Visual system diversity in coral reef fishes.

Coral reefs are one of the most species rich and colourful habitats on earth and for many coral reef teleosts, vision is central to their survival and reproduction. The diversity of reef fish visual systems arises from variations in ocular and retinal anatomy, neural processing and, perhaps most easily revealed by, the peak spectral absorbance of visual pigments. This review examines the interplay between retinal morphology and light environment across a number of reef fish species, but mainly focusses on visual adaptations at the molecular level (i.e. visual pigment structure). Generally, visual pigments tend to match the overall light environment or micro-habitat, with fish inhabiting greener, inshore waters possessing longer wavelength-shifted visual pigments than open water blue-shifted species. In marine fishes, particularly those that live on the reef, most species have between two (likely dichromatic) to four (possible tetrachromatic) cone spectral sensitivities and a single rod for crepuscular vision; however, most are trichromatic with three spectral sensitivities. In addition to variation in spectral sensitivity number, spectral placement of the absorbance maximum (λmax) also has a surprising degree of variability. Variation in ocular and retinal anatomy is also observed at several levels in reef fishes but is best represented by differences in arrangement, density and distribution of neural cell types across the retina (i.e. retinal topography). Here, we focus on the seven reef fish families most comprehensively studied to date to examine and compare how behaviour, environment, activity period, ontogeny and phylogeny might interact to generate the exceptional diversity in visual system design that we observe.