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some sharks also “glow” in the dark

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According to new research, three species of sharks living in twilight depths are able to glow in the dark. Measuring almost two meters in length, one of these fish is also one of the largest bioluminescent vertebrates in the world.

Bioluminescence abilities are developed in many other organisms. These include mushrooms, fish and phytoplankton. Naturally, these characteristics appear to be more useful for life forms that evolve in the dark. In the aphotic zone (or mesopelagic zone) between 200 and 1000 meters below the surface of the ocean, bioluminescence is also the norm, playing an important role in the structuring of the largest ecosystem on our planet. Over 90% of species there produce their own lights in one way or another.

However, in sharks, bioluminescence is not well documented. As part of a recent study, marine biologists Jérôme Mallefet and Laurent Duchatelet, from the Catholic University of Louvain (Belgium), therefore tried to find out a little more.

Three bioluminescent species

With this in mind, they collaborated with the National Institute for Water and Atmosphere Research (NIWA) in New Zealand. Scientists were then able to capture several hundred specimens. Of this sample, twenty-five specimens were capable of bioluminescence.

Thanks to their work, we now know that the shark kite (Dalatias licha) also glows in the dark. Researchers had suspected it for about 40 years, but we had no definitive proof so far. With its 1.8 m long on average, it is one of the larger bioluminescent vertebrates known to the planet.

Two other species are also concerned, namely the black bellied lantern shark, or lucifer shark (Etmopterus lucifer), and the southern lantern shark (E. granulosus). These two fish are smaller. The first measures about forty-seven centimeters on average and the second about sixty centimeters.

The Kite Shark (top) and the Black-bellied Lantern Shark (bottom). Credits: Credits: Mallefet et al., Front. Tue Sci., 2021

A great way to camouflage yourself?

Unsurprisingly, biologists have isolated photophores (electroluminescent organs) in the skins of all three species. More interesting: the researchers found that the light emission in these sharks was controlled by different hormones. This is a first. First, the glow is triggered by melatonin, and then the alpha-melanocyte stimulates it. Finally, adrenocorticotropic hormones turn it off.

As to why these sharks glow in the dark, it could be a tactic to lure a mate, lure prey, or camouflage themselves. For now, the question remains open, but biologists are leaning more towards the third option. At these depths, sunlight penetrates a little longer, sometimes revealing the blue sky. By a backlighting effect (the belly of these sharks lights up in blue) and these fish could therefore go unnoticed from certain angles.

Details of the study are published in the journal Frontiers in marine sciences.



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