Corals have fluorescent colours to attract their prey, according to a study by a team of researchers from the Tel Aviv University in Israel. The study, published in the journal Communications Biology showed that, even at depths over 40m, corals rely on fluorescent light to find plankton.
A team of researchers from the University of Tel Aviv, in collaboration with the Interuniversity Institute for Marine Sciences and the Steinhardt Museum of Natural History in Eilat, managed to prove for the first time that the fluorescent colours found in coral in deep reefs serve to attract their prey. It turns out that even at great depths some fish recognise and follow these fluorescent colours.
The phenomenon of fluorescence in nature has been the subject of fascination for centuries. This is common in reef corals, but its purpose is still a mystery. Over the years, researchers have tested many suggestions, from protection against radiation to a way to improve photosynthesis, but the real reason remains elusive.
To answer this question, the team wanted to determine whether plankton (small organisms in the water) can recognise fluorescence. They used the crustacean Artemia saline — commonly used in lab experiments — and gave them a choice between a green or orange fluorescence and a clear non-fluorescent control target. These small crustaceans showed a significant preference for fluorescent light. If the crustaceans only had two clear non-fluorescent options, the researchers didn’t notice any particular preferences. In contrast, fish — which are not considered coral food — did not exhibit the same behaviour and, in fact, avoided the fluorescent targets.
In the second phase, the researchers moved from the lab to the actual reef, about 40m deep in the sea. In this location, the green and orange fluorescent traps attracted twice as much plankton as the clear traps.
“We conducted an experiment in the depths of the sea to examine the possible attraction of diverse and natural collections of plankton to fluorescence under the natural currents and light conditions that exist in deep water. Since fluorescence is ‘activated’ principally by blue light (the light of the depths of the sea), at these depths, the fluorescence is naturally illuminated, and the data that emerged from the experiment were unequivocal, similar to the laboratory experiment,” said Dr or ben-Zvi.
Curiously, the team found that corals with green fluorescence were about 25% more successful than those that exhibited yellow fluorescence. The real reason for this is unknown.
“Many corals display a fluorescent colour pattern that highlights their mouths or tentacle tips, a fact that supports the idea that fluorescence, like bioluminescence (the production of light by a chemical reaction), acts as a mechanism to attract prey. The study proves that the glowing and colourful appearance of corals can act as a lure to attract swimming plankton to ground-dwelling predators, such as corals, and especially in habitats where corals require other energy sources in addition or as a substitute for photosynthesis (sugar production by symbiotic algae inside the coral tissue using light energy),” said Prof Yossi Loya.
The authors suggested that this hypothesis — which they called the light trap hypothesis — can also apply to other fluorescent organisms, and this phenomenon may be more prevalent than previously thought.
Ben-Zvi, O., Lindemann, Y., Eyal, G. et al. (2022) Coral fluorescence: a prey-lure in deep habitats. Commun Biol 5, 537. https://doi.org/10.1038/s42003-022-03460-3