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 — where gases are exchanged — more often than in its gills.

Five toothlike plates, called "Aristotle's lantern," surrounds an urchin's mouth on the underside of its test. 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.

Traditionally, fisheries have sought the red sea urchin for its roe. As landings of red urchin declined, many fisheries have turned their attention to the purple sea urchin, but because it’s small and yields less roe, a larger commercial demand hasn't developed.

Sea urchin behavior can signal changes in water parameters and deteriorating water quality — it’s one of the first animals to show stress during algal blooms, unseasonal temperature changes, and in polluted water. Signs of stress include a lack of movement and drooping (or dropped) spines. Eventually, poor water quality kills the 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 move on. Nearby forests might not be affected.

• Sea otters, sunflower stars and California sheephead prey on the purple sea urchin. Sea otter predation on the purple sea urchin helps protect kelp forests from destruction. Sea otters that regularly eat the purple sea urchin are easily detected — their bones and teeth turn sea-urchin purple!
• When sea otter populations decline, urchin populations go unchecked and can decimate entire kelp forests in a short period of time. This is called an urchin barren — an example of how healthy ecosystems depend on keeping predator/prey populations in balance.
• In the intertidal zone, a purple sea urchin will decorate itself with shells, rocks and pieces of algae. Scientists think this behavior protects the urchin 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.