Thursday, November 26, 2015         


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Sea urchins the perfect janitors to keep Kaneohe coral clean

By Susan Scott


The state has found employment for Hawaii's sea urchins. With the help of a canoe club, state workers placed about 1,000 baby collector urchins this month on a patch of coral cleaned of an obnoxious alien seaweed with the state's "super sucker" vacuum cleaner. Now it's the urchins' job to keep it clean.

The probability that these marine invertebrates will excel in their new career is good. In 2009, researchers tested the urchins' hoovering skills in a yearlong study. When collector urchins were placed on a vacuumed coral patch, the urchins kept it spic-and-span by gobbling up new seaweed sprouts. On a vacuumed patch without urchins, however, the bad seaweed quickly regrew to coral-smothering densities.

Sea urchins hold a special place in my heart because they helped launch me into the wonderful world of marine biology. When I saw my first one decades ago, I thought it was a plant, some sort of sea cactus. "That's an animal?" I said when Craig (my future husband) corrected me. "Why do they call them urchins? What do they eat? How do they walk?" He didn't know. I was hooked.

Over the years, I've come to think of sea urchins as flowers of the reef. Several lovely species, called wana in Hawaiian, are pokier than rose thorns and can cause painful punctures if you step on one. Most urchins, though, are as harmless as pansies. Many names describe their appearance such as red pencil, shingle and banded urchin.

The urchins the state put to work are native collector urchins (hawa'e maoli in Hawaiian), black, softball-size creatures with stubby, blunt spines. These are safe to touch in Hawaii (please be gentle), but in other parts of the world, a similar-looking urchin can deliver a nasty sting.

The charm of a collector urchin is its inclination to collect. Using sticky tube feet that can stretch beyond its hard body spines, the urchin picks up items small and light enough to transfer, conveyor-belt style, onto its back. Any item will do. Besides the usual seashells, seaweeds and pebbles, I've seen collector urchins wearing plastic silverware and fishing lures.

Researchers don't know the reason for this behavior, but it has to do with light. Some collector urchins cover themselves more in the long days of summer than in the shorter winter days, and tend to drop their cover at night. In a lab experiment, when biologists projected a narrow band of light across one side of a collector urchin's body, the creature moved its small stones from three different directions to the illuminated strip.

Still, it's common to see collector urchins sitting on the ocean floor bare-naked in the bright summer sun. Only the urchins know why they sometimes cover up and sometimes don't.

All collector urchins graze on seaweed, but they'll eat anything that can't run away. Walking on their suction-cup tube feet up to 3 feet per day, these urchins also dine on dead fish, worms, snails and even other urchins.

And the name? Today the term urchin means a mischievous child. In old England, however, an urchin was another bristly animal: a hedgehog.

If the new collector urchins keep the evil seaweed in check as expected, thousands more will be hand-raised and put to work for the state. These new jobs might not reduce Hawaii's unemployment rate, but hopefully they will reduce the seaweed creep that plagues Kaneohe Bay.


Susan Scott can be reached at

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