Surprising culprit found that killed 95% of a sea urchin population

The mystery killer behind a recent mass die-off of a once-common sea urchin species has been identified as a parasitic microorganism called a ciliate.

Long-spined sea urchins (Diadema antillarum) once peppered Caribbean reefs in the millions, but in 1983 urchins began losing their spines, dying and vanishing from the reef within a matter of days. By the following year, 98 per cent of Caribbean long-spined sea urchins were gone.

The urchins had been making a slow recovery in the 40 years since, until the mystery killer struck again in January 2022, this time wiping out up to 95 per cent of the remaining population in the Caribbean. “We’re probably looking at millions [of urchin deaths] across the entire region,” says Ian Hewson at Cornell University in New York.

To investigate, Hewson’s collaborators in the Caribbean collected both healthy and diseased urchins from 23 different reef sites. They sent urchin tissue samples to Hewson’s lab in New York, where he and his colleagues looked for evidence of viruses and pathogens – common culprits of mass die-offs – on a molecular level.

At first, nothing stood out. Then, they looked for genetic signals of microorganisms like fungi and ciliates – tiny organisms covered in hair-like structures that help them move and eat. Hewson noticed that the ciliate Philaster apodigitiformis was abundant in sick urchins and absent from healthy ones.

Researchers then added the living ciliate to tanks with healthy sea urchins in the lab. “After a few days, 60 per cent of the urchins lost their spines and looked identical to the animals that were dying in the field,” says Hewson, suggesting P. apodigitiformis was the cause.

Hewson says the results were “a bit of a surprise” because ciliates are typically thought of as simple degraders that munch bacteria and decaying tissue. While related ciliates have been known to infect sharks, this is the first time it has been found to kill sea urchins.

“The cause of [long-spined sea urchin] die-offs in the Caribbean has long been a mystery,” says Michael Sweet at the University of Derby in the UK. “What this group did was nothing short of amazing.”

Researchers still don’t know what triggers a P. apodigitiformis outbreak in urchins, but hope the work is the first step in developing ways to control its spread, a task Hewson says will be extremely challenging in an aquatic environment.