Crustacea (crabs, shrimps, woodlice, water fleas, cyclops, barnacles, etc.)

Crustacea overview

There are about 45 000 described Crustaceans, but it is thought that many more await description. The body plan varies considerably and in some the head and some thoracic segments have become fused into a cepalothorax, and in others the carapace covers most of the body.

The cuticle is mainly composed of calcareous material, with some chitin and protein, and this no doubt restricts their colonisation of water with a low pH.

There are many leg modifications, e.g. walking legs, paddles, food collection, and claspers for mating. Although some are terrestrial, e.g. land crabs, all except the woodlice must return to the water to breed.

In the smaller aquatic crustaceans there is a tendency for gaseous exchange to take place over the entire body.

They are separated into nine Classes. On this page (below) is the Malocostraca, the largest class, see the table above left for links to the other classes.

Chromatophores.

These are pigment containing cells, and can be quite large. However the pigment can be concentrated in a tiny area so small as to be invisible, or dispersed throughout the cell making it visible. The cells usually have irregularly shaped sides so that they fit together like a jig-saw making a number of colour combinations possible that allow them to change colour rapidly, it is thought that colour change is one form of communication.

Class Malacostraca

This is the largest Crustacean class with about 29,000 species described so far. There is great diversity of body form. It includes all the Crustaceans consumed by humans, e.g. crabs, (see the coconut crab on the left and the edible crab on the right) shrimps and lobsters; as well as the only truly terrestrial Order, the Isopoda (woodlice).

Many in this Class have a fore gut where food is ground into particles. Most have well-developed compound eyes.

In crabs and lobsters the cuticle is stiffened by calcification, i.e. the deposition of calcium carbonate.

The edible crab

The edible crab is commonly found in rock pools when the tide goes out. It is not a very good swimmer. Note that in the ventral view, the tail is curled under the body.

Macrocheira kaempferi, the giant spider crab, Japanese spider crab

On the right is Macrocheira kaempferi, the giant spider crab, Japanese spider crab. The leg span can reach four metres, but the body diameter is usually less than 40 cm. It can weigh as much as 20 kg, and is found in the Pacific Ocean around Japan, usually between 300 - 400 metres deep. It eats carrion and shellfish. Its life span is thought to be as much as 100 years. Eggs are laid in spring in water around 50 metres deep. The crab is a delicacy in Japan and is eaten both raw and cooked - it is delicious.