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Marine invertebrates

Crabs

Common shore crab and hermit crab in a rock poolCommon shore crab and hermit crab in a rock pool Crabs are crustaceans, the same group to which lobsters, shrimp and and barnacles belong. Crabs have round, flat bodies with a shell, known as a carapace, four pairs of jointed legs, two pairs of sense receptors, called antennae, on their heads, and a pair of pinching claws that are used both for feeding and defence. They have thick external skeletons, called exoskeletons. Most crabs live on the seabed, but some live in sandy burrows on the seashore or on land. Crabs feed mostly on algae, molluscs, worms and other crustaceans, but some also eat the rotting bodies of dead animals and plants.

Blue crabBlue crab 
Baby crabs on a beach in Christmas Island Baby crabs on a beach in Christmas Island

Red crabs

The red crabs of Christmas Island in Oceania usually live in the island's forests, but they still return to the sea to mate each November. The crabs face a number of obstacles along the way, crossing roads and even scrambling through houses. Males reach the beaches first and dig burrows in the sand, ready to mate with females. Females stay in the burrows while their eggs develop. When ready, they crawl to the sea at high tide and shake the eggs into the water. About 25 days later, the tiny crabs, just 5 millimetres (0.2 inches) across, crawl out of the sea and head inland.
 
 

There are nearly 7000 species of crab.

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