sea urchins feed mainly on algae, but can also feed on sea cucumbers, and a wide range of invertebrates such as mussels, polychaetes, sponges, brittle stars and crinoids. sea urchins are one of the favorite foods of sea otters and wolf eels. without predators to eat them, urchins breed in large numbers and eat almost all the nearby kelp, creating an "urchin barren", devoid of macroalgae and associated fauna. sea otters have re-entered british columbia, dramatically improving coastal ecosystem health by eating sea urchins.