According to a new Tel Aviv University study conducted in collaboration with the Steinhardt Museum of Natural History and the Interuniversity Institute for Marine Sciences in Eilat, the magical phenomenon in deep reefs in which corals display glowing colors (fluorescence) is intended to serve as a mechanism for attracting prey. The study found that the marine animals that corals prey on recognize and are drawn to fluorescent colors.
Dr. Or Ben-Zvi led the study, which was co-led by Yoav Lindemann and Dr. Gal Eyal, and was overseen by Prof. Yossi Loya of Tel Aviv University’s School of Zoology and the Steinhardt Museum of Natural History.
The researchers explain that for centuries, naturalists and scientists have been fascinated by the ability of sea creatures to glow. The phenomenon is very common in reef-building corals, but its biological role is still being debated. Several hypotheses have been tested over the years, including: Does this phenomenon protect against radiation? To improve photosynthesis? Is there an antioxidant activity? To protect the corals from herbivores or to attract symbiotic algae? According to the latest research, the fluorescence of corals serves as a lure for prey.
We conducted an experiment in the depths of the sea in order 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, at these depths the fluorescence is naturally illuminated, and the data that emerged from the experiment were unequivocal, similar to the laboratory experiment.Dr. Or Ben-Zvi
The researchers tested their hypothesis in the study, first determining whether plankton (small organisms that drift in the sea with the current) are attracted to fluorescence in the laboratory and at sea. The predatory abilities of mesophotic corals (corals that live between the shallow coral reef area and the deep, completely dark zone of the ocean) with different fluorescent appearances were then quantified in the lab.
The researchers used the crustacean Artemia salina, which is used in many experiments and as food for corals, to test the planktons’ potential attraction to fluorescence. The researchers noted that when the crustaceans were given a choice between a green or orange fluorescent target versus a clear “control” target, they showed a significant preference for the fluorescent target.
Moreover, when the crustaceans were given a choice between two clear targets, its choices were observed to be randomly distributed in the experimental setup. In all of the laboratory experiments, the crustaceans vastly exhibited a preferred attraction toward a fluorescent signal. Similar results were presented when using a native crustacean from the Red Sea. However, unlike the crustaceans, fish that are not considered coral prey did not exhibit these trends, and rather avoided the fluorescent targets in general and the orange targets in particular.
In the second phase of the study, the experiment was carried out in the corals’ natural habitat, about 40 meters deep in the sea, where the fluorescent traps (both green and orange) attracted twice as many plankton as the clear trap. Dr. Or Ben-Zvi says, “We conducted an experiment in the depths of the sea in order 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.”
In the last part of the study, the researchers examined the predation rates of mesophotic corals that were collected at 45 m depth in the Gulf of Eilat, and found that corals that displayed green fluorescence enjoyed predation rates that were 25 percent higher than corals exhibiting yellow fluorescence.
Professor Loya: “Many corals have a fluorescent color pattern that highlights their mouths or tentacle tips, lending credence to the idea that fluorescence, like bioluminescence (the production of light by a chemical reaction), serves as a predator-attraction mechanism. The research shows that the glowing and colorful appearance of corals can act as a lure to attract swimming plankton to ground-dwelling predators like corals, especially in habitats where corals require other energy sources in addition to or as a substitute for photosynthesis (sugar production by symbiotic algae inside coral tissue using light energy).”
Dr. Ben-Zvi concludes, saying: “Despite gaps in existing knowledge regarding plankton visual perception of fluorescence signals, the current study provides experimental evidence for fluorescence’s prey-luring role in corals. We propose that this hypothesis, dubbed the “light trap hypothesis,” may also apply to other fluorescent organisms in the sea and that this phenomenon may play a larger role in marine ecosystems than previously thought.”