Corals that glow in fluorescent colors dozens of meters below the surface of the sea do so to lure prey, an Israeli study has shown for the first time.
Fluorescence is common in reef-building corals, including those that live between the shallow reef area and the deep, completely dark zone of the sea.
But to date, nobody has been able to pinpoint why.
Among the hypotheses put forward have been that the phenomenon protects against radiation, optimizes photosynthesis (by the algae that live within the corals and give them their color), enhances immunity to disease, protects from herbivores or attracts symbiotic algae.
However, this latest study, published last month in the peer-reviewed Communications Biology, shows that the function of the corals’ fluorescence is actually to serve as a lure for prey.
Fluorescence is different from bioluminescence, which results from a reaction between a light-emitting molecule and an enzyme, usually luciferase.
It is found in living things ranging from marine creatures to insects such as fireflies and some species of bacteria and fungi.
Some deep sea fish, for example, have a bioluminescent lure to entice prey. But other species may glow for camouflage, mimicry or to attract mates.
Biofluorescence, by contrast, occurs in living things that emit light having already absorbed energy.
To test whether the true function of the fluorescence in deep sea corals was to lure prey, the researchers first sought to determine whether plankton (small organisms that drift in the sea along with the current) were attracted to fluorescence, both in the laboratory and at sea.
Then, in the lab, the researchers quantified the predatory capabilities of the corals being studied.
Among other creatures, the researchers used the small brine shrimp, Artemia salina, which corals eat. When the shrimps were given a choice in the laboratory between a green or orange fluorescent target versus a clear control target, they showed a significant preference for the fluorescent one.
A native crustacean from the Red Sea showed a similar preference for fluorescence.
By contrast, fish that are not considered coral prey avoided the fluorescent targets in general and the orange ones in particular.
In the second phase of the study, the experiment was carried out in the corals’ natural habitat, about 40 meters (130 feet) below the surface of the Red Sea in southern Israel. There, the fluorescent traps (both green and orange) attracted twice as many plankton as the clear trap.
Dr. Or Ben-Zvi, who lead the research, said, “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,” he said.
In the last part of the study, the researchers examined the predation rates of corals that were collected 45 meters (150 feet) below the surface of the Gulf of Eilat.
They found that corals with green fluorescence enjoyed 25 percent higher predation rates than those that glowed yellow.
Yossi Loya, a marine scientist and Tel Aviv University Emeritus Professor, who supervised the study, said it supported the idea that the corals were trying to attract prey.
“Many corals display a fluorescent color 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,” Loya said.
“The study proves that the glowing and colorful 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),” he said.
The research was carried out in collaboration with Yoav Lindemann and Gal Eyal, the Steinhardt Museum of Natural History and the Interuniversity Institute for Marine Sciences in Eilat.