthparts have further receptors which are sensitive to particular chemicals. Crabs rely on a combination of these sense organs to find food and mates and flee predators.

Distribution
There are more than 50 families of crabs and 1000s of species wordwide. Most are marine, occuring in all oceans from the edge of the sea down to the greatest depths of the ocean. A few species live in fresh water. The greatest diversity of crabs is in tropical seas and in southern Australia more than 100 species are known.

Home

Main menu

Crab Gallery