TAU researchers identify pathogen causing mass death of sea urchins in the Red Sea

Fish feeding on an urchin carcass in the Mediterranean Sea. Photo: Tel Aviv University.

Epidemic has spread to the Indian Ocean and poses imminent threat to coral reefs

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A continuing study from Tel Aviv University (TAU) has found that a deadly epidemic affecting sea urchins discovered last year has spread across the Red Sea and into the Indian Ocean, which has essentially wiped out Eilat’s most abundant and ecologically significant urchins. The researchers say that what appeared at first to be a severe but local epidemic has quickly spread through the region, and now threatens to become a global pandemic.

Using molecular-genetic tools, the research group at TAU was able to identify the pathogen responsible for the mass mortality of sea urchins of the species Diadema setosum in the Red Sea: a scuticociliate parasite most similar to Philaster apodigitiformis. The researchers explain that this unicellular organism was also responsible for the reoccurring mass mortality of Diadema antillarum in the Caribbean about two years ago, following the notorious 1983 sea urchin population collapse there which led to the a catastrophic phase shift of the coral reef.

The study was led by Dr. Omri Bronstein from TAU’s School of Zoology and the Steinhardt Museum of Natural History (SMNH), together with research students Lachan Roth, Gal Eviatar, Lisa Schmidt, and May Bonomo, as well as Dr. Tamar Feldstein-Farkash from the SMNH. Research partners throughout the region and Europe also took part in the study, which encompassed thousands of kilometers of coral reefs. The results were published on May 23, 2024, in the journal Current Biology.

The researchers estimate that the epidemic has annihilated most of the sea urchin populations of the species affected by the disease in the Red Sea since it broke out in December 2022, as well as an unknown number of sea urchins, estimated at hundreds of thousands, worldwide. Sea urchins are considered the “gardeners” of coral reefs, feeding on the algae that compete with the corals for sunshine. Their disappearance can severely and globally impact the delicate balance on coral reefs. The researchers note that since the discovery of the epidemic in Eilat’s coral reefs, the two species of sea urchins previously most dominant in the Gulf of Eilat have vanished completely.

“This is a growing ecological crisis, threatening the stability of coral reefs on an unprecedented scale,” Dr. Bronstein says. “Apparently, the mass mortality we identified in Eilat back in 2023 has spread along the Red Sea and beyond, to Oman and even as far as Reunion Island in the Indian Ocean.

The deadly pathogen is carried by water and can affect vast areas in a very short time. Even sea urchins raised in seawater systems at the Interuniversity Institute for Marine Sciences in Eilat, or at the Underwater Observatory, were infected and died, after the pathogen got in through the recirculating seawater system.

“In our study we also demonstrated that the epidemic is spreading along routes of human transportation in the Red Sea,” Dr. Bronstein adds. “The best example is the wharf in Nueiba in Sinai, where the ferry from the Jordanian city of Aqaba docks. When we published a report last year, we already knew of sea urchin mortalities in Aqaba, but had not yet identified signs of it in Sinai. The first spot in which we ultimately did identify mortality in Sinai was next to this wharf in Nueiba. Two weeks later the epidemic had already reached Dahab, about 70km further south.

“The scene underwater is almost surreal: seeing a species that was so dominant in a certain environment simply erased in a matter of days. Thousands of skeletons rolling on the sea bottom, crumbling and vanishing in a very short time, so that even evidence for what has occurred is hard to find.”

According to Dr. Bronstein, there is currently no way to help infected sea urchins or vaccinate them against the disease. Dr. Bronstein says that researchers should quickly establish broodstock populations of endangered species in cultivation systems disconnected from the sea so that in the future they can be reintroduced into the natural environment.

“Unfortunately, we cannot repair nature, but we can certainly change our own behavior,” Dr. Bronstein concludes. “First of all, we must understand what caused this outbreak at this time. Is the pathogen transported unknowingly by seacraft? Or has it always been here, erupting now due to a change in environmental conditions? These are precisely the questions we are working on now.”

"This is a growing ecological crisis, threatening the stability of coral reefs on an unprecedented scale."