It’s an iconic picture: whale sharks (Rhincodon typus) traveling vast distances to feed on plankton and other small prey, their white-and-blue spots often jutting out of the water due to their enormous size. And it’s not just their traveling around global tropical oceans that breaks records – the whale shark also has one of the longest vertical ranges of any sea creature! Filter-feeding from the ocean’s bright surface to about 6562 feet (2000 meters) deep, where sunlight does not penetrate this eternal darkness, scientists were baffled when they first discovered this. There is no clear explanation for why whale sharks dive so deep although some believe the behavior may be associated with mating while others point to foraging. The range is impressive, even more so when you consider whale shark vision is critical, and recent studies suggest their eyes are highly protected.
But can they see when they are down in the pitch-black depths?
A study published last month in the Proceedings of the National Academy of Sciences show they can! While their eyes should strain to deal with the contrast of light in their vertical range, scientists have recently discovered the genetic secret that prevents this from happening. An unusual genetic mutation in these gentle giants makes a pigment in their retina more sensitive to temperature changes. Thus, sharks are able to prioritize different parts of their vision at different depths by activating them in the cold deep sea and deactivating them when they return to the warmer surface (where they can see a spectrum of colors instead of only blue). If you have a medical background and this sounds familiar, it’s because the genetic mutation is surprisingly similar to the one that causes night blindness in humans by degrading pigments in our retina. The condition of night blindness (nyctalopia) in humans results in one’s inability to see clearly at night or in poor light conditions, such as in a movie theater or in a dimly-lit restaurant.
In the dark waters of our ocean, whale sharks are able to navigate with the help of a light-sensing pigment in their retinas called rhodopsin. In bright environments, these pigments are less useful, but in dim ones, they help many vertebrates detect light. This includes us humans! But the rhodopsin pigments inside whale sharks’ eyes have been specifically calibrated to see blue light, which is the only color that reaches these depths in the deep sea. Whale sharks are not the only ones to have these pigments in their eyes, with past research showing that cloudy catsharks (Scyliorhinus torazame) sport them as well! The difference is these sharks – which can be found off the coasts of Japan, Korea, China and the Philippines – are solely bottom dwellers.
An evolutionary biologist from the National Institute of Genetics and his colleagues at the National Institute of Genetics conducted experiments on the eyes of zebra sharks (Stegostoma tigrinum), a close relative of whale sharks to figure out what makes whale shark vision so versatile in both light and dark waters. Inhabiting coral reefs, although these sharks are known to be nocturnal foragers, they have a much smaller vertical range than their larger spotted relatives. Dr. Shigehiro Kuraku and his team compared the genetic information they extracted from the zebra sharks to previously published genomic data on whale sharks in order to pinpoint where there were differences between the two sharks’ genetic code. There were two spots – known as sites 94 and 178 in their DNA – where the rhodopsin protein was altered due to mutations that altered its amino acid composition. “In this study, our spectroscopy of recombinant whale shark rhodopsin (RHO) mutants revealed that this blue shift is caused dominantly by an unprecedented spectral tuning site 94,” the authors stated in their paper. “RHOs having the natural substitution at site 94 are also found in some Antarctic fishes, suggesting that the blue shift by the substitution at the CSNB site associated with the reduction in thermal stability might be allowed in cold-water deep-sea habitats.”
This is just another secret the world’s largest living fish has – and it couldn’t come at a better time, since worldwide their numbers are declining. In July 2016, the International Union for the Conservation of Nature upgraded the conservation status of whale sharks from ‘Threatened’ to ‘Endangered’ due to environmental stressors and human threats. These slow-moving creatures still have much to teach us… like perhaps understanding more about nyctalopia and how to prevent it in humans!