What caused a massive wipe out of sea urchins in the Caribbean?

By Stephen Beech via SWNS

A deadly parasite caused a mass die-off of sea urchins in the Caribbean Sea - with "devastating" consequences for coral reefs and marine ecosystems.

What was responsible for the huge number of deaths has remained a mystery until now.

Long-spined sea urchins serve as vital herbivores that graze on algae, say scientists.

The algae which will outcompete corals for space and blanket them, block light and kill them if left unchecked.

They explained that, by feeding on algae, the sea urchins are essential to maintaining the health of coral and balance in the marine ecosystem.

Large numbers of sea urchin deaths were first reported at St. Thomas in the US Virgin Islands in late January last year.

By late March 2022, the condition was found across the Lesser Antilles, Jamaica and the Mexican Caribbean.

And by last June, it had been detected in most of the Greater Antilles, Florida and Curacao.

Scientists have been trying to identify the cause of the mysterious illness, which led to urchin population declines of between 85 percent and 95 percent in affected areas.

When sea urchins die, they lose their spines and detach from their anchors.

Now, an international team of 42 scientists has identified the cause as Philaster apodigitiformis, a unicellular eukaryote that is part of a group of 8,000 species called ciliates. P. apodigitiformis, a known parasite in fish.

Study lead author Professor Ian Hewson, a marine ecologist at Cornell University in the United States, said: “Rarely are we afforded the opportunity to understand marine disease events in this detail, where we can actually work out a cause of it."

He said that, although scientists do not yet know how to treat P. apodigitiformis infections, discovering the parasite’s identity may help them design ways of protecting Diadema sea urchins that are being raised for restocking efforts across the region.

Prof. Hewson said: “Knowing the pathogen's identity may also help mitigate risk to untouched Diadema through such things as boat traffic, dive gear, or other ways it may be moved around."

In the early 1980s, long-spined sea urchins were almost completely wiped out in the Caribbean by an unknown cause, leading to around 98 percent declines from previous numbers.

Their populations rebounded 30 years later, but only by an estimated 12 per cent from pre-epidemic numbers.

That die-off led to rapid degradation of many coral reefs around the region that persist today, with some coral species becoming extremely rare.

The cause of the early 1980s outbreak was never determined, although Prof Hewson and his colleagues may now investigate whether P. apodigitiformis can be detected in Diadema museum samples from that time and region.

For the new study, published in the journal Science Advances, the research team collected Diadema samples from 23 sites around the Caribbean.

Prof. Hewson and his team then ran lab tests to identify viral or bacterial pathogens in the tissues using state-of-the-art molecular biological and veterinary pathological techniques.

However, the results were inconclusive.

Prof. Hewson said: “They initially did not show any sort of unusual or candidate microorganisms at all.

“We were a bit at a dead end.”

Prof Hewson decided to investigate genomic signals of eukaryotic microorganisms - such as fungi, ciliates and dinoflagellates.

He said: “Immediately when I did that, I had an enormous signal of this scuticociliate Philaster."

Prof. Hewson ran to the labs and pulled out fluid samples from Diadema that people in the field had collected and put those under the microscope.

He said: “I saw this ciliate was actually very, very abundant. That was the big ‘aha’ moment.”

He said the ciliates were not present in samples from the control sites.

While P. apodigitiformis has been known to infect fish, Prof. Hewson said this is the first time it has been associated with mass mortality in an invertebrate.

Using fresh samples collected from infected Diadema from the Florida Keys and aquacultured Diadema, which had never been exposed to any pathogens, obtained from The Florida Aquarium’s Centre for Conservation, the researchers infected the aquarium Diadema with ciliates isolated from the body fluid of infected Florida sea urchins.

Prof. Hewson said: “Animals that were treated with the ciliate became sick and died in 60 per cent of the cases."

The scientists were then able to isolate and identify the very same P. apodigitiformis ciliate from those newly diseased animals, proving it was responsible for the disease.

Prof. Hewson added: “Almost never are we able in a wildlife setting, at least in marine habitats, to prove that a microorganism is actually responsible for disease."