Crustaceans fact sheet

What are crustaceans?

Crustaceans are invertebrates that belong to the phylum Athropoda. They have a hard exoskeleton with a segmented body, two-parted legs and two pairs of antennae.

Most crustacean species live in aquatic environments such as crabs and lobsters, however there are some terrestrial species such as slaters and pill bugs. Terrestrial means living on the land.

Crustaceans have three body segments – head, thorax and abdomen. In some species such as this slater, the head and part of the thorax are fused together to form a cephalothorax.

Crustaceans such as soldier crabs are characterised by having more than four pairs of jointed legs and a strong exoskeleton called a carapace.

Most crustaceans are omnivores and across the group can have a broad diet, however most are scavengers eating algae and the scraps of other animals. Omnivore means eating plants and animals.

There are more than 50,000 species of crustaceans in the world. They range in size from the 0.2 millimetres microscopic water flea to the 4.3 metre leg span of the Japanese spider crab.

Case study – pill bug

Appearance

The pill bug, or rollie pollie, gets its name from being able to roll into a tight ball. It has a carapace that looks like armour which can vary in colour from white to dark grey. Carapace means hard, outer shell.

Pill bugs roll into a tight ball to protect themselves.

Habitat

Pill bugs live in wet or damp terrestrial environments. They are found under logs and rocks and amongst decaying vegetation.

Logs create habitat for pill pugs. The underside of the log is usually moist. Pill bugs need moist conditions as they breathe through gills.

Diet

Pill bugs eat algae, moss, bark, fungi as well as decaying plant and animal material.

Pill bugs look for food such as fungi and decaying leaves.

Role in the ecosystem

Pill bugs clean up the environment by helping to recycle and decompose old plant and animal matter. They are also food for centipedes, spiders, ants, birds and amphibians.

Pill bugs help to break down and recycle dead plant material such as logs and leaf litter.

Pill bug anatomy

Cephalothorax

The head and first segment of the thorax are joined together. This is called the cephalothorax. Two pairs of antennae are attached to the cephalothorax, one long pair on the top and one shorter pair underneath.

Thorax

The thorax includes the region between the 2nd and 7th segment. This region is called the pereon.

Abdomen

The abdomen is made up of five condensed segments called the pleon.

The hard exoskeleton has seven armoured plates which protect the body. They are made of chitin.

The hard exoskeleton of a pill bug provides protection from predators.

Related fact sheets

More invertebrate fact sheets

• Invertebrate fact sheets – overview
• Annelids fact sheet – segmented worms that also live in soil and leaf litter.
• Myriapod fact sheet – centipedes and millipedes that share log and litter habitats and prey on pill bugs.
• Arachnids fact sheet – spiders and other arachnids that feed on crustaceans like pill bugs.

Habitats and ecosystems

• Leaf litter habitat fact sheet
• Field of Mars Reserve fact sheet