You are currently viewing Corals in deep water glow to attract prey – study in Israel

Corals in deep water glow to attract prey – study in Israel

Fluorescent-colored corals that glow tens of meters below the sea surface do so to deceive their prey, an Israeli study has shown for the first time ever.

Fluorescence is a common phenomenon in coral reefs – especially for those corals that live in shallow reefs and those that find themselves submerged in the total darkness of the sea floor.

But until now, no one had been able to determine the reason for this phenomenon.

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Among the hypotheses that had been put forward, that this phenomenon would protect corals from radiation, that it would optimize photosynthesis (by the algae that live within them and give them their color), that it would improve immunity to diseases, that it would protect them from herbivores or that it would attract symbiotic algae.

However, the new study, which was published last month in the peer-reviewed journal Communications Biologydemonstrates that the function of coral fluorescence is, ultimately, to mislead prey.

Fluorescence is different from bioluminescence which results from a reaction between a light-emitting molecule and an enzyme – usually luciferases.

It is found in all kinds of living species, in marine species or in insects – as is the case with glowworms – through certain species of bacteria and fungi.

Some fish living in the depths of the sea, for example, deceive their prey thanks to bioluminescence. But other species may glow for camouflage, imitation, or to attract peers.

The gigantactis is a deep-sea fish with a dorsal fin whose first filament has become very long and inclined, with a bioluminescent photophore lure. (Credit: Theodore W. Pietsch, University of Washington, CC BY 3.0, Wikimedia Commons)

Biofluorescence, in contrast, occurs in living species that emit light having already absorbed energy.

To determine that the true function of fluorescence in deep-sea corals was to mislead prey, the researchers first sought to prove that plankton (small organisms that move through the sea based on currents) were themselves attracted to fluorescence – tests they conducted in the lab and in the sea.

READ: Plankton, a small ocean creature with a colossal impact

Then, in the laboratory, the researchers quantified the predation capacities of the corals examined in this context.

Among other species, the researchers used brine shrimp – a small crustacean eaten by corals. When brine shrimp were given the choice in the laboratory between a green or orange fluorescent target and a transparent control target, the animals showed a significant preference for the fluorescent target.

This was also the case with a small crustacean native to the Red Sea.

Shiny-tipped coral tentacles. (Credit: Tel Aviv University)

In contrast, fish that the researchers say are not among the prey prized by corals avoided fluorescent targets in general and orange ones in particular.

In the second phase of the study, the experiment was conducted in the natural habitat of corals, about 40 meters below the surface of the Red Sea, in southern Israel. And there, the fluorescent traps (green and orange) or attracted twice as many planktons as the transparent trap.

“We conducted an experiment in the depths of the sea to examine the possible attractiveness, for various planktons, of fluorescence under natural currents and in the light conditions specific to deep waters,” said Dr. Or Ben-Zvi, who led the research.

The installation of the necessary traps in the Red Sea to examine which species were attracted by the fluorescent colors of the corals. (Credit: Tel Aviv University)

“Since fluorescence is primarily ‘activated’ by blue light (light from the depths of the sea), at these depths fluorescence is naturally illuminated and the data obtained in this experiment was unequivocally similar to those found in the laboratory”, he comments.

In the final part of the study, the researchers looked at the predation rate of corals collected 45 meters below the surface of the Gulf of Eilat.

They found that green fluorescence improved a coral’s predation abilities by 25% compared to yellow fluorescence.

Yossi Loya, a marine scientist and professor emeritus from Tel Aviv who supervised the study, said he believes the corals are trying to attract prey.

“Many corals display a fluorescent color pattern that emphasizes their mouths or the end of their tentacle – which supports the idea that this fluorescence, like bioluminescence (the production of light by chemical reaction), acts as a mechanism of prey attraction,” he says.

“The study proves that the bright and colorful appearance of corals can act as a decoy to attract swimming plankton to predators that are on the ground, such as corals, and especially in habitats where corals need other sources of energy in addition to or instead of photosynthesis (the production of sugar, by the symbiotic algae found inside the corals, using light energy),” he continues.

The research was conducted in collaboration with Yoav Lindemann and Gal Eyal, the Steinhardt Museum of Natural History and the Eilat Interuniversity Institute of Marine Sciences.