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The Crab
Said to be closely related to shrimps, crabs can be described as decapod crustaceans of the infra-order Brachyura which typically have a very short projecting tail where their reduced abdomen is entirely hidden under the thorax.
They are generally covered with a thick exoskeleton (also called a carapace) and are armed with a single pair of chelae (claws).
Over 6,700 species of crab are known to man and can be found in all of the world’s oceans. Additionally, there are many freshwater and terrestrial crabs, particularly in tropical regions.
Researchers have claimed that about 850 species of crab are freshwater or terrestrial species and they are found throughout the world’s tropical and semi-tropical regions.
They were in fact previously thought to be a closely related group but are now believed to represent at least two distinct lineages.
However crabs vary in size from the pea crab which is only a few millimetres wide, to other species like the Japanese spider crab (the largest known arthropod) that have a leg span of about four metres (13 ft). They are said to live on the floor of the North Pacific Ocean.
The biggest land crab is the Coconut crab with a leg span of up to 75 centimetres and they are found mostly on islands in the Pacific Ocean.
As invertebrates or crustaceans without a backbone, Crabs are able to protect themselves from predators with their carapace, which is in fact an outer shell that also provides them with support. These decapod crustaceans or 10-legged omnivores are able to walk only sideways.
The front pair of legs are specialised with enlarged claws, called pincers or chelipeds so they are commonly described as eight-legged. They have a flattened body, two feelers (antennae) and two eyes located at the ends of stalks.
Marine crabs breathe underwater using gills which are located in two cavities under the carapace while land crabs have enlarged modified cavities that act like lungs so that they can breathe air.
But although crabs can be found 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.
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.
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.
Some crabs feed primarily on algae and others consume any type of food including molluscs, worms, other crustaceans, fungi, bacteria and detritus, depending on their availability and the crab species.
Many crabs have a mixed diet of plant and animal matter which results in them having fast growth and great fitness. Their 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, and in the females, it is broad and round and most obvious when eggs are being carried.
Female crabs usually lay their eggs shortly after copulating but can also store sperm for many months. The eggs are fertilised 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 larva spends its life swimming in the plankton moulting several times until it reaches a stage ready to settle on the sea floor again.
However, while the life of crabs is said to be very intriguing, they make up 20 per cent of all marine crustaceans that are caught and farmed worldwide, with over half of a million tonnes being consumed annually.
(Source: The Wikepedia Free Online Encyclopedia)
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