Another Killer on Caribbean Reefs

Long-spined sea urchins, or Diadema antillarum, once blanketed many Caribbean reefs, grazing on the algae that grow on corals.

But a year ago, they began displaying strange symptoms - their sharp spines drooping or falling off, their suction-cup feet losing their grip. And they started dying in droves, from the Virgin Islands to Puerto Rico to Florida. It was a repeat disaster, similar to one in the 1980s that slashed sea urchin populations by around 98 percent.

Before the 1980s die-off, U.S. Virgin Islands' reefs were blanketed with the healthy spiny creatures. Now, they are choked by algae, struck by coral disease, and stressed from rising temperatures.

Last year, the die-off began to repeat itself, throwing the reef ecosystem into chaos again. Scientists think they have solved the mystery. Reporting in the journal Science Advances, they have found a tiny single-celled parasite - a ciliate - that shows up only in sick urchins. They don't know how to treat it, but at least they know the culprit.

What to do about it is the next mystery to solve.