The dark depths of the ocean are still shrouded in mystery for us humans. Despite our species’ tenacious approach to figuring it all out, we have yet to understand so many facets and elements of the planet we call home.
That said, curious researchers from the School of Zoology, the Steinhardt Museum of Natural History at Tel Aviv University, and the Interuniversity Institute for Marine Sciences in Eilat have lifted the veil on one of the questions that have befuddled scientists for years: Why do corals glow?
Going with the glow
Scientists have known for a long time now that corals that dwell 148 feet below the ocean’s surface, in what is called the twilight mesophotic zone, glow a radiant green or orange around their mouth area (yes, they have mouths!) or on the tips of their tentacles.
Coral experts have also known that corals give off light after they absorb energy. This is called biofluorescence—not bioluminescence, which is an effect created by a reaction between an enzyme such as luciferase and a light-releasing molecule.
What researchers didn’t know was why corals at this depth glow. Some of their best hypotheses are that the fluorescence protects the corals from heat or light after bleaching (something like sunscreen for corals) and that the fluorescence somehow moves the process of photosynthesis along.
As great as these ideas are, once researchers tested these hypotheses on corals that live between 98 and 492 feet below sea level, neither holds true.
The glow of satisfaction
Since no evidence was found to support past hypotheses, a research team led by experts at the University of Tel Aviv put forth a new suspicion: What if the corals glow to attract prey?
The scientists then set about testing their hypothesis with a three-step study.
Step one
To figure out whether common coral prey was attracted to fluorescence in the first place, the team looked at the Artemia salina, a crustacean. They found that the crustacean had no presence when exposed to two clear targets, but definitely favored green or orange fluorescent targets over a clear one.
The team repeated this experiment with different Red Sea crustaceans and got the same results.
Step two
Once the team determined that crustaceans are partial to green and orange fluorescents, they decided to test out the effect they would have on plankton at a depth of approximately 131 feet (which is the corals’ natural habitat). The plankton, too, were more attracted to the green or orange glow emitted by the researcher’s traps. Compared with the clear traps, the glowing ones caught twice as many planktons.
Step three
To finish the job, the researchers collected corals from around 148 feet in the Gulf of Eilat in the Red Sea and found that the corals glowing green caught 25 percent more prey than those glowing yellow.
“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,” explains study leader Dr. Or Ben-Zvi of Tel Aviv University in the press release. “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.”
Why is this important?
Like so many of our planet’s creatures, corals are under massive threat. Deep corals, like the ones at the center of this study, are especially vulnerable to threats such as bottom-trawl fishing, offshore oil and gas production, deep sea mining, and the climate crisis.
Thus, any information we can get on the mysterious lives of coral can help us find new ways to protect and conserve them for enhancing biodiversity and for future generations to marvel at and enjoy.
Source study: communications biology—Coral fluorescence: a prey-lure in deep habitats
