Since July, more than 5,000 black-blue sea urchins (Echinothrix diadema) have been found dead on the beaches of western Reunion Island, an excess mortality that could be attributed to a parasite, but scientists remain cautious.
The first alert was raised by divers on July 24 after a hundred sea urchins were spotted dead off the west coast.
“The peak was reached on the weekend of August 15,” said AFP Brice Cauvin of the Reunion Marine Reserve.
The dead sea urchins were found “everywhere in the northern part of the six to eight kilometer long coral reef,” he notes.
This excessive mortality affects only one of the 41 species of sea urchins that live in Reunion Island waters.
These invertebrates act as ecosystem regulators by feeding on algae that grow on corals. This situation worries scientists.
“The increase in deaths can lead to disruption of the ecosystem and the proliferation of algae,” explains Jean-Pascal Quod, marine biologist specializing in medical oceanology.
This excess mortality is due to the fact that the coral reefs of this French Indian Ocean island 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 excessive mortality.
But “the presence of the parasite Philaster sp, identified for the first time on the island, was confirmed after analyzing the dead sea urchins,” notes Jean-Pascal Quod.
This presence, a first on Reunion Island, represents “the most likely hypothesis to explain this excessive mortality,” estimates the scientist.
But “until we have completed the molecular biology work, we cannot guarantee that it is actually this parasite that is the main cause of these deaths,” he points out.
This parasite, which has been detected in several regions and countries, particularly in the West Indies and Israel, “compromises the immunity of the sea urchin by causing a disturbance in its physiology. The sea urchin loses vitality, its spines fall off and it eventually dies.” , describes Jean-Pascal Quod.
“We don’t know at the moment how the parasite got on the island: it can be caused by emptying the water tanks of boats, algae and microplastics,” he enumerates.
If the parasite’s responsibility for the excessive mortality is confirmed, “there is no way to fight it,” the biologist notes. “We have to accept this pathology, the sea urchins have to adapt,” he warns.