Green Sea Urchin — like a hedgehog

The green sea urchin (Strongylocentrotus droebachiensis) comes from the class of animals called  Echinoidea, which means "like a hedgehog." Can you guess why?

Green sea urchin at the New England Aquarium

Young hedgehog (Photo: By Calle Eklund/V-wolf, via Wikimedia Commons)

Sea urchins are related to sea stars, sand dollars and sea cucumbers. You can find urchin species in oceans around the world (and in the Edge of the Sea Touch Tank here at the Aquarium!). The green sea urchin prefers shallow, rocky areas. They snack on algae and dead matter by scraping with five pointed teeth.

Urchins can destroy kelp beds and are sometimes seen as pests! An urchin's mouth structure is also called an Aristotle's Lantern. It contains the five teeth and also has 40 different skeletal parts and 60 muscles.

Urchins can move at a rate of six to seven feet per hour. They use their spines and tube feet, also called podia, to get around. Each spine is attached with ball and socket joint.

 Look closely and you can see the podia in this picture.

Podia are long and slender with terminal suckers at the end. Tube feet operate hydraulically by means of a water vascular system.

 Urchins often cover themselves with pebbles, shells, and bits of seaweed for camouflage.

Sea urchins have a long list of predators, and people are tops on that list. Marine birds, arctic foxes, sea otters and starfish also eat urchins. Wolffish swallow urchins whole. Triggerfish blow them over with a jet of water. Gulls pick the urchins from a tide pool, fly overhead, and drop the animals onto the rocks. The shell breaks open on impact and the gull flies down for its meal.

 Group of green sea urchins in the Aquarium's Edge of the Sea exhibit

Conservation Notes: Sea urchin roe (actually both the male and female gonads), called uni in Japan, is considered a delicacy. During the "Green Gold Rush" of the 1980's and 1990's, millions of pounds of urchins were harvested for their roe. Overharvesting amid a lack of regulation caused the urchin population to crash. Regulations now prevent overharvesting of urchins, but populations have been slow to recover.

On your next visit to the Aquarium, look for a sea urchin at the Edge of the Sea touch tidepool. You can also see urchins in the cuttlefish exhibit and the Northern Waters gallery.

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