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 Malacostraca; 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.

Birus latro, coconut crab, robber crab

Class Malacostraca

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 sea grass 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 using its hugely flattened front claws. It eats snails and its pincers are strong enough to break the snail shell.

box crab, Cryptopodia fornicata
On the right 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
On the right is Carcinoplax longimanus, it is found around japan, to the South China Sea, and around South Africa. Carcinoplax longimanus
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