Mystery pathogen is stripping sea urchins of their flesh and turning them to skeletons — and it's spreading fast

A sudden and deadly epidemic sweeping across the Red Sea has killed an entire species of sea urchin, stripping their flesh and turning them into skeletons. 

Just two months ago, thousands of black sea urchins (Diadema setosum) lived in the Gulf of Aqaba, in the northern tip of the Red Sea, keeping the corals there healthy by snacking on excess algae. Now, only their skeletons remain, after their tissue was consumed by a mysterious pathogen. 

"It's a fast and violent death: within just two days a healthy sea urchin becomes a skeleton with massive tissue loss," Omri Bronstein, a senior lecturer in Zoology at Tel Aviv University, said in a statement. "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."

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Researchers spotted the first signs of the urchin plague in the Mediterranean Sea at the beginning of the year, when an invasive species of urchin began falling sick in waters around Greece and Turkey. From there, the disease appears to have spread southward through the Suez Canal to the Red Sea. 

Scientists are unsure of the exact disease causing the mass die-off, but they suspect it is a pathogenic ciliate parasite — a single-celled microorganism — which in 1983 eliminated the Caribbean’s entire sea urchin population. Before the parasite plague, the Caribbean was home to thriving tropical reefs, but since losing the sea urchins the reefs have been smothered by algal blooms that multiplied unchecked, blocking out sunlight and destroying around 90% of the region’s coral. 

The disease was only identified after a second wave hit the Caribbean in 2022, an event which gave scientists a second opportunity to study it.

"The sea urchins are the reef's 'gardeners' — they feed on the algae and prevent them from taking over and suffocating the corals that compete with them for sunlight," Bronstein said. "Unfortunately, these sea urchins no longer exist in the Gulf of Eilat [Aqaba] and are quickly disappearing from constantly expanding parts of the Red Sea further south."

The imperilment of the region’s corals is significant on both a local and global level. The Gulf of Aqaba is known for its numerous diving spots and is a popular tourist destination. And because the corals there evolved to high temperatures and salinity over millions of years, they are more resistant to climate change-driven temperature fluctuations that are killing off other coral reefs around the world. 

—'Gardens and graveyards' of coral discovered in hidden canyons off Australia's coast

—Massive 'pristine' reef of giant rose-shaped corals discovered lurking unusually deep off the coast of Tahiti

—400-year-old 'Muga dhambi' is one of the largest and oldest corals in the Great Barrier Reef

"It must be understood that the threat to coral reefs is already at an all-time peak, and now a previously unknown variable has been added," Bronstein said. "This situation is unprecedented in the entire documented history of the Gulf of Eilat [Aqaba]."

Bronstein says that to prevent the entire sea urchin populations of the Red Sea and the Mediterranean from being eradicated, urgent action is needed to establish uninfected broodstock populations of the urchins so that they can be returned to the oceans once the epidemic is over.

"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," Bronstein added.

The researchers published their findings May 24 in the journal Royal Society Open Science.