Whale sharks' night vision is aided by the same mutation that causes human night blindness.

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• shark
• pigments
• retina
• eyesight
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• evolutionary

A whale shark can't be underestimated, not even with a fisherman's yarn. The largest of its kind, these fish may grow to be 18 metres in length and weigh as much as two elephants. The list of superlatives continues with the fact that whale sharks have one of the deepest foraging ranges of any marine animal, diving approximately 2,000 feet to filter feed.

It's unlikely that sharks could adapt to a life where they often swim from the light surface waters to the pitch dark depths of the ocean. Yet scientists have figured out the genetic code that stops it from occurring.

This week's issue of PNAS has a research that identifies the specific genetic mutation in the whale shark that increases the sensitivity of a retinal pigment to temperature fluctuations. Hence, the sharks may prioritise various elements of their eyesight at different depths by activating the pigments—which perceive blue light in gloomy environments—in the cool deep water and deactivating them when they return to the warm surface to dine. The genetic variant is very similar to one that causes night blindness in humans by accelerating the degradation of retinal pigments.

The deep dives of whale sharks are a mystery. The lack of food at such depths suggests that this behaviour is related to mating. No matter how they get about, sharks depend on rhodopsin, a light-sensing pigment in their retinas. Many animals, including humans, rely on these pigments to sense light in low-light conditions, while they are of little utility in sunny surroundings. Whale sharks can only detect blue light, which is why their rhodopsin pigments are well tuned for deep water vision.

Similarly calibrated pigments have been found in the eyes of bottom-dwelling cloudy catsharks (Scyliorhinus torazame), according to previous study. While whale sharks are the only sharks known to exhibit these colours in the shallows, little sharks are perfectly satisfied in the depths. Although these blue light-sensing pigments would be a problem in brighter seas, whale sharks can still see well enough to navigate and hoover up fish with ease.

Shigehiro Kuraku, an evolutionary biologist at Japan's National Institute of Genetics, and his colleagues dissected the eye of a zebra shark, a close relative of whale sharks that frequents coral reefs and has a much more restricted vertical range, to determine what makes whale shark vision so versatile in both light and dark waters. The researchers compared the genomic data they obtained from the zebra shark with that of the whale shark, which had already been published.

Sites 94 and 178 on the sharks' DNA were identified as having mutations that affected the rhodopsin protein's amino acid makeup. It was fascinating to see the mutation at location 94. Black rockcod, a deep-sea inhabitant found in Antarctica, also has the ability to sense blue light due to a similar change in an amino acid. The researchers deduced that the mutation at site 94 is mostly responsible for the "blue shift" in whale shark eyesight.

Mutations at site 94 are not exclusive to whale sharks and black cod. Congenital stationary night blindness is caused by a mutation in this region of human DNA, which reduces the stability and efficiency of rhodopsin pigments in the retina. People affected struggle to see in dim lighting.

Scientists think that whale sharks' retinas are undergoing a similar development. Rhodopsin pigments in whale and zebra sharks were shown to be less stable and breakdown at higher temperatures when amino acids at sites 94 and 178 were manipulated in the lab. This suggests that the blue light-sensing pigments of these fish are more sensitive to the cooler temperatures of deeper waters than to the warmer surface waters.

Because their blue light-sensing pigments are activated and deactivated based on temperature, whale sharks' vision is constantly shifting as they move up and down the water column. According to Kuraku, "whale sharks' blue eyesight is activated in deep waters and deactivated near the surface." Sharks can perceive a wider range of hues, not only blue, by tuning out certain pigments at lesser depths.

Noting that many other deep dives, including sperm whales, use a similar method, visual ecologist from the University of Tampa Jeffry Fasick points out that these animals have tuned their pigments to filter blue light at dark depths.

However, a mutation that degrades pigments gives whale sharks a distinct visual advantage. Fasick finds the phenomenon "intrigueing" since "it shouldn't be beneficial for an animal to develop to have that mutation," but the mutation has spread across the species. They adjust their level of sensitivity based on the amount of available light.

Citation

doi: 10.1126/science.adh9610