Surprising culprit discovered that killed 95% of sea urchin population TNA

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

Long-spined sea urchins (Diadema antillarum) once dotted the reefs of the Caribbean by the millions, but in 1983 the sea urchins began to lose their spines, dying and disappearing from the reef within days. The following year, 98% of the Caribbean’s longspine urchins were gone.

The kids had made a slow recovery over the next 40 years, until the mysterious killer struck again in January 2022, this time wiping out up to 95% of the remaining population in the Caribbean. “We’re probably looking at millions [of urchin deaths] throughout the region,” explains Ian Hewson at Cornell University in New York.

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

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

The researchers then added the live ciliate to tanks containing healthy sea urchins in the lab. “After a few days, 60% of the sea urchins lost their spines and looked identical to the animals dying in the field,” Hewson says, suggesting P. apodigitiformis was the cause.

Hewson says the results were “a little surprising” because ciliates are generally thought of as simple degraders that nibble away at bacteria and decaying tissue. While related ciliates have known to infect sharksthis is the first time he has killed sea urchins.

“The cause of [long-spined sea urchin] dead in the Caribbean has long been a mystery,” says Michael Doux at the University of Derby in the UK. “What this group did was just amazing.”

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