Pandemic ‘beneath the surface’ has been quietly wiping out sea urchins around the world

(Press-News.org) Sea urchins are ecosystem engineers, the marine equivalent of mega-herbivores on land. By grazing and shredding seaweed and seagrass, they control algal growth and promote the survival of slow-growing organisms like corals and some calcifying algae. They are likewise prey for a plethora of marine mammals, fish, crustaceans, and sea stars. However, when they become overabundant, for example when these predators are overhunted or overfished, sea urchins can also inflict substantial damage to marine habits and form so-called ‘urchin barrens’.

Now, a study in Frontiers in Marine Science has revealed that over the last four years, an unrecognized pandemic that has been wiping out sea urchins around the world has hit the Canary Islands. The consequences on marine ecosystems aren’t yet fully known, but likely profound.

“Here we show the spread and impacts of a ‘mass mortality event’ which severely hit populations of the sea urchin Diadema africanum in the Canary Islands and Madeira through 2022 and 2023,” said Iván Cano, a doctoral student at the University of La Laguna on Tenerife in the Canaries Islands, Spain.  

“At approximately the same time, ther Diadema species have been observed to be dying off in the Caribbean, the Mediterranean, the Red Sea, the Sea of Oman, and the western Indian Ocean.”

The genus Diadema comprises eight species inhabiting subtropical and tropical waters around the globe. These include D. africanum, which used to thrive on rocky reefs off western Africa and the Azores, between five meters and 20 meters deep. In the Canary Islands, its numbers had been increasing since the mid-1960s, likely due to overfishing of predators and global warming. At some sites in the archipelago, its increased numbers even caused ‘urchin barrens’ in the past, which prompted unsuccessful efforts at biological control between 2005 and 2019.

Sea change

In February 2022, Cano and colleagues observed that D. africanum had begun to die en masse off the islands of La Palma and Gomera in the western Canaries. Spreading eastwards across the archipelago over the course of that year, the disease caused the sea urchins to move less and in an abnormal way, become unresponsive to stimuli, and lose their flesh and spines before dying.

The scientists recognized these symptoms, because this was not the first outbreak of such mass mortality events in the islands. In early 2008, and again in early 2018, a disease killed an estimated 93% of D. africanum individuals off Tenerife and La Palma, and 90% off the islands in the neighboring Madeira archipelago.

But the 2022 outbreak was different: while many affected populations had recovered – sometimes surprisingly fast – after the 2008 event, this didn’t seem to happen in 2022. Rather, a second wave of mass mortality struck the Canary Islands over the course of 2023.

To assess the impact of the die-off, Cano et al. surveyed D. africanum numbers at 76 sites across the archipelago’s seven main islands between the summer of 2022 and the summer of 2025, comparing these to historical data. The authors also invited professional divers to give information on its relative abundance at their usual dive sites in 2023 and between 2018 and 2021. They then used traps to collect dispersing larvae at four sites off eastern Tenerife in September 2023, the annual peak of the spawning season. Finally, they quantified the number of newly settled juveniles at the same sites in January 2024.

“Our analyses showed that the current abundance of D. africanum across the Canary Islands is at an all-time low, with several populations nearing local extinction,” said Cano.

“Moreover, the 2022-2023 mass mortality event 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.”

Prickly issue

The authors also concluded that since the 2022-2023 event, effective reproduction of D. africanum has mostly ceased on the eastern coast of Tenerife: only negligible numbers of larvae were caught in the traps, and no early juveniles were observed in any of the shallow rocky habitats surveyed.

“Reports from elsewhere suggest that the 2022-2023 die-off in the Canary Islands was another step in a broader marine pandemic, with serious consequences for these key reef grazers,” concluded Cano.

“We don’t yet know for certain which pathogen is causing these die-offs. Mass mortality events of Diadema elsewhere in the world have been linked to scuticociliate ciliates in the genus Philaster, a kind of single-celled parasitic organisms,” said Cano.

“Previous die-offs in the Canary Islands were associated with amoebae such as Neoparamoeba branchiphila and followed episodes of strong southern swells and unusual wave activity, similar to what we saw again in 2022. Without a confirmed identification, we cannot say whether the agent arrived from the Caribbean by currents or shipping, or whether climate change is to blame.”

“We aren’t yet sure how this pandemic will evolve. So far, it seems to have not spared to other Diadema populations in Southeast Asia and Australia, which is good news – but we cannot rule out the possibility that the disease will reappear and potentially spread further.”

END