Deep-sea fish in the Red Sea have been found to possess a unique visual system, challenging long-held biological assumptions about how vertebrates see, according to research published this week in the journal Science Advances.
For over a century, biology textbooks have described vertebrate vision as relying on two distinct types of photoreceptor cells: rods, for low-light vision, and cones, for bright light and color perception. However, scientists have discovered a new type of visual cell in the larvae of three deep-sea fish species – Maurolicus mucronatus, Vinciguerria mabahiss, and Benthosema pterotum – that combines characteristics of both.
The research, led by Lily Fogg, a postdoctoral researcher in marine biology at the University of Helsinki, Finland, identified cells that possess the elongated shape of rods, optimized for capturing even the faintest light, but also contain the molecular machinery and genes typically found only in cones. Maurolicus mucronatus retains these hybrid cells throughout its life, while the other two species transition to the standard rod and cone dichotomy as they mature.
These fish, measuring between 3 and 7 centimeters in length as adults, inhabit a “crepuscular” marine environment where sunlight barely penetrates. The discovery was made while examining the retinas of larvae collected from depths of 20 to 200 meters in the Red Sea. “We found that, in the larval stage, these deep-sea fish primarily utilize a hybrid type of photoreceptor that combines different characteristics,” Fogg explained. “These cells look like rods – long, cylindrical, and optimized to capture as many photons as possible. But they use the molecular machinery of cones, activating genes normally found only in cones.”
The researchers note that in low-light environments, both rods and cones are typically active in vertebrate retinas, but neither functions optimally. These fish appear to have evolved a solution to this challenge. Fabio Cortesi, a marine biologist and neuroscientist at the University of Queensland, Australia, and co-author of the study, described the finding as “a exceptionally interesting discovery that shows that biology doesn’t fit neatly into boxes.”
The three species studied also exhibit bioluminescence, producing blue-green light from organs on their undersides, a common camouflage technique known as counter-illumination to avoid predators in the deep sea. These small fish play a crucial role in the marine ecosystem, serving as a food source for larger predatory fish like tuna and marlin, as well as marine mammals and seabirds.
these fish undertake one of the largest daily migrations in the animal kingdom, ascending to near-surface waters at night to feed on plankton-rich waters before returning to the depths during the day to evade predators. Cortesi added, “The deep sea remains a frontier for human exploration, a box of mysteries with the potential for significant discoveries. We should care for this habitat with the utmost attention to ensure that future generations can continue to marvel at its wonders.”
