Coral reefs under threat

Parasite causes mass death of sea urchins worldwide

Countless dead sea urchins on a beach on Reunion Island

(Bild: AFP/Richard Bouhet)

A mass death of sea urchins caused by a parasite is increasingly turning into a global pandemic. Researchers believe this is threatening the underwater world - especially coral reefs, which are already suffering greatly from climate change.

The deadly disease can now also be detected in the Indian Ocean, reports a team of researchers in the journal "Current Biology". Photographs show countless dead sea urchins on a beach on Reunion Island. The outbreak poses an immediate threat to coral reefs worldwide: Sea urchins eat algae that would otherwise overgrow and kill corals.

Sea urchins in the Mediterranean also affected
A mass mortality of diademed sea urchins caused by ciliates was first noticed in the Virgin Islands in January 2022. In the months that followed, similar observations were made in large parts of the Caribbean. The Mediterranean and soon the Red Sea were also affected.

It is estimated that most of the populations of affected sea urchin species in the Red Sea and hundreds of thousands of sea urchins worldwide have been destroyed since December 2022. In the reef system near the Israeli coastal town of Eilat, for example, the two sea urchin species that were previously most common in the Gulf of Aqaba have completely disappeared.

Ciliatesto blame for mass mortality
A team led by Omri Bronstein from Tel Aviv University has now identified the pathogen responsible for the mass mortality of common diadem sea urchins (Diadema setosum) - long-spined, black sea urchins - in the Red Sea: a ciliate similar to the species Philaster apodigitiformis. The unicellular parasite was also the cause of the mass extinction of the Atlantic diademed sea urchin (Diadema antillarum) in the Caribbean about two years ago.

A devastating collapse of the population in this region had already been observed in 1983. Both the coral and sea urchin populations in the Caribbean have never fully recovered, the scientists report.

It is assumed that the pathogen that has now been identified was also the cause back then. According to the researchers, the ciliate also infects Echinothrix sea urchins, a group of species closely related to Diadema.

The parasite turns the animals into a tissue-less shell (pictured left) within just two days.

(Bild: AFP/Richard Bouhet)

Researchers see coral reefs under serious threat
The parasite turns the animals into a tissue-less shell (top left in the picture) within two days - unless predators prey on the weakened animals before then. The deadly pathogen is transmitted via the water and can infest large areas in a very short time, it was said. The stability of coral reefs is threatened to an unprecedented extent, said Bronstein.

The disease is spreading along human transportation routes, as data from the Red Sea has shown. It is uncanny to see thousands of sea urchins on the seabed turn into skeletons and disappear within a very short space of time, said Bronstein. So far, there is no way to help infected sea urchins.

Changed environmental conditions the trigger?
Breeding populations of endangered species in breeding systems separated from the sea are urgently needed so that healthy animals can later be released back into the wild. The factors that led to the outbreak also need to be investigated. One possible reason is a change in environmental conditions.

Ciliates consist of just one cell and have hairs on their surface with which they can move. They are often found in water and are usually harmless. However, relatives of the ciliates that have now been found have already been blamed for mass deaths of other marine animals such as sharks.