Story: Crabs, crayfish and other crustaceans

Crustaceans are diverse – some are less than a millimetre long and others are over half a metre, there are lobsters and those shaped like worms, and some are thin discs and others are totally enclosed in shells. Belonging to the same group as insects, spiders, mites and scorpions, they are the bugs of the sea.

Story by Niel Bruce and Alison MacDiarmid
Main image: A squat lobster

Story summary

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What is a crustacean?

Some common crustaceans are crabs, crayfish, barnacles and slaters. A crustacean has:

  • a hard outside shell, called an exoskeleton
  • legs with joints
  • four antennae
  • gills
  • at least seven pairs of appendages (for example, claws and legs) for walking, swimming, eating and breathing.

Crustaceans live in the sea, rivers, lakes and on land. There are at least 67,000 kinds of crustaceans in the world. In New Zealand 2,682 different kinds have been identified.

How do crustaceans grow?

As crustaceans’ bodies grow they need to get rid of the hard shell, which becomes too small for them. All crustaceans moult – they cast off the old shell and there is a new one underneath. At first the shell is soft, so they aren’t protected from predators.

Crayfish have marks on their shells that stay each time they moult. Like fingerprints, the marks on each crayfish shell are unique.


Crabs have wide, flat bodies, and their head and chest are joined together. Among their different appendages, they have two claws and four pairs of legs. Some other crustaceans look like crabs but they only have three pairs of legs, so they are not real crabs.

The giant spider crab is the biggest crab in New Zealand. Like a spider it has long legs compared to the size of its body.


Crayfish have lots of features that help them explore their surroundings. They can turn their antennae in all directions to explore rocks and to poke or frighten predators. They also have a pair of feelers that can smell chemicals in the water. Their eyesight is very good under water, but it can be damaged by bright light.

Crayfish hatch in the water from eggs, as tiny larvae. For one or two years the larvae float on the currents, where they eat and moult. When they are about 2.5 centimetres long they swim back to the coast. Adult crayfish live among rocks and eat kina (sea urchins), crabs and shellfish.


From the outside, barnacles don’t look like other crustaceans because they are totally covered by a shell. But inside, their bodies look like shrimps.

Most barnacles attach themselves to rocks or floating wood. Some barnacles live in or on other animals, like those that stick to whales’ heads.


Slaters and sand hoppers make up 20% of crustaceans and live on land and in the sea. Slaters come in many more shapes than other crustaceans. Some are flat and round shapes, some are like worms, and some are bugs that can roll up in a ball. Most of them are very small – 1 centimetre or less.

How to cite this page:

Niel Bruce and Alison MacDiarmid, 'Crabs, crayfish and other crustaceans', Te Ara - the Encyclopedia of New Zealand, (accessed 14 July 2024)

Story by Niel Bruce and Alison MacDiarmid, published 12 June 2006