Caribbean and Indian Oceans Face Deadly Sea Urchin Plague
A deadly pathogen is ravaging sea urchin populations across multiple marine ecosystems.

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In a study led by scientists from Tel Aviv University, researchers have confirmed that the same scuticociliate parasite responsible for past Caribbean die-offs is now wiping out sea urchins in the Red Sea and Indian Ocean. As these crucial "reef gardeners" disappear, coral reefs face a growing threat of algal overgrowth, pushing entire ecosystems toward collapse.
The study was published in Ecology.
The silent crisis unfolding in coral reefs
Sea urchins, often referred to as the "gardeners" of coral reefs, help maintain the delicate balance of underwater ecosystems. By grazing on algae that compete with corals for sunlight, sea urchins prevent overgrowth that could suffocate coral colonies.
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Subscribe for FREEThis delicate balance is now under threat from a deadly pandemic decimating the sea urchin population worldwide, first observed in the Caribbean in 1983. An unidentified pathogen wiped out vast numbers of Diadema antillarum (D. antillarum), triggering an ecological cascade. Without sea urchins to manage algae growth, the coral-dominated reefs transitioned into algae-dominated landscapes, which blocked sunlight and disrupted the ecosystem's stability. Decades later, neither the coral reefs nor the urchin populations have fully recovered.
The crisis resurfaced in 2022, this time targeting the remaining Diadema populations in the Caribbean. Soon after, similar outbreaks were detected in other critical marine ecosystems. A scuticociliate parasite, identified by researchers at Cornell University, was pinpointed as the culprit.
In 2023, scientists from Tel Aviv University documented new mass mortality events in the Red Sea and Indian Ocean.
“Until recently, this was one of the most common sea urchins in Eilat’s coral reefs – the familiar black urchins with long spines. Today, this species no longer exists in significant numbers in the Red Sea. The event was extremely violent: within less than 48 hours, a healthy population of sea urchins turned into crumbling skeletons. In some locations in Eilat and the Sinai, mortality rates reached 100%,” said corresponding author Dr. Omri Bronstein, a senior lecturer in the school of zoology at Tel Aviv University.
Bronstein and his team worked to identify the pathogen responsible for the mass mortality of sea urchins across multiple regions. They conducted a comprehensive study spanning thousands of kilometres of coral reef ecosystems and collected samples from infected sea urchins across multiple regions, including the Red Sea, the Gulf of Aqaba and the Indian Ocean.
Identifying the cause of mass sea urchin deaths
The recent study focused on the outbreak in Réunion Island, where Diadema setosum and Echinothrix diadema populations were devastated – to investigate its spread, potential causes, and ecological consequences for coral reef ecosystems. Researchers documented symptoms similar to those seen in previous outbreaks: infected sea urchins first exhibited sluggish movement, followed by loss of tube feet, detachment of spines and rapid tissue decay.
Using molecular-genetic tools, they successfully identified the pathogen responsible for the 2023 outbreaks: a scuticociliate parasite – the same agent behind the catastrophic decline of Diadema antillarum in the Caribbean.
“This is the first genetic confirmation that the same pathogen is present in all these locations. Now it’s a global event, a pandemic. The Caribbean, Red Sea and the Indian Ocean are critical regions for the world’s coral reefs, and mortality rates for sea urchins in these areas are very high – over 90%. This is a first-rate ecological disaster,” said Bronstein.
Could the sea urchin plague spread?
“As of now, we have no evidence of this pathogen in Pacific Ocean sea urchins, but this is something we are actively investigating. Although we’ve developed genetic tools for the specific identification of the pathogen, it’s difficult to monitor such rapid extinction events in the vast underwater environment. We are terrestrial creatures, and some reefs are located in deep or remote areas. If we miss the mortality event by even a couple of days, we might find no trace of the extinct population,” said Bronstein.
With sea urchin populations collapsing at unprecedented rates, the research community is urgently working to understand the mechanisms behind the outbreak and explore possible mitigation strategies. The implications of this crisis extend far beyond the loss of a single species – coral reefs, already under immense stress from climate change and human activity, now face an additional existential threat. If the unchecked spread of the pathogen leads to widespread reef degradation, the cascading ecological consequences could be profound, affecting biodiversity, fisheries and coastal protection.
“For populations that are already infected, we really have no tools to help them. There is no Pfizer or Moderna for sea urchins – not because we don’t want one, but because we simply can’t treat them underwater,” said Bronstein.
How did this deadly pathogen spread?
The team has put forward two main hypotheses for why the pandemic has erupted at this particular time. The first is the transportation hypothesis, which suggests that human activity has unintentionally introduced the pathogen to new regions. Large cargo ships take in water from one location and release it in another, potentially carrying microscopic marine organisms across oceans.
"Incidentally, if this hypothesis is correct, we would expect to see mortality events in West Africa as well. Indeed, just in the past few weeks, we’ve discovered widespread mortality events in West Africa, as we predicted, and we’ve managed to obtain a limited number of samples collected during these events, which we are currently analyzing in the lab," said Bronstein.
The second hypothesis – the climate-triggered virulence hypothesis – is even more alarming. This theory proposes that the pathogen has always existed in these environments but has only recently become lethal due to changes in ocean conditions, such as rising temperatures or altered water chemistry.
“That’s a challenge of an entirely different magnitude, one that we, as marine biologists, have very limited means to address,” said Bronstein.
We cannot repair nature, but we can change our behavior
In response to the crisis, researchers are working to develop new strategies for early detection and population restoration. One initiative is the breeding nucleus project at the Israel Aquarium in Jerusalem, which serves as a refuge for unaffected sea urchins. Unlike other aquariums in Eilat, which became contaminated through seawater circulation, this facility is completely isolated from the ocean – providing a controlled environment where healthy sea urchins can be raised.
"We genetically test the urchins transferred to the nucleus to ensure they are not carriers of the disease and that they genetically belong to the Red Sea population, enabling us to rehabilitate the population in the future," said Bronstein.
“At the same time, we are using them to develop sensitive genetic tools for early disease detection from seawater samples – essentially creating ‘underwater COVID tests’ for sea urchins,” he added.
"Unfortunately, we cannot repair nature, but we can certainly change our own behavior. 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," said Bronstein.
Reference: Quod J, Séré M, Hewson I, Roth L, Bronstein O. Spread of a sea urchin disease to the Indian Ocean causes widespread mortalities—Evidence from Réunion Island. Ecology. 2025;106(1):e4476. doi: 10.1002/ecy.4476
This article is a rework of a press release issued by Tel-Aviv University. Material has been edited for length and content.