Abstract: Snailfishes (Liparidae) span a continuous vertical gradient from sun-lit surface waters across the continental slope to the hadal trenches, experiencing a multi-scale transition from broad-spectrum, high-intensity light to extreme photon limitation. This natural gradient provides an unparalleled system for dissecting how vertebrate visual systems adapt to changing light regimes. We combined histological and ultrastructural examinations with comparative eye transcriptomics in four species occupying distinct depth zones: the shallow-water Liparis tanakae, the deep-sea Careproctus colletti and C. furcellus, and the hadal Pseudoliparis swirei. Whereas the shallow-water retina retains a sizeable cone population, deep-sea snailfish retinas are almost entirely rod-dominated, featuring stacked rod outer-segment tiers and only trace cones. This anatomical shift indicates the loss of bright-light and color vision, with object detection relying instead on highly sensitive scotopic pathways—an adaptation to the dim deep sea. Transcriptomic profiling revealed pronounced depth-related expression signatures: shallow-water species up-regulate phototransduction and circadian-rhythm genes, whereas deep-sea and hadal species prioritize energy metabolism, protein processing, autophagy and stress-response pathways. The cone-specific short-wavelength opsin sws2 is restricted to the shallow-water species, whereas the rod-specific rh1 is significantly elevated in deep-sea and hadal taxa. Together, these data demonstrate that snailfishes have functionally traded color vision for high-sensitivity scotopic vision during their vertical expansion from surface to abyss, offering a clear example of how teleosts adapt to extreme light environments.

Key words: Liparidae, deep-sea adaptation, visual system, phototransduction pathway, visual adaptation