Species-specific development of retinal architecture in elopomorph fishes: adaptations for harvesting light in the dark

Elopomorph fishes are distinguished in part by a shared leptocephalus larval form. These glass-clear, ribbon-like larvae are remarkably similar across the elopomorpha, yet they mature to radically different forms as adults. Along the timecourse of development, they transition through a variety of spectrally distinct habitats, occupying very different temporal niches as adults—some diurnal predators in high-light environments, others crepuscular, and others nocturnal. We found that in concert with these changes, the retinas of a variety of elopomorph fish change in similarly dramatic ways over the course of development. While the mature bonefish [Albula vulpes (Linnaeus, 1758)] retina is specialized for visual tasks in a high-light environment, the Atlantic tarpon (Megalops atlanticus Valenciennes, 1847) and ladyfish (Elops saurus Linnaeus, 1766) exhibit multiple specializations for function at night or otherwise in very dim light conditions. These include stacked rod photoreceptors that are gathered into massive bundles, retinomotor movement of photoreceptor outer segments, and a highly reflective tapetum. In the adult form, the speckled worm eel (Myrophis punctatus Lütken, 1852) maintains a retina with nearly all rods. The dramatic divergence among taxa over the course of development produces species with distinctly specialized visual capabilities. Moreover, the ability to change over the course of development may underlie the capacity for resilience in the face of anthropogenic insults, including light pollution in increasingly developed coastal zones. On the other hand, if light history can drive retinal change, exposure to artificial light at night may be detrimental to the survival of individuals that move between light-polluted and naturally dark locations.