Photoreceptor Pigments of the Blueback Herring (Alosa aestevalis, Clupeidae) and the Atlantic Silverside (Menidia menidia, Atherinidae)

Variation in opsin transcript expression explains intraretinal differences in spectral sensitivity of the northern anchovy

Vertebrate retinal photoreceptors house visual pigments that absorb light to begin the process of vision. The light absorbed by a visual pigment depends on its two molecular components: protein (opsin) and chromophore (a vitamin A derivative). Although an increasing number of studies show intraretinal variability in visual pigment content, it is only for two mammals (human and mouse) and two birds (chicken and pigeon) that such variability has been demonstrated to underlie differences in spectral sensitivity of the animal. Here, we show that the spectral sensitivity of the northern anchovy varies with retinal quadrant and that this variability can be explained by differences in the expression of opsin transcripts. Retinal (vitamin A1) was the only chromophore detected in the retina, ruling out this molecular component as a source of variation in spectral sensitivity. Chromatic adaptation experiments further showed that the dorsal retina had the capacity to mediate color vision. Together with published results for the ventral retina, this study is the first to demonstrate that intraretinal opsin variability in a fish drives corresponding variation in the animal’s spectral sensitivity.

Variability in mitochondria of zebrafish photoreceptor ellipsoids

Ultrastructural examination of photoreceptor inner segment ellipsoids in larval (4, 8, and 15 days postfertilization; dpf) and adult zebrafish identified morphologically different types of mitochondria. All photoreceptors had mitochondria of different sizes (large and small). At 4 dpf, rods had small, moderately stained electron-dense mitochondria (E-DM), and two cone types could be distinguished: (1) those with electron-lucent mitochondria (E-LM) and (2) those with mitochondria of moderate electron density. These distinctions were also apparent at later ages (8 and 15 dpf). Rods from adult fish had fewer mitochondria than their corresponding cones. The ellipsoids of some fully differentiated single and double cones contained large E-DM with few cristae; these were surrounded by small E-LM with typical internal morphology. The mitochondria within the ellipsoids of other single cones showed similar electron density. Microspectrophotometry of cone ellipsoids from adult fish indicated that the large E-DM had a small absorbance peak (∼0.03 OD units) and did not contain cytochrome-c, but crocetin, a carotenoid found in old world monkeys. Crocetin functions to prevent oxidative damage to photoreceptors, suggesting that the ellipsoid mitochondria in adult zebrafish cones protect against apoptosis and function metabolically, rather than as a light filter.

Regional variations in the outer retina of atherinomorpha (Beloniformes, Atheriniformes, Cyprinodontiformes: Teleostei): photoreceptors, cone patterns, and cone densities

The outer retinae of adults of 13 atherinomorph species, representing nine different families, were examined by both light and electron microscopy. The retinae were investigated with respect to photoreceptor types, cone densities, and cone patterns. All data were composed to eye maps. This procedure allows an interspecific comparison of the regional differences within the outer retina among these shallow-water fish. Furthermore, for a more detailed pattern analysis nitro-blue tetrazolium chloride- (NBT)-stainings in the retina of Melanotaenia maccullochi are presented. Apart from rods, eight morphologically different cone types could be identified: short, intermediate, and long single cones, double cones (equal and unequal), triple cones (triangular and linear), and in Ameca splendens one quadruple cone. Dimensions and occurrence of photoreceptors vary among the respective species and within the retinal regions. In the light-adapted state, the cones are arranged in highly ordered mosaics. Five different cone tessellation types were found: row patterns, twisted row patterns, square patterns, pentagonal patterns, and, exclusively in Belone belone, a hexagonal pattern. In Melanotaenia maccullochi the different spectral photoreceptor classes correspond well with the distribution of morphological photoreceptor classes within the mosaic. Double cone density maxima together with a highly ordered cone arrangement usually occur in the nasal and/or ventral to ventrotemporal retina. In most of the species that were examined these high-density regions are presumed to process visual stimuli from the assumed main directions of vision, which mainly depend on feeding behavior and predator pressure. Our findings are discussed with respect to the variable behavioral and visual ecology and phylogeny of the respective species.