Abstract The retinas of the vast majority of vertebrate species are termed “duplex” – that is, they contain both rod and cone photoreceptor neurons in different ratios. The retina of Little skate ( Leucoraja erinacea ) is a rarity among vertebrates because it contains only rod photoreceptors and is thus “simplex”. This unique retina provides us with an important comparative model and an exciting opportunity to study vertebrate rod circuitry within the context of a functional, evolutionarily optimized system, all without the concern about artifacts from genetically modified rod-only mouse models. Perhaps even more importantly, the Leucoraja retina is able to function under both scotopic and photopic ranges of illumination with a single complement of photoreceptors. It is currently unknown what structural characteristics mediate this remarkable functional plasticity. To address this question, we performed serial block-face electron microscopy imaging and examined the structure of rods and their post-synaptic partners. We find that skate rods exhibit ultrastructural characteristics that are either common to rods or cones in other vertebrates (e.g., outer segment architecture, synaptic ribbon number, terminal extensions), or are uniquely in-between those of a typical vertebrate rod or cone (e.g., number of invaginating contacts, clustering of multiple ribbons over a single synaptic invagination). We therefore hypothesize that the unique hybrid rod-cone structure of skate rods and their post-synaptic partners is correlated with the ability of the skate visual system to function across scotopic and photopic ranges of illumination. These findings have the potential to reveal as yet undescribed principles of vertebrate retinal design. Significance statement The vast majority of vertebrate retinas are duplex and have mixed rod-cone populations of photoreceptors in varying ratios. The processing of visual information in a duplex retina tends to be separated between rod and cone systems, which mediate function under scotopic and photopic lighting conditions, respectively. However, the cartilaginous fish Little skate ( Leucoraja erinacea ) has a simplex retina, comprised solely of rod photoreceptors. Skate rods are also unusual because they have the ability to retain function over a full range of lighting conditions. We have little knowledge about the ultrastructural anatomy of the skate retina, and we hypothesize that this functional plasticity can be traced back to morphological adaptations at the level of individual photoreceptors and the downstream retinal circuitry, thus illuminating new pathways for the processing of visual information among vertebrates.
