U is for Urchin

Close relatives of sand dollars, sea cucumbers and sea stars, these spiny echinoderms are some of the ocean's most unique inhabitants. There are hundreds of sea urchin species worldwide, and they run the gamut of shape, size and color.

Some have thick, pencil-wide spines jutting from their shells. Others look like pincushions covered with thin, needlelike projections. It's no wonder that sea urchins, with their spiny exterior, were once nicknamed sea hedgehogs. Some of the ocean's largest grow to nearly 7 inches in diameter with 2-to-3-inch-long spines.

There are purple, blue, chartreuse green, brown, black, white and ochre sea urchins. The fire urchin has thick clusters of multicolored spines in brick red and dandelion yellow, tipped with venomous blue points.

Collector sea urchins—native to the Indo-Pacific, Hawaii and the Red Sea—have projectile pincers. When a predator approaches, they launch a swarm of the tiny jaws, which clamp down and release toxins.

Despite their obvious means of defense, sea urchins still have a few predators to watch out for. They are a favorite snack for sea otters, wolf eels and sunflower sea stars.

A sea urchin's spines surround its round inner shell, called a test, and rotate on ball-and-socket joints. They are coated in cilia, tiny hair-like structures, that wave to create a water current carrying food to the urchin's mouth and washing away waste.

Sea urchins eat sea plants, such as algae and kelp, but also crunch on the occasional invertebrate or scavenge dead fish. The urchin's mouth is hidden underneath its body at the base of its shell. It has five tiny, triangular "teeth" commonly referred to as Aristotle's lantern.

Urchins employ 60 muscles to move their mouths. They use their spines and teeth to dig holes in rocks to hide, sometimes staying so long that they outgrow their dugouts and end up stuck for good.

Little hydraulically operated tube feet extend from the urchin's underside between its spines. The long, flexible tentacles end in tiny suction cups and are not strictly used for locomotion. Lacking lungs or gills, sea urchins breathe through their tube feet. The eye-less creatures also use these appendages to sense light. In addition to absent eyes and gills, a sea urchin has no central brain or heart.

Despite their seemingly simple anatomy, sea urchins are one of the first animals to show signs of stress in polluted waters, and are therefore great indicators of water quality.