Purple Sea Urchin

Sea Urchin

Arbacia punctulata

Color: Dark purple to reddish brown.

Size: Up to 2 inches in diameter. The spines can be up to an inch long.

Habitat: Attached to rocks and shells in tide pools, on seaweeds, along rocky bottoms.

Seasonal Appearance: All year.

Sensitivity to Human Action                               







Distinguishing Features and Behaviors:

     Sea urchins are squat, round creatures with hard exoskeleton usually covered in spines. They belong to the group of echinoderms, meaning “spiny shin,” that also includes sea stars and sand dollars. The hard exoskeleton, known as a test, is composed of calcium carbonate plates called ossicles. The test can often be found among other shells on the shore, and the sutures of the plates are noticeable along with rows of bumps and pin-sized holes.

     While alive, the exoskeleton resembles a pin cushion, covered in spines. Spines are absent in a small area near the top where pores lead to reproductive organs and on the bottom where the mouth is located.

     In tidal areas, urchins use their spines to dig small depressions among the rocks and sand. These holes hold water even after the tide has gone out; the holes also protect the urchin from waves at high tide.

     Urchins have an organ in their mouths, called “Aristotle’s Lantern,” which they use for feeding. This structure resembles a bird beak, with five plates rather than two and is used to scrape algae from rocks. Purple sea urchins are omnivorous and will feed on sponges, algae, small invertebrates, and detritus.

Relationship to People:

     Urchins are sensitive to light and hide in rock crevices during daylight; however, this has not prevented divers from finding and harvesting them. Sea urchins are primarily exported to Japan for food. Urchins are harvested along both the Pacific and Atlantic coasts, but many areas have been closed in California and the Gulf of Main as a result of over fishing. Purple sea urchins are common along the rocky shores of Narragansett Bay. Stepping on or touching the spines of an urchin can cause a painful cut.