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Evolution gifted bioluminescent sharks with eyes perfect for deep-sea environment

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Animals evolve so they can adapt to their surroundings and increase their odds for survival and this apparently applies in the case of sharks that live in deep sea environment. A new study has found that bioluminescent sharks living deep underwater have evolved special eyes that allow them to see complex light patterns in the dark parts of the sea.

For the new study published the journal PLOS ONE on Aug. 6, Julien Claes, from the Earth and Life Institute of the Catholic University of Louvain in Neuve, Belgium, and colleagues examined the eyes of five species of bioluminescent sharks that include four lantern sharks namely E. splendidus, E. spinax, Etmopterus lucifer, andTrigonognathus kabeyai, and the kitefin shark Squaliolus aliae, which live in the mesopelagic twilight zone, a region 650 to 3,300 feet deep in the sea where only weak light from the sun manages to penetrate.

Using light microscope and other optical instruments to study the eyes of the animals and then comparing them with those of non-bioluminescent sharks, the researchers found that the glowing sharks possess light-sensitive cells in their eyes that were higher in density compared with those in the non-glowing sharks, a characteristic that the researchers said give the deep-sea sharks  better temporal resolution or "faster vision" , which allows them to see quickly changing light patterns including those that they use to interact with each other and hunt for food.

"A higher temporal resolution could facilitate bioluminescent signaling within species of the Etmopteridae, which would require the capacity to detect and follow small glowing areas of conspecifics during dynamic behaviors such as cohesive swimming and hunting," the researchers wrote.

Claes and colleagues likewise found that the upper socket of the eyes of the lantern sharks has a transparent region, which apparently helps the sharks to adjust the illumination of their eyes and camouflage themselves against their predators.

The researchers said that although further studies are still needed to understand the relationship, their research has found evidence that the visual systems of the glowing sharks evolved together with their ability to produce light and that these deep-sea sharks have adaptations that allow them to cope in the dark mesopelagic twilight zone.

"Every bioluminescent signal needs to reach a target photoreceptor to be ecologically efficient. Here, we clearly found evidence that the visual system of bioluminescent sharks has co-evolved with their light-producing capability, even though more work is needed to understand the full story," Claes said.

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