Beginning in late 2022, sea urchins died en masse at the northern end of the Gulf of Aqaba, Jordan. Already at the end of April 2023 - 150 kilometers to the south, along the Sinai Peninsula, Egypt. All individuals were found dead there within a few days. Now the death of sea urchins is already spreading into the Indian Ocean. The problem was raised in a recent study by scientists from Tel Aviv University, published in Current Biology.
— Currently, the disease is registered in several species of sea urchins of the genus Diadema. These are tropical animals with long, movable spines and live primarily on coral reefs. In the eastern Mediterranean, the disease is spreading in the population of the invasive species Diadema setosum, which entered here through the Suez Canal from the Red Sea. The question remains open whether it can spread to other species of urchins living off the coast of Turkey and Greece, in the Red Sea, explains the head of the Laboratory of Morphology and Ecology of Marine Invertebrates of the Institute of Ecology and Evolution of the Russian Academy of Sciences, Doctor of Biological Sciences, Professor Temir Britaev.
What is unusual about the situation?
This is not the first time such a pestilence has arisen, but this time the epidemic is spreading very actively, which forces scientists to sound the alarm.
“This epidemic is not a new phenomenon; it was first observed in related long-spined urchins living in the Caribbean Sea (Diadema antillarum) more than 40 years ago, when in the early 80s the pathogenic bacterium also wiped out urchins on Caribbean reefs,” says the leading scientist employee of the Department of Benthos Ecology of the Federal Research Center InBYUM, Doctor of Biological Sciences Konstantin Tkachenko.
What if there are no more sea urchins?
In early 2022, the plague returned to the Caribbean. It is not by chance that biologists are drawing attention to the threat: the plague of sea urchins can have great consequences for the environment and ultimately affect the lives of people - everything on the planet is interconnected.
“It might seem strange, why do we care about some sea urchins that we practically never encounter? But in fact, sea urchins play a very important role in marine ecosystems, especially coral reef ecosystems. The fact is that corals compete with algae, and urchins feed on the latter. If you remove the urchins, the bottom becomes overgrown with algae, which displaces the corals. This is exactly the situation that has developed in the Caribbean Sea: after the mass death of urchins, the area occupied by coral reefs began to rapidly decrease. In addition, many types of urchins are actively used by humans, as a gourmet product and as a source of biologically active substances in pharmacology and cosmetics,” says Temir Britaev.
“Many planktonic organisms, including fry of commercial fish, feed on sea urchin larvae. The fertility of sea urchins is quite high: females of some species during spawning can lay up to 30 million eggs, each of which can develop into a planktonic larva - Echinoplutheus. Of course, not all the larvae will survive to settle, otherwise the entire seabed would be populated by sea urchins, and there are about 800 species of them in the ocean. Some live more than 100 years. These echinoderms are sources of valuable biologically active substances. Medicines, for example, histochrome, are obtained from them,” adds Anatoly Drozdov, chief researcher at the National Scientific Center for Marine Biology, Far Eastern Branch of the Russian Academy of Sciences, Doctor of Biological Sciences, Professor.
Several thousand species of marine animals, including those vital to humans, depend on coral reefs for their livelihoods. The disappearance or degradation of reefs will set off a chain reaction of environmental changes, the consequences of which are difficult to even imagine. Therefore, in fact, enormous efforts are being made around the world to preserve corals, which are already being destroyed by rising global temperatures and ocean acidification. Added to this is a new problem.
“In the study for the Red Sea, the pathogen is clearly identified - single-celled ciliates from the genus Scuticociliata, which cause necrosis of the soft tissues of some species of sea urchins. Not all of them, but, unfortunately, the two most abundant, representing the most important functional group that maintains balance in the coral-dominated reef ecosystem. These are long-spined black urchins from the genus Diadema and another species from the genus Echinothrix,” explains Konstantin Tkachenko.
The authors of the Israeli study note that this is the same pathogen that was wiping out sea urchins in the Caribbean in 2022. The scientists write that this is not surprising, but "but the speed with which it spread [to the Red Sea] was unexpected."
Who will replace sea urchins?
