The whale shark is an awe-inspiring creature, surpassing even the wildest fisherman’s tales. With a length of up to 18 meters and a weight that could rival two elephants, it holds the title of the largest fish in the world. What’s more, these majestic creatures have an incredible vertical range, spanning from the ocean surface to depths of almost 2000 meters where they filter-feed.
One might think that swimming between the bright waters of the surface and the dark abyss would be too much for the whale shark’s eyes, but recent research has shed light on how they manage this feat.
In a study published this week in the Proceedings of the National Academy of Sciences, scientists have discovered a genetic mutation that makes a visual pigment in the whale shark’s retina more sensitive to temperature changes. This adaptation enables the pigment to sense blue light in the dark depths and be deactivated when the shark returns to the warm surface waters to feed. This allows the sharks to prioritize different aspects of their vision at different depths.
Interestingly, this genetic modification is similar to one that causes night blindness in humans by degrading pigments in their retinas.
The reason behind why whale sharks dive to such deep depths remains a mystery. It is speculated that this behavior may be linked to mating, as prey is scarce at these depths. Regardless of their purpose, the sharks rely on the rhodopsin pigment in their retinas to navigate the darkness. This pigment helps many vertebrates, including humans, detect light in dim environments, and is specifically calibrated in whale sharks to detect blue light, the only color that penetrates these depths.
Interestingly, previous studies have shown that bottom-dwelling catsharks also possess these pigments calibrated to spot blue light. However, these small sharks are content in the deep, making whale sharks the only known sharks to use these pigments in shallower waters. Although these pigments could potentially hinder vision in lighter waters, whale sharks have no trouble maneuvering and hunting.
To uncover the secret behind the versatility of whale shark vision in both light and dark waters, evolutionary biologist Shigehiro Kuraku and his team dissected the eye of a zebra shark, a close relative of the whale shark that frequents coral reefs and has a limited vertical range. By comparing the genetic information of the zebra shark’s tissue to previously published whale shark genomic data, they identified the differences in the two sharks’ genetic code.
The genetic mutations that alter the amino acid composition of rhodopsin protein in whale sharks were found by researchers at two sites on the sharks’ DNA, site 94 and site 178. The team concluded that the “blue shift” in the sharks’ vision is mainly due to the mutation at site 94, as it also occurs in black rockcod, a deep-sea creature that can see blue light. This mutation is similar to one that causes congenital stationary night blindness in humans, decreasing the stability and effectiveness of rhodopsin pigments in the retina.
The researchers discovered that the rhodopsin pigments in whale shark eyes become less stable and degrade at warmer temperatures when they manipulated the amino acids at sites 94 and 178 in whale and zebra shark tissues in the lab. Therefore, the sharks’ blue light-sensing pigments are more effective in deeper and colder waters than at the warmer surface, and they become activated and deactivated by changing temperatures as the sharks migrate up and down the water column.
This state of flux allows whale sharks to see a range of colors available in shallower depths rather than just blue light. Similar strategies are employed by other deep divers, such as sperm whales, to tune their pigments to filter blue light at dark depths. However, 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,” says Jeffry Fasick, a visual ecologist at the University of Tampa who was not involved in the study. “They match their sensitivity to what light is available.”