Excerpt:
Researchers are restoring the Caribbean’s surprising, spiky custodians, which gobble up the algae smothering coral reefs.
South of Tampa Bay, Florida, wedged between a quiet neighborhood and a mangrove forest, custom-designed aquariums are home to thousands of sea urchin larvae that tumble and drift through the water. Scientists with The Florida Aquarium and the University of Florida care for the little urchins, checking them daily under microscopes for signs that they’re maturing into juveniles, which look like miniature versions of the adults. Few will make it. For every one million embryos conceived in the lab, only about 100,000 become larvae. Of those, only up to 2,000 become adults.
And at this particular moment, coral reefs in the Caribbean need all the urchins they can get.
Long-spined sea urchins (Diadema antillarum) play a vital role in Caribbean coral ecosystems. While overpopulated urchins elsewhere are treated as villains—in California, for instance, divers smash purple urchins with hammers to keep them from mowing down kelp forests—Diadema are the Caribbean’s unsung heroes. Dark and rotund with spines radiating in all directions, some as long as knitting needles, the urchins eat massive amounts of algae that would otherwise smother corals or prevent coral larvae from affixing to rocks and growing into colonies.
“They’re very simple animals, but they’re very effective at what they do,” says Alex Petrosino, a biologist at The Florida Aquarium and a member of the urchin lab team. Where their radiating spines converge, urchins have delicate, bulbous skeletons with holes for wriggly tube feet and bumps where spines attach. Their mouths—equipped with limestone plates for scraping algae off hard surfaces—are in the middle of that skeleton, on the animal’s underside. Petrosino calls Diadema the janitor of the reef because it’s so efficient at cleaning reef surfaces.
In the 1980s, however, an unknown ailment killed about 97 percent of Diadema urchins across the Caribbean and as far north as Bermuda. A later outbreak caused by a single-celled organism known as a ciliate further decimated urchins.
As a result, algae have taken over spaces that were once home to coral; the amount of live coral cover in the Caribbean has altogether plummeted by more than 80 percent since the 1970s. Disease, declining water quality, climate change, and overfishing all play a role, but the lack of urchins has worsened the problem, particularly in Florida where nutrient runoff—from sewage, fertilizers, and soil—feed algae, and increasingly warm summers encourage them to grow. While fish and other animals also typically eat algae, overfishing has left many reefs without enough grazers. Urchins have returned to some spots, but most reefs simply don’t have enough janitors left to keep them clean.
To tackle this problem, The Florida Aquarium has teamed up with University of Florida aquaculture researchers to bring more sea urchins into the world. The team is raising long-spined sea urchins, and partners are releasing them into struggling reefs in Florida and beyond with the goal of developing methods that can be applied at a large scale.
If it can be done efficiently and at scale, raising urchins in labs may jump-start populations of wild urchins in places where they haven’t been able to recover on their own. (Sometimes that’s because there aren’t enough adults left to reproduce, or because less coral leaves less urchin habitat, or because there are predators like crabs hiding in the algae that eat young urchins.) Researchers in Puerto Rico and the Caribbean island of Saba, a municipality of the Netherlands, are also working on urchin repopulation. And the idea is of interest beyond the Caribbean as well, now that another Diadema species in the Red Sea and the Indian Ocean is also being pummeled by a ciliate.
Raising Diadema, however, is no easy task…
