By Peter de Kruijff, ABC
Greenland sharks can live for up to 400 years, so some sharks alive today may have been born before the British colonised Australia. Photo: ABC/SUPPLIED
Swimming at a crawl with cloudy eyes and mottled skin, the Greenland shark looks like it's seen better days.
The shark's eyes were thought to be barely functional, as it spends most of its time in pitch-black waters up to 3000 metres deep.
Its unearthly look is often accentuated by the presence of tiny crustacean parasites hooked into the corneas on its eyes, but the Greenland shark (Somniosus microcephalus) can live up to 400 years in the chilly North Atlantic and Arctic waters, making it one of the longest-living vertebrates on Earth.
According to new research, published in Nature Communications, its seemingly undead eyes are fully functioning and barely deteriorate, even after a century.
Unravelling the shark's anti-ageing secrets may benefit human eye health.
Australian marine biologist Lily Fogg studies the visual systems of fish, including the Greenland shark. Photo: ABC/SUPPLIED
Australian marine biologist and lead study author Lily Fogg, from the University of Basel, said some sort of mechanism kept these ocular organs in such good shape.
"That's something humans can't do, so if we can study that and work out what that mechanism is, they could have biomedical applications," she said.
Anatomy of a shark eye
To understand more, the researchers looked at eyes from 10 dead Greenland sharks with estimated ages between 100-134 years old. Their eyes are about 5-6cm in diameter and surrounded by thick protective tissue.
Human eyes have two types of cells for sensing light - cone cells, which are good for bright light and detecting colour, and rod cells, which work better in dim light.
Greenland shark range
Greenland shark range. Photo: ABC/SUPPLIED
Greenland shark eyes only have rod cells.
"In most deep-sea fish, they only have the cell that's good for dim light and we see the same thing in Greenland sharks," Dr Fogg said.
Study senior author Dorota Skowronska-Krawczyk, from the University of California, Irvine, said this meant the sharks saw their world in black and white.
"[As a Greenland shark] you don't have high resolution, you see light and darkness, but you really don't see the shapes very well or you cannot distinguish probably fast movements," she said.
Greenland sharks, which grow to about seven metres in length, are found from the sea surface to depths below thousands of metres.
"They're probably not using the vision right down in the deep," Dr Fogg said. "It's possible that they're using it to find the right depth... but they're obviously still using their visual system for something.
"Evolution is very efficient. If you don't need something, you usually get rid of it."
Macquarie University neurobiologist Laura Ryan, who was not involved in the study, said their vision would still function at shallow depths, especially at night, but they might be blinded by brighter light.
"Similar to how humans struggle with sudden bright light, after being in darkness, Greenland sharks show extreme low-light specialisation," she said.
Greenland shark eye. Photo: ABC/SUPPLIED
Remarkably, parasitisation of the eyes by small crustaceans didn't seem to hinder the shark from detecting light.
The species of copepods (Ommatokoita elongata) that lives on the eyes of Greenland sharks grows to about 3cm in length and resembles a white tassel.
"The copepod parasites attached to the cornea probably reduce image clarity, but the study indicates that the retina and visual pathways remain intact and functional," she said.
"Vision may be partially compromised, but the sharks are not blind - they can still detect light, contrast, and movement in the deep sea."
Even with copepods attached, what really stood out in the eyes studied was the lack of degeneration in any of them.
"The structure of the eye looks beautiful," Dr Skowronska-Krawczyk said. "I mean, it's basically pristine."
How shark eyes can help human health
The researchers suggest that a set of DNA repairing genes - ERCC1 and ERCC4 - may be related to a potential mechanism that underpins the long-term health of the retina, the part of the eye containing the rods.
"The high expression of DNA repair genes suggests a powerful molecular mechanism that helps maintain retinal health over centuries - a cool finding," Dr Ryan said.
Greenland shark. Photo: ABC/SUPPLIED
University of Melbourne visual neuroscientist Patricia Jusuf, who was also not involved in the study, said the research findings opened up promising new avenues for counteracting age-related retinal degeneration in people.
"The same genes are also functioning in DNA repair pathways in humans," she said. "When these genes do not work properly in mammals, we see detrimental effects associated with premature ageing.
"The fact that these greater than 100-year-old sharks showed little sign of retinal degeneration suggest that these DNA repair pathways might hold the key for maintaining retinal health and vision over a long life span."
No piece of Greenland shark is wasted by researchers who study its ability to live so long. Photo: ABC/SUPPLIED
Dr Jusuf said rods were often the first to be impacted in human eyes by age-related macular degeneration and retinitis pigmentosa.
"Being able to manipulate DNA repair pathways to slow down or halt degeneration of these rod photoreceptors in humans holds great benefits for the more than 200 million people affected by visual loss due to these degenerative conditions.
"Nature comes up with incredibly powerful and unique solutions, and tapping into these to use bio-inspired approaches for human health represents exciting avenues."
Further research into understanding the DNA repair mechanism of the eyes could lead to genetic or molecular therapies for people, Dr Skowronska-Krawczyk said.
Could Australian species help unlock genetic secrets?
Greenland sharks come from the family of fish Somniosidae, also known as 'sleeper sharks'.
So far we know little about the southern sleeper shark (Somniosus antarcticus) and Pacific sleeper shark (Somniosus pacificus), which are found in waters around Australia and New Zealand.
There may be more genetic secrets that could benefit humans hiding in these Southern Hemisphere sharks, but we won't know until we do the research.
Dr Skowronska-Krawczyk said it was important to do basic biological studies without having an outcome in mind. Only through the curiosity of researchers was the long life-span of the Greenland shark discovered.
"Only then could we start working and thinking about [medical] applications," Dr Skowronska-Krawczyk said.
"It's very important to study basic science and to fund basic science."
- ABC