Cellular mechanisms for color-coding in holostean retinas and the evolution of color vision

Why different regions of the retina have different spectral sensitivities: A review of mechanisms and functional significance of intraretinal variability in spectral sensitivity in vertebrates

Vision is used in nearly all aspects of animal behavior, from prey and predator detection to mate selection and parental care. However, the light environment typically is not uniform in every direction, and visual tasks may be specific to particular parts of an animal’s field of view. These spatial differences may explain the presence of several adaptations in the eyes of vertebrates that alter spectral sensitivity of the eye in different directions. Mechanisms that alter spectral sensitivity across the retina include (but are not limited to) variations in: corneal filters, oil droplets, macula lutea, tapeta, chromophore ratios, photoreceptor classes, and opsin expression. The resultant variations in spectral sensitivity across the retina are referred to as intraretinal variability in spectral sensitivity (IVSS). At first considered an obscure and rare phenomenon, it is becoming clear that IVSS is widespread among all vertebrates, and examples have been found from every major group. This review will describe the mechanisms mediating differences in spectral sensitivity, which are in general well understood, as well as explore the functional significance of intraretinal variability, which for the most part is unclear at best.

A spitting image: specializations in archerfish eyes for vision at the interface between air and water

Archerfish are famous for spitting jets of water to capture terrestrial insects, a task that not only requires oral dexterity, but also the ability to detect small camouflaged prey against a visually complex background of overhanging foliage. Because detection of olfactory, auditory and tactile cues is diminished at air-water interfaces, archerfish must depend almost entirely on visual cues to mediate their sensory interactions with the aerial world. During spitting, their eyes remain below the water's surface and must adapt to the optical demands of both aquatic and aerial fields of view. These challenges suggest that archerfish eyes may be specially adapted to life at the interface between air and water. Using microspectrophotometry to characterize the spectral absorbance of photoreceptors, we find that archerfish have differentially tuned their rods and cones across their retina, correlated with spectral differences in aquatic and aerial fields of view. Spatial resolving power also differs for aquatic and aerial fields of view with maximum visual resolution (6.9 cycles per degree) aligned with their preferred spitting angle. These measurements provide insight into the functional significance of intraretinal variability in archerfish and infer intraretinal variability may be expected among surface fishes or vertebrates where different fields of view vary markedly.

Retinal parallel pathways: seeing with our inner fish

The general organization of the vertebrate retina is highly conserved, in spite of structural variations that occur in different animal classes. The retinas of cyprinid fish, for example, differ in many aspects from those of primates. However, these differences are in the same order of magnitude as those found among mammalian species. Therefore, it is important to consider whether these changes are minor variations on the same theme or whether they lead to fundamentally different functions. In this light, we compare the retinal organization of teleost fish and mammals as regards parallel processing and discuss their many similarities.

Photoreceptors and visual pigments in the retina of the fully anadromous green sturgeon (Acipenser medirostrus) and the potamodromous pallid sturgeon (Scaphirhynchus albus)

Green sturgeon and pallid sturgeon photoreceptors were studied with scanning electron microscopy (SEM), microspectrophotometry and, in the case of the green sturgeon, retinal whole-mounts. The retinas of both species contain both rods and cones: cones comprise between 23% (whole-mount) and 36% (SEM) of the photoreceptors. The cone population of both species is dominated by large single cones, but a rare small single cone is also present. In both species, most rods have long outer segments of large diameter. A rod with a relatively thin outer segment is present in the pallid sturgeon retina. Mean cone packing density for the entire green sturgeon retina is 4,690+/-891 cones/mm2, with the dorsal retina 14% more dense than the ventral. There is evidence for a horizontal visual streak just above and including the optic disc. Mean rod packing density is 16,006+/-1,668 rods/mm2 for the entire retina, and fairly uniform throughout. Both species have rods with peak absorbance near 540 nm, as well as short-wavelength-sensitive cones (green: 464.5+/-0.7 nm; pallid: 439.7+/-3.5 nm); middle-wavelength-sensitive cones (green: 538.0+/-1.4 nm; pallid: 537.0+/-1.7 nm); and long-wavelength-sensitive cones (green: 613.9+/-3.0 nm; pallid: 617.8+/-7.6 nm).

Goldfish color vision sensitivity is high under light-adapted conditions

The wavelength discrimination threshold of three goldfish was examined in a series of behavioral experiments. Using an auto-shaping technique, detection thresholds were established for 531 and 648 nm spectral increments presented on a 6.6 cd m(-2) white background. Next, discrimination between the wavelengths was established at equal, suprathreshold, intensities. Finally, the intensities of the two stimuli were reduced to establish the intensity threshold for the wavelength discrimination. The results indicate that goldfish, like several mammalian species, can discriminate wavelength at detection threshold intensity. This finding suggests that high color sensitivity is not confined to mammals or dependent upon a very high percentage of wavelength opponent ganglion cells. Rather, high color vision sensitivity may be based upon an inherent sensitivity advantage of wavelength opponent receptive fields compared to non-wavelength opponent receptive fields and be an important selective advantage of wavelength opponency and color vision.

