MORPHOLOGICAL TYPES AND ARRANGEMENT OF CONE CELLS, AND THE VISUAL ACUITY OF SUTCHI CATFISH PANGASIANODON HYPOPHTHALMUS

Abstract

The density and spatial arrangement of photoreceptor cells in the retina reflect the visual environment of a fish. The density of photoreceptor cells also determines the visual acuity. In this study, the morphological types and arrangement of cone cells, and the visual acuity of sutchi catfish Pangasianodon hypophthalmus were determined to obtain fundamental understanding of its vision. The left eyes of the adult sutchi catfish were enucleated, fixed in Bouin’s solution for 24 hours and then preserved in 70% ethanol. The fixed retinae were cut into 17 regions. The nine major regions were the dorso-nasal (DN), dorsal (D), dorso-temporal (DT), nasal (N), bottom (B), temporal (T), ventro-nasal (VN), ventral (V), and ventro-temporal (VT). The 17 regions were then immersed separately in a series of ethanol (from 70% to 100%), cleared with histolene, embedded in paraffin, cut into 6 μm thick tangential sections, and stained with haematoxylin-eosin. The density of cone cells per 0.01 mm2 in each region was counted from the stained sections. Visual acuity was then calculated using cone cell densities and lens radii. Only one type of cone cells, which is the single cone cell, was identified and these single cone cells were closely spaced. The area around the bottom region showed tendency of higher density of single cone cells. These findings provide the fundamental understanding on the adaptation of retinal structure of sutchi catfish to its feeding behaviour.