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We studied the topography of retinal ganglion cells (GCs) and estimated spatial resolving power (SRP) in the pajama cardinalfish Sphaeramia nematoptera (Bleeker, 1856), a relatively small brightly colored fish inhabiting coral reefs and lagoons in the Western Pacific. S. nematoptera is an active night predator feeding on near‐bottom animal plankton and benthos. DAPI staining was used to label nuclei of GCs and non‐GCs in the inner plexiform and ganglion cell layers. Non‐GCs were distinguished from GCs in Nissl‐stained retinal wholemounts based on cell size, shape, and staining intensity. The proportion of displaced amacrine cells (DACs) varied from 15.46 ± 1.12 (visual streak [VS]) to 17.99 ± 1.06% (dorsal periphery) (mean ± S.E.M., N = 5); the respective proportions of glial cells were 6.61 ± 0.84 and 5.89 ± 0.76%. Thus, 76%–78% of cells in the ganglion cell layer and inner plexiform layer were GCs. The minimum spatial coverage of GCs (3600–4600 cells/mm2) was detected in the dorsal and ventral periphery. It gradually increased toward the central retina to form a moderate VS. The maximum GC density (11,400–12,400 cells/mm2) was registered in the central portion of the VS. No pronounced concentric retinal specializations were found. The total number of GCs ranged within 595.2–635.9 × 103. The anatomical spatial resolving power was minimum in the ventral periphery (4.91–5.53 cpd) and maximum in the central portion of the VS (8.47–9.07 cpd). The respective minimum separable angles were 0.18–0.20° and 0.11–0.12°. The relatively high spatial resolving power and presence of the VS in the pajama cardinalfish are in line with its highly visual behavior. [ABSTRACT FROM AUTHOR]

The Green Weaver ants, Oecophylla smaragdina are iconic animals known for their extreme cooperative behaviour where they bridge gaps by linking to each other to build living chains. They are visually oriented animals, build chains towards closer targets, use celestial compass cues for navigation and are visual predators. Here, we describe their visual sensory capacity. The major workers of O. smaragdina have more ommatidia (804) in each eye compared to minor workers (508), but the facet diameters are comparable between both castes. We measured the impulse responses of the compound eye and found their response duration (42 ms) was similar to that seen in other slow-moving ants. We determined the flicker fusion frequency of the compound eye at the brightest light intensity to be 132 Hz, which is relatively fast for a walking insect suggesting the visual system is well suited for a diurnal lifestyle. Using pattern-electroretinography we identified the compound eye has a spatial resolving power of 0.5 cycles deg−1 and reached peak contrast sensitivity of 2.9 (35% Michelson contrast threshold) at 0.05 cycles deg−1. We discuss the relationship of spatial resolution and contrast sensitivity, with number of ommatidia and size of the lens. [ABSTRACT FROM AUTHOR]

The distribution of retinal ganglion cells (RGCs) was observed in the retinal wholemount of native chicken (Gallus gallus domestricus) of Bangladesh by using light microscopy. We considered five different anatomic regions (central, nasal, temporal, dorsal, and ventral) of Nissl stained wholemount, and the RGCs were counted, plotted, and measured accordingly. The average area of the retina was 431.75 mm2 while the total number of ganglion cells was 2124431 on average. Only the central area of the retina was the peak density (10400 cells/mm2 ) area, signifying the acute visual area, whilst the maximum spatial resolving power was 11 cycles/degree. The overall concentration of RGCs gradually declined towards the periphery but the size of cells generally decreased towards centrally. The size of ganglion cell was not uniform (12 to 180 µm2 ), specifically the central retina, just above the optic disc was packed with tiny-sized cells. The number, topographic distribution, and size of RGCs in native chicken signified their domesticated or terrestrial characters, uneven visual acuteness, and possibly only the central retina was the area for fine vision as the function of RGCs. [ABSTRACT FROM AUTHOR]

