A silent ocean pandemic is wiping out sea urchins worldwide

When sea urchin populations grow too large, however, especially if their natural predators are heavily hunted or overfished, the balance can flip. In these cases, intense grazing by urchins can strip seafloors of plant life, damaging marine habitats and creating so-called "urchin barrens."

Global sea urchin pandemic reaches the Canary Islands

A recent study published in Frontiers in Marine Science reports that, over the past four years, a previously unrecognized pandemic killing sea urchins worldwide has also struck the Canary Islands. Scientists are still working to understand the full ecological consequences, but expect the effects on marine ecosystems to be significant.

"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."

Diadema africanum from overabundant grazer to sudden collapse

The genus Diadema consists of eight known species that live in warm subtropical and tropical seas around the world. One of these, D. africanum, historically flourished on rocky reefs along the coast of western Africa and around the Azores, generally at depths between five meters and 20 meters.

In the Canary Islands, numbers of D. africanum have been climbing since the mid-1960s, probably driven by a combination of overfishing of its predators and ongoing global warming. At several locations in the archipelago, this population boom was so intense that it led to extensive urchin barrens. From 2005 to 2019, managers attempted biological control measures to reduce urchin numbers, but these efforts ultimately did not succeed.

Rapid sea urchin die-off spreads across the archipelago

In February 2022, Cano and colleagues noticed that D. africanum had started to die off in large numbers off La Palma and Gomera, islands in the western part of the Canary archipelago. Over the rest of that year, the disease moved eastward across the islands. Infected sea urchins became less active, moved in unusual ways, failed to respond to external stimuli, and eventually lost their flesh and spines before dying.

The team recognized these symptoms from earlier episodes. This was not the first time Diadema populations in the region had suffered severe die-offs. In early 2008, and again in early 2018, a disease killed about 93% of D. africanum off Tenerife and La Palma, and about 90% off the islands of neighboring Madeira.

The 2022 outbreak, however, showed a crucial difference. After the 2008 event, many urchin populations rebounded, sometimes relatively quickly. Following the 2022 mortality, that kind of recovery did not appear to be happening. Instead, a second wave of mass mortality swept through the Canary Islands during 2023.

Surveys reveal historic lows for Diadema africanum

To understand how severe these losses were, Cano et al. carried out extensive field surveys of D. africanum at 76 sites across the seven main islands of the archipelago. They worked from the summer of 2022 through the summer of 2025 and compared their counts with historical records.

The researchers also gathered information from professional divers, asking them to report on the relative abundance of D. africanum at their regular dive locations in 2023 and between 2018 and 2021. To investigate reproduction and early life stages, the team set traps to capture drifting larvae at four sites off eastern Tenerife in September 2023, when spawning typically peaks each year. They then measured how many newly settled juveniles appeared 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."

Reproduction collapse and local extinction risk

The authors concluded that, following the 2022-2023 crisis, D. africanum is barely reproducing along the eastern coast of Tenerife. Only extremely small numbers of larvae were caught in the traps, and no early juveniles were detected in any of the shallow rocky habitats that were 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.

Mystery pathogen and uncertain future for 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," 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 populationsin Southeast Asia and Australia, which is good news -- but we cannot rule out the possibility that the disease will reappear and potentially spread further."