Sea Urchin, a prickly-looking marine animal, spherical in shape and covered with long movable spines. It somewhat resembles a hedgehog and is sometimes called the “hedgehog of the sea.” Sea urchins are found on the ocean bottom, usually near rocky shores. Sea urchins may be brown, black, purple, green, white, or red. Most are about two to four inches (5 to 10 cm) in diameter, including the spines.

In addition to its spines, the sea urchin also has pedicellariae (three-jawed pincers atop slender stalks) and tiny tube feet projecting from its body surface. The movable spines (which in some species are solid and in others hollow and filled with poison) are used for locomotion and protection. The pedicellariae (which in some species contain poison glands) are used for defense and for cleaning the body by removing larval animals and small crustaceans. The tube feet are hollow, muscular projections ending in suckers. They are flexible and can be extended beyond the spines to grip objects on the ocean floor.

The sea urchin feeds on seaweed and other organic matter. On its undersurface is a mouth with five strong teeth used in feeding. Some sea urchins bore holes with their teeth in rocks along the shore and then use the rocks as hiding places. Sea urchins reproduce sexually by means of eggs and sperm. The eggs are used as food in many European and Asian countries.

Who Has the Longest Spines?

Sea urchins have the longest spines of any echinoderm. Like sea stars, sea urchins have spines over much of their bodies. Their spines stick out in all directions from their bony plates—just like little spears. And these spines are often poisonous.

A sea star and a sea urchin are easy to tell apart. A sea urchin doesn't have arms. A sea star does. A sea urchin's body is round like a ball, while a sea star's body is mostly flat. A sea urchin also has something that no sea star has—teeth.

A sea urchin has bony plates just below its skin. These plates come together like the slices of an orange to form a tough skeleton called a test. Not all plates have spines attached to them, though. Every other plate has many tiny holes, through which a sea urchin can wiggle its tube feet.

Why Is the Sea Urchin a Master of Disguise?

Some sea urchins use their tube feet to pick up small rocks, bits of shell, or seaweed. The animals arrange these objects so that they cover their bodies. By doing this, a sea urchin can blend in with its surroundings. This is a form of camouflage (KAM uh flahzh). It helps the sea urchin hide from enemies.

Like most other spiny-skinned animals, sea urchins use their tube feet to move along the ocean floor. But sea urchins also use their spines to help them get around.

A sea urchin sometimes squeezes into a hole between rocks. If a hole is too small, a sea urchin will use its teeth and spines to make it bigger. By carving out a bigger hole, a sea urchin can make its own little home. And it's usually a home that's too small for unwelcome visitors, such as sea stars and other enemies.

Sea urchins belong to the class Echinoidea.