Deadly pandemic threatens worldwide wipe out of sea urchins

By Stephen Beech

Sea urchins face extinction worldwide due to a deadly "pandemic" beneath the waves, warns new research.

Scientists say the consequences for marine ecosystems aren’t yet fully known, but are likely to be "profound."

A report shows that an unrecognized disease, which has been wiping out sea urchins around the globe over the last four years, has hit the Canary Islands.

Researchers explained that the iconic species are ecosystem engineers, the marine equivalent of "mega-herbivores" - such as elephants - on land.

By grazing and shredding seaweed and seagrass, they control the growth of algae and promote the survival of slow-growing organisms such as corals and some calcifying algae.

They are also prey for several marine mammals, fish, crustaceans, and sea stars.

But when they become overabundant, for example when their predators are overhunted or overfished, sea urchins can also inflict substantial damage to marine habits and form so-called "urchin barrens".

Study co-author Iván Cano said: “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.

“At approximately the same time, the 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, including D. africanum, which used to thrive on rocky reefs off western Africa and the Azores.

Its numbers had been rising since the mid-1960s in the Canary Islands, which the research team say was likely due to overfishing of predators and global warming.

Its increased numbers even caused "urchin barrens" in the past, which prompted unsuccessful efforts at biological control between 2005 and 2019.

In February 2022, the research team 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 researchers recognised the symptoms, because it wasn't 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 urchins off Tenerife and La Palma, and 90% off the islands in the neighbouring Madeira archipelago.

But the researchers found the 2022 outbreak was different.

While many affected populations had recovered – sometimes surprisingly fast – after the 2008 event, that didn’t seem to happen in 2022.

Instead, a second wave of mass urchin deaths struck the Canary Islands over the course of 2023.

To assess the impact of the die-off, the research team 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 the data to historical data.

They also invited professional divers to give information on its relative abundance at their usual dive sites in 2023 and between 2018 and 2021.

The research team also used traps to collect dispersing larvae at four sites off Tenerife in September 2023, the annual peak of the spawning season.

They quantified the number of newly settled juveniles at the same sites in January 2024.

Cano, a doctoral student at the University of La Laguna on Tenerife, said: “Our analysis showed that the current abundance of D. africanum across the Canary Islands is at an all-time low, with several populations nearing local extinction.

“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.”

The research team 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, according to the findings published in the journal Frontiers in Marine Science, and no early juveniles were seen in any of the shallow rocky habitats surveyed.

Cano said: “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.

“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.

“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.”

He added: “We aren’t yet sure how this pandemic will evolve.

"So far, it seems to have not spared to other Diadema populations in South East Asia and Australia, which is good news – but we cannot rule out the possibility that the disease will reappear and potentially spread further.”