Tsunami in Tiny Form: How a Microscopic Parasite Shook Caribbean Coral Reefs

It’s amazing something so little could do such large damage. Over 40 scientists, working on research funded by a National Science Foundation RAPID (rapid response research) grant and other organizations, have discovered that a tiny parasite is behind a titanic die-off of long-spined sea urchins (Diadema antillarum) in the Caribbean Sea. The Diadema deaths were first reported in the U.S. Virgin Islands in January 2022. Two months later, urchin fatalities were found across the Lesser Antilles, Jamaica, and the Mexican Caribbean. By June of the same year, the die-offs were detected in most of the Greater Antilles, Florida, and Curacao. 

The problem is that without the herbivore-eating urchins, algae grows unchecked and covers, blocks life-sustaining light and eventually kills corals. The researchers found that the culprit is a microscopic protozoan called Philaster apodigitiformis, an organism known in fish. As of December 2022, it seemed the urchins had stopped dying and new ones were repopulating. However, this April, new reports have come from the U.S. Virgin Islands and the Cayman Islands of urchins dying. The bad news is that scientists don’t know how to treat P. apodigitiformis infections. But they hope that discovering the parasite’s identity may put them one step closer to keeping Diadema sea urchins healthy and the coral reefs they live on.

Update, August 2026: More than three years after this outbreak was first identified, the picture has both improved and expanded. In 2024, researchers with the U.S. Geological Survey tested a range of compounds against the scuticociliate in laboratory conditions and found two that killed the parasite within 15 minutes in vitro — the first real treatment lead since the parasite was identified. That said, the study’s authors were careful to note their efficacy in actual infected urchins still needs to be tested, so a field-ready treatment isn’t here yet.

The bigger surprise is geographic. The same parasite, or a very close relative, has since been linked to mass die-offs far outside the Caribbean: Diadema setosum urchins in the Mediterranean Sea in 2022, and again in the Sea of Oman in April 2023. What looked like a regional crisis in 2022 now looks like a pattern spreading between ocean basins.

Recovery in the Caribbean itself remains slow and uneven. A study near Saba found that a year after the die-off, urchins were reestablishing on a shelter-rich artificial reef at meaningfully higher densities than on nearby natural reefs — evidence that fish predation and a lack of hiding spots, more than a shortage of new larvae, are what’s holding wild populations back. It’s a hopeful sign for reef-restoration projects using artificial structure, even as scientists still don’t have a cure for the parasite itself.

Carol Bareuther has written for ALL AT SEA since 2004.