Hybrid photoreceptor cells in deep-sea fish are reshaping scientific understanding of how vertebrate vision works, according to new research published in Science Advances.
For decades, biology textbooks have described vision as relying on two distinct retinal cell types: rods for dim light and cones for bright light and colour. However, scientists studying deep-sea fish larvae in the Red Sea have identified cells that blend features of both.
The research focused on three species: the hatchetfish (Maurolicus mucronatus), the lightfish (Vinciguerria mabahiss), and the lanternfish (Benthosema pterotum).
In their larval stages, these fish displayed hybrid photoreceptors that structurally resemble rods but use the molecular machinery typically associated with cones. The hatchetfish retained these hybrid cells into adulthood, while the other two species transitioned to the traditional rod-cone system later in life.
“These cells look like rods, long and cylindrical, but they activate genes usually found in cones,” said Lily Fogg, lead author and marine biologist at the University of Helsinki.
The vertebrate retina contains photoreceptors that convert light into signals sent to the brain. Traditionally, rods and cones were considered clearly separate cell types.
The new findings suggest that photoreceptors may be more flexible than previously thought. In dim environments, such as depths between 65 and 650 feet, where the larvae were studied, both rods and cones typically function suboptimally. The hybrid cells appear to provide an adaptive advantage.
“Our results challenge the longstanding idea that rods and cones are two fixed, clearly separated cell types,” Fogg said.
Senior author Fabio Cortesi of the University of Queensland noted that the discovery highlights the evolutionary adaptability of vertebrate visual systems. He suggested that similar hybrid cells could exist in other vertebrates.
The studied fish inhabit low-light ocean layers where sunlight barely penetrates. All three species use bioluminescence for camouflage, producing blue-green light that blends with faint surface illumination a strategy known as counterillumination.
These small fish also undertake daily vertical migrations, rising toward the surface at night to feed and descending during daylight hours to avoid predators. Researchers emphasise that the deep sea remains one of Earth’s least explored environments. Discoveries like this underscore the complexity and adaptability of life in extreme habitats.
