Sea Urchins Face Local Extinction in the Canary Islands

Sea urchins have been vanishing beneath the waves, well away from beaches and promenades, and initially the change drew little notice.

Divers were the first to realise that something was wrong. Fishers soon began reporting the same bare areas.

Scientists then compared observations and survey results, finding that a familiar animal which had once covered rocky seafloors had abruptly disappeared. There were not merely fewer of them: they were absent.

The ecological role of sea urchins

Sea urchins seldom attract much attention, yet they perform an understated but continual function in maintaining the balance of marine ecosystems.

When they disappear, algae rapidly proliferates. Corals find it harder to persist, while fish lose the environments on which they rely. What began as a local issue has become a warning of global significance.

The Canary Islands, located off Africa's north-west coast, have become the most alarming example so far.

Researchers' findings there indicate something that is rarely recorded while it is taking place: local extinction.

A recurring pattern

Sea urchins graze throughout the day on algae growing across rocks and reefs. This continual feeding prevents algae from dominating these habitats. In their absence, algae can form dense growth that blocks sunlight, harming corals and other marine organisms.

This ecological equilibrium has collapsed previously. During the 1980s, most sea urchins disappeared from parts of the Caribbean, and the reefs did not fully recover.

That earlier event is relevant today because an apparently similar pattern is now emerging more quickly and over greater distances.

The Canary Islands situation is distinct not only because adult sea urchins have been lost, but also because juveniles are missing. If no new generation emerges, recovery is almost impossible.

Global evidence of Diadema sea urchin decline

Professor Omri Bronstein of Tel Aviv University led the research alongside an international team from Spain and the Canary Islands.

The study focused on Diadema, the long-spined black sea urchins commonly encountered by people swimming along rocky shores.

At the Canary Islands location, the team established that this genus can no longer produce offspring. This finding is highly significant, suggesting that the population has reached a tipping point rather than simply undergoing decline.

The investigation brought together local citizen-science reports, underwater surveys, satellite data and samples taken directly from the seafloor.

Combined, the evidence exposed a mass mortality event that had initially passed unnoticed in mid-2022.

A disease continuing to spread

Professor Bronstein outlined the broader sequence of events. “In 1983-84, a mass mortality event of Diadema sea urchins was recorded in the Caribbean islands in the western Atlantic Ocean.”

He described the major ecological transformation caused by that die-off. “With the sea urchins – the habitat’s primary algae grazers – gone, vast algal fields spread, blocking sunlight and causing severe, irreversible damage to coral reefs in the region.

Professor Bronstein noted that another mortality event hit the Caribbean in 2022, when the pathogen responsible for the fatal disease was identified for the first time.

“This epidemic spread to the Red Sea by 2023, and by 2024 it was also detected in the Western Indian Ocean, off the coast of Reunion.”

The event in the Canary Islands seems to bridge a missing part of this spread, linking outbreaks in different oceans and demonstrating the speed and distance over which the disease has travelled.

When sea urchins can no longer reproduce

Sea urchins depend on large numbers for their survival. They release eggs and sperm into open seawater, relying on chance encounters to produce larvae. These larvae float for days or weeks before eventually settling on the seabed.

“Sea urchins reproduce by releasing sperm and eggs into the seawater, where fertilization produces millions of embryos that drift as plankton in the water column. After several days to weeks (depending on the species), the larvae settle on the seabed and develop into juvenile urchins – a process known as ‘recruitment,'” explained Professor Bronstein.

“In this study, we discovered that for the first time in history, there are no new juvenile urchins being recruited across several Canary Islands, indicating that the recruitment process has halted since the extensive mortality event that took place there.”

Put differently, Professor Bronstein said that the adult die-off has been so extensive that the species can no longer generate a subsequent generation.

Without recruitment, the available time rapidly diminishes. Surviving adults grow old and die, with nothing to take their place.

Sea urchins near local extinction

Sea urchin numbers commonly fluctuate, and this is normal. Researchers are concerned now by the extent and apparent permanence of the loss. Rather than a decline followed by recovery, it appears to be an endpoint.

The researchers caution that the same result may occur in other areas already affected by mass mortality events, including the Red Sea coast and the Gulf of Eilat. Once algae becomes dominant, undoing the resulting damage is exceptionally difficult.

“In this study, we identified a mass mortality event of sea urchins that occurred in mid-2022 in the Canary Islands,” said Professor Bronstein.

“In its aftermath, it became clear that the affected species is no longer capable of successfully reproducing in this area – a finding that may lead to local extinction, which is expected to have severe ecological consequences.”

“A likely outcome would be the uncontrolled proliferation of algae, which would affect the entire ecosystem – although at this stage, it is difficult to predict exactly in what way.”

The complete study was published in the journal Frontiers.