Retinal ganglion cell topography and spatial resolution estimation in the Japanese tree frog Hyla japonica (Gunther, 1859)

Tree frogs are an interesting and diverse group of frogs. They display a number of unique adaptations to life in the arboreal environment. Vision plays a crucial role in their ecology. The topography of retinal ganglion cells (GCs) is closely related to a species' visual behavior. Despite a large amount of research addressing GC topography in vertebrates, there is scarce data on this subject in tree frogs. I studied the topography of GCs in the retina of the Japanese tree frog Hyla japonica. The GC density distribution was locally fairly homogeneous, with spatial density increasing gradually from the dorsal and ventral periphery towards the equator. A moderately pronounced visual streak was found close to the equator in the dorsal hemiretina, with a distinct area retinae temporalis in the dorsotemporal quadrant potentially subserving binocular vision. The minimum GC density (mean +/- SEM, n = 5) was 3060 +/- 60 and the maximum 12 800 +/- 170 cells/mm(2). The total number of GCs was 292 +/- 7 x 10(3). The theoretical anatomical spatial resolution estimated from GC densities and eye optics was lowest in the ventral periphery (ca. 0.9 and 1.3 cycles/degree in air and water, respectively) and highest in the area retinae temporalis (ca. 2.1 and 2.8 cycles/degree). The relatively high GC density and presence of specialized retinal regions in Hyla japonica are consistent with its highly visual behavior. The present findings contribute to our understanding of the relative role of common ancestry and environmental pressure in GC topography variation within Anura.