Sea urchins are a small, round, spiny creature related to the starfish. They move very slowly on hundreds of small tentacle-like legs that use suction to stick to rocks. Their mouth is at the bottom of the sea urchin, and they eat algae as they move around.

Different kinds of sea urchin have different colours and different lengths of spines. The spines are hinged and can move around. Sea urchin shells don't have spines and look very different to a live sea urchin.