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Sea Life, Animals
& Exhibits

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Purple Sea Urchin

(Arbacia punctulata)

A purple sea urchins pin cushion appearance comes from its round inner shell, called a “test.” The test is covered with pincers (pedicellariae), tube feet and purple spines that move on ball-and-socket joints.The spines spear food and protect an urchin from predators. Tiny hairs (cilia) covering the spines create a water current that carries food to the urchin and washes away wastes. An urchin uses its many tube feet to move along rocks, sand or other surfaces.They are only able to move at a rate of about 1-2 inches per minute. Surprisingly, an urchin also “breathes” through its tube feet—that’s where gases are exchanged, instead of in gills or lungs. If turned upside-down, they can right themselves by using their tube feet and specialized movements of their spines. The spines on the oral side (bottom) of their bodies are spatula-shaped spines that mostly aid in movement across sandy or rocky bottoms. If flipped upside down, urchins can right themselves through specialized movements of these spines.


The purple sea urchin is usually found in intertidal and sub-tidal zones. They prefer shallow water on hard substrates like jetties, or pilings. They can be found along the coasts of North America from Massachusetts, southward to the Yucatan Peninsula.


The sea urchins are very important to their habitat because they help maintain the amount of algae in the ocean. They are actually considered the “grazers of marine algae.” They are omnivorous and will feed on seaweed and algae, sponges, coral polyps, and dead animals including dead urchins. Five tooth-like plates, called “Aristotle’s lantern, “surround an urchin’s mouth on the bottom of its shell. If food lands on an urchin’s back, the tube feet pass the food down to the urchin’s mouth like a bucket brigade.


Purple sea urchins breed yearly from January through March and are ready to do so when they have turned two year of age. This reproduction process occurs through external fertilization during which males release their gametes into the ocean and fertilize the female’s eggs at random. The surf definitely aids in the transportation of the gametes, but too strong of a surf can damage the ova and spermatozoon. After fertilization, the egg matures and grows into a sea urchin. The test protects the sea urchins internal organs. However, the strengthening of the test must happen quickly to protect the sea urchin in its vulnerable state.


The spines on their dorsal (top) side are sharp and spike-like and are most often used in defense. Although sharp, these spines are quite brittle, can frequently break off during run-ins with potential predators. When shadows pass over an urchin, they have been observed to point their spines in the direction of the shadow, to protect itself from a potential predator. Sea gulls, triggerfish, otters, sea stars and wolf eels are all predators to the sea urchin.