Sea urchin mortality is spreading

Remains of dead sea urchins washed up on the beach of Reunion Island in the Indian Ocean. © Jean-Pascal Quod

The “gardeners” of coral reefs could fall victim to a global pandemic, researchers warn: A disease discovered in 2022 has already decimated sea urchins in the Red Sea and has already spread to the Indian Ocean, their study shows. They have now also been able to identify the cause of the mass extinction more precisely: a parasitic single-cell organism that is apparently spread by shipping.

They keep the weeds in the underwater gardens under control: sea urchins feed on algae that grow on coral structures and thus protect the sensitive cnidarians from rampant competition. If there are too few of the prickly “gardeners”, coral reefs can literally suffocate under the green burden. The importance of sea urchins was first dramatically demonstrated in the Caribbean in 1983: a then mysterious disease had wiped out the echinoderms there in large numbers – with devastating ecological consequences. Since then, there have been further waves of disease, which has prevented the sea urchin populations and the affected reefs from fully recovering.

The last time mass deaths occurred there was in 2022. In this case, however, the disease was apparently not limited to the Caribbean: First, sea urchins died en masse in the Mediterranean and then the disease spread to the Red Sea. Researchers led by Omri Bronstein from Tel Aviv University reported on this last year. Since then, they have devoted themselves intensively to further research into the disease and its spread.

Already reached the Indian Ocean

As the researchers report in their current study, the significance of the epidemic has become increasingly clear: in the Gulf of Aqaba, where the die-off first became apparent in December 2022, the two most important sea urchin species have now completely disappeared. But the disease has also spread far south beyond this northern extension of the Red Sea: it has now penetrated far into the Indian Ocean. “The die-off now extends across the Red Sea, the Gulf of Oman and the West Indian Ocean, with population declines reaching 100 percent in some places,” the scientists write. They also present images of beaches on the island of Reunion, east of Madagascar, which are littered with the remains of dead sea urchins.

“In our study, we also showed that the epidemic has spread along transport routes. The best example is the area around Nueiba in Sinai, where the ferry from the Jordanian city of Aqaba docks. When we published our report last year, we already knew about the sea urchin die-off in Aqaba, but had not yet found any signs of the infestation in the Sinai area. The first place where we finally detected sea urchin die-off was next to the jetty in Nueiba. Two weeks later, the epidemic had already reached Dahab, about 70 kilometers further south.” The sea urchins there were then wiped out within a few days. “Masses of the remains rolled around on the seabed,” says Bronstein.

A deadly parasite

Using molecular genetic methods, the team was able to confirm the previous suspicion that the same pathogen that caused the mass deaths in the Caribbean is responsible for the epidemic. It is a parasitic member of the ciliates. Camera recordings of infected sea urchins also documented exactly how the disease progresses. The deadly pathogen infects the sea urchins through the water. Within two days, the animals’ tissue then disintegrates and they lose their spines. They are often eaten by predators during the process because they can no longer defend themselves. In their study, the researchers were also able to show that more sea urchin species are affected by the disease than previously thought.

There is therefore a fear that mass extinction could now reach other marine regions of the world, destroying the important “cleaning power” of coral reefs. “This is a growing ecological crisis that threatens the stability of coral reefs on an unprecedented scale,” says Bronstein. But what can be done about it? Unfortunately, Bronstein says there is currently no way to help infected sea urchins or vaccinate them against the disease. However, it might be useful to breed reserve populations in facilities that are separated from the sea. After the pandemic subsides, these animals could then be used to repopulate the reefs, the researchers explain. “We also need to continue researching the factors that led to this outbreak,” Bronstein concludes.

Source: Tel Aviv University, professional article: Current Biology, doi: 10.1016/j.cub.2024.04.057

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