Sea urchins are spiny, radial invertebrates that live on the ocean floor. Over 700 species of sea urchins currently exist, the largest of which may have toxic spines that reach up to 12 inches in length. When a sea urchin dies, however, all of its spines fall off leaving only its hard shell, called a test.

View a second image of a sea urchin spine.

Members of the phylum Echinodermata, sea urchins belong to the same taxonomic group as sea stars, sand dollars, sea lilies and sea cucumbers. The barbed creatures possess five bands of pores through which they can extend their tubular feet, and from nodules on their tests, movable spines arise. Between the spines are small, pincer-like organs called pedicellariae, which they use to clean and defend themselves. Although sea urchins generally use their feet to cling to hard surfaces, they can move fast if needed by means of their feet or spines.

Sea urchins consume both plant and animal matter, including kelp, decaying matter, algae, dead fish, sponges, and mussels. The mouths of sea urchins are located on the underside of their bodies, consisting of a complex dental system and five radially arranged teeth called Aristotle’s lantern. The teeth are most often utilized to scrape food from rocks, but some species can use them to dig holes in which they can hide.