Vision plays a crucial role in the life of the vast majority of vertebrate species. The spatial arrangement of retinal ganglion cells has been reported to be related to a species' visual behavior. There are many studies focusing on the ganglion cell topography in bony fish species. However, there are still large gaps in our knowledge on the subject. We studied the topography of retinal ganglion cells (GCs) in the Japanese smelt Hypomesus nipponensis, a highly visual teleostean fish with a complex life cycle. DAPI labeling was used to visualize cell nuclei in the ganglion cell and inner plexiform layers. The ganglion cell layer was relatively thin (about 6‐8 μm), even in areas of increased cell density (area retinae temporalis), and was normally composed of a single layer of cells. In all retinal regions, rare cells occurred in the inner plexiform layer. Nissl‐stained retinae were used to estimate the proportion of displaced amacrine cells and glia in different retinal regions. In all retinal regions, about 84.5% of cells in the GC layer were found to be ganglion cells. The density of GCs varied across the retina in a regular way. It was minimum (3990 and 2380 cells/mm2 in the smaller and larger fish, respectively) in the dorsal and ventral periphery. It gradually increased centripetally and reached a maximum of 14,275 and 10,960 cells/mm2 (in the smaller and larger fish, respectively) in the temporal retina, where a pronounced area retinae temporalis was detected. The total number of GCs varied from 177 × 103 (smaller fish) to 212 × 103 cells (larger fish). The theoretical anatomical spatial resolution (the anatomical estimate of the upper limit of visual acuity calculated from the density of GCs and eye geometry and expressed in cycles per degree) was minimum in the ventral periphery (smaller fish, 1.46 cpd; larger fish, 1.26 cpd) and maximum in area retinae temporalis (smaller fish, 2.83 cpd; larger fish, 2.75 cpd). The relatively high density of GCs and the presence of area retinae temporalis in the Japanese smelt are consistent with its highly visual behavior. The present findings contribute to our understanding of the factors affecting the topography of retinal ganglion cells and visual acuity in fish. [ABSTRACT FROM AUTHOR]

In this study, we assessed eye morphology and retinal topography in two flamingo species, the Caribbean flamingo (Phoenicopterus ruber) and the Chilean flamingo (P. chilensis). Eye morphology is similar in both species and cornea size relative to eye size (C:A ratio) is intermediate between those previously reported for diurnal and nocturnal birds. Using stereology and retinal whole mounts, we estimate that the total number of Nissl‐stained neurons in the retinal ganglion cell (RGC) layer in the Caribbean and Chilean flamingo is ~1.70 and 1.38 million, respectively. Both species have a well‐defined visual streak with a peak neuron density of between 13,000 and 16,000 cells mm−2 located in a small central area. Neurons in the high‐density regions are smaller and more homogeneous compared to those in medium‐ and low‐density regions. Peak anatomical spatial resolving power in both species is approximately 10–11 cycles/deg. En‐face images of the fundus in live Caribbean flamingos acquired using spectral domain optical coherence tomography (SD‐OCT) revealed a thin, dark band running nasotemporally just dorsal to the pecten, which aligned with the visual streak in the retinal topography maps. Cross‐sectional images (B‐scans) obtained with SD‐OCT showed that this dark band corresponds with an area of retinal thickening compared to adjacent areas. Neither the retinal whole mounts, nor the SD‐OCT imaging revealed any evidence of a central fovea in either species. Overall, we suggest that eye morphology and retinal topography in flamingos reflects their cathemeral activity pattern and the physical nature of the habitats in which they live. [ABSTRACT FROM AUTHOR]

Bats are nocturnal mammals known for their ability to echolocate, yet all bats can see, and most bats of the family Pteropodidae (fruit bats) do not echolocate – instead they rely mainly on vision and olfaction to forage. We investigated whether echolocating bats, given their limited reliance on vision, have poorer spatial resolving power (SRP) than pteropodids and whether tongue click echolocating fruit bats differ from non-echolocating fruit bats in terms of visual performance. We compared the number and distribution of retinal ganglion cells (RGCs) as well as the maximum anatomical SRP derived from these distributions in 4 species of bats: Myotis daubentonii, a laryngeal echolocating bat from the family Vespertilionidae, Rousettus aegyptiacus, a tongue clicking echolocating bat from the family Pteropodidae, and Pteropus alecto and P. poliocephalus, 2 non-echolocating bats (also from the Pteropodidae). We find that all 3 pteropodids have a similar number (≈200,000 cells) and distribution of RGCs and a similar maximum SRP (≈4 cycles/degree). M. daubentonii has fewer (∼6,000 cells) and sparser RGCs than the pteropodids and thus a significantly lower SRP (0.6 cycles/degree). M. daubentonii also differs in terms of the distribution of RGCs by having a unique dorsal area of specialization in the retina. Our findings are consistent with the existing literature and suggest that M. daubentonii likely only uses vision for orientation, while for pteropodids vision is also important for foraging. [ABSTRACT FROM AUTHOR]

Tree frogs are an interesting and diverse group of frogs. They display a number of unique adaptations to life in the arboreal environment. Vision plays a crucial role in their ecology. The topography of retinal ganglion cells (GCs) is closely related to a species' visual behavior. Despite a large amount of research addressing GC topography in vertebrates, there is scarce data on this subject in tree frogs. I studied the topography of GCs in the retina of the Japanese tree frog Hyla japonica. The GC density distribution was locally fairly homogeneous, with spatial density increasing gradually from the dorsal and ventral periphery towards the equator. A moderately pronounced visual streak was found close to the equator in the dorsal hemiretina, with a distinct area retinae temporalis in the dorsotemporal quadrant potentially subserving binocular vision. The minimum GC density (mean ± SEM, n = 5) was 3060 ± 60 and the maximum 12 800 ± 170 cells/mm2. The total number of GCs was 292 ± 7 × 103. The theoretical anatomical spatial resolution estimated from GC densities and eye optics was lowest in the ventral periphery (ca. 0.9 and 1.3 cycles/degree in air and water, respectively) and highest in the area retinae temporalis (ca. 2.1 and 2.8 cycles/degree). The relatively high GC density and presence of specialized retinal regions in Hyla japonica are consistent with its highly visual behavior. The present findings contribute to our understanding of the relative role of common ancestry and environmental pressure in GC topography variation within Anura. [ABSTRACT FROM AUTHOR]

