A purple sea urchin's pin cushion appearance comes from its round inner shell, called a "test." The test is covered with pincers (pedicellariae), tube feet and purple spines that move on ball-and-socket joints. Young urchins sport green spines. The spines spear food and protect an urchin from predators. Tiny hairs (cilia) covering the spines create a water current that carries food to the urchin and washes away wastes. An urchin uses its many tube feet to move along rocks, sand or other surfaces. And if food lands on an urchin's back, all those tube feet pass the food down to the urchin's mouth like a bucket brigade. Surprisingly, an urchin also "breathes" through its tube feet—that's where gases are exchanged, instead of in gills or lungs.
Five toothlike plates, called "Aristotle's lantern," surround an urchin's mouth on the bottom of its shell. An urchin uses its teeth and spines to dig holes in stones, which become the sea urchin's hideaway. Sometimes a sea urchin grows larger than its dugout and is "in for life"—then it must depend on food drifting to it. An urchin's teeth and spines can even drill through steel pilings by flaking away the rust that coats them.
In the past, fisheries sought red sea urchins to supply the market with urchin roe. Because the urchins were overfished and preyed upon by sea otters, this commercial fishery is no longer sustainable. Fisheries have turned their attention to purple sea urchins, but because they are small and yield less roe, a large fishery hasn't developed.
Sea urchin behavior can signal poor water quality—they're one of the first animals to show stress in polluted water. Signs of stress include a lack of movement and drooping spines. Eventually, poor water quality kills urchins, and other sea life, too.
Unlike intertidal sea urchins that live solitary lives in crevices, waiting for a piece of kelp to drift by, subtidal urchins live together in hordes. One of these hordes can devastate a giant kelp forest. The urchins attack the base of the kelp, often eating through the entire stem (stipe) of the plant. Eventually, the area becomes a barren desert, and the purple sea urchins mysteriously disappear. Nearby forests might not be affected.
Sea otters, sunflower stars and California sheephead prey on purple sea urchins. Sea otter predation on purple sea urchins helps protect kelp forests from destruction. Sea otters that regularly eat purple sea urchins are easily detected—their bones and teeth turn sea-urchin purple!
In the intertidal zone, purple sea urchins decorate themselves with shells, rocks and pieces of algae. Scientists think this behavior protects urchins from drying out, getting eaten by gulls or being damaged by the sun's ultraviolet rays.
When a sea star strives to get near an urchin, the urchin moves its spines aside and lets the sea star's arm get really close. Then the pincers chomp on the sea star's tube feet. The sea star immediately backs off. However, purple sea urchins' pincers can't defend against sunflower stars. If a sunflower star approaches, an urchin waves its spines and pincers and retreats. If the urchin doesn't move away fast enough, the sunflower star swallows the urchin whole.