Local Species: Green Sea Urchin

This post is part of an occasional series on local marine species compiled by New England Aquarium aquarist Peter Gawne. Unless otherwise noted, all photos are Peter's. When he's not diving locally for these posts or researching coral reefs in Belize, Peter takes care of the touch tank's sharks and rays and the jellies exhibit. Today's post focuses on a local tidepool regular.

Green sea urchin (Strongylocentrus droebachiensis)

Green sea urchins are members of the phylum Echinodermata, and the order Echinoida. The phylum Echinodermata also includes sea stars, sea biscuits, and sea cucumbers.  Members of this phylum are named for their spiny shell from the Greek ekhînos, “hedgehog,” and dérma “skin.”

The green sea urchins’ body plan, while typical for their phylum, remains highly unusual when compared to other species in the animal kingdom. Sea urchins have five-fold symmetry, called pentamerism, with five equally sized parts radiating from a central axis. The animal is covered by hundreds of sharp protective spines that are interspersed with projecting suction-cups, called tube feet. To move, sea urchins utilize both their spines and tube feet to pull their body across the sea floor.
A green urchin on exhibit at the Aquarium

The lower portion of the sea urchin, the part in contact with the sea floor, is referred to as the oral 
surface because it contains the mouth. The mouth is centrally located, and has five tooth-like structures, which are comprised of a mosaic of two different kinds of calcite crystals: fibers and curved plates. The crystals are arranged crosswise to one another, and bound together with an incredibly hard cement of calcite nanoparticles. Between the crystals are layers of weaker organic material. These weak spots may allow parts of the teeth to tear away, shedding worn or damaged areas and exposing a new, sharper edge. Sea urchin teeth grow constantly, so this process allows them to consistently have sharp teeth. Many sea urchins bore through rock and other hard substances, so well-honed teeth are essential. 
Green sea urchin jaws are centrally located in a fleshy mouth, surrounded by short spines and numerous tube feet.

Sea urchins use these strong teeth to eat a variety of food sources. Most sea urchins, green urchin included, are voracious herbivores. Sea urchins in general can be ravenous in their pursuit of plant material for energy. Large aggregations of sea urchins sweep through undersea kelp forests and algae-rich areas, consuming all available material. The urchins’ powerful teeth scrape rock and stone, often removing the very last traces of kelp and algae in the environment, leaving little or nothing to regrow. Such devastated areas are referred to as “urchin barrens” due of the lack of life left in their wake.
The sea urchin shell is made of calcium carbonate, and its spherical body is protected by hundreds of spines.  Between the spines are tiny suction cups, or tube feet, which are used for locomotion.

Sea urchins are eaten by a variety of predators, including sea stars, crabs, fish, mammals, birds, and even humans. Here in New England, Maine’s green sea urchin fishery accounts for roughly 22% of sea urchin landings in the United States. This fishery, while dating back to as early as 1929, really took off in the mid-1970’s to meet the interest from Japanese markets, where sea urchins are prized for their edible gonads. Sea urchin, or uni as it is known in Japanese cuisine, has a unique flavor. It has the consistency of custard cream, and a flavor that is unmistakably from the ocean.  

Pictured here is a serving of uni offered at a local Cambridge restaurant.

Savoring this delicacy is said to induce euphoria as sea urchin roe contains a neurotransmitter called anandamide – a naturally occurring nerve-signaling molecule. The anandamide molecule’s name is derived from the Sanskrit word ananda, meaning “bliss.” This molecule interacts with a receptor that plays a role in controlling pain, mood, appetite, memory, and fertility. Anandamide is very quickly broken down by the body’s enzymes, explaining why it doesn’t produce a perpetual natural high. 

A 2001 assessment judged the stock status of Maine’s green sea urchins to be 90% down from their virgin, unfished biomass. Fortunately, declines in the biomass estimates appear to have slowed in recent years. Green sea urchin populations are inherently resilient. Each individual produces several million eggs per spawning event, reaches maturity relatively early—at about 5 years, and may live more than 30 years. Finally, the fishery for green sea urchins is accomplished mostly through hand collection by divers, so depths greater than practical on scuba gear act as a reserve for the urchins.

Read previous entries in Peter's series on local species:

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