Deep-sea fish reveal an alternative developmental trajectory for vertebrate vision.

Vertebrate vision relies on two photoreceptor types: cones for bright light and rods for dim light. The current dogma is that vertebrates develop cone-dominated retinas first, adding rods later. Here we show that larval deep-sea fishes have "hybrid" photoreceptors, expressing cone-specific genes in rod-like cells. Through development, they either retain these rod-like cones (in Maurolicus mucronatus) or transition to true rods expressing rod-specific genes and transcription factors (in Vinciguerria mabahiss and Benthosema pterotum). Unlike most marine fish larvae, which inhabit brightly lit waters, deep-sea fish larvae experience deeper and dimmer environments. By combining rod-like morphology with cone-like molecular machinery, hybrid photoreceptors likely maximize visual performance under such conditions. Consistently, spectral maxima predictions and environmental light estimations suggest tuning to the prevailing light environment throughout life. Our findings provide molecular, morphological, and functional evidence for the evolution of an alternative developmental trajectory for vertebrate vision.