Taurine, amino acid transmitters, and related molecules in the retina of the Australian lungfish Neoceratodus forsteri: a light-microscopic immunocytochemical and electron-microscopic study

The morphology of the retina of the Australian lungfish Neoceratodus forsteri was investigated by means of light- and electron microscopy, whilst immunocytochemical studies were performed to determine the cellular distributions of the major amino acid neurotransmitters and other amino acids. The distributions of glycine and GABA were similar to those previously described for teleost, amphibian and mammalian retinae. Labelling was abundant in amacrine cells, whilst GABA was also present in one layer of horizontal cells and some bipolar cells. Taurine was present in both rods and cones, but, unlike the mammalian or avian retina, was absent from other cellular structures, including glial elements. Unexpectedly, the photoreceptor terminals lacked an apparent content of the excitatory amino acid transmitter glutamate. The glutamate that was present in the rods and cones occupied a crescentic arc corresponding to the location of glycogen-rich paraboloids. Asparagine was also present in rods, albeit in the modified mitochondria that formed the elipsoids of the rod inner segments. Arginine, the precursor for formation of nitric oxide, was present in glial cells, and in the paraboloids of both rods and cones.

Immunocytochemical reactivity of rod and cone visual pigments in the sturgeon retina

Microspectrophotometry and immunocytochemistry with several antivisual pigment antibodies were used to study visual cells of the Siberian sturgeon, Acipenser baeri Brandt. The retina contained rods and three morphological types of cones: large cones with oil drops, small cones with oil drops, and cone-like cells without oil drops. Rods and cone-like drop-free cells were found to possess porphyropsin-549, while the large oil drop-bearing cones contained red-sensitive (P613), green-sensitive (P542), and blue-sensitive (P462) visual pigments. The immunocytochemical staining pattern with three antibodies to visual pigment proteins also revealed one visual pigment in rods and three visual pigments in cones. Rods were labeled with all three antibodies, while the majority of large cones (type I), presumably the red-sensitive ones, were negative with the polyclonal serum AO against bovine opsin. A less-frequently occurring large cone type (type II) was stained by all three antibodies including mAb COS-1 specific to middle-to-long-wave visual pigments in birds and mammals, and is thought to be green-sensitive. An even less-frequent large cone type (type III, probably the blue-sensitive one) did not bind COS-1. The small cones with oil droplets showed immunoreactivities similar to either type II or type III cones. The oil drop-free small photoreceptor exhibited a staining pattern identical with that of rods. These results indicate that the immunocytochemical approach can be used to reveal photoreceptor-specific neural connections in the sturgeon retina.

Spectral characteristics of photoreceptors and horizontal cells in the retina of the Siberian sturgeon Acipenser baeri Brandt

In the retina of Siberian sturgeon, three spectral classes of photoreceptors were identified by microspectrophotometry. These were rods, oil drop-containing and oil drop-free cones possessing P549, and oil drop-containing cones possessing P613 and P465. With intracellular recordings, rod-driven, cone-driven, and mixed horizontal cells of luminosity type were found, as well as color-opponent horizontal cells of, at least, 6 kinds. Biphasic R/G cells received hyperpolarizing input either from rods or from green cones; in R/B cells, it was from blue cones. Other three types comprised biphasic G/B (or Y/B), RG/B, and triphasic G/BR cells. So the Chondrostean retina has the color-processing circuitry common for all ray-finned fishes.

Response properties of C-type horizontal cells in the retina of the bowfin

Intracellular recordings were obtained from biphasic- and triphasic-type horizontal cells (C cells) in the retina of the bowfin. For steady-state responses, both cell types displayed a linear stimulus-response function for responses up to at least 20% of maximum. In the linear range, responses to red/green mixtures were well predicted from the assumption that opposed inputs combine by simple summation. Action spectra were measured in the linear range for 30 biphasic and 12 triphasic cells. Biphasic cells showed their peak hyperpolarization near 530 nm and peak depolarization near 680 nm. Triphasic cells showed peak hyperpolarization near 450 nm, peak depolarization near 570 nm and small hyperpolarizing responses to deep red flashes (greater than 670 nm). The response to deep red test flashes was reduced by chromatic backgrounds which either depolarized or hyperpolarized the cell, in contrast to past findings in carp triphasic cells. In both classes of cells, the depolarizing input mechanism had a shorter latency than the hyperpolarizing mechanism, a result not previously observed in other fish retinas. Color opponency was maintained in both classes of C cells for stimuli of small diameter. The findings in bowfin and other species suggest that both feedback and direct pathways shape the depolarizing response of C cells.