Ocular morphology, topography of ganglion cell distribution and visual resolution of the pilot whale (Globicephala melas)

Abstract

The ocular morphology, morphological characteristics and topography of ganglion cell distribution were studied in four eyes of Globicephala melas to estimate the retinal resolution. The ganglion cell layer was composed of a single row of ganglion cells with a primarily round shape and a cell size which varied from 10 to 75 µm (mean 33.5 µm) in diameter. The typical feature was that 65 % of ganglion cells had a diameter larger than 25 µm, with a similar average size in all regions of the retina. The total number of ganglion cells (183,000–218,000; mean 203,000) was distributed in several isodensity lines with two definite areas of high cell density: one area was located in the temporal retinal area, and the other one in the nasal retina, at 65° ± 5° from the optic disk. A surprising result was the presence of a third cell density peak in the dorsal region of one retina. The mean peak cell densities of three retinas were 268 and 267 cells/mm2 in the nasal and temporal areas, respectively, and 287 cells/mm2 in the third peak of the dorsal region. Finally, the underwater retinal resolution, calculated from posterior nodal distance and the peak cell density, was 10.9′ (2.8 cycles/degree) in both nasal and temporal retinal areas, whereas the aerial resolution was 14.5′ (2.1 cycles/degree). These data suggest that G. melas has a visual acuity similar to other cetaceans investigated so far.