Meeting Abstract
Coral reef fishes, one of the most speciose assemblage of vertebrates, display an enormous diversity in terms of habitat preferences, diel activity patterns, and trophic specializations. Such large ecological diversity is coupled with disparate demands for vision, making coral reef fish a uniquely well-suited group to study evolutionary interactions between photic environments and the visual system. For example, fishes feeding on small zooplankton have frequently been hypothesized to have higher visual acuity than their relatives feeding on benthic prey, but existing evidence is inconclusive. We tested this hypothesis by following predictions of physiological optics, and collected data on lens diameter (a proxy of focal length) and the density of retinal ganglion cells. Both features can be combined into a physiological estimate of visual acuity. Our results for 19 labrid and 7 pomacentrid species, representing six independent transitions between plankton/benthic feeding, demonstrate that visual acuity does not differ between trophic specialists. However, most of the analyzed species, zooplanktivore or not, are theoretically able to detect zooplankton at a normal strike distance, as shown by optical modeling. In addition, the majority of the zooplanktivores (all plankton-feeding pomacentrids and 1 out of 3 plankton-feeding labrids) have a centrally located area centralis as opposed to the horizontal streak of high density ganglion cells commonly seen in benthic feeders. While our results suggest differences in retinal topography associated with trophic specialization, the evolution of visual acuity is likely controlled by additional factors other than diet.
