Retinal ganglion cell topography in elasmobranchs

Retinal wholemounts are used to examine the topographic distribution of ret inal cells within the ganglion cell layer in a range of elasmobranchs from different depths, The retina is examined for regional specializations for a cute vision in six species of selachians, Galeocerdo cuvieri, Hemiscyllium ocellatum, Scyliorhinus canicula, Galeus melastomus, Etmopterus spinax, Isi stius brasiliensis, one species of batoid, Raja bigelowi and one species of chimaera, Hydrolagus mirabilis. These species represent a range of lifesty les including pelagic, mesopelagic and benthic habitats, living from shallo w water to the sea bottom at a depth of more than 3000 m. The topography of cells within the ganglion cell layer is non-uniform and changes markedly a cross the retina. Most species possess an increased density of cells across the horizontal (dorsal) meridian or visual streak, with a density range of 500 to 2,500 cells per mm(2) with one or more regional increases in densit y lying within this specialized horizontal area. It is proposed that the hi gher spatial resolving power provided by the horizontal streak in these spe cies mediates panoramic vision in the lower frontal visual field, Only I. b rasiliensis possesses a concentric arrangement of retinal iso-density conto urs in temporal retina or an area centralis, thereby increasing spatial res olving power in a more specialized part of the visual field, an adaptation for its unusual feeding behavior. In Nissl-stained material, amacrine and g anglion cell populations could be distinguished on the criteria of soma siz e, soma shape and nuclear staining. Quantitative analyses show that the pro portion of amacrine cells lying within the ganglion cell layer is nonunifor m and ranges between 0.4 and 12.3% in specialized retinal areas and between 8.2 and 48.1% in the peripheral non-specialized regions. Analyses of soma area of the total population of cells in the ganglion cell layer also show that the pelagic species possess significantly smaller soma (9-186 mu m(2)) than benthic and/or deep-sea species (16-338 mu m(2)), and that a number o f different morphological classes of cells are present including a small po pulation of giant ganglion cells. Copyright (C) 2000 S. Karger AG. Basel.