SCANNING ELECTRON-MICROSCOPY AND MICROSPECTROPHOTOMETRY OF THE PHOTORECEPTORS OF ICTALURID CATFISHES

The retinal photoreceptors from larval channel catfish (Ictalurus punc tatus) were studied using single cell, in situ microspectrophotometry. Rods appear at 5 days after hatch; cones are present from day one. Th e rods contain a visual pigment which absorbs light maximally at 540 n m. The cones contain either a green sensitive visual pigment with peak absorbance at 535 nm or a red sensitive visual pigment with peak abso rbance at 608 nm. All pigments are based on vitamin A2. Visual pigment complement does not change with age, as photoreceptors from adult I. punctatus, I. catus and 1. melas contain visual pigments virtually ide ntical to those of the larval I. punctatus. Regardless of age, no visu al pigment with peak absorbance in the short wavelength region of the spectrum was ever observed. Scanning electron microscopy of adult I. p unctatus retinas showed large rods with long, cylindrical outer segmen ts and smaller cones with short, tapered outer segments. The myoids of both rods and cones are extensable. The rods, embedded in a granular tapetal material, comprise from 50 to 60% of the photoreceptors. Only single cones are present. The data are consistent with the idea that t he ictalurid catfishes spend their entire lives in an environment defi cient in blue light.