Scientists Solve a Sea Urchin Murder Mystery

A microscopic parasite that has been killing the Caribbean creatures since last year might also be at fault for a population collapse four decades ago

purple sea urchins with long spines on rocks underwater
Diadema antillarum sea urchins in the Caribbean Sea Reinhard Dirscherl / ullstein bild via Getty Images

Since last year, long-spined sea urchins in the Caribbean Sea have been dropping dead with no known cause. Now, scientists may finally have found the culprit.

The species, called Diadema antillarum, has faced dire times before: Between 1983 and 1984, roughly 98 percent of them mysteriously died. Their defensive needles that shield their bodies fell off completely, leaving them easy prey for surrounding predators.

Then, the negative effects cascaded: Without as many sea urchins, coral reefs became more vulnerable to algae take-overs and deterioration. Typically, the urchins eat algae, which competes with coral and grows all over it, blocking sunlight. Since that mass urchin die-off, only 12 percent of the original population has been restored—and urchins and coral reefs alike have suffered as a result.

So, in 2022, when the sea urchins suddenly began dying at rapid rates once again, researchers quickly searched for an explanation for the collapse—which could hold serious consequences for the whole aquatic ecosystem.

After four months of sleuthing, scientists reported in Science Advances in April that they successfully solved the sea urchin murder mystery. The killer: a microscopic parasite that appears “cute,” says Mya Breitbart, the team’s lead researcher, to Matt Cohen of the Tampa Bay Times.

“I never thought I would be able to solve a mystery like this in my career,” Breitbart, a biological oceanographer at the University of South Florida, tells the Tampa Bay Times. “We so rarely actually figure out the cause. So just being a part of that was really incredible.”

When Breitbart started seeing the sea urchin deaths increase, she put together a team of 48 scientists from 12 different countries. They worked together tirelessly to crack the case, which happened much faster than they’d anticipated. 

“Science never works like this,” says Breitbart to the Times.

Massive Caribbean sea urchin die-off caused by parasite

The culprit—a parasite known as a ciliate—is a single-celled, fuzzy-looking amoeba that uses its many hairs to swim. Ciliates can commonly be found inside living organisms, including sea urchins, but they are usually harmless to the spiny creatures. This particular type of ciliate, however, called a scuticociliate, has caused mass die-offs of other marine species before, according to a statement.

While the ciliate’s involvement was unexpected, after scientists studied urchins from 23 sites in the Caribbean, there was no denying what had been the murderer. They collected urchins from sites where illness had struck, picking up animals that looked sick and some that looked healthy. For comparison, they also collected healthy urchins from sites that had not seen die-offs. When researchers noticed the parasite was present in the sick urchins’ muscles, they tested whether adding it into a tank would make healthy sea urchins sick. Sure enough, the ciliate infected the animals in the lab, and within just days, they had lost their spines and become weak.

Researchers speculate that this amoeba could be the same parasite that drove the steep sea urchin decline 40 years ago, but because they do not have sea urchin samples from that time period, they can’t confirm this idea.

In the future, climate change could play a role in the parasite-driven demise of other species, Ian Hewson, lead author of the study and a marine ecologist at Cornell University, tells the Atlantic’s Ed Yong. It has put creatures under additional stresses, lowering the strength of their immune systems. Organisms that never interacted before are crossing paths due to climate change forcing them into new areas, resulting in an exchange of pathogens

“Things that maybe weren’t related to mass mortality in the past will start to cause new diseases,” says Hewson to the Atlantic. “I’d fully expect us to see more of this kind of thing in the future.”

While pinpointing the sea urchins’ killer is essential in the process of halting their death, scientists are still trying to figure out the best way forward to save them, and in turn, save the suffering coral reefs.

“Coral reefs in the Caribbean are in trouble,” Don Levitan, a marine biologist at Florida State University who was not involved in the study, tells the Associated Press’ Maddie Burakoff. He remembers a time before the sea urchin die-off, when coral reefs were thriving and the spiky creatures had a large presence in Caribbean waters. “We’re at a different place than we were 30, 40 years ago.”

Although scientists are not yet sure how to solve this parasitic problem, the quick turnaround of the new study has put them ahead of the game for the next step: figuring out solutions.

“This time,” Don Behringer, a co-author and marine ecologist at the University of Florida, says in the statement, “we know the culprit.”

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