Ordered Chaos: Photoreceptor Mosaics in the Larval and Adult Turquoise Killifish (Nothobranchius furzeri).

An organism's vision is dependent on the photoreceptor types in the eye and how these cells are organized across the retina. The turquoise killifish (Nothobranchius furzeri) is an annual killifish species that lives in the turbid waters of ephemeral pools. Although turquoise killifish are gaining popularity as a model, we know little about their photoreceptors. Here, the turquoise killifish retina was investigated for photoreceptor subtype identity and mosaic organization at the adult and larval stages to assess specialization for their light environment. Turquoise killifish were determined to have cone-dominant retinas, with expression of UV, blue, green, and red cone opsins, as well as rhodopsin. Adult fish were found to have an ordered square mosaic, wherein double cones formed squares with single cones in the center and corners of the squares. Double cones could be comprised of a red and green or two red cone units, while the single cones could be UV, blue, red, or - very rarely - green in identity. Although ordered, the mosaic did not have regularity of cone subtype orientation across the retina. Larvae were found to have retinal regionalization, with a dorsonasal region of increased short-wavelength cone density. In both larvae and adult retinas, red cones were the predominant cone subtype. This study presents the first description of the photoreceptor types and mosaic in an annual killifish, and provides a foundation for future work investigating killifish photoreceptor biology and visual function.