A group of shy sharks shine bright – in green – perhaps as a way of finding mates underneath the waters, reveals an exciting U.S. study.

The deep waters are the best places to experience darkness in daytime. Although the amount of light, including sunlight, that penetrates the waters depends on water depth and the level of light, in general, sunlight is significantly decreased once the waters are more than 200 meters (656 feet) deep. Meanwhile, no light goes through the aphotic zone, which is already 1,000 meters (3,281 feet).

The decreased penetration of light may be one of the reasons why "glowing" marine species such as sharks and turtles are hardly studied until today.

U.S. marine researchers have studied the phenomenon of biofluorescence on two shark species namely swell shark (Cephaloscyllium ventriosum) and chain catshark (Scyliorhinus rotifer) and learned not only how these animals exhibit biofluorescence but also how they show it differently depending on their sex and type.

The animals in spotlight are small catshark species that like to spend most of their time around 500 meters (1640 feet) under the waters. Because of the distance from the surface, only blue light reaches the sharks' habitats, and they don't emit blue but rather green through their skin.

To understand how this happens, the researchers then studied the animals' retinas, and they discovered that they possess long rods that allow sharks to see clearly despite insufficient light, as well as one cone that can detect shades of blue and green.

Not content, the team also dove at Scripps Canyon to observe the sharks in their natural environment, armed with a specially designed camera that's meant to mimic the way their eyes work.

They then noticed that the biofluorescence can already be detected within 38 meters (125 feet). Their skin also glowed in green, but the patterns differ depending on the species' sex and type.

Swell sharks, in general, feature "bright green fluorescent spots," but "the females also have a unique 'face mask' with light spots in the center on each side and more dense ventral spotting that extends further anteriorly than in males," according to the study.

The chain sharks, on the other hand, do not have the dark spots but feature reticulated patterns on their skin in dark and light shades, which are more pronounced among the females. Males have glowing pelvic claspers, which they use for mating.

The study now in Nature doesn't fully explore the reason for biofluorescence, but one of the possible theories is that it's used to help these species see each other in the darkness, hopefully landing a mate. In the meantime, it provides them cover against predators.

But perhaps the biggest learning for researchers is their discovery that biofluorescence is actually not uncommon among marine species and has, in fact, evolved, which may suggest that there's more to it than allowing them to glow.

While there's still exploring biofluorescence among marine creatures, David Gruber, lead author, has a suggestion. "This work forces us to take a step out of the human perspective and start imagining the world through a shark's perspective. Hopefully it will also inspire us to protect them better," he said.

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