New study challenges theory about deep-sea fish mystery

The researchers examined the retinas of fish larvae caught at deep depths NewsScienceNew study challenges theory about deep-sea fish mysteryThe researchers examined the retinas of fish larvae caught at deep depths Will Dunham Wednesday 18 February 2026 11:57 GMTBookmarkCommentsGo to commentsBookmark popoverRemoved from bookmarksClose popoverMariana snailfish swim at 8km deep in the oceanYour support helps us to tell the storyRead moreSupport NowFrom reproductive rights to climate change to Big Tech, The Independent is on the ground when the story is developing. Whether it's investigating the financials of Elon Musk's pro-Trump PAC or producing our latest documentary, 'The A Word', which shines a light on the American women fighting for reproductive rights, we know how important it is to parse out the facts from the messaging.At such a critical moment in US history, we need reporters on the ground. Your donation allows us to keep sending journalists to speak to both sides of the story.The Independent is trusted by Americans across the entire political spectrum. And unlike many other quality news outlets, we choose not to lock Americans out of our reporting and analysis with paywalls. We believe quality journalism should be available to everyone, paid for by those who can afford it.Your support makes all the difference.Read moreFor over a century, biology textbooks have asserted that vertebrate vision, including our own, is constructed from two distinct cell types: rods for processing dim light and cones for bright light and colour. However, groundbreaking new research involving deep-sea fish is now challenging this long-held understanding.Scientists have identified a novel type of visual cell in these deep-sea creatures that remarkably blends the physical form of rods with the molecular machinery and genes typically associated with cones. This hybrid cell, uniquely adapted for sight in the perpetually gloomy conditions of the deep ocean, was discovered in the larval stages of three distinct deep-sea fish species inhabiting the Red Sea.The species studied included a hatchetfish (Maurolicus mucronatus), a lightfish (Vinciguerria mabahiss), and a lanternfish (Benthosema pterotum). While the hatchetfish retained these hybrid cells throughout its life, the other two species transitioned to the conventional rod-cone dichotomy as adults.These small fish, with adults typically measuring between 3-7 cm, and their larvae even smaller, thrive in a marine realm where sunlight struggles to penetrate the watery depths. open image in galleryScientists have identified a new type of visual cell in deep-sea fish that blends the shape and form of rods with the molecular machinery and genes of cones. This hybrid type of cell, adapted for sight in gloomy light conditions, was found in larvae of three deep-sea fish species in the Red Sea (Dr. Wen-Sung Chung)The vertebrate retina, the sensory membrane at the back of the eye responsible for detecting light and converting it into signals for the brain, traditionally possesses two main types of light-sensitive visual cells, known as photoreceptors, named for their characteristic rod and cone shapes."The rods and cones slowly change position inside the retina when moving between dim and bright conditions, which is why our eyes take time to adjust when we flick on the light switch on our way to the restroom at night," said Lily Fogg, a postdoctoral researcher in marine biology at the University of Helsinki in Finland and lead author of the research published in the journal Science Advances."We found that, as larvae, these deep-sea fish mostly use a mix-and-match type of hybrid photoreceptor. These cells look like rods - long, cylindrical and optimized to catch as many light particles - photons - as possible. But they use the molecular machinery of cones, switching on genes usually found only in cones," Fogg said.The researchers examined the retinas of fish larvae caught at depths from 65 to 650 feet (20 to 200 meters). In the type of dim environment they inhabit, rod and cone cells both are usually engaged in the vertebrate retina, but neither works very well. These fish display an evolutionary remedy."Our results challenge the longstanding idea that rods and cones are two fixed, clearly separated cell types. Instead, we show that photoreceptors can blend structural and molecular features in unexpected ways. This suggests that vertebrate visual systems are more flexible and evolutionarily adaptable than previously thought," Fogg said.open image in galleryTwo deep-sea fish species, Maurolicus muelleri and Maurolicus mucronatus from the Red Sea, that have hybrid photoreceptors in their eyes as larvae and adults (via REUTERS)"It is a very cool finding that shows that biology does not fit neatly into boxes," said study senior author Fabio Cortesi, a marine biologist and neuroscientist at the University of Queensland in Australia. "I wouldn't be surprised if we find these cells are much more common across all vertebrates, including terrestrial species."All three