Researchers found bioluminescent sharks have a higher rod density in their eyes than non-bioluminescent sharks.
The study, published in the August 6 edition of the open access journal PLOS ONE, suggested this adaptation helps the sharks perceive bioluminescent light in order to communicate with each other, camouflage into their surroundings, and track down prey.
The mesopelagic twilight zone, located between 200 and 1,000 meters below the sea; even at the zone's shallowest depths the water is extremely dim. As the water gets deeper more and more light is replaced by point-like bioluminescent emissions.
To gain insight into bioluminescent predators in the region researchers analyzed their eye shape, structure, and retinal value of five deep sea predators. The species included in the study consisted of four Lanternsharks (Etmopteridae) and one kitefin shark (Dalatiidae).
The researchers found the shark's eyes contained a translucent area, which could work to adjust counter-illumination or to provide camouflage. The team also identified a gap between the lens and iris that allowed extra light into the retina, which has not been identified before in sharks. Data comparisons revealed bioluminescent sharks possess higher rod densities on their eyes, which could provide them with temporal resolution; this could be effective in social interactions.
"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," said Julien M. Claes, postdoctoral researcher from the FNRS at Université catholique de Louvain.
The findings highlight the extraordinary diversity of photon capture strategies and adaptations that help creatures adjust to the twilight zone.
The paper is titled "Photon Hunting in the Twilight Zone: Visual Features of Mesopelagic Bioluminescent Sharks."
