Sea urchins are eaten by birds, sea stars, cod, lobsters and foxes. Purple sea urchins are also eaten by otters in the northwest. Sea urchin eggs, also called roe, are eaten as a delicacy by people in Asia.
Most sea urchins are equipped with long, pointy spines that help them to deter predators. Sea urchins that are found in south Florida have sharp poisonous spines that penetrate the skin and break off. Those that are found in cool northern waters, such as purple or green sea urchins, are typically free of spines.
Underneath the spines of a sea urchin is a hard outer body called a test. It resembles the hard outer skeleton of other sea creatures that also belong to the phylum Echinodermata, such as sea stars, sand dollars, sea lilies and sea cucumbers. The outer shell is constructed of 10 fused plates that surround the entire body in the same structure as the slices of an orange.
Every other section of the outer body contains holes that allow sea urchins to extend their tubed feet using a water vascular system when they need to walk or hang onto objects when they feed. Sea urchins can also walk using their spines or their teeth.