You might not feel particularly motivated to get to know the sea urchin. And I get it! Their spiny bodies, slow pace and generally muted colors don’t exactly make them the most charismatic critters in the sea. But I guarantee that the more you learn about sea urchins, the more you will love them. You’ll never look at urchins the same way again!
First things first: sea urchins are echinoderms, which is a phylum of marine invertebrates that includes sea stars, brittle stars and sea cucumbers. Echinoderms are known for their five-point radial symmetry and water vascular system. The water vascular system involves several small tube feet that become stiff when water is pushed into them, allowing the animals to move on a conveyor belt-like rotation of feet. They typically have a tough, spiny surface, which inspired their name (in Greek, echinos means “spiny” and derma means “skin”). Of course, sea urchins’ spiny exterior is a little more obvious than the others, but a close look at the skin of a sea cucumber or sea star will show distinctive bumps.
Sea urchins are further classified in the class Echinoidea, which includes the classic “sea urchins” you’re picturing now, as well as sand dollars and sea biscuits (of the non-equine variety).
There are more than 900 species of sea urchins around the world from the shallows to the deep sea. Urchins live at depths of more than 24,000 feet. Hidden beneath their spines they have a hard, outer skeleton called a test. If you’ve ever walked along the beach and seen a circular purple, white or green-ish shell with dots all over it, it might be a sea urchin test! Each raised dot represents where there used to be a spine when the urchin was alive.
Sea urchins also have a hard mouthpiece called an Aristotle’s lantern. It’s named after the philosopher Aristotle, who described the urchin’s mouth as a “horn lantern” in his book The History of Animals. They have five calcium carbonate teeth that grind and scrape algae, plankton and kelp.
Sea urchins plan an important role in the ecosystem. They are grazers that help keep algae in check and are a favorite food source of many ocean species. We’ve seen what can happen if we don’t keep sea urchin populations in check. In the Caribbean, for example, there was a massive die-off of the black sea urchin (Diadema antillarum) in the 1980s. Algae began to take over without those urchins grazing on the reef, which made it harder for corals to get the light they needed to thrive.
Next time you’re in the ocean or at the aquarium, don’t overlook the prickly but precious sea urchin!