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The majority of holocephalans live in the mesopelagic zone of the deep ocean, where there is little or no sunlight, but some species migrate to brightly lit shallow waters to reproduce. This study compares the retinal morphology of two species of deep-sea chimaeras, the Pacific spookfish (Rhinochimaera pacifica) and the Carpenter's chimaera (Chimaera lignaria), with the elephant shark (Callorhinchus milii), a vertical migrator that lives in the mesopelagic zone but migrates to shallow water to reproduce. The two deep-sea chimaera species possess pure rod retinae with long photoreceptor outer segments that might serve to increase visual sensitivity. In contrast, the retina of the elephant shark possesses rods, with an outer-segment length significantly shorter (a mean of 34 µm) than in the deep-sea species, and cones, and therefore the potential for color vision. The retinal ganglion cell distribution closely follows that of the photoreceptor populations in all three species, but there is a lower peak density of these cells in both deep-sea species (215–275 cells/mm2 vs. 769 cells/mm2 in the elephant shark), which represents a significant increase in the convergence of visual information (summation ratio) from photoreceptors to ganglion cells. It is evident that the eyes of deep-sea chimaeras have increased sensitivity to detect objects under low levels of light, but at the expense of both resolution and the capacity for color vision. In contrast, the elephant shark has a lower sensitivity, but the potential for color discrimination and a higher visual acuity. [ABSTRACT FROM AUTHOR]

ABSTRACT Megachiropteran bats (megabats) show remarkable diversity in microhabitat occupation and trophic specializations, but information on how vision relates to their behavioral ecology is scarce. Using stereology and retinal wholemounts, we measured the topographic distribution of retinal ganglion cells and determined the spatial resolution of eight African megachiropterans with distinct roosting and feeding ecologies. We found that species roosting in open microhabitats have a pronounced streak of high retinal ganglion cell density, whereas those favoring more enclosed microhabitats have a less pronounced streak (or its absence in Hypsignathus monstrosus). An exception is the cave-dwelling Rousettus aegyptiacus, which has a pronounced horizontal streak that potentially correlates with its occurrence in more open environments during foraging. In all species, we found a temporal area with maximum retinal ganglion cell density (∼5,000-7,000 cells/mm2) that affords enhanced resolution in the frontal visual field. Our estimates of spatial resolution based on peak retinal ganglion cell density and eye size (∼6-12 mm in axial length) range between ∼2 and 4 cycles/degree. Species that occur in more enclosed microhabitats and feed on plant material have lower spatial resolution (∼2 cycles/degree) compared with those that roost in open and semiopen areas (∼3-3.8 cycles/degree). We suggest that the larger eye and concomitant higher spatial resolution (∼4 cycles/degree) in H. monstrosus may have facilitated the carnivorous aspect of its diet. In conclusion, variations in the topographic organization and magnitude of retinal ganglion density reflect the specific ecological needs to detect food/predators and the structural complexity of the environments. J. Comp. Neurol. 525:186-203, 2017. © 2016 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Retinal ganglion cell (RGC) isodensity maps indicate important regions in an animal's visual field. These maps can also be combined with measures of focal length to estimate the theoretical visual acuity. Here we present the RGC isodensity maps and anatomical spatial resolving power in three budgerigars ( Melopsittacus undulatus) and two Bourke's parrots ( Neopsephotus bourkii). Because RGCs were stacked in several layers, we modified the Nissl staining procedure to assess the cell number in the whole-mounted and cross-sectioned tissue of the same retinal specimen. The retinal topography showed surprising variation; however, both parrot species had an area centralis without discernable fovea. Budgerigars also had a putative area nasalis never reported in birds before. The peak RGC density was 22,300-34,200 cells/mm in budgerigars and 18,100-38,000 cells/mm in Bourke's parrots. The maximum visual acuity based on RGCs and focal length was 6.9 cyc/deg in budgerigars and 9.2 cyc/deg in Bourke's parrots. These results are lower than earlier behavioural estimates. Our findings illustrate that retinal topography is not a very fixed trait and that theoretical visual acuity estimations based on RGC density can be lower than the behavioural performance of the bird. [ABSTRACT FROM AUTHOR]

Eye morphology and the retinal topography of animals that live in either 'open' (e.g., grassland) or 'enclosed' (e.g., forest) terrestrial habitats show common adaptations to constraints imposed by these different habitat types. Although relationships between habitat and the visual system are well documented in most vertebrates, relatively few studies have examined this relationship in birds. Here, we compare eye shape and retinal topography across seven species from the family Phasianidae (Galliformes) that are diurnally active in either open or enclosed habitats. Species from enclosed habitats have significantly larger corneal diameters, relative to transverse diameters, than species from open habitats, which we predict serves to enhance visual sensitivity. Retinal topography, however, was similar across all seven species and consisted of a centrally positioned area centralis and a weak horizontal visual streak, with no discernible fovea. In the Japanese quail ( Coturnix japonica), there was also a dorso-temporal extension of increased neuron density and, in some specimens, a putative area dorsalis. The total number of neurons in the retinal ganglion cell layer was correlated with retinal whole-mount area. Average and peak neuron densities were similar across species, with the exception of the Japanese quail, which had greater average and peak densities. Peak anatomical spatial resolving power was also similar among species, ranging from approximately 10-13 cycles/°. Overall, the pattern of retinal topography we found in phasianids is associated with ground-foraging in birds and presumably facilitates the identification of small food items on the ground as well as other visually guided behaviors, irrespective of habitat type. [ABSTRACT FROM AUTHOR]

The visual abilities of sharks show substantial interspecific variability. In addition, sharks may change their habitat and feeding strategy throughout life. As the eyes of sharks continue to grow throughout the animal's lifetime, ontogenetic variability in visual ability may also occur. The topographic analysis of the photoreceptor and ganglion cell distributions can identify visual specializations and assess changes in visual abilities that may occur concurrently with eye growth. This study examines an ontogenetic series of whole-mounted retinas in two elasmobranch species, the sandbar shark, Carcharhinus plumbeus, and the shortspine spurdog, Squalus mitsukurii, to identify regional specializations mediating zones for improved spatial resolution. The study examines retinal morphology and presents data on summation ratios between photoreceptor and ganglion cell layers, anatomically determined peak spatial resolving power, and the angular extent of the visual field. Peak densities of photoreceptors and ganglion cells occur in similar retinal locations. The topographic distribution of neurons in the ganglion cell layer does not differ substantially with eye growth. However, predicted peak spatial resolution increases with eye growth from 4.3 to 8.9 cycles/deg in C. plumbeus and from 5.7 to 7.2 cycles/deg in S. mitsukurii. The topographic distribution of different-sized ganglion cells is also mapped in C. plumbeus, and a population of large ganglion cells (soma area 120-350 μm²) form a narrow horizontal streak across the retinal meridian, while the spatial distribution of ordinary-sized ganglion cells (soma area 30-120 μm²) forms an area in the central retina. Species-specific retinal specializations highlight differences in visually mediated behaviors and foraging strategies between C. plumbeus and S. mitsukurii. [ABSTRACT FROM AUTHOR]