The photoreceptors of the “four‐eyed” fish, Anableps anableps L

Abstract

The photoreceptors of the adult Anableps anableps have been investigated by light microscopy, scanning and transmission electron microscopy. The fish is a surface swimmer and the eye is divided by the water meniscus. In general, the photoreceptors (rods, single cones, and double equal cones) resemble those of other vertebrates, but there are several unusual features: (1) The outer segment discs of the double cones differ in the two members. (2) All cones have a prominent accessory outer segment derived from the single connecting cilium, and there is no second centriole. (3) The exterior of the inner segments is ridged and grooved longitudinally, most markedly so in the cones. (4) A membranebounded oil droplet is present in the distal cone inner segment, formed from mitochondria which enlarge, fuse and transform in a vitreal‐scleral gradient. (5) There are knob‐like invaginations of rod cytoplasm into the cones immediately scleral to the external limiting membrane. (6) Subsurface cisterns underlie apposed plasma membranes of double cone inner segments and direct rod‐cone inner segment contacts. (7) Fine “fins” on the cones interdigitate, with Müller cell cytoplasm between, just scleral to the external limiting membrane. (8) In the rod spherule there is a greater density of vesicles and the cytoplasm is darker than in the cone pedicle. The well‐defined cone mosaic has a linear pattern peripherally and a square pattern centrally. The photoreceptors undergo photomechanical movements. Photoreceptor ultrastructure is alike in both dorsal and ventral parts of the retina, but the ventral retina contains more cells and is thicker than the dorsal retina. The adjustments necessary for simultaneous air and water vision are found mainly in lens shape, corneal thickness and curvature, and the greater number of cells in the ventral retina.

Pronounced heritable variation and limited phenotypic plasticity in visual pigments and opsin expression of threespine stickleback photoreceptors

“…for the dystrophic lake populations were 14.4±0.95nm and 9.3±1.8nm (N=63 cones, 6 fish), and 14.7±0.89nm and 9.2±1.6nm (N=75 cones, 8 fish) for the transplanted populations. These numbers, which are in the range reported for other fishes (Borwein and Hollenberg, 1973;Collin et al, 1996), indicate no changes in outer segment morphology as a result of transplantation.…”

The Ocular Morphology of the Southern Hemisphere Lamprey <i>Mordacia mordax</i> Richardson with Special Reference to a Single Class of Photoreceptor and a Retinal Tapetum

“…Membrane-bound and filled with a dense amorphous matrix that is insoluble in organic solvents, these globules appear to be mature ellipsosomes as described in the cones of a number of teleost fishes [Borwein and Hollenberg, 1973;MacNichol et al, 1978;Nag and Bhattarcharjee, 1995]. An increase in the density of mitochondrial cristae surrounding the ellipsosome, the similar osmiophilic staining of both structures and the putatively colorless nature of the ellipsosome suggest that it is of mitochondrial origin.…”

“…Of mitochondrial origin, similar structures have been described in the cone photoreceptors in a number of vertebrate classes, including teleosts [Walls, 1942;Ishikawa and Yamada, 1969;Borwein and Hollenberg, 1973;Kunz and Regan, 1973;Kunz and Wise, 1973;Anctil and Ali, 1976;Nag andBhattacharjee, 1989, 1995;Nag, 1995] and mammals [Knabe et al, 1997] including primates [Bowmaker, 1991]. Although the photoreceptors in M. mordax have not been definitively characterized as rod-like, such confirmation in the future would mean that M. mordax provides the first report of an ellipsosome within a rod.…”

“…Moreover, the conical shape of the synaptic terminals and the presence of a large ellipsosome in the sclerad region of the inner segment also suggests a cone-like receptor. Described by Franz [1932] as false oil droplets, these large ellipsosomes are of mitochondrial origin and have previously been described only in the cones of teleosts [Borwein and Hollenberg, 1973;Anctil and Ali, 1976;MacNicol et al, 1978;Nag andBhattacharjee, 1989, 1995;Nag, 1995]. Despite the conflicting morphological evidence, we support the contention of both Franz [1932] and Walls [1942] that the photoreceptors in M. mordax are rod-like because the characteristics of this type of photoreceptor would be of value to a nocturnal habitat.…”

Cone photoreceptor regeneration in adult fish retina: phenotypic determination and mosaic pattern formation

“…viously in the paired cones of other teleost species (Berger, 1967;Borwein and Hollenberg, 1973;Cantino et al, 1986) were never observed in rods or SCs. The fundamental rhombic pattern in stretched retina was evident in the electron micrographs (dotted lines, Fig.…”