Corals collected from deep in the Gulf of Eilat were found to better attract prey when displaying green fluorescence as opposed to yellow. Coral Photo: Tel Aviv University.

Israeli researchers say they have explained one of the ocean’s most beautiful and striking scientific mysteries: glowing coral in the depths of the sea.

A recent study by researchers from Tel Aviv University and the Interuniversity Institute for Marine Sciences in Eilat found that marine organisms swim towards fluorescent colors, suggesting that corals glow in order to lure their prey. Published last month in Nature’s Communications Biology journal, the paper sought to resolve the debate over the biological role and function of coral fluorescence.

“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,” said Yossi Loya, of Tel Aviv University. “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.”

The glowing phenomenon is common in reef-building corals, but its biological role has long been the subject of debate, with proposed functions including protection against radiation, facilitating photosynthesis, or attracting symbiotic algae.

In new study, researchers initially tested whether plankton — small organisms that drift in the sea along with the current — are attracted to fluorescence, both in the laboratory and at sea. They used the crustacean Artemia salina, or brine shrimp, as food for corals. In the lab, the crustaceans were attracted to fluorescent cues when given a choice between a green or orange fluorescent target versus a control target. Similar results were observed when using a native crustacean from the Red Sea, the researchers said.

At the same time, fish which are not coral prey did not show the same response, avoiding the fluorescent targets in general and the orange targets in particular. A similar experiment was then carried out 130 feet deep in the sea, in the corals’ natural habitat, where the fluorescent traps lured twice as many plankton as a clear trap.

“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,” said Or Ben-Zvi, who led the study.

In the final phase of the study, researchers looked at corals collected some 150 feet deep in the Gulf of Eilat, at Israel’s southern tip — finding that corals signaling green fluorescence showed predation rates 25 percent higher than those exhibiting yellow fluorescence.

“Despite the gaps in the existing knowledge regarding the visual perception of fluorescence signals by plankton, the current study presents experimental evidence for the prey-luring role of fluorescence in corals,” Ben-Zvi concluded. “We suggest that this hypothesis, which we term the ‘light trap hypothesis’, may also apply to other fluorescent organisms in the sea, and that this phenomenon may play a greater role in marine ecosystems than previously thought.”