Last year, sea urchins in the Caribbean began to fall ill, shed their spines, die, and throw coral reef ecosystems into chaos. Now, scientists think they have caught the killer in this marine murder mystery.
Researchers reported Wednesday in the journal A tiny, single-celled parasite responsible for the massive death Science advances.
“Case closed,” said study author Mia Breitbart, a marine microbiologist at the University of South Florida.
Long-spined sea urchins, or Diadema antillarum, are spiky black creatures that hide in coral reefs in the Caribbean. They act like “lawn mowers” for coral reefs, gobbling up algae growing on the reef, Breitbart said.
But in January 2022, these animals began showing strange symptoms—their sharp spines droop and fall off, their sticky feet losing their grip—before dying in droves, from the Virgin Islands to Puerto Rico to Florida.
For marine scientists, it was deja vu: Another wave swept the region in the 1980s and reduced sea urchin populations by about 98%.
This issue has not been resolved. But this time, an international team of researchers sprang into action, sampling sick and healthy urchins from across the Caribbean to search for genetic evidence.
They saw no signs of viruses or bacteria, said study author Ian Hewson, who researches marine diseases at Cornell University. But they did find traces of tiny single-celled organisms called ciliates, which only appeared in sick hedgehogs.
Hewson said that while most ciliates do not cause disease, this species has been linked to other aquatic disease outbreaks, making it a prime suspect.
To confirm the capture of the killer, the scientists placed the parasites in tanks with intact, captive-grown hedgehogs to see how they would react. Of the 10 hedgehogs pitted against the young critters, 60% died—after showing the same symptoms the researchers saw in the wild.
It’s possible that this same parasite also caused death in the 1980s, Breitbart said, but scientists can’t be sure.
And they haven’t figured out a way to treat sick hedgehogs. But they hope knowing the source of the deaths will help preserve coral reefs, especially once more is learned about how the parasites spread, Breitbart said.
These urchin deaths and other stresses have already changed coral reefs, added Don Levitan, a Florida State University marine scientist who was not involved in the study.
Back before the first sea urchin eruption, Levitan recalled seeing coral reefs in the US Virgin Islands covered in spiny creatures. Now, those reefs look a lot different—suffocated by algae, battered by coral diseases and stressed by rising temperatures.
“The coral reefs in the Caribbean are in trouble,” Levitan said. “We are in a different place than we were 30 or 40 years ago.”
Ian Hewson et al, scuticociliate causes mass mortality of Diadema antillarum in the Caribbean Sea, Science advances (2023). DOI: 10.1126/sciadv.adg3200
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