An Army of Sea Urchins Could Help Save Coral Reefs

This article was originally published by Hakai Magazine .

South of Tampa Bay, in 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 1 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. Whereas 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 a delicate, bulbous skeleton with holes for wriggly tube feet and bumps where spines attach. Their mouth—equipped with limestone plates for scraping algae off hard surfaces—is 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 mature Diadema urchins in parts of the Caribbean, with the die-off reaching as far north as Bermuda. A later outbreak caused by a single-celled organism known as a ciliate further decimated urchins.

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—feeds algae, and ever-warmer summers encourage them to grow. Although fish and other animals also typically eat algae, overfishing may have left many reefs without enough grazers. Urchins have returned to some spots, but many reefs simply don’t have enough janitors left to keep them clean.

[ Read: The great sea urchin crisis ]

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 fewer corals leave less urchin habitat, or because predators such as crabs hide in the algae and 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 threatened by a ciliate.

Raising Diadema , however, is no easy task.

The urchin-rearing efforts share space with other projects run by the Florida Aquarium’s Conservation Campus. In one bright, spacious lab, rescued sea turtles with illnesses and injuries peer through windows at the sides of colossal tanks, awaiting veterinary care. In a nearby greenhouse, corals that may one day replenish Caribbean reefs quietly grow in broad, shallow tanks. Tucked between these charismatic creatures is a multitude of sea urchins in various stages of life.

“We have larvae in there right now,” the postdoctoral researcher Aaron Pilnick says, pointing to one of the tanks. There are thousands of baby sea urchins in the 40-liter tank, but they’re so tiny, I see nothing but seawater through the glass.

The aquarium tank is an odd shape, with one side curved and the other straight. Water flows in loops inside it, taking the microscopic urchin larvae on a nonstop ride. Diadema larvae aren’t good swimmers, so they’d sink and die without continuously flowing water. The water can’t flow too fast, though, or the larvae will run into one another—a problem for creatures as fragile as these. Each has two long arms jutting out from its tiny body, and if an arm breaks, the larva dies. Some larvae have arms that are four millimeters long and a body that’s only about half a millimeter wide. “That’s eight times the width of the body!” says Josh Patterson, an aquaculture expert and urchin-lab lead at the University of Florida. We both take a minute to consider what life would be like with arms that long.

“Their larval stage is extremely sensitive,” Patterson adds. He’s grown other types of urchins in an ordinary bucket, but Diadema need special care and superb water quality. Once, unclean water sickened a batch of larvae; a veterinarian prescribed antibiotics and a full change of the water, which helped the larvae recover. To avoid dosing larvae with antibiotics again, Patterson and his team improved the water-cleansing system and added a huge UV filter to kill bacteria.

Another “crazy thing” about Diadema , Patterson says, is that “the larval stage is nothing like the adult stage.” Inside each larva, a small urchin grows, waiting to metamorphose like a caterpillar about to turn into a butterfly. Or maybe, Patterson muses, it’s more like a spaceship carrying a little alien inside it.

If all goes well, the larvae turn into miniature versions of the adults in four to six weeks. Patterson shows me a photo of a new urchin under a microscope, pointing out the minute skeleton ball, radiating spines, and comically enormous tube feet that the urchin will hopefully grow into. Altogether, it’s about one millimeter across—no bigger than the point of a pencil.

After larvae transform into tiny urchins, researchers move them to broad, shallow tanks in the greenhouses next door—the next stop in their journey to the sea—where they’ll grow without the threat of predators. In the greenhouses, the urchins sometimes share tanks with small coral colonies to help keep the coral algae-free. Pilnick points out a tank peppered with year-old urchins whose bodies measure less than 10 centimeters across, their spines as long as pens.

The tanks include blocks of rock and pieces of sliced PVC pipe that look like little urchin carports where the animals can shelter. Researchers hope the urchins will use these structures to behave nocturnally, hiding during the day and coming out at night to feed. In the wild, this instinct helps urchins avoid being munched on by crabs, fireworms, and queen triggerfish “like little candy morsels,” as Pilnick puts it. When we peer into the tank, however, some urchins are sheltering in the carports or under rocks and others aren’t, suggesting that not all of the lab-raised urchins have the instinct to hide.

“If you’re kept in a fish tank, you behave differently as an urchin than you would on the reef,” Pilnick explains. “That could have some really big implications for things like predation or migration.”

An adjacent tank is brimming with about a dozen fully grown urchins collected from patch reefs near the Florida Keys. Pilnick picks one up by slipping beneath it a large two-pronged fork, a device designed specifically to move urchins around without getting pricked by poisonous spines. This is one of the parents of all the young urchins raised at the lab—5,403 of them as of this past April. The number isn’t yet high enough to restore entire ecosystems, but Pilnick says it’s “leaps and bounds” ahead of where they started in 2018.

On a whiteboard is a Diadema scorecard, a list of all the cohorts raised in the lab. Several of the first attempts, in 2018 and 2020, failed to produce any urchins, but the following year, the team successfully raised 100 adults. By mid-2022, they were consistently producing urchins; a cohort from late 2023 had more than 1,800.

Now that researchers have figured out how to raise Diadema , the next step is to learn what happens to those lab-raised urchins—and the ecosystems they support—in the wild.

When researchers poured a cohort of young urchins into the shallow water of the middle Florida Keys in 2021, the spiky orbs scuttled about rocky and sandy patches of seafloor in search of shelter. They zoomed toward cracks between rocks and crowded below branching staghorn corals. Since then, urchin-lab partners have released other cohorts and are studying how lab-raised urchins react in the wild, whether they make a difference on reefs, and what strategies may help more survive. It’s not easy to track where the specific urchins go, however. Urchins can’t be tagged like other wildlife and are hopefully hidden during the day, making them hard to find.

[ Read: The sea-urchin murderer has finally been apprehended ]

“We’ve come a long way but obviously still have a ton more to do,” Patterson says. Despite the unknowns, he’s optimistic. He and Pilnick found that even sparse adult urchins—just one urchin for every six and a half square meters of reef—can curtail algae. “I mean, these things eat a lot. It’s kind of amazing.”

But a lack of urchins and other grazers is just one of many problems affecting reefs. Marine heat waves, now supercharged by climate change, are a particularly grave threat. In 2023, unprecedented ocean temperatures in Florida and in the Caribbean caused widespread coral bleaching and mortality. This was the start of a global bleaching event, the fourth ever documented and the second in the past decade alone.

Successful restoration of reefs, including urchins, isn’t an excuse to not tackle climate change, Patterson notes. Voracious urchins won’t prevent marine heat waves or protect corals from bleaching. Still, the urchins could help reefs bounce back after a heat wave, buying time while we reduce fossil-fuel emissions. “We’re doing this one little thing over here to try to keep things together while these much larger issues get fixed,” Patterson says.

When corals die during a heat wave, space opens up on the reef that, without urchins, is soon covered by algae. The same is true in the greenhouse aquariums. Without urchins, the surfaces in the tanks would be coated with fuzzy green algae, thwarting baby corals from growing. In one of the greenhouse tanks, though, Pilnick points out an urchin about the size of a pea that has scoured the algae from around a small coral. The urchin is just eating, as all animals do, but it’s also creating space for the coral to grow. With a little help, both creatures may one day be part of a wild reef with habitats for a diverse array of life.