Even a fisher’s yarn would sell a whale shark short. These fish—the biggest on the planet—stretch up to 18 meters long and weigh as much as two elephants. The superlatives don’t end there: Whale sharks also have one of the longest vertical ranges of any sea creature, filter feeding from the surface of the ocean to nearly 2000 meters down into the inky abyss.
Swimming between bright surface waters and the pitch black deep sea should strain the shark’s eyes, making their lifestyle impossible. But researchers have now uncovered the genetic wiring that prevents this from happening.
The study, published this week in the Proceedings of the National Academy of Sciences, pinpoints a genetic mutation that makes a visual pigment in the whale shark’s retina more sensitive to temperature changes. As a result, the pigments—which sense blue light in dark environments—are activated in the chilly deep sea and deactivated when the sharks return to the balmy surface to feed, allowing them to prioritize different parts of their vision at different depths. Ironically, the genetic alteration is surprisingly similar to one that degrades pigments in human retinas, causing night blindness.
It remains unclear why whale sharks dive so deep. Because prey is scarce at these depths, the behavior may be linked to mating. But whatever they do, the sharks rely on a light-sensing pigment in their retinas called rhodopsin to navigate the dark waters. Although the pigments are less useful in sunny habitats, they help many vertebrates, including humans, detect light in dim environments. In the deep sea, the rhodopsin pigments in whale shark eyes are specifically calibrated to see blue light—the only color that reaches these depths.
Previous research has revealed bottom-dwelling cloudy catsharks (Scyliorhinus torazame) have similarly calibrated pigments in their eyes to spot blue light. But these small sharks are content in the deep, making whale sharks the only known sharks to sport these pigments in the shallows. In lighter waters, these blue light–sensing pigments could act as a hindrance to seeing other kinds of light, but whale sharks are still able to maneuver with ease as they vacuum up seafood.
To figure out what makes whale shark vision so versatile in both light and dark waters, 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. They compared the genetic information they extracted from the zebra shark’s tissue with previously published whale shark genomic data to pinpoint where the two sharks’ genetic code differed.
They pinpointed two spots—site 94 and 178 on the sharks’ DNA—where there were mutations that altered the amino acid composition of the rhodopsin protein. The mutation at site 94 was particularly intriguing. The same amino acid substitution occurs in black rockcod, a deep-sea denizen found around Antarctica that can also see blue light. This led the team to conclude that the “blue shift” in whale shark vision is primarily linked to the mutation at site 94.
Whale sharks and blackcod are not the only organisms with a mutation at site 94. In humans, a genetic alteration here decreases the stability and effectiveness of rhodopsin pigments in the retina, sparking a condition known as congenital stationary night blindness. Those afflicted have trouble seeing in low-light situations.
The researchers propose that a similar process is unfolding in the retinas of whale sharks. By manipulating the amino acids that occur at sites 94 and 178 in both whale and zebra shark tissues in the lab, they found that the fishes’ rhodopsin pigments become less stable and degrade at warmer temperatures. This means their blue light–sensing pigments are likely more effective in deeper, chillier waters than at the warmer surface.
As a result, the vision of whale sharks is in a state of flux as they migrate up and down the water column because the changing temperatures activate and deactivate their blue light–sensing pigments. “Blue vision of whale sharks is turned on in deep waters, while it is turned off near the surface,” Kuraku says. Tuning these pigments out at shallower depths allows the sharks to see a spectrum of available colors instead of focusing on only blue light.
Several other deep divers such as sperm whales employ a similar strategy by tuning their pigments to filter blue light at dark depths, notes Jeffry Fasick, a visual ecologist at the University of Tampa who was not involved in the new study.
But what sets whale sharks apart is that they benefit visually from a mutation that degrades pigments. “It’s intriguing because that shouldn’t be advantageous for an animal to evolve to have that mutation but it’s spread throughout the species,” Fasick says. “They match their sensitivity to what light is available.”
