Scientists investigating a widespread and sudden die-off of long-spined sea urchins that spread nonstop for much of 2022 across the Caribbean and Florida have identified a tiny parasite as the killer.
But while the breakthrough came quickly thanks to social media, avid divers and advances in DNA, figuring out how to protect the species — which is critical to the health of reefs — remains a challenge.
The parasite has been known to infect fish, but scientists had never seen such a lethal infection in urchins.
"The thing about this disease is that once it arrived somewhere, it started killing them so fast that if you weren't there within two or three weeks, it was over," said Don Behringer, a marine pathologist at the University of Florida who studies emerging diseases and an author of the study published this month. “They were dead, they were gone, and there was nothing left except little piles of spines.”
As it raced around the Caribbean and north to Florida, the outbreak alarmed scientists, who worried that struggling urchins might finally be dealt a lethal blow.
In the early 1980s, a disease that has never been diagnosed wiped out 98% of the region’s long-spined urchins in just 13 months. Only about 12% rebounded. Efforts have been underway to breed urchins in labs and repopulate the reefs. But numbers remain low.
“To identify a die-off like this and figure out what it is as quickly as we did is remarkable and it's critical.”Don Behringer, marine pathologist at the University of Florida
“Then this appears almost 40 years later exactly, so it was striking,” Behringer said.
While not as plentiful as other urchins, long-spined sea urchins play a critical role on the reef. Their voracious appetite for algae helps keep the reef clean and healthy for coral, which are also fighting to survive as climate change raises water temperatures, increases ocean acidity and makes coral more vulnerable to diseases like stony coral disease.
The die-off first appeared off the U.S. Virgin Islands in January 2022. It quickly spread to Jamaica, then far to the west in the Mexican Caribbean off Cozumel, then to Saba and St. Eustacious in the Windward Islands, Behringer said.
Scientists raced to muster their resources. They posted alerts on dive web sites, scoured social media for reports of dead urchins and enlisted divers participating in an ongoing network that helps assess reef health. Emails - something that didn’t exist in the 1980s — meant information was exchanged almost instantly. Behringer received a rapid response grant from the National Science Foundation that allowed divers to rush to sites to collect samples.
READ MORE: 'Nothing Is Accidental In Nature.' Why Scientists Are Racing To Save Sea Urchins
With the samples in hand, scientists were then able to take advantage of advances in DNA technology.
“Just zipping a sample off and getting its DNA sequence and then being able to match that because now there's libraries of sequences out there,” Behringer said. “So you get sequences from something and you basically just check it against everything that's already known and boom, you find out it's 96% similar to this thing that already exists.”
The culprit was identified as a microscopic protozoan parasite. To prove it was the killer, scientists first identified it by comparing its DNA to the DNA library, then infected healthy lab-bred urchins, which quickly died.
“To identify a die-off like this and figure out what it is as quickly as we did is remarkable and it's critical,” Behringer said.
The next, more difficult challenge will be figuring out how to stop it, he said.
“These are wild animals in the sea and so you can't vaccinate something that doesn't have an adaptive immune system in the same sense that we do,” he said. “Really, the goal is to figure out what might have triggered [the disease] to emerge. Is it something that's always been there? Is it something that infects other organisms and might have jumped to these urchins? What caused it to become pathogenic.”
Stressors like rising ocean temperatures, increasing salinity and ocean acidification could be causes, he said. It might also be related to the size of the urchins killed or the way they congregate. It’s possible only dense populations got wiped out. It could be tied to water quality, especially near ports where researchers suspect ballast water has helped spread deadly stony coral disease.
“So is there something that maybe compromised the urchins in those areas and gave this parasite a leg up and got it going,” he said.
Experiments are now ongoing to root out those factors. One theory is that the parasite experienced sudden and explosive growth at several locations and quickly dispersed in water, carried by currents, sargassum or even fish. So far, it appears to be especially hardy, Behringer said
This past summer, the die-off appeared to be waning, with no new reports. But just this month, divers reported dead urchins littering the sea floor off Cayman Brac, the easternmost island in the Caymans, south of Cuba.
“So I don't think we're necessarily out of the woods,” Behringer said. “The timing is very similar to last year, right? The urchin epizootic was still underway. And so it's not surprising that this time of year, if it was going to crop back up, it would do it now.”
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