Big fish can now detect blue light even in the darkest depths, thanks to changes in their eyesight genes.
A whale shark is worthless even at the end of a fisherman's thread. However, they have many features waiting to be discovered. The largest in the world, these fish can reach 18 meters in length and weigh as much as two elephants. The impressive features of whale sharks do not end there; Feeding by gliding from the ocean surface to the pitch black at a depth of about 2000 meters, these fish have one of the widest vertical ranges among marine animals.
Sharks' eyes must be forced to swim between the bright surface waters and the pitch blackness of the deep sea, making their mode of existence untenable. But scientists have now identified the genetic makeup that prevents that from happening.
The research, published this week in the journal Proceedings of the National Academy of Sciences, identifies a genetic mutation that increases the sensitivity of a pigment in the whale shark's retina to temperature changes. In gloomy environments, pigments that detect blue light are active in the cold deep sea to prioritize different areas of vision at various depths, and are deactivated when sharks return to the warm surface to feed. Ironically, the genetic modification is strikingly similar to the modification that causes night blindness in humans by disrupting the pigments in the retina.
Why whale sharks dive so deep remains a mystery. Prey is scarce at these depths, so this activity may be related to mating. But whatever they do, sharks use rhodopsin, a light-sensing pigment in their retinas, to find their way through dark waters. Although less useful in sunny environments, the pigments help many species, including humans, detect light in low-light conditions. The rhodopsin pigments in whale shark eyes are specially tuned to detect blue light in the deep sea because it is the only color that reaches these depths.
Previous studies have shown that the pigments in the eyes of bottom-dwelling cloudy cat sharks (Scyliorhinus torazame) are similarly tuned to detect blue light. Whale sharks are the only sharks known to display these pigments in shallow waters because these tiny sharks are content to live at the depths. These blue light-sensing pigments may make it difficult for whale sharks to detect other types of light in more open waters, but they can still maneuver without difficulty while foraging for their prey.
By examining the eye of a zebra shark, a close relative of whale sharks, a frequent visitor to coral reefs and a much more restricted vertical range, Shigehiro Kuraku, an evolutionary biologist at Japan's National Institute of Genetics, and colleagues analyzed the whale shark's vision in both light and dark waters. He tried to find out what made it so versatile. To identify genetic differences between the two sharks, they matched genetic data from zebra shark tissue with previously published genomic information for whale sharks.
Regions 94 and 178 in the sharks' DNA were identified as the sites of two mutations that changed the amino acid structure of the rhodopsin protein. Interestingly, the mutation at position 94 was found. The black goby, a deep-sea creature found in Antarctica that can detect blue light, also exhibits the same amino acid change. This led the team to conclude that the mutation in region 94 was mainly responsible for the "blueshift" in whale shark vision.
Black cod and whale sharks, as well as other animals, have a mutation at position 94. A genetic variation in humans causes congenital fixed night blindness by reducing the stability and effectiveness of rhodopsin pigments in the retina. People with this condition have trouble seeing in dim light.
According to the researchers, a similar process takes place in whale shark retinas. They discovered that by altering amino acids at positions 94 and 178 in both whale and zebra shark tissues, the fish's rhodopsin pigments became less stable and degraded at warmer temperatures. Accordingly, the probability of benefiting from blue light-sensing pigments in deeper and colder waters is higher than in warmer surfaces.
As a result of fluctuating temperatures that activate and deactivate their blue light-sensing pigments, whale sharks' vision is constantly changing as they move up and down the water column. According to Kuraku, whale sharks' blue vision is clear in deep waters and closed near the surface. Because these pigments are set at lower levels, sharks can detect various hues rather than just blue light.
Some other deep divers, such as sperm whales, are adopting a similar method by changing their pigments to filter out blue light at depth, according to 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 have undergone a mutation that destroys pigments and allows them to see better. This is remarkable as an animal should not evolve to have this mutation, but Fasick notes that the mutation spreads across species. They adjust their sensitivity to the available light.
Source: science.org/content/article
📩 25/03/2023 22:36