A sea urchin lives on rocks or on the sea bed. It moves using hundreds of tiny, transparent, tube feet set between the protective spines. Sea urchins are related to sea cucumbers and distantly related to sea stars, and brittle stars (that have five arms).

Sea urchins feed mainly on tiny green plants that float in the sea or coat the rocks. They are known as algae. They will also feed on mussels, sponges and brittle stars.

Sea urchins are one of the favourite foods of many sea otters, lobsters, crabs, triggerfish and others. These animals are specially adapted to cope with the spines. The urchins are well protected against almost all other animals.

Sea urchins are naturally kept in check by these animals, but if they are not eaten, they develop in huge numbers and eat everything in sight, leaving the area barren. So when lobsters, crabs and so on are caught by fishermen in too great numbers, there can be an explosion in sea urchin numbers.

Most sea urchins live between the surface and a few tens of metres because the algae they eat need sunlight to grow.