Glowing sharks in the deep-sea have a number of adaptations to cope with the mesopelagic twilight zone. They have evolved special eyes that are adapted to produce and perceive complex light patterns in the deep-sea. According to a study published in the journal PLOS ONE, these bioluminescent sharks this light to camouflage against predators, communicate and find prey.

Bioluminescent sharks have a very complex mechanism to produce light

The glowing sharks have a very high density of light-sensitive cells in their retinas. Many other species of sharks in the mesopelagic twilight zone have developed other adaptations for the same purposes. The mesopelagic twilight zone is the deep-sea where little sunlight penetrates. At about 200-1,000 meter deep in the sea, the sunlight is increasingly replaced by point-like bioluminescent emissions.

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Julien Claes, a biologist at the at The Catholic University of Louvain in Belgium, told LiveScience that there are 50 different shark species that produce light. Claes and his colleagues found that bioluminescent sharks use a complex mechanism involving hormones. In contrast, the glowing bony fish species use brain signalling chemicals such as melatonin.

Besides protection and camouflage, sharks use their light to recognize members of their own species to find mates or hunting partners. What surprised scientists was the sensitivity to light and resolution of the sharks. There is always a trade-off between resolution and sensitivity. In most deep-sea animals, their vision is tuned for sensitivity to light rather than resolution. But the visual systems of glowing sharks have both.

Glowing sharks have a higher density of light-sensitive cells in their eyes

To find out the reason, Claes studied the eye structure, shape and mapping of retinal cells in five species of bioluminescent sharks. And then they compared eyes of these sharks with those of non-bioluminescent ones. Researchers found that the eyes of glowing sharks have much larger density of light-sensitive cells called rods. It gives them faster vision. As a result, they can easily see the quickly changing light patterns that they use to interact with each other.

Claes said the visual system of these glowing sharks has evolved together with their ability to produce light.