A mass die-off of sea urchins is rapidly becoming a global pandemic. The deadly disease has now also been detected in the Indian Ocean, reports a research team in the journal “Current Biology”. Footage shows 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.
A mass death of diadem sea urchins caused by ciliates was initially noticed in the US Virgin Islands in January 2022. In the following months, similar observations were made across large parts of the Caribbean. Then the Mediterranean and quickly the Red Sea were also affected.
Entire populations disappear
The researchers estimate that since December 2022, most of the populations of affected sea urchin species in the Red Sea, as well as hundreds of thousands of sea urchins worldwide, have been destroyed. In the reef system near the Israeli coastal city of Eilat, for example, the two sea urchin species that were previously most common in the Gulf of Aqaba have completely disappeared. The team led by Omri Bronstein from Tel Aviv University has now identified the pathogen responsible for the mass death 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 single-celled parasite is also the cause of the mass extinction of the Atlantic diadem sea urchin (Diadema antillarum) in the Caribbean about two years ago.
40 years – and still not fully recovered
A devastating collapse of the population in this region was observed as early as 1983. Both the coral and sea urchin populations in the Caribbean have never fully recovered, according to the researchers. It is suspected that the pathogen that has now been identified was the cause even then. According to the research team, the ciliated animal also affects Echinothrix sea urchins, a group of species closely related to Diadema.
The parasite causes the animals to become a tissueless shell within two days – unless predators prey on the weakened animals beforehand. The deadly pathogen is transmitted via water and can affect 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 transport routes, data from the Red Sea showed.
Breed to preserve the species
It is scary to see thousands of sea urchins on the sea floor become skeletons and disappear in a very short space of time, says Bronstein. There is currently no way to help infected sea urchins. Breeding populations of endangered species in breeding systems separated from the sea are urgently needed so that healthy animals can be released back into the wild later. Research must also be carried out into the factors that led to the outbreak. Changes in environmental conditions are considered to be a possible cause.
Ciliates consist of just one cell and have hairs on their surface that allow them to move. They are often found in water and are often harmless. However, relatives of the ciliates now discovered have already been blamed for mass deaths in other marine animals such as sharks.
