A team of researchers from the University of La Laguna and the Institute of Tropical Diseases and Public Health of the Canary Islands has demonstrated that a mass mortality event between 2022 and 2023 has led to the local near-extinction of the sea urchin Diadema africanum.
This latest outbreak has had a greater impact than those that occurred in 2008 and 2018, as on this occasion the production of larvae and the recruitment of juveniles has practically ceased, the ULL details in a statement.
The pathogens that caused the disease are not yet known with certainty, but similar sea urchin deaths have been reported almost simultaneously from the Caribbean to the western Indian Ocean.
Ecosystem Engineers
Sea urchins are ecosystem engineers, the marine equivalent of terrestrial megaherbivores. By grazing and shredding algae and seagrasses, they control algal growth and promote the survival of slow-growing organisms like corals and some calcifying algaeThey are also prey to a large number of marine mammals, fish, crustaceans, and starfishHowever, when they become overabundant, or when these predators are overfished or overhunted, sea urchins can also cause damage to marine habitats and form so-called 'urchin barrens'
Now, a study published in Frontiers in Marine Science has revealed that, in the last four years, an unrecognized pandemic that has been exterminating sea urchins worldwide has affected the Canary Islands.
The consequences for marine ecosystems are not yet fully known, but they are likely to be profound, point out researchers Carlos San Gil, José Carlos Hernández, and Jacob Lorenzo, who have demonstrated the spread and impacts of a mass mortality event that severely affected populations of the sea urchin *Diadema africanum* in the Canary Islands and Madeira between 2022 and 2023.
Almost in parallel, the disappearance of other Diadema species has been observed in the Caribbean, the Mediterranean, the Red Sea, the Sea of Oman, and the western Indian Ocean
The genus Diadema
The genus Diadema comprises eight species that inhabit subtropical and tropical waters worldwide. Among them is D. africanum, which used to thrive on rocky reefs off West Africa and the Azores, at depths of between five and twenty meters.
In the Canary Islands, its population had been increasing since the mid-1960s, likely due to overfishing of predators and global warming
In some places in the archipelago, the increase in their population even caused 'urchin barrens' or 'urchin deserts' in the past, which motivated unsuccessful biological control efforts between 2005 and 2019.
In February 2022, researchers observed that the African sea urchin Diadema africanum had begun to die off en masse in La Palma and La Gomera.
As the disease spread eastward across the archipelago that year, it caused sea urchins to move less and abnormally, become insensitive to stimuli, and lose their spines before dying
Scientists recognized these symptoms, as this was not the first outbreak of this type of mass mortality on the islands.
In early 2008, and again in early 2018, a disease wiped out approximately 93% of individuals of Diadema africanum on the coasts of Tenerife and La Palma, and 90% on the islands of the neighboring archipelago of Madeira.
But the 2022 outbreak was different: while many affected populations recovered, sometimes with surprising speed, after the 2008 event, this did not seem to happen in 2022Instead, a second wave of mass mortality struck the Canary Islands throughout 2023
To assess the impact of the mortality, researchers analyzed populations of Diadema africanum at 76 points across the archipelago's seven main islands between the summers of 2022 and 2025, comparing them with historical data.
The authors also invited professional divers to provide information and subsequently used traps to collect dispersing larvae at four points off the east coast of Tenerife in September 2023, the annual peak of the spawning season.
Finally, they quantified the number of newly settled juveniles in the same locations in January 2024.
Researchers' analyses have shown that the current abundance of Diadema africanum in the Canary Islands is at its lowest level, with several populations on the verge of local extinction.
Furthermore, the massive mortality event of 2022-2023 affected the entire population of the species across the archipelago. For example, since 2021, there has been a 74% decrease in La Palma and a 99.7% decrease in Tenerife
Reports from elsewhere suggest that the 2022-2023 die-off in the Canary Islands was a further step in a broader marine pandemic, with serious consequences for these key reef herbivores, points out Iván Cano.
This researcher indicates that it is not yet known with certainty what pathogen is causing these deaths, which in other parts of the world have been linked to scuticociliate ciliates of the genus Philaster, a type of unicellular parasitic organism.
Previous deaths in the Canary Islands were associated with amoebas such as Neoparamoeba branchiphila and occurred after episodes of strong southern swells and, in general, unusual wave activity, similar to what was observed again in 2022.
"Without confirmed identification, we cannot determine if the agent came from the Caribbean, nor if climate change is responsible," he admits
So far, it appears it has not spread to other Diadema populations in Southeast Asia and Australia, which is good news.
However, "we cannot rule out the possibility that the disease may reappear and spread further," Cano points out
