Don Levitan, a professor of biological sciences at Florida State University, captures a sea urchin known as Diadema antillarum. Levitan is researching the cause and impact of two massive dieoffs of the Caribbean species.

FL/Carib - Sea urchin die-off threatens reefs from Florida to Caribbean. Scientists hope to revive them

These days, long-spined sea urchins are known as the gardeners of the sea. They tend the algae on the coral reefs they call home, making sure it never overwhelms their hosts. Spotting one on the Florida reef tract is a good sign that nearby corals are doing OK.

Decades ago, their reputation was a little different. They were viewed as damaging nuisances — to divers and to reefs.

The first time marine scientist Don Levitan saw the reefs near the U.S. Virgin Islands, they were blanketed in black — the coral covered by thousands of urchins spiked with sharp, poisonous spines.

“It was so dense it looked like a reef of sea urchins,” said Levitan, a professor at Florida State University. “You couldn’t even walk into the water.”

That was in 1983, six months before a mysterious disease all but wiped out the population that reached throughout the Caribbean, including Florida’s reef tract. The average mortality rate across the Caribbean, he said, was 95%. In the following years, it would become clear that too few urchins would turn out to be worse for reefs than too many of them.

Now, after decades of gradual recovery, the population of this specific type of sea urchin, known formally as Diadema antillarum, has dramatically declined again. A recent paper led by Levitan, published in the journal of the Proceedings of the National Academy of Sciences, found that a die-off that began in early 2022 was equally devastating: 98% of the Diadema population was wiped out, once again.

That looms as another blow to struggling coral reefs across the entire region, including Florida.

Diadema are known as the “billy goats of the sea.” Their favorite food is the macroalgae that can clump along coral, cutting off the oxygen it needs to survive. Diadema are prolific grazers, and if there are enough of them around, they create little algae-free zones— also called halos — around coral that help them survive.

The corals also give back to the urchins. They provide nooks and crannies to hide from hungry predators like triggerfish or hogfish.

But Florida’s corals aren’t in great shape these days. Climate change has made ocean water hotter and more acidic, causing coral bleaching. The widespread and devastating stony coral tissue loss disease has weakened scores of once-strong reefs. And plumes of pollution from leaky septic tanks and sewage spills are choking out coral with too many nutrients.

That’s why, Levitan said, they need all the help they can get from Diadema. But unlike other spots in the Caribbean, Florida saw some of the slowest recovery between the first die-off in the 1980s and the second recent one, so the natural population is nearly nonexistent.

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