Special coral eaters in the Red Sea

The Red Sea crown-of-thorns starfish now bear the name Acanthaster benzeii. © PD Dr. Oliver Voigt, LMU Geobiology

Targeting notorious spiked creatures: Researchers have identified the Red Sea crown-of-thorns starfish as a distinct species among coral eaters notorious for their destructive “campaigns”. Delineating the different species can now benefit research into these problematic reef dwellers in threatened marine ecosystems, say the scientists.

Spiny, venomous, and gluttonous, anyone interested in the world's coral reefs is probably familiar with the images of armies of crown-of-thorns starfish (Acanthaster spp.) that sometimes haunt the underwater world. The hallmark of these marine animals, which can be up to 40 centimeters in size, are their poisonous spines, which they use to protect themselves from attackers. They also cover up to 20+ arms, which these starfish use to attack the polyps of hard corals throughout the Indo-Pacific. In recent decades, these creatures have multiplied more and more frequently, which can then literally devastate coral reefs. This is a major threat to these ecosystems, which are already under severe pressure from climate change and other human-made impairments. It is assumed that the mass proliferation of the crown-of-thorns starfish is also a consequence of the systems being thrown out of balance. An important factor is probably the disappearance of their natural enemies, partly due to overfishing.

Not just a pest

Due to their important role, the crown-of-thorns starfish are comparatively well researched. But surprisingly, until now there has been a lack of clarity about their diversity. "For a long time it was assumed that the first described species of the genus, Acanthaster planci, is distributed from the Red Sea across the Indian Ocean to the entire Pacific," says first author Gert Wörheide from the Ludwig Maximilian University in Munich. But then clearly different lines emerged among the animals. So far, three species have been identified that colonize the reefs in different parts of the Indo-Pacific: Acanthaster planci in the northern Indian Ocean, Acanthaster mauritiensis in the southern Indian Ocean and Acanthaster cf. solaris in the Pacific Ocean.

However, there have also been indications that the crown-of-thorns starfish in the Red Sea could represent another species. Wörheide and his colleagues have now specifically pursued this lead as part of their study. "In the case of crown-of-thorns starfish from the Red Sea, individual peculiarities had already been observed earlier, for example a more nocturnal lifestyle or probably less toxicity of the spines, but we did not yet know that it is actually a separate species," says Wörheide. However, through their genetic and morphological investigations, the scientists have now been able to clearly show for the first time that the crown-of-thorns starfish native to the Red Sea are actually a so-called endemic species. It occurs exclusively in this side sea of ​​the Indian Ocean between north-east Africa and the Arabian Peninsula.

Thinner spikes and fewer arms

The species now described as Acanthaster benziei therefore differs significantly from the other species of the A. planci species complex. "This once again highlights the importance of the Red Sea as an ecosystem with a unique fauna and numerous endemic species," says Wörheide. In addition to characteristic sequences in the mitochondrial DNA, the special features also include morphological features such as a smaller number of arms and thinner, differently shaped venom spines, the scientists report. "Now that we know that it is a distinct species, we can now turn our attention to the biology, ecology and toxicology of A. benzeii and the other acanthaster species," says Wörheide.

The focus is also on the differences in the problematic aspect of these marine animals. "By clearly delineating the different species of coral-eating crown-of-thorns starfish, we can explore in even more detail the dynamics of mass eruptions and thus another of the multiple stressors affecting tropical reefs - ultimately a step towards better management of reef ecosystems," says Woerheide.

According to the researchers, it is interesting that the crown-of-thorns starfish in the Red Sea are showing a lower tendency to mass outbreaks. "These are mainly known from Acanthaster cf. solaris from the western Pacific and regularly cause considerable damage to the Great Barrier Reef, while the phenomenon seems to occur less violently in the Red Sea - whether this also has something to do with species-specific characteristics could be a question be the subject of future investigations,” says Wörheide.

Source: Bavarian State Natural Science Collections, specialist article: ZOOTAXA, doi: 10.11646/zootaxa.5209.3.7

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