Sea urchins are dying in the Mediterranean – and the corals in the Red Sea are in danger as a result

On Tuesday, Tel Aviv University posted a video to its YouTube channel in which zoologist Omri Bronstein tells a story “that begins in the Mediterranean”. The video is accompanied by dramatic music – and Bronstein’s story is indeed dramatic.

He tells how, starting in the Mediterranean, a deadly epidemic is spreading among diademed sea urchins, threatening the coral reefs of the Red Sea in particular. The zoologist says urgently at the end of the video:

The common sea urchin with diadem (Diadema setosum) is considered a key species throughout its range Coral reefs, according to a scientific paper Bronstein and his team published Wednesday.

To the Diadema setosum belong to two genetically closely related subspecies. One is mainly found in the western Pacific and the eastern coast of Africa, the other was until recently confined to the Red Sea and the Persian Gulf. In 2006, individuals of the second group were first observed in the Mediterranean. Since 2018 they have multiplied exponentially, which is why they can now be found all over the Levantine basin. They can be found on the coasts of Greece, Lebanon, Israel, Egypt and Libya.

But now, since July 2022, a disease has been spreading among the population in the Mediterranean and northern Gulf of Aqaba, with the infected animals dying within 48 hours, the researchers write. The publication even speaks of a “mass extinction”.

The epidemic is destroying entire stocks locally. This phenomenon was first documented last summer: on July 22, 2022 in the Anthony Quinn Bay near Rhodes the first deaths of apparently healthy diademed sea urchins were reported, and by August 1, the population in the bay was already completely extinct. There are now three places where the epidemic has completely wiped out the local population, and several coastal areas face the same fate. Cases of apparently healthy but dead sea urchins have been reported to the coast of southern Turkey – so the cases of the disease already extend to more than 1000 kilometers of coastline.

Also in Gulf of Aqaba the disease has already spread. According to the zoologist, there are only diademed sea urchin skeletons and no living individuals in the northern part near the Israeli coastal town of Eilat. Particularly concerning: Even the sea urchins kept in aquariums in Eilat for research purposes got sick and died.

According to the researchers’ report, the reason for the frequent deaths of diademed sea urchins in the Mediterranean Sea and Gulf of Aqaba has not yet been clearly identified.

However, local environmental pollution as a driver of the epidemic could be ruled out with a high degree of probability. The researchers justify this statement by saying that the dead also fall into deep water. Local environmental pollution would normally not have such serious consequences as in shallow water.

Laboratory observations and footage revealed dead and dying individuals from all affected areas a unique pathology: The first observable symptom is that the dying sea urchins lose their legs, which they use to attach themselves to the bottom. Soon after, tissue loss begins and the spines fall out, leaving bald spots clearly visible. Finally, the sea urchin dies. It takes about two days from the first symptoms to death. “It’s a quick and violent death,” Bronstein told Eurek Alert.

The researchers therefore assume that the continuing wave of deaths in the Mediterranean Sea and in the Gulf of Aqaba is caused by pathogens. Bronstein says:

The researchers suspect that the suspected pathogen could be a parasite. Because one of these wiped out the entire sea urchin population in the Caribbean back in the 1980s.

This thesis could also explain why animals died at the research facility in Eilat, Bronstein told Eurek Alert. The pathogens could have entered the aquariums via the pump systems. This supports the assumption that the pathogen is not only transmitted through contact between animals, but also through water.

There are also other ways that pathogens can spread rapidly within marine ecosystems. To begin with, baitfish carry the skeletons of dead sea urchins and disperse them that way. On the other hand, sick or dead sea urchins also drift from place to place with the current. Finally, there is a third possibility: the spread of pathogens via shipping.

All of these routes could be used to transport pathogens relatively quickly from the Levant through the Suez Canal or through the Gulf of Aqaba to the rest of the Red Sea – and eradicate the diadem sea urchins that are native and predominant there. The researchers warn that this could have “catastrophic consequences” for the coral world in the Red Sea.

Sea urchins are the marine gardeners: they constantly graze on algae on rocks and corals. On the one hand they create space for fish larvae to settle and on the other hand they keep algae growth in check. In this way they prevent slower growing organisms, such as corals, from being displaced by algae. The slow-growing corals compete with the fast-growing algae for sunlight.

When sea urchins disappeared from the Caribbean in the 1980s, the coral reef suffered irreversible damage. One of the most devastating events in the history of marine ecology. A group of researchers from Cornell University identified the cause of death of sea urchins in the Caribbean: a pathogenic ciliate. Bronstein suspects that the pathogen in the Mediterranean Sea and Gulf of Aqaba could also be a sickening ciliated animal.

The disease broke out again in the Caribbean last year, killing off surviving sea urchin populations. The corals there are therefore still considered endangered, Bronstein explains. And such a catastrophe must be avoided for the Red Sea, one of the most biodiverse marine ecosystems.

Tel Aviv University scientists emphasize that there is still hope, but only if immediate action is taken.

If nothing is done, «in a short time the entire population of these sea urchins in both the Mediterranean Sea and the Red Sea will be extinct. The Red Sea will get sick and die,” Bronstein predicts.

He therefore calls for immediate cooperation between affected countries and territories so that the pathogen can be identified as quickly as possible and then monitored in real time.

As a first and most urgent measure, a breeding population for the diadem sea urchins must be established so that they can be released back into the wild if necessary in the future.