The eyes of whale sharks, the biggest fish on the planet, are able to see in the pitch-black deep sea due to a mutation in their retinas that causes night blindness in humans, a team of Japanese researchers has said.

The genetic mutation in the whale shark's eyes activates visual pigments that sense blue light, the only color that reaches the deep sea, based on temperature changes, according to researchers from the National Institute of Genetics, Osaka Metropolitan University, Okinawa Churashima Foundation and the Riken Center for Biosystems Dynamics Research.

Whale sharks are known to swim near the surface of the ocean to feed on plankton, but they also dive into deep waters at depths of nearly 2,000 meters and much of their biology remains a mystery.

To better understand how the creatures are able to see in both bright surface waters and the darkness of the deep sea, researchers focused on rhodopsin, a protein in the retina of the eye that enables vision in dimly lit environments.

They found that rhodopsin, normally sensitive to green light, had mutated to become sensitive to blue light in whale sharks, with the amino acid composition of the protein unique to the species.

A genetic alteration in the same amino acid site in humans causes congenital stationary night blindness, a condition that impairs vision in low-light situations.

The findings, published in the Proceedings of the National Academy of Sciences on March 21, suggest that whale sharks' blue light-sensing pigments adapt according to temperature.

"While the rhodopsin in whale sharks degrades when exposed to heat, it functions effectively in the chilly waters of the deep sea," said Shigehiro Kuraku, a professor at the National Institute of Genetics involved in the study.