Purple Sea Urchin

Facts about the purple sea urchin, inhabitant of tidepools and kelp forests of the Pacific Coast.

by Allison Titus

The watery world of kelp forests and tidepools often looks like something out of a sci-fi movie. Giant green sea anemones wave their tentacles, ghostly fish glide in and out of rocks and brightly colored nudibranchs move slowly across the sea floor, making up a galaxy of strange marine creatures. The purple sea urchin fits right in to this otherworldly ecosystem; with its bright purple color and sharp spines, it’s hard to tell whether it’s a plant or an animal, let alone a species of this planet.

The purple sea urchin is actually an animal, despite its extraterrestrial look. Relative of the sea star, this spiky two-to-five inch echinoderm is found along the Pacific Coast from Alaska all the way down to Baja California. It lives amongst the rocky shores and sea floors of the coast of Planet Earth in shallow areas affected by the crashing waves.

How do these small animals survive in the turbulent ocean? Well, it certainly isn’t their brain power; in fact, like all other echinoderms, purple sea urchins do not have brains. They communicate through a simple nervous system that extends from their mouths to the small, tubular, barely visible structures on their spines called tube feet. Tube feet actually cover the whole spiny external part of the sea urchin and enable it to move slowly using hydraulics.

Sea urchins feed mostly on algae and decayed organic matter, although they are known to eat kelp and sponges as well. The mouth of the sea urchin, located on the underside of its body, is named “Aristotle’s lantern” due to Aristotle’s description of the urchin mouth in his book History of Animals. Aristotle observed the symmetry of the adult sea urchin found in his area (not the purple sea urchin we see in the Pacific) and noted the five calcium carbonate teeth that served as the mouth of the animal. The sea urchin actually uses these teeth to “dig” into the rock, where it lodges itself for an extended period of time. In fact, if a sea urchin carves out a hideout for itself at a young age, it can grow too large over time and get stuck in its own hole (remember: the poor things don’t have a brain).

Sea urchins are both social and independent organisms. They are not known to congregate in groups, but a good food source can attract a crowd of sea urchins. Nothing like some good food and good company, huh? Unfortunately, large groups of sea urchins can actually be detrimental to kelp forests. Sea urchins feed upon kelp and often eat the stems of the plant found near the rocky ocean floor. This effectively kills the entire kelp plant. It is often sea otter predation on the sea urchins that keeps sea urchin populations in check and preserves the kelp forests. How’s that for some ecosystem dynamics?

Otters aren’t the only predators of the purple sea urchin; spiny lobsters and sheephead fish make a meal of them as well. However, the sea urchin is not defenseless against these hungry predators. Its first line of defense is its sharp spines, which many divers can tell you are no joke. The next line of defense is the tiny stinging structures found in their spines, called pedicellarines. Pedicellarines are poisonous, and can be released into prey or attacking predators.

Lastly, purple sea urchins are actually an indicator species. They are very sensitive to changes in their environment, and usually are one of the first in their ecosystems to show signs of distress when the water quality starts to decline. Stress in sea urchins often shows itself as drooping spines and a lack of movement. This is a species to watch as ocean acidification, ocean temperatures and human-caused pollution continue to increase.

You can tell if a sea otter preys on purple sea urchins because the otters’ teeth and bones actually turn a tell-tale purple.

Part of the sea urchin’s scientific name, Strongylocentrotus, literally means “spiky creature.”

Check out this scary video about Sea Urchins, and learn more at The Shape of Life.