Photoreceptors and visual pigments in a cichlid fish, Nannacara anomala

Correlation of visual pigment content and photoreceptor cell structure is studied in the cichlid fish Nannacara anomala the retina of which is characterized by simple organization of the outer plexiform layer. The visual cell types consist of long and bulky rods and only two types of cones: equal double cones and short single cones. Both types of cones differ form other known teleost cones in that their outer segments are very slender yet twice as long as the inner segments. The double cones form a square mosaic with four doubles along the sides an a single cone in the center. Four rods usually surround the single cone and a few others lie in the corners of the square. The population densities of photoreceptor cells in the central part of the retina is 45,600 cones and 52,600 rods per mm2. The cone to rod ratio is 1:1.15; the double cone to single cone ratio is 4.07:1. The outer segments of photoreceptors contain four spectroscopic types of pigment: one type in rods with lambda max = 498 +/- 5 nm, one type in single cones with lambda max = 460 +/- 5nm, and one in each member of the double cones with lambda max of 555 +/- 5 and 600 +/- 10 nm. Single cones are found to contain only the blue-absorbing pigment. The double cones, whenever both members are successfully recorded from, appear to use two spectrally different pigments. Based on spectral bandwidth determination, the four pigments are probably of the prophyropsin class, although some admixture (10--20%) of rhodopsin-class pigments is also indicated, especially in double cones.