Sea urchins belong to the phylum Echinodermata--the same group as sea stars, sand dollars, sea lilies and sea cucumbers. Although difficult to see through all the spines, sea urchins also have a hard outer body like that of its relatives. Their outer skeleton--called a test--is made up of ten fused plates that encircle the the sea urchin like the slices of an orange. Every other section has holes through which the sea urchin can extend its tubed feet. These feet are controlled by a water vascular system. By changing the amount of water inside, the animal can extend or contract the feet to move about. Sea stars move around the same way (to view a movie of how sea stars move (~1.6M), click here). Mainly sea urchins use their feet to hang on to the bottom while feeding, but they can move fast, walking on their feet, their spines, or even their teeth.
When a sea urchin dies, all its spines fall off, leaving only the test. If you look carefully at a test, you can see tiny bumps covering it where the spines were once connected. The base of the spines once fit over the bump like a snug-fitting cap. The spines can rotate extensively around this bump. In a live sea urchin, skin and muscle cover the test and can be pulled on to move the spines.
Sea urchins mainly congregate in colder, offshore waters, but sometimes travel into shallower waters looking for
food. The green sea urchin of New England is often found in tide pools and below the low-tide line.
They also tend to shy away from light. Their tests grow to a size of three or four inches.
Sea urchins will eat just about anything that floats by. Its sharp teeth can scrape algae off rocks, and grind up plankton, kelp, periwinkles, and sometimes even barnacles and mussels.
Sea urchins are sought out as food by birds, sea stars, cod, lobsters, and foxes. In the northwest, sea otters are common predators of the purple sea urchin. Humans also seek out sea urchin eggs, or roe, for food. The eggs are considered a delicacy in Asia.
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