Retinal ganglion cell topography and spatial resolution in the smelt Hypomesus japonicus (Brevoort, 1856)

The present study deals with the topography of retinal ganglion cells (GCs) and spatial resolution in the smelt Hypomesus japonicus. The eyes and retinae were examined by light microscopy and computerized tomography. DAPI labelling was used to visualize cell nuclei in the ganglion cell and inner plexiform layers. Two zones of increased GC density in the nasal and temporal retina were bridged by a horizontal streak with the GC density ranging from 5600 to 8000 cells/mm2. The maximum cell density (area retinae temporalis) ranged from 9492 to 14,112 cells/mm2, and the total number of GCs varied from 286 x 103 to 326 x 103 cells in three individuals. The theoretical anatomical spatial resolution (the anatomical estimate of the upper limit of visual acuity) was minimum in the ventral periphery (smaller fish, 1.43 cpd; larger fish, 1.37 cpd) and maximum in area retinae temporalis (smaller fish, 2.83 cpd; larger fish, 2.41 cpd). The relatively high density of GCs and presence of the horizontal streak and area retinae temporalis in the H. japonicus are consistent with its highly visual behaviour. The present findings contribute to better understanding of the factors affecting the topography of retinal ganglion cells and mechanisms of visual adaptation in fish.