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

Crustacea characteristics and fast facts

Bilaterally symmetrical
Calcareous exoskeleton
The body plan varies but generally a head, leg bearing segments, a trunk with a variable number of segments, and a terminal telson
A mouth, through gut and anus
Appendages (legs, antennae, etc.) mainly biramous (two branches).
Two pairs of antennae
Simple and compound eyes
Gaseous exchange by gills or across the body surface
Sexes are usually separate, but some are hermaphrodite
Females may have brood pouches
Mainly marine, but some freshwater and a few terrestrial
Hugh size range from tiny water fleas just 0.25 mm long to crabs with a body width of 300 mm and a leg span of 3600 mm!

Crustacea overview

There are about 52,000 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 cephalothorax; 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 colonization 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 (see the menu above). On this page is the Malocostraca; the largest class.

Humans eat more Crustacea than any other invertebrate Phylum. It is often the remains of these meals, the middens, that indicate sites of past human habitation.


Chromatophores 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 the animal to change colour rapidly. It is thought that colour change is one form of communication.

Class Malacostraca

Birus latro, coconut crab, robber crab

The Malacostraca 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 above right 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.

Birgus latro, the robber crab or coconut crab

The coconut crab (above right) can be up to 1 m in length and is the largest land crab. Its claws are strong enough to open the husk and shell of coconuts so that it can feed off the flesh. And it is said that larger species can move rock up 25 kg in weight.

It is found in the tropical islands of the South Pacific. It was once common but is now rare as it is regarded as a delicacy. Adults moult annually, and juveniles more often. It is solitary, nocturnal, and it cannot swim.

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.

edible crab dorsal
edible crab ventral

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.

Many spider crabs attach seaweed and sponges to their body so that they blend into the background.

Macrocheira kaempferi, giant spider crab, Japanese spider crab

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

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

box crab, Cryptopodia fornicata