Deep Sea Shark Eyes are Adapted to Hunt Photons in the Twilight Zone

The deep sea is home to some unusual and startling creatures that rely on bioluminescence to navigate and communicate in their dark world. Now, scientists have found that the eyes of deep-sea bioluminescent sharks have a higher rode density than their non-bioluminescent counterparts. The findings reveal a way in which sharks can perceive bioluminescent light in order to communicate, find prey and camouflage themselves against predators.

The deep see in this case is known as the mesopelagic twilight zone, which is about 200 to 1,000 meters below the surface of the ocean. It's a vast, dim habitat where sunlight is progressively replaced by bioluminescent emissions. This makes it crucial for predators, like sharks, to be able to perceive this light in order to navigate in and hunt in this dim world.

The researchers analyzed the eye shape, structure and retinal cell mapping in the visual systems of five deep-sea bioluminescent sharks. This included four lantern sharks and one kitefin shark. In the end, the scientists found that the sharks' eyes possessed a translucent area present in the upper eye orbit of the lantern sharks, which may aid in adjusting to counter-illumination or in using bioluminescence to camouflage the fish. The researchers also found a gap between the lens and the iris that allows extra light to the retina. In addition, these sharks possess higher rod densities than non-bioluminescent sharks, which may provide them with improved temporal resolution.

"Every bioluminescent signal needs to reach a target photoreceptor to be ecologically efficient," said Julien Claes, one of the researchers, in a news release. "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."

The findings reveal a bit more about deep-sea sharks' unique adaptations to their low-light world. In addition, it reveals that these sharks have evolved more than one photon-hunting strategy in order to cope with the twilight zone.

The findings are published in the journal PLOS One.