“Infection occurs through direct contact of animals with each other (urchins often form clusters) or with fish that feed on living and dead tissues of urchins and willingly eat the tissues of sick individuals. The pathogen can be transported over long distances by currents or in the ballast water of ships. In geographically remote areas the disease progresses similarly. In urchins, the ambulacral legs with which they are attached to the ground stop contracting, echinoderms are torn from the ground by the current, begin to dangle in the bottom layer of water, and their accumulations form in places of local gyres. Then the long needles fall off, and finally the tissues are separated from the skeleton in layers. Moreover, all these phenomena develop rapidly and literally two days after the visible manifestations of the disease, the urchins die,” says Temir Britaev.
There are opinions that sea urchins can be functionally replaced by fish - some types also feed on algae. However, here too the picture is spoiled by man: massive and uncontrolled fishing, both in the Caribbean and in the Red Sea, does not allow us to hope for this solution.
“On the Caribbean reefs in the early 80s, a phase shift occurred from the dominance of corals to the dominance of macroalgae due to the fact that the main regulator of algae growth, these same urchins, was no longer present. And the number of secondary regulators - herbivorous fish - was maximally undermined by long-term unregulated fishing. So the reefs degraded when such an important functional group was removed from the ecosystem. In the Red Sea, the situation with the abundance of herbivorous fish is much better than in the Caribbean, but still their abundance does not compensate for the regulation of algae growth that urchins provided. And on reefs with significant anthropogenic load, for example, East Africa or Southeast Asia, such an epidemic promises enormous changes. I’m currently working on an expedition in Vietnam, and the coral reefs here have a lot of problems: thermal anomalies, and outbreaks in the number of the predatory crown-of-thorns starfish. This leads to large-scale mass mortality of corals, and human activity makes everything even worse. If this causative agent of sea urchin disease comes here, it will be very bad,” says Konstantin Tkachenko.
Temir Britaev agrees with him:
“The situation is alarming. Coral reefs are rapidly declining, and if this infection enters the tropical waters of the Indian and Pacific Oceans, the beautiful underwater gardens could disappear forever.”
To avoid the spread of the epidemic, Israeli scientists propose to analyze the ballast water of ships arriving from infected regions to determine whether they contain infection. However, no measures have been taken so far and whether they will be taken is unknown.
Among other things, sea urchins are a popular delicacy. Many seafood lovers are concerned about the question: is it possible to eat these echinoderms now?
“Sea urchins use gonads for food. In infected individuals they may decrease. It is better not to eat gonads raw. It must be subjected to heat treatment: frozen, heated, dried,” emphasizes Anatoly Drozdov.”
Does this apply to Russia?
Is the epidemic relevant for Russia and can it affect the water resources of our country?
“This has nothing to do with Russia at all. In Russian seas there are no coral reefs and their inhabitants, to which these urchins belong,” explains Konstantin Tkachenko.
“So far, in the Barents Sea and in the Far Eastern seas (Japan and Okhotsk), everything is fine with sea urchin populations. There are both coastal and deep-sea species. Deaths caused by environmental causes (desalination, toxic algae outbreaks) are sometimes observed, but then populations recover. In the 90s, in the Far Eastern seas and on the coast of the Kuril Islands, there was a sharp decrease in the number of spherical sea urchins due to their intensive fishing, but then the number recovered,” comments Anatoly Drozdov.
The population of sea urchins destroyed by the plague in the Caribbean Sea also recovered in the 80s, although not completely. However, this amazing sea creature is still not safe.
“The death of sea urchins can also be caused by environmental reasons. For example, by desalination - echinoderms are very sensitive to a decrease in salinity. They cannot live in the Black and Baltic seas due to the low salinity of the water, as well as in the Caspian. In the Aral Sea, on the contrary, the salinity is unbearably high for echinoderms. They also belong to stenothermic marine animals, that is, their optimum is a narrow temperature range. An increase in temperature beyond what is permissible for them cannot be tolerated. The death of marine biota can also be caused by toxic substances entering the environment either from the outside or due to an outbreak of toxic algae,” says Anatoly Drozdov.
The sea urchin plague provides an important lesson about the fragility of ecosystems. Even the smallest animal plays a critical role in maintaining the balance of the environment, and interfering with an ecosystem without taking into account all the relationships can have very dangerous consequences.