How Do Sharks See the World? A Deep Dive into Shark Vision
SHOW NOTES
Dr Lily Fogg’s field of study might seem a bit unusual to some; she is fascinated by how different fish species see the world. And it started with a drive to get back into the great outdoors [11.05]. “I had good grades when I was a kid, but I didn’t really know what I wanted to do. So, the careers advisor said, well, maybe you should be a doctor!” Lily recalls. This led her to move across the world from her home country of Australia to a much colder and rainier Scotland, to study medical sciences. “And then I realised partway through that I actually just really like being outside and that I really love the natural world. So that’s when I decided to switch.”
While looking for a PhD related to medical research, Lily came across the perfect answer: a project looking at vision in coral reef fishes, with fieldwork in French Polynesia [12:45]. “I was like, sold! That is the one for me,” she laughs.
Now based at the University of Helsinki, Finland, Lily’s research centres around one core question: “If you drop the same basic vertebrate eye design into completely different light environments, what does evolution do with it?” [13:52]. In other words, how does the visual system of one animal living in a brightly lit, complex environment – like a coral reef – differ from that of one living in a dark, murky habitat? And the answer is that evolution gets incredibly inventive.
When it comes to fish, the basic structure of their eyes isn’t all that different from our own [14.37]. Both have lenses, corneas and retinas with rods and cones — the cells responsible for detecting light. But living underwater means fish have to use these shared building blocks a little differently. “Humans focus an image by changing the shape of our lens. But most bony fishes actually move the whole lens backwards and forwards inside the eye to focus. Kind of like adjusting a camera,” explains Lily. “In humans the cornea does a lot of focusing work too. But the cornea and water actually have really similar optical properties, so it’s really hard to use the cornea for focusing light underwater. So that’s why fish mostly use their lenses.”
Another key difference is light sensitivity [17.45]. Many fishes can detect parts of the light spectrum humans cannot, including ultraviolet light. Damselfishes, for example, look yellow to us, but in actual fact have unique ultraviolet patterns on their faces. “[It] is a bit like having an invisible face tattoo,” says Lily. These hidden markings may help fish recognise members of their own species — or even specific individuals.
A significant part of Lily’s research is trying to understand how the visual systems of fishes have evolved to cope with vastly different light conditions [20.42]. Coral reefs are vibrant and bright, but below about 1,000 metres, sunlight disappears completely. In response, deep-sea fishes have evolved extraordinary visual adaptations to capture even the faintest light. Some species stack layers of light-sensitive cells to increase the chance of detecting a photon, while others develop tubular eyes aimed upwards to capture the last traces of light from the surface. Perhaps the most extreme example is the barrel-eye fish, which has a transparent head to maximise the amount of light that can reach its bizarre tube-like eyes. “It’s definitely [some] of the wackiest eyes in the sea,” Lily laughs.
But how about sharks and rays – the cartilaginous fishes [24.50]? Despite diverging from bony fishes hundreds of millions of years ago, their eye structure remains surprisingly similar.
However, sharks appear to prioritise low-light vision over colour perception. “Most of the shark species that we’ve actually studied only have one type of cone cell… which means they’re probably colour blind”, Lily explains. The ocean through a shark’s eyes might actually be in black-and-white, rather than the vibrant, technicolour landscape seen by a reef fish.
But one shark in particular has surprised scientists with its visual capabilities – and it’s not the one you might think [34.30]. Lily’s most recent published study explores the vision of the Greenland shark — the longest-lived vertebrate on Earth.
After reviewing one of her papers on fish vision, another researcher invited Lily to analyse Greenland shark eyes collected during their fieldwork in the Arctic. Soon, she received an unusual delivery…“It was a bit crazy opening a box full of eyes!”
It had previously been assumed that Greenland sharks had quite poor eyesight, due to their extraordinarily long lifespans and habit for hanging out in dim, murky environments [40.41]. They are also famous for the parasitic copepods that often attach to the surface of their eye. “For a really long time, people assumed that if you’ve got something physically attached to your eye, especially for [a] period of decades, that it had to damage the window into the eye, which would block light and therefore hinder your ability to see,” Lilly says. But her research revealed a different story.
First, the parasites did not block much light [43.39]. “We shone a light [through the eye], and measured how much came out the other side. And we found that it did, in substantial proportions, so actually the majority of light still gets through,” explains Lily. Even more surprising, the retinas of extremely old sharks appeared healthy and intact: “all of the layers were preserved… even in sharks estimated to be over a century old.” Their eyes were also perfectly attuned to the dim Arctic depths in which they reside. Lily’s research found that their visual system was entirely built around rod cells – the cells used for seeing in dim light. We use our rod cells at night; when you wake up in a dark room, you can’t see colour or detail, but you do have better sensitivity and can detect shapes and movement. A Greenland shark, it seems, experiences the world like that all the time. “Their retina is essentially a low-light camera,” says Lily.
But the biggest mystery may be how Greenland sharks maintain good eye health for centuries [45.00]. Typically, in humans and most animals, ageing is associated with an accumulation of DNA damage and a gradual decline in tissue health. The retina is particularly susceptible, and degeneration of the retina is very common. So if humans, with our very meagre lifespans, suffer from a reduction in eye health, why doesn’t the world’s longest living vertebrate?
The answer may be in their very DNA. Lily’s team found signs that genes involved in DNA repair are particularly active in Greenland shark eyes — a possible explanation for how these animals maintain tissue health for such extraordinary lifespans. It’s an area that requires more research, and the answers could be relevant to our own health: “They’re pushing the limits of what we think is biologically possible. And trying to understand how they do that can teach us a lot about how life adapts to extreme conditions and could also have biomedical relevance. So if you can keep an eye functioning for centuries, then that challenges the idea that age-related vision loss is inevitable. And you can try and understand what that mechanism is that keeps the eye intact for so long. And then maybe in the future, not anytime soon, but maybe one day we can apply that, a similar idea to extend the healthy lifespan of human vision.”
But for now, these findings simply add another layer to the mystery of one of the ocean’s longest-lived and most fascinating animals – and adds another item to the long list of how sharks keep pushing the boundaries of what is physiologically possible.
ABOUT OUR GUEST
DR LILY FOGG
Dr. Lily Fogg is an Australian biologist fascinated by the remarkable ways animals adapt to the world around them. Her research dives into the crazy world of fish vision, uncovering how some species have evolved to see on colourful coral reefs while others have adapted to the near-darkness of the deep sea. More recently, Lily turned her attention to the enigmatic Greenland shark, the longest-lived vertebrate on Earth, to discover how its eyes withstand centuries in the cold, dark Arctic deep sea. Like the animals she studies, Lily has adapted to a variety of environments, earning her PhD at the University of Queensland in sub-tropical Australia, then moving to mountainous Switzerland for a postdoc at the University of Basel. Nowadays you can find her at the University of Helsinki in Finland, learning to embrace the Arctic winters.
Papers mentioned in this episode:
https://www.nature.com/articles/s41467-025-67429-6
