Eyes From the 'Twilight Zone': A Study of Deep-Sea Sharks

There isn't a lot that researchers know about the deep sea. In fact, researchers discovered only very recently that at least one species of deep sea octopus takes a whopping four years to hatch from its egg, all the while being guarded by its mother.

In an effort to learn more about the mysterious deep, researchers are taking a closer look at one of its primary predators, deep sea sharks.

In a study recently published in the journal PLOS ONE, lead researcher Julien M. Claes and his colleagues decided to stare into - or rather, at - the eyes of the lanternshark, a bioluminescent shark that stalks waters 200 to 1,000 meters below the ocean's surface.

This region is interestingly called the mesopelagic twilight zone, and is largely devoid of light, save for the bioluminescence of local species.

Analyzing the eye shape, structure, and retinal cell mapping in the visual systems of four Lanternsharks (Etmopteridae) and one kitefin shark (Dalatiidae), Claes and his team hoped to build an understanding of strategies used by bioluminescent predators.

According to the study, a comparison between these sharks' eyes and non-bioluminescent sharks reveals that bioluminescent sharks possess higher rod densities in their eyes, which might provide them with improved sensitivity to light shifts. This could prove particularly useful for bioluminescent communication during social interactions, and my not be a predatory adaptation.

However, the five sharks' eyes were also found to possess a translucent area in the upper-eye which might help the predators compensate for counter-illumination used by prey.

"Every bioluminescent signal needs to reach a target photoreceptor to be ecologically efficient," Claes said in a statement. "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."