Fish eye growth rate and vertical habitat: A maximum for eye growth rate investment in the mesopelagic zone.

Vision is one of the most important means by which animals perceive their environment. In the pelagic ocean, there is an enormous gradient of available light from the well-lit surface to the deep bathypelagic zone. Fish inhabiting different depths of the pelagic ocean must adapt to these conditions. One of the ways to optimize sensitivity (i.e., the ability to detect light) in dim environments is to increase eye size, increasing the probability of a photon hitting the retina. Here, we investigate how fish eye investment (i.e., the relative eye vs. body growth rate) varies based on the maximum residence depth of fishes. We measured eye diameter, eye area and lengths from 5639 unique individuals, belonging to 551 species and 37 orders, and extracted ecological traits for these species. We found that there was a wide variety of fish eye investment strategies across depths. These strategies, however, were bounded by a depth-dependent maximum eye investment that peaked at around ~500 m before decreasing as fish resided in deeper depths. These results suggest that, although a multitude of strategies are viable at each depth, there is a depth-dependent limit to how advantageous eye investment is. The depth at which a high eye investment is the most beneficial is located in the middle of the mesopelagic zone in very dim conditions - but where sunlight still penetrates and where bioluminescence occurs. All individual fish measurements and all species traits are available as supplementary material, representing a valuable resource for future pelagic fish trait-based studies.