The Greenland shark, a creature of the deep, often appears to be on its last legs, moving with a slow crawl, its eyes cloudy and skin mottled. These ancient mariners, spending much of their lives in the crushing darkness of waters up to 3,000 metres deep, were long believed to possess rudimentary vision, if any at all. Their otherworldly appearance is often further amplified by the tiny crustacean parasites that latch onto their corneas, giving the impression of perpetual blindness.
However, new research is shedding light on the remarkable resilience of these deep-sea dwellers. The Greenland shark ( Somniosus microcephalus ) boasts an extraordinary lifespan, potentially reaching up to 400 years, making it one of the longest-living vertebrates on our planet. Contrary to previous assumptions, its seemingly decrepit eyes are, in fact, fully functional and show minimal deterioration even after a century of use. This discovery opens up exciting possibilities for understanding and potentially improving human eye health.
The Anatomy of Ancient Vision
To delve into the secrets of the Greenland shark’s remarkable ocular longevity, researchers examined the eyes of ten deceased Greenland sharks, with estimated ages ranging from 100 to 134 years. These eyes, measuring approximately 5–6 centimetres in diameter, were enveloped by robust protective tissue.
Unlike human eyes, which possess two types of light-sensing cells – cone cells for bright light and colour perception, and rod cells for low-light conditions – Greenland shark eyes are equipped solely with rod cells. This suggests a visual system finely tuned for the dim, perpetual twilight of their deep-sea habitat.
- Rod Cells: These are the primary photoreceptor cells in Greenland shark eyes, enabling them to detect light and darkness.
- Absence of Cone Cells: This means Greenland sharks likely perceive their world in shades of grey, with limited ability to discern fine details, shapes, or fast movements.
As Dr. Lily Fogg, an Australian marine biologist and lead author of the study from the University of Basel, explains, “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.” This adaptation allows them to navigate and survive in their challenging environment.
Functionality in the Dark Depths
The research indicates that while Greenland sharks may not possess high-resolution vision, their eyes are far from useless. Dr. Dorota Skowronska-Krawczyk, a senior author from the University of California, Irvine, noted that these sharks “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.”
Despite their immense size, reaching up to 7 metres in length, Greenland sharks inhabit a vast range of depths. While they may not rely on vision for hunting or complex navigation in the deepest trenches, their visual system likely plays a role in other critical functions.
- Depth Regulation: Vision might assist in finding and maintaining optimal depths within their preferred temperature and pressure zones.
- Environmental Awareness: Even a basic ability to detect light and darkness can provide crucial information about their surroundings.
Dr. Fogg emphasises the efficiency of evolution: “Evolution is very efficient. If you don’t need something, you usually get rid of it.” This suggests that their visual system, even if simplified, serves a purpose.
The Impact of Parasites on Vision
A peculiar aspect of Greenland shark vision is the presence of Ommatokoita elongata, a species of copepod parasite that attaches to the shark’s cornea. These parasites, growing up to 3cm and resembling a white tassel, have long been thought to severely impair vision.
Laura Ryan, a neurobiologist at Macquarie University not involved in the study, commented, “The copepod parasites attached to the cornea probably reduce image clarity.” However, the study’s findings suggest that despite this parasitic burden, the sharks’ visual pathways remain functional.
- Retina and Visual Pathways: These core components of the visual system were found to be intact and operational.
- Light and Contrast Detection: The sharks can still detect light, contrast, and movement, crucial for survival in their environment.
“Vision may be partially compromised, but the sharks are not blind — they can still detect light, contrast, and movement in the deep sea,” Ryan added. The remarkable lack of degeneration in the studied eyes, even with these parasites present, is what truly captivated the researchers. “The structure of the eye looks beautiful. I mean, it’s basically pristine,” stated Dr. Skowronska-Krawczyk.
Unlocking the Secrets of Anti-Ageing Eyes
The enduring health of the Greenland shark’s retina, the light-sensitive tissue at the back of the eye, has led researchers to explore potential molecular mechanisms behind its longevity. Two DNA-repairing genes, ERCC1 and ERCC4, have emerged as key suspects.
The high expression of these genes in the sharks’ retinas suggests a potent molecular defence system that actively repairs cellular damage, thereby preserving retinal health over extended periods. “The high expression of DNA repair genes suggests a powerful molecular mechanism that helps maintain retinal health over centuries — a cool finding,” remarked Dr. Ryan.
Patricia Jusuf, a visual neuroscientist at the University of Melbourne, highlighted the implications for human health. “The same genes are also functioning in DNA repair pathways in humans,” she noted. “When these genes do not work properly in mammals, we see detrimental effects associated with premature ageing.”
The observation of minimal retinal degeneration in sharks over 100 years old points towards these DNA repair pathways as a potential key to maintaining vision throughout a long lifespan.
Broader Implications for Human Vision
Age-related macular degeneration (AMD) and retinitis pigmentosa are common conditions that affect millions worldwide, often impacting the rod photoreceptors first. Dr. Jusuf believes that understanding and potentially manipulating the DNA repair mechanisms seen in Greenland sharks could offer significant therapeutic benefits.
“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,” she explained. “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 these genetic mechanisms could pave the way for novel genetic or molecular therapies to combat vision loss in humans.
Australian Connection: Uncovering More Genetic Secrets?
Greenland sharks belong to the Somniosidae family, commonly known as “sleeper sharks.” While much is understood about their Arctic cousins, the southern sleeper shark (Somniosus antarcticus) and Pacific sleeper shark (Somniosus pacificus), found in waters around Australia and New Zealand, remain less studied.
These Southern Hemisphere sleeper sharks may harbour their own unique genetic secrets that could benefit human health. Dr. Skowronska-Krawczyk stressed the importance of fundamental biological research, even without an immediate application in mind. “It was only through the curiosity of researchers that the long life-span of the Greenland shark was discovered,” she stated. “And only then could we start working and thinking about [medical] applications. So it’s very important to study basic science and to fund basic science.”
The ongoing investigation into the Greenland shark’s extraordinary longevity and vision serves as a powerful reminder of the untapped biological wonders that exist within our oceans and the potential for groundbreaking discoveries when we commit to exploring them.
