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Scientists have solved the mysterious death of Caribbean sea urchins

Last year there seemed to be a deadly plague lurking beneath the crystal blue waters of the Caribbean. Killing sea urchins at a rate not seen for decades. For months, no one knew what caused it.

Now, scientists say they have identified the mystery killer.

The giant problem was caused by none other than an organism so small, it was made up of just a single cell – a tiny parasite known as a ciliate.

A team of researchers has uncovered the mystery, with long-spined sea urchins losing their spines in just a few days and dying “in groups,” according to a University of South Florida press release. Dive shops first began reporting the situation in February, but it is believed that the “urchin graveyard” spanning thousands of miles from the US Virgin Islands and the Caribbean to Florida’s east coast began a month earlier.

“This project in particular is a bit like a mystery novel, basically Whodunit? Who’s Killing the Urchins?” said Ian Hewson, Cornell professor of microbiology and co-author of the study.


Pins showing locations where diseased sea urchins were observed in 2022.

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Scientists were immediately concerned about the incident, as sea urchins — vital marine animals that eat algae that otherwise destroy coral reefs — are still recovering from another mass die-off in the area that occurred 40 years ago. The incident killed 98% of the region’s long-spined sea urchin population, scientists said. The cause of death in the early 80s has yet to be determined.

“When urchins are removed from an ecosystem, essentially the corals cannot survive because they are overgrown by algae,” Hewson said in the Cornell press release, a problem that is increasingly important to address as coral bleaching is expected to increase due to global warming. The incidence is so high that the United Nations says it will “Catastrophic” for reef systems.

Maya Breitbart, lead author of the study, published Wednesday in Science Advances, said her team was “not thrilled” but “shocked” to realize what happened so quickly. What would normally take decades to determine, his team pulled off in just four months.

“At the time we didn’t know if these deaths were caused by pollution, stress, anything else — we just didn’t know,” Hewson said in a USF release.

To solve the mystery, they looked at urchins from 23 different sites across the Caribbean. And there was a clear similarity between them that was affected by the event – ciliates. Ciliates are tiny organisms covered in cilia, which look like tiny hairs, which help them move around and eat.

“They are found almost anywhere there is water,” according to a press release from the University of South Florida, where Breitbart works. “Most are not disease-causing agents, but this one is. It’s a specific type called a scuticosylate.”


On the left, a ciliate culture is viewed under the microscope. Right, an infected sea urchin is seen next to a healthy one in St. John’s in April 2022.

Mackenzie Kerr, USF College of Marine Science (left) and Ian Hewson, Cornell University (right)

The ciliate breed has been linked to mass die-offs of other marine species, the scientists said, but this is the first time it has been linked to the massive decline of sea urchins.

Researchers were excited to figure out the cause, but even though they figured out who the mystery culprit was, they still didn’t understand how or why it started in the first place.

One theory is that ciliates have seen an “explosive growth,” the researchers said in their study. But more research is needed to determine if this was a major factor.

The finding may also help answer other questions about what’s happening underwater nearby. Microbiologist Christina Kellogg says there is some overlap between where urchins are dying and where disease is destroying stony coral tissue. Coral populations.

“Almost never can we prove in a wildlife environment, at least in marine habitats, that a microorganism is actually responsible for disease,” Hewson said. “…Knowing the identity of the pathogen can also help reduce the risk of uncontacted diadem through things like boat traffic, dive gear, or other means that turn it around.”

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