Deep-Sea Fish Reveal New Insights into Vertebrate Vision

Recent research has uncovered a novel type of visual cell in deep-sea fish, challenging the long-held belief that vertebrate vision is solely based on two distinct cell types: rods and cones. This study, led by Lily Fogg, a postdoctoral researcher at the University of Helsinki, focused on larvae from three species of deep-sea fish found in the Red Sea: the hatchetfish (Maurolicus mucronatus), the lightfish (Vinciguerria mabahiss), and the lanternfish (Benthosema pterotum). The research, published in the journal *Science Advances*, indicates that these fish possess hybrid photoreceptors that combine features of both rods and cones, specifically adapted for low-light environments.

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The study revealed that the larvae of these fish primarily utilize these hybrid cells, which resemble rods in shape—long and cylindrical—yet utilize the molecular machinery typical of cones. This adaptation allows them to maximize light capture in the dimly lit depths of the ocean, where sunlight penetration is minimal. Notably, the hatchetfish retains these hybrid cells throughout its life, while the other two species transition to the conventional rod-cone structure as adults.

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The findings suggest that the vertebrate visual system is more flexible and evolutionarily adaptable than previously understood. Fogg stated, "Our results challenge the longstanding idea that rods and cones are two fixed, clearly separated cell types." Senior author Fabio Cortesi, a marine biologist at the University of Queensland, emphasized the significance of this discovery, noting that it indicates a potential prevalence of similar hybrid cells across various vertebrate species, including those on land.

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These deep-sea fish are not only fascinating from a biological perspective but also play a crucial role in marine ecosystems. They are abundant and serve as a primary food source for larger predatory fish, marine mammals, and birds. The species studied exhibit bioluminescence, producing blue-green light that aids in camouflage through a strategy known as counterillumination. This adaptation allows them to blend with the faint sunlight filtering from above, thereby avoiding predators.

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Cortesi highlighted the importance of the deep sea as a largely unexplored frontier, stating, "The deep sea remains a frontier for human exploration, a mystery box with the potential for significant discoveries." He urged for careful stewardship of this habitat to ensure that future generations can continue to explore and appreciate its wonders.

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• “Our results challenge the longstanding idea that rods and cones are two fixed, clearly separated cell types.” — Lily Fogg, Postdoctoral Researcher, University of Helsinki

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• “It is a very cool finding that shows that biology does not fit neatly into boxes,” — Fabio Cortesi, Senior Author, University of Queensland

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• “Small fish like these fuel the open ocean. They are plentiful and serve as food for many larger predatory fishes, including tuna and marlin, marine mammals such as dolphins and whales, and marine birds,” — Fabio Cortesi, Senior Author, University of Queensland

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• “We should look after this habitat with the utmost care to make sure future generations can continue to marvel at its wonders.” — Fabio Cortesi, Senior Author, University of Queensland

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