Even a fisher’s yarn would sell a whale shark short. These fish—the biggest on the planet—stretch up to 18 meters long and weigh as much as two elephants. The superlatives don’t end there: Whale sharks also have one of the longest vertical ranges of any sea creature, filter feeding from the surface of the ocean to nearly 2000 meters down into the inky abyss.
Swimming between bright surface waters and the pitch black deep sea should strain the shark’s eyes, making their lifestyle impossible. But researchers have now uncovered the genetic wiring that prevents this from happening.
The study, published this week in the Proceedings of the National Academy of Sciences, pinpoints a genetic mutation that makes a visual pigment in the whale shark’s retina more sensitive to temperature changes. As a result, the pigments—which sense blue light in dark environments—are activated in the chilly deep sea and deactivated when the sharks return to the balmy surface to feed, allowing them to prioritize different parts of their vision at different depths. Ironically, the genetic alteration is surprisingly similar to one that degrades pigments in human retinas, causing night blindness.
It remains unclear why whale sharks dive so deep. Because prey is scarce at these depths, the behavior may be linked to mating. But whatever they do, the sharks rely on a light-sensing pigment in their retinas called rhodopsin to navigate the dark waters. Although the pigments are less useful in sunny habitats, they help many vertebrates, including humans, detect light in dim environments. In the deep sea, the rhodopsin pigments in whale shark eyes are specifically calibrated to see blue light—the only color that reaches these depths.
Previous research has revealed bottom-dwelling cloudy catsharks (Scyliorhinus torazame) have similarly calibrated pigments in their eyes to spot blue light. But these small sharks are content in the deep, making whale sharks the only known sharks to sport these pigments in the shallows. In lighter waters, these blue light–sensing pigments could act as a hindrance to seeing other kinds of light, but whale sharks are still able to maneuver with ease as they vacuum up seafood.
To figure out what makes whale shark vision so versatile in both light and dark waters, 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. They compared the genetic information they extracted from the zebra shark’s tissue with previously published whale shark genomic data to pinpoint where the two sharks’ genetic code differed.
They pinpointed two spots—site 94 and 178 on the sharks’ DNA—where there were mutations that altered the amino acid composition of the rhodopsin protein. The mutation at site 94 was particularly intriguing. The same amino acid substitution occurs in black rockcod, a deep-sea denizen found around Antarctica that can also see blue light. This led the team to conclude that the “blue shift” in whale shark vision is primarily linked to the mutation at site 94.
Whale sharks and blackcod are not the only organisms with a mutation at site 94. In humans, a genetic alteration here decreases the stability and effectiveness of rhodopsin pigments in the retina, sparking a condition known as congenital stationary night blindness. Those afflicted have trouble seeing in low-light situations.
The researchers propose that a similar process is unfolding in the retinas of whale sharks. By manipulating the amino acids that occur at sites 94 and 178 in both whale and zebra shark tissues in the lab, they found that the fishes’ rhodopsin pigments become less stable and degrade at warmer temperatures. This means their blue light–sensing pigments are likely more effective in deeper, chillier waters than at the warmer surface.
As a result, the vision of whale sharks is in a state of flux as they migrate up and down the water column because the changing temperatures activate and deactivate their blue light–sensing pigments. “Blue vision of whale sharks is turned on in deep waters, while it is turned off near the surface,” Kuraku says. Tuning these pigments out at shallower depths allows the sharks to see a spectrum of available colors instead of focusing on only blue light.
Several other deep divers such as sperm whales employ a similar strategy by tuning their pigments to filter blue light at dark depths, notes Jeffry Fasick, a visual ecologist at the University of Tampa who was not involved in the new study.
But what sets whale sharks apart is that they benefit visually from a mutation that degrades pigments. “It’s intriguing because that shouldn’t be advantageous for an animal to evolve to have that mutation but it’s spread throughout the species,” Fasick says. “They match their sensitivity to what light is available.”