Mass mortality of sea urchins threatens to destroy Eilat coral reef

TAU researchers: a parasite is killing the sea urchins essential to the sustainability of coral reefs

 Fish feeding on a dying D. setosum urchin in the Mediterranean Sea (photo credit: Courtesy of Tel Aviv University)
Fish feeding on a dying D. setosum urchin in the Mediterranean Sea
(photo credit: Courtesy of Tel Aviv University)

"The mass mortality of sea urchins in the Mediterranean Sea has spread to the Gulf of Eilat and threatens to destroy the coral reef,” warns a Tel Aviv University research team led by Dr. Omri Bronstein.

Sea urchins in general, and particularly black sea urchins, known as Diadema setosum, are essential for coral reefs’ healthy functioning. Without them, a coral reef can become dominated and overcome by algae.

This story began two decades ago, when these sea urchins invaded from the Red Sea to the Mediterranean Sea off the coast of Turkey. Bronstein and his team followed the urchin invasion for about 15 years, including when the first one was discovered off the coast of Tel Aviv in 2016. Then, between 2018 and 2019, the situation started to change, and the small population of invading black sea urchins grew exponentially.

“They grew so much that, in some sites in southern Turkey, there are now populations of tens of thousands of individuals,” Bronstein explained. “This is called a population outbreak.”

Bronstein and the team were preparing to file a paper on why the Mediterranean environment was ripe for these species. But as they completed that report, they started hearing about mass sea urchin mortalities from colleagues in Turkey and Greece.

“As we worked on studies summarizing the invasion of sea urchins in the Mediterranean, we began to receive reports on sudden extensive mortality,” he said. “Supposedly, the extinction of an invasive species is not a bad thing, but we must be aware of two major risks: First, we don’t yet know how this mortality and its causes might impact local species in the Mediterranean; and second, and much more critical, the geographic proximity between the eastern Mediterranean and the Red Sea might enable the pathogen to quickly cross over to the natural population in the Red Sea. As we feared and predicted, this appears to have happened.”

Disappearance of sea urchins in the Caribbean

Bronstein and the team immediately started following this story, and the first thing they noticed was that something felt familiar about the situation. The mass mortality reminded the TAU team of the famous disappearance of sea urchins in the Caribbean.

“Until 1983, the Caribbean coral reef was a thriving tropical reef, quite similar to the coral reef in the Gulf of Eilat,” Bronstein said. “Once the sea urchins disappeared, the algae multiplied without control, blocked the sunlight from reaching the corals, and the entire reef changed irreversibly – from a coral reef to an algae field.

“A group of researchers from Cornell University identified the cause of mortality in the Caribbean: a pathogenic ciliate parasite. The pathology observed in the sea urchins dying in Greece and Turkey is identical to the pathology in the Caribbean, and it’s also the pathology we see in the sea urchins dying here in the Red Sea,” he said.

Bronstein described sea urchins as the “gardeners or lawnmowers” of the coral reef. As vegetarians, they eat large amounts of plant materials, ensuring a balance that allows the corals to thrive against the algae. Unfortunately, the situation has not improved four decades after the Caribbean pandemic.

“At first, we thought it was some kind of pollution or poisoning, or a local chemical spill, from the industry and hotels in the north of the Gulf of Eilat, but when we examined additional sites in Eilat, Jordan and Sinai, we quickly realized that this was not a local incident,” Bronstein stressed. “All findings pointed to a rapidly spreading epidemic. Similar reports are coming in from colleagues in Saudi Arabia.

“Even sea urchins that we grow for research purposes in our aquariums at the Interuniversity Institute and sea urchins at the Underwater Observatory Marine Park in Eilat contracted the disease and died, probably because the pathogen got in through the pumping systems,” he continued. “It’s a fast and violent death: Within just two days, a healthy sea urchin becomes a skeleton with massive tissue loss. While some corpses are washed ashore, most sea urchins are devoured while they are dying and unable to defend themselves, which could speed up contagion by the fish who prey on them.”

Last week, the first in a series of studies from the university was published in Frontiers in Marine Science and a third report is underway, Bronstein said. The papers show how the entire population of black sea urchins in Eilat was wiped out over a few months, with similar extensive mortality also occurring in Jordan, Egypt, Saudi Arabia, Greece and Turkey.

The team has also submitted their report and recommendations for emergency steps to save the coral reefs to the Israel Nature and Parks Authority.

“We must understand the seriousness of the situation: In the Red Sea, mortality is spreading at a stunning rate and already encompasses a much larger area than we see in the Mediterranean,” Bronstein said. “In the background, there is still a great unknown: What is actually killing the sea urchins? Is it the Caribbean pathogen or some new unfamiliar factor? Either way, this pathogen is clearly carried by water, and we predict that in just a short time, the entire population of these sea urchins in both the Mediterranean and the Red Sea will get sick and die.

“In my opinion, we must urgently establish a brood stock population for these sea urchins so that, if needed, we will be able to return them to nature in the future,” he said. “As with COVID-19, at this point, no one knows what will happen – will this epidemic disappear by itself, or will it stay with us for many years and cause a dramatic change in coral reefs? However, unlike the COVID-19 pandemic, in this case, we have no way of vaccinating or treating the sea urchins, and so we must focus all efforts on prevention.”

Bronstein warned that the window of preserving a healthy population of this species in Eilat has already closed, and if a brood stock is going to be established, it needed to be done yesterday.

“This is a complex task, but it is absolutely necessary if we wish to ensure the survival of this unique species that is so critical to the future of coral reefs,” he said. “The question is: Will decision-makers be faster than the disease? I am doing my best to make sure this is the case.”