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Crustaceans

Crustaceans

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. 

close up of a terrestrial slate showing its antennae, segmented body and legs
Image: 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.
Soldier crab wading through the sand showing its exoskeleton and two-parted legs
Image: Crustaceans like 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. 

There are more than 50,000 species of crustaceans in the world. They range in size from the 0.2mm microscopic water flea to the 4.3m 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. 

Close up of a pill bug rolled into a ball covered by its exoskeleton on the soil
Image: Pill bugs will 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.

A short log with a hollow through the middle
Image: Logs create excellent habitat. 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 bug crawling through the soil and leaf litter
Image: Pill bugs will look for food in the place where they live. Rotting logs are an excellent source of food.

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. 

Rotting log on forest floor with a hollow through the middle
Image: Pill bugs help to break down and recycle dead plant material such as logs and leaf litter.

Pill bug anatomy

drawing of a pill bug showing an exoskeleton and antennae
Image: The hard exoskeleton of a pill bug provides protection from predators.

close up drawing pill bug head showing one pair of antenna
Image: Pill bug cephalothorax

Cephalothorax

The head and first segment of the thorax are joined together. This is called the cephalothrax.

Two pairs of antennae are attached to the cephalthorax, one long pair on the top and one shorter pair underneath. 

close up drawing of a pill bug thorax showing armoured plates of an exoskeleton
Image: Pill bug thorax

Thorax

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

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

 

close up drawing of a pill bug abdomen
Image: Pill bug abdomen

Abdomen

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

Invertebrate explorer book
Image: Invertebrate explorer digital book

Find out more

The invertebrate explorer digital book explores the incredible world of Australian invertebrates.

Students can use the book to investigate classification, features, adaptations and habitats of a variety of Australian invertebrates through narrated videos, stunning images, interactive activities and detailed text.

This book was designed by teachers to support the NSW Science and Technology K-6 syllabus and NSW English K-6 syllabus.

Content supports living world, Australian animals and class studies on invertebrates.

Download for free from Apple Books.

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