The eyes of mysterious glowing sharks that live in a shadowy "twilight zone" in the ocean depths have evolved to let them see complex light patterns in the gloomy waters, researchers have found.
The creatures, known as bioluminescent sharks, possess more light-sensitive retinal cells known as rods than their non-glowing shark relatives possess, the researchers say.
That and some other adaptations in their vision allow them to signal each other, camouflage themselves and seek out their prey in what is known as the mesopelagic twilight zone.
That zone, from 650 feet down to 3,200 feet, is a dim environment where sunlight progressively gives way to bioluminescent light the deeper one goes.
In an effort to understand how predator species operate at such depths, Belgian researchers looked at eye shapes, structures and configurations of retinal cells in deep-sea types of bioluminescent sharks, examining four Lanternshark species and one species of kitefin shark.
"There are about 50 different shark species that are able to produce light -- about 10 percent of all currently known sharks," says study author and biologist Julien Claes at the Catholic University of Louvain. Claes's study was published in the open-access journal PLOS ONE.
Adaptation in bioluminescent sharks includes a translucent region of the eye's upper orbit that may help them adjust their illumination as camouflage against their own predators by blending their light with weak sunlight coming from above.
In addition, the researchers found some specialization in the sharks' eyes, including a gap between the iris and lens of the eye that allows more light to reach the retina, something previously unknown among any shark species.
Finally, the higher density of rod cells in the eyes helps improve the resolution capability in detecting dim lights, which could be especially beneficial in receiving bioluminescent signals used in social interactions such as finding mates or selecting hunting partners, the researchers said.
As an example of this, they said, lantern sharks in particular have evolved sexual organs with light-producing structures helping males and females locate each other as they move in the almost total darkness of their marine environment.
The researchers used light microscopes and a number of optical instruments to study the structure and configuration of the eyes of bioluminescent sharks and, by comparison, those of non-bioluminescent species.
"Every bioluminescent signal needs to reach a target photoreceptor to be ecologically efficient," Claes says. "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."
