Sea Potatoes

If you dig into the sand below the tide line along many of the world’s beaches, there’s a chance you’ll dig up some potatoes. They’re small, fuzzy, and fragile. But they’re not the kind of spuds you serve with butter and sour cream. Instead, they’re sea potatoes—a type of sea urchin—smaller cousins of the ones with long, painful spines.

A typical sea potato is maybe two or three inches long. It’s encased in a thin shell, known as a test. The shell is shaped a bit like a heart—hence the potato’s other name, heart urchin. A pattern on one side of the shell forms the outline of a sea star—another of the sea potato’s relatives.

Soon after it hatches, a sea potato burrows a few inches into the sand at the bottom of shallow coastal waters. It then spends the rest of its life there. It excavates a small “den,” with side pockets for air and wastes. The den is lined with a layer of mucus.

Small, yellow hair-like spines lie flat against the sea potato’s back. They trap air so the animal doesn’t drown. “Tube feet” push it through the sediments at about an inch per day. As it moves, the critter funnels the sand into its mouth. Its digestive system then filters out the goodies—algae, the remains of dead animals, and nutrients from the sand itself.

The shells of thousands of dead sea potatoes sometimes wash up on the beach—the result of extreme weather or some other big shock. Otherwise, they’re seldom seen—hidden in the sand just off-shore.

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