Crustacea

Crustacea, a group of invertebrate animals belonging to the phylum Arthropoda. It includes not only large shellfish such as crabs, shrimps, and lobsters, but also smaller animals like copepods and krill that are the most numerous and important animals of the plankton. They have a hard outer skeleton or carapace, which in many species is hardened with calcium carbonate, and they have to shed this in order to grow. In crabs mating can only occur after the female has moulted and still has a soft carapace. In some species like the shore crab (Carcinus maenas) once a crab matures it no longer moults, and so can only mate once. In the spring the male common spider crabs (Maia squinado) assemble in huge piles in the centre of which are the moulting females. In contrast the edible crab (Cancer pagurus) and species of lobster (Homarus) continue to moult and grow, so mating is less of a crisis event and individual animals can attain much larger sizes. The heaviest on record is a 1.06-metre (3-ft 6-in.) long American lobster, which weighed 20.14 kilograms (44 lb 6 oz). The record for size is held by a giant spider crab (Macrocheira kaempferi) found off Japan, which had a leg span of over 3.7 metres (12 ft). Many of the larger crustaceans spend their larval stages in the plankton. The planktonic larvae of lobsters are highly transparent and leaflike, and are called phyllosoma; they spend nearly a year in the plankton during which time they may drift across an ocean.

Many types of crustacean live permanently in the plankton. One group, the copepods, make up 70% of the animals caught in nets and are probably the most numerous invertebrates on the planet. They are a key link in the food web between the phytoplankton and larger animals such as fish and the smaller seabirds. Another important planktonic group are the euphausiids. These shrimplike animals range in adult size from 1 to 3 centimetres (0.5–1.5 in.).

There are many important commercial species of crustaceans. Crabs and lobsters are trapped in baited pots and prawns are trawled from the seabed. In tropical countries sea-water ponds are carved out of mangrove swamps to make ponds in which prawns are cultured. The stocking of these ponds relies heavily on larvae from the wild populations that are introduced when the ponds are opened to the sea. Nearly 6.5 million tonnes of crustaceans are caught each year, and a nearly another million tonnes are produced by aquaculture. In all crustaceans contribute 7% to global fisheries.

M. V. Angel