Scientists finally figured out why deep sea corals glow in the dark

(ORDO NEWS) — The oceans are full of magical marine life that has given rise to ecosystems teeming with biodiversity. Corals, which come in all shapes, sizes and colors, are no exception. Some species even glow in the dark.

Now a team of Israeli scientists have figured out why that might be the case. Deep sea corals with luminous green and yellow tentacles can glow in bright colors to lure prey to them for a snack.

“Despite gaps in existing knowledge regarding plankton’s visual perception of fluorescence signals, this study provides experimental evidence for the role of fluorescence in corals in attracting prey,” says Tel Aviv University coral reef researcher Or Ben-Zvi, who led the study.

Most reef-building corals bask in shallow water so that their algae can capture sunlight from the ocean surface. These are coral reefs with their photosynthetic zooxanthellae that we know and love.

But other fearless coral species manage to grow at great depths, up to 6,000 meters (20,000 feet) below the surface in the dark, cold, deep sea. (Unfortunately, even they cannot escape human exposure.)

The researchers behind the new study speculated that these deep-sea corals, many of which are fluorescent, may use light to attract their prey, such as small plankton, into their waters – similar to other deep-sea creatures that emit bioluminescence.

But they needed to test this theory, which they called the “light trap” hypothesis.

“Many corals have fluorescent coloration that accentuates their mouth or tentacle tips,” explains marine ecologist and senior study author Yossi Loya of Tel Aviv University.

This ability to fluoresce and attract prey seems like a pretty important adaptation for corals stuck on the seafloor, and “especially in habitats where corals require other sources of energy in addition [to] or instead of photosynthesis,” adds Loya.

However, many other ideas have been proposed to explain why corals fluoresce. For example, the “sunscreen” hypothesis suggests that fluorescence may protect bleached corals from further heat stress and light damage. Another possible explanation could be increased photosynthesis.

But mesophotic corals that grow in low, bluish light are a little different – there is no evidence yet that their fluorescence provides any kind of protection or energy boost.

So Ben-Zvi and her colleagues turned their attention to coral species that grow at depth in low light and rely more on predation than photosynthesis for food.

In a series of lab experiments, the team tested whether juvenile shrimp (Artemia salina) preferred a green or orange fluorescent target to clear, reflective, or frosted targets placed on the opposite side of the aquarium.

Indeed, the shrimp were attracted and swam towards the fluorescent signal.

Similar results were obtained when researchers set up experiments in the Gulf of Eilat, located at the northern tip of the Red Sea. A native crustacean that falls prey to corals in the bay, Anisomysis Marisrubri preferred fluorescent signals to reflective targets, while larvae of an introduced fish species did not.

Finally, the researchers compared levels of predation among different colored Euphyllia paradivisa corals that were collected from the Gulf of Eilat at a depth of 45 meters (148 feet) and taken to the lab.

It turned out that the fluorescent green corals had higher levels of predation than their yellow fluorescent counterparts, eating more A. salina shrimp in 30 minutes. And when the experiment was repeated under red rather than blue light, which does not excite coral fluorescence, there was no difference in the amount of shrimp eaten.

“In its natural habitat in the mesophotic reefs of Eilat, the yellow morph of E. paradivisa was found to be the least abundant, which can now potentially be explained by the lower prey attractiveness of this color found in the present study,” write Ben-Zvi and colleagues.

Of course, it is important to note that only one species of mesophotic coral was considered in this study. More research is also needed to better understand how plankton and other coral-supporting crustaceans perceive color, which is likely to differ across species, habitats and life stages.

But regardless, the results of the study highlight why corals, which are the backbone of biodiverse ocean ecosystems, are so important to protect. Fortunately, we know how to do it.

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