Seeing a coral reef is a magical experience. Its colors, its shapes and the incredible biodiversity it houses leave no one indifferent. There are some that even glow in the dark. But why? A team of Israeli scientists appears to have discovered why.
Most reef-building corals are located in shallow water so that the algae that reside in it can capture sunlight as it filters in from the ocean surface. These are the reefs that we popularly know. However, there are also other species of coral that grow deeper , up to 6000 meters below the sea surface and many of them are fluorescent.
When the researchers wondered why the corals glowed in the dark, what they thought was that, like other creatures of the deep, the reason is to attract prey. However, they need to test their theory, which they dubbed the “light trap hypothesis.”
“Many corals display a fluorescent color pattern that highlights their mouths or the tips of their tentacles,” explains marine ecologist and study lead author Yossi Loya of Tel Aviv University. “This ability to fluoresce and attract prey appears to be a fairly essential adaptation for corals trapped on the seafloor, and especially in habitats where corals need other energy sources in addition to or in lieu of photosynthesis,” Add.
Other reasons that scientists have given to the fluorescence of corals would be, on the one hand, to protect themselves from the sun and, on the other, to promote photosynthesis . The sunscreen hypothesis suggests that fluorescence would protect bleached corals from thermal stress and light damage. As for the photosynthesis hypothesis, there is still no evidence that fluorescence offers mesophotic corals (they live between 30 and 150 meters deep) any kind of energy boost.
The team of researchers focused on studying coral species that live at depths where little light reaches and that depend more on predation than photosynthesis to survive.
Scientists did laboratory experiments in which they found that the tiny Artemia salina shrimp would target a green or orange fluorescent target rather than a clear, reflective or matte one. The researchers obtained similar results when they conducted experiments in the Gulf of Eilat in the Red Sea. Anisomysis Marisrubri , an indigenous crustacean that preys on Gulf corals, preferred fluorescent cues to reflective targets, but an introduced species of fish larvae did not. Finally, the researchers compared predation rates between Euphyllia paradivisa corals of different colors that were collected from the Gulf of Eilat at depths of 45 meters and brought to the laboratory. What they found was that the fluorescent green corals ingested more A. salina shrimp in 30 minutes than the fluorescent yellow ones. When the experiment was repeated using red lights, not blue ones, which do not activate the corals’ fluorescence, there was no difference in the amount of shrimp consumed by one or the other.
“In its natural habitat on the mesophotic reefs of Eilat, the yellow morph of E. paradivisa was found to be the least abundant, which can now potentially be explained by the lower prey attraction to this color found in the present study,” the researchers write. .
It must be taken into account that only one species of mesophilic coral has been analyzed in the study. More research is also needed to better understand how plankton and other crustaceans that feed corals perceive color, which likely differs between species, locations and life stages.
Referencia: Ben-Zvi, O., Lindemann, Y., Eyal, G. et al. 2022. Coral fluorescence: a prey-lure in deep habitats. Communications Biology. DOI: https://doi.org/10.1038/s42003-022-03460-3
