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North Carolina Aquariums
Toward a greater understanding of North Carolina's aquatic resources . . .
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I found a round, purple, prickly ball about the size of a lime at Cape Lookout jetty. It looked like a pin cushion. Can you tell me what it is?

June 12, 2006


Our best bet is that it’s a sea urchin. Surprisingly, sea urchins are related to sea stars and sand dollars.


Two types of urchins are common in our waters: the purple urchin, with long spines, and the white urchin, with short spines. The spines aren’t venomous, and serve mainly to protect and aide the urchin wedging itself securely into cracks and crevices.


Urchins move on hundreds of tiny tube feet. They live in rocky outcrops, tidal pools, jetties, pilings, seaweed and eel grass beds. They can also be found in deep water and are frequently hauled up in fishermen’s nets. They move slowly and graze on algae, small invertebrates and carrion.

Often called pin cushions or sea porcupines, sea urchins are classified as echinoderms.


Beneath the urchin’s prickly armor is an intricate shell, visible only after the animal has died and its spines have fallen off. Called a “test,” the shell is beautifully designed with small, perfectly aligned, raised circles and five equally spaced elongated triangles. The raised circles are where the spines attach. The tiny holes in the circles are where the hundreds of thread-like tube feet protrude. The tube feet operate on a suction principle. This makes for slow going, but the strong grip of the hundreds of feet can hold the urchin fast, even in strong currents.


The urchin’s mouth is on the bottom of its shell, surrounded by a small ring of soft tissue. Resembling a small beak, the mouth consists of five small teeth. The teeth scrape minute plants and animals from rocks, shells and other solid surfaces. The hole in the top of the shell is for excreting waste.


Many animals prey on sea urchins, including sea stars, conchs, oystercatchers, gulls, crabs and fish. Humans also have a taste for urchins in the form of roe. The roe is eaten raw and is highly prized in the Orient as a gourmet food.