Crabs 1, 2

Spider crab, Mithrax spinosissimus

Spider crab

The Spider crab, above, is also known as the West Indian spider crab, the Channel clinging crab, the Roof crab, and the Coral crab. It is found off Florida and in the Caribbean from 3 m deep down to about 40 m in caves and beneath overhangs. It is active mainly at night, and shelters during the day.

The Spider crab has a tough exoskeleton to deter predation by octopus, fish and other crabs. Its carapace can be up to 18 cm wide and is almost circular with spines all over its upper surface. Its leg span can reach 60 cm. It can weight up to 2 kg. The colour is red/brown with purple.grey claws.

The coconut crab, Birgus latro

Birgus latro, the coconut crab

Above is the Coconut crab, a terrestrial hermit crab also known as the robber crab and Palm thief. It is the largest terrestrial arthropod and can weigh up to 4.1 kg and have a leg span of 1 m. It is found on the islands in the Indian and Pacific oceans. Although it is a hermit crab adults do not need to find a shell to give them protection as their exoskeleton has hardened enough. However juveniles do behave like other hermit crabs and inhabit a shell of a dead animal.

Coconut crabs cannot swim, in fact if they are submerged for more than an hour they would drown, but as with all crabs the females must return to water to release their eggs. The mating is done on land (usually from May to september). The male deposits a spermatophore on the female's abdomen. The female carries her eggs (from 50,000 - 138,000 per spawn) around with her for a few months attached to the underside of her body, and only when the eggs are about to hatch does she return to the ocean. She will wait until it is high tide to go down to the ocean to release the hatching larvae into the water. The larvae pass through various stages in the ocean, and during this time many are eaten. However those that survive to moult into juvenile crabs and find their way to the shore and a shell. Eventually they leave the water permanently. They are solitary animals, and tend to stay in a burrow or crevice during the day to prevent water loss, emerging at night to feed.

It takes around 5 years for a coconut crab to be sexually mature, and they can live as long as 60 years. They feed on nuts, seeds, but will also eat dead animals, and almost anything they can find. Although it is possible for a large individual or a group to open up a coconut, coconuts do not form a large part of their diet normally.

Even after reaching adulthood the crabs still moult annually. During this time they are very vulnerable and so they dig a burrow, and moult in it. Then they wait for another 1 - 3 weeks for the exoskeleton to harden before emerging.

It is thought that coconut crabs may have eaten the remains and dispersed the bones of Amelia Earhart and her navigator after they had been forced to land on an uninhabited island.

The Box crab, Cryptopodia fornicata (also known as the Elbow crab, Domed elbow crab) is below, can be found around Japan, India, the Australian coast and the Persian Gulf in sea grass beds and muddy or sandy bottoms where it can dig in.

box crab, Cryptopodia fornicata

Its carapace completely hides its thin legs. It has tiny eyes. It burrows into the sand using its hugely flattened front claws. It eats snails and its pincers are strong enough to break the snail shell.

Below is Neosarmatium meinerti, found in mangrove swamps of the shores around the Indian Ocean. Although these crabs are omnivores, their main food consists of leaves fallen on the mangrove swamp surface. They are therefore valued for their recycling abilities.

Neosarmatium meinerti, mangrove swamp crab

Scylla serrata, the Mangrove crab

Mangrove crab, Scylla serrata

The Mangrove crab, above, also known as the Mud crab and Black crab, is found in estuaries and mangrove swamps of Africa, Asia and Australia. Their flesh is tasty and their growth rate is high making them an economically important species. Since the 1970s they have been reared commercially at low densities in ponds with free swimming shrimp and fish, each crab in its own plastic container as they can be cannibalistic. In the wild they dig deep burrows emerging at night to feed. Adults and juveniles eat mainly molluscs and small crabs.

The shell colour ranges from deep green to dark brown. A fully grown adult carapace can reach 24 cm across the widest part, and weight can reach 3.5 kg. It has paddle-like back legs that help it swim. Females can release 6 million eggs at a single spawn, and produce three batches of eggs from each mating. It is cannibalistic on moulting individuals of the same species.