The first alert was given on July 24 by divers, after the observation of a hundred dead sea urchins off the west coast.
"The peak was reached on the weekend of August 15," Brice Cauvin, of the Marine Nature Reserve of Reunion, told AFP.
The dead sea urchins were found "all over the northern part of the coral reef, which is six to eight kilometers long," he notes.
This excess mortality affects only one of the 41 species of sea urchins living in Reunion waters.
These invertebrates act as regulators of the ecosystem by feeding on algae growing on corals, the situation worries scientists.
"The multiplication of deaths can lead to a disruption of the ecosystem and the proliferation of algae," says Jean-Pascal Quod, a marine biologist specializing in medical oceanology.
This excess mortality comes as the coral reefs of this French island in the Indian Ocean are increasingly threatened by "nitrate and phosphate pollution", "an ecological imbalance with algae that deprive corals of light", laments Brice Cauvin.
Scientists have not yet determined the reasons for this excess mortality.
But "the presence of the parasite "Philaster sp", identified for the first time on the island, was confirmed after analysis of dead sea urchins," notes Jean-Pascal Quod.
This presence, a first in Reunion, is "the most likely hypothesis to explain this excess mortality," says the scientist.
But "until we do the molecular biology work, we will not be able to guarantee that it is this parasite that is mainly responsible for these deaths," he insists.
Identified in several regions and countries, including the West Indies and Israel, this parasite "jeopardizes the immunity of the sea urchin by causing a disruption of its physiology. The sea urchin loses vitality, its spines fall and it eventually dies," says Jean-Pascal Quod.
Sea urchins washed up on the beach of Cap Boucan Canot, August 23, 2023 in Reunion © Richard BOUHET / AFP
"We do not yet know how the parasite arrived on the island: it can come from emptying water tanks of boats, algae, micro-plastics," he lists.
If the responsibility of the parasite in excess mortality was confirmed, "no solution exists to fight it," notes the biologist. "We will have to accept this pathology, sea urchins will have to adapt," he warns.
© 2023 AFP