The test and tube feet
Sea urchin spines are joined to bony plates that come together like the slices of an orange to form a tough skeleton called a test. Some plates have tiny holes through which sea urchins can wiggle their hollow tube feet. The tube feet have suckers and can extend beyond the spines to grip objects and the ocean floor.
Masters of disguise
Sea urchins are generally nocturnal and usually hide in holes or burrow in sand during the day. Some sea urchins use their tube feet to pick up items such as small rocks, pieces of shell, and bits of seaweed that they can use to disguise their bodies and blend in with their surroundings. Some urchins even carry live soft corals or anemones. This form of camouflage helps sea urchins hide from predators.
Teeth as tools
Sea urchins mainly feed on decomposing matter and algae on coral rocks. They ingest food through a beak-type mouth called Aristotle’s lantern that has five tooth-like plates used to scrape rocks clean of algae. Sea urchins can also use their “teeth” to expand holes in rocks until the holes are big enough for them to hide in.
Tropical sea urchin species are important grazers that help maintain the health of coral reefs. Sea urchins eat algae that would otherwise smother the reefs and they leave behind clean places for new coral larvae to settle.
People and other predators
Fish, birds, crabs, sea otters and humans all feed on sea urchins. Overfishing of sea urchins for a delicacy called “uni” is a threat to sea urchin populations.