Purple sea urchin | Urchin, Sea, Spines, Urchins, Feet

Purple sea urchin

Purple sea urchin (Arbacia punctulata) The purple sea urchin is so named because of the purple color of both the outer skeleton and the spines. The skeleton, or test as it is technically called, is about 2 inches in diameter and is made up of 10 plates fused together to enclose the body. This outer skeleton of the urchin is covered by long spines that are about 1 inch long.

NOAA image / DeWet-Oleson

When an urchin dies, the spines fall off and the empty skeletons may wash up on shore. These skeletons look like small, purple somewhat flattened balls that are covered by tiny holes and small bumps. If you look closely at the skeleton, you might see the holes in the plates of the urchin where the tube feet stuck out from. The spines of the urchin attach to the bumps on the skeleton and are used to protect themselves from predators like seabirds, fish and sea stars. Urchins also use these spines as well as their tube feet to walk around. Tube feet are small sucker-like protrusions that operate by a water vascular system within the urchin. This system transports water through the body and by adjusting the flow of water, the urchin controls the movement of the tube feet. Sea urchins also have a feeding structure called Aristotle’s lantern, which is a beak-like formation made up of 5 hard plates. The mouth is on the underside of the animal and this “beak” is used to scrape algae off rocks to eat. They will also eat dead animals on the sea floor and some small invertebrates, and when urchin teeth get worn down they can even regrow them! Urchins are either male or female. After the females release eggs, millions of them at a time, the males release their sperm and fertilize the eggs.

Purple sea urchins are common inhabitants of tidepools and jetties, but they can also be found in deeper water to over 700 feet. They prefer to settle on rocks and shells but sometimes hide in crevices or under rocks. The spines of the sea urchin are fragile and in rough seas or if attacked by a predator, they may break off. This is not a big problem for the urchins because they can grow their broken spines back!

Many species of sea urchins are harvested by people for food. While the soft body is commonly eaten, in some countries the eggs are considered quite a delicacy. Sea urchins can be dangerous to handle because of their long spines. Be extremely careful if you do try to pick one up and be very careful not to step on one!

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