The Nubian ibex (Capra nubiana) occurs in information‐rich visual habitats including the edges of cliffs and escarpments. In addition to needing enhanced spatial resolution to find food and detect predators, enhanced visual sampling of the lower visual field would be advantageous for the control of locomotion in such precarious terrains. Using retinal wholemounts and stereology, we sought to measure how the ganglion cell density varies across the retina of the Nubian ibex to reveal which portions of its surroundings are sampled with high resolution. We estimated a total of ~1 million ganglion cells in the Nubian ibex retinal ganglion cell layer. Topographic variations of ganglion cell density reveal a temporal area, a horizontal streak, and a dorsotemporal extension, which are topographic retinal features also found in other artiodactyls. In contrast to savannah‐dwelling artiodactyls, the horizontal streak of the Nubian ibex appears loosely organized possibly reflecting a reduced predation risk in mountainous habitats. Estimates of spatial resolving power (~17 cycles/degree) for the temporal area would be reasonable to facilitate foraging in the frontal visual field. Embedded in the dorsotemporal extension, we also found an unusual dorsotemporal area not yet reported in any other mammal. Given its location and spatial resolving power (~6 cycles/degree), this specialization enhances visual sampling toward the lower visual field, which would be advantageous for visually guided locomotion. This study expands our understanding of the retinal organization in artiodactyls and offers insights on the importance of vision for the Nubian ibex ecology. [ABSTRACT FROM AUTHOR]

A micro-channel plate detector with wedge and strip anode can be used for visible and ultraviolet light imaging. In this paper, the imager electronics are designed in order to process specific output charge characteristics. The original signal output from the detector is shaped into a Gaussian pulse signal, and a digital Constant Fraction Discriminator is used to judge the arrival time of the pulse. The resolution of the electronic output image is 1024 × 1024, and the position decoding accuracy is 1 pixel. The experimental imaging resolving power of the detector is as high as 70 µm. [ABSTRACT FROM AUTHOR]

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: Investigating retinal specializations offers insights into eye functionality. Using retinal wholemount techniques, we investigated the distribution of retinal ganglion cells in the Little skate Leucoraja erinacea by (a) dye‐backfilling into the optic nerve prior to retinal wholemounting; (b) Nissl‐staining of retinal wholemounts. Retinas were examined for regional specializations (higher numbers) of ganglion cells that would indicate higher visual acuity in those areas. Total ganglion cell number were low compared to other elasmobranchs (backfilled: average 49,713 total ganglion cells, average peak cell density 1,315 ganglion cells mm−2; Nissl‐stained: average 47,791 total ganglion cells, average peak cell density 1,319 ganglion cells mm−2). Ganglion cells fit into three size categories: small (5–20 µm); medium (20–30 µm); large: (≥ 30 µm), and they were not homogeneously distributed across the retina. There was a dorsally located horizontal visual streak with increased ganglion cell density; additionally, there were approximately three local maxima in ganglion cell distribution (potential areae centrales) within this streak in which densities were highest. Using computerized tomography (CT) and micro‐CT, geometrical dimensions of the eye were obtained. Combined with ganglion cell distributions, spatial resolving power was determined to be between 1.21 and 1.37 cycles per degree. Additionally, photoreceptor sizes across different retinal areas varied; photoreceptors were longest within the horizontal visual streak. Variations in the locations of retinal specializations appear to be related to the animal's anatomy: shape of the head and eyes, position of eyes, location of tapetum, and shape of pupil, as well as the visual demands associated with lifestyle and habitat type. [ABSTRACT FROM AUTHOR]

The river hippopotamus ( Hippopotamus amphibius), one of the closest extant relatives to cetaceans, is a large African even-toed ungulate (Artiodactyla) that grazes and has a semiaquatic lifestyle. Given its unusual phenotype, ecology, and evolutionary history, we sought to measure the topographic distribution of retinal ganglion cell density using stereology and retinal wholemounts. We estimated a total of 243,000 ganglion cells of which 3.4% (8,300) comprise alpha cells. The topographic distribution of both total and alpha cells reveal a dual topographic organization of a temporal and nasal area embedded within a well-defined horizontal streak. Using maximum density of total ganglion cells and eye size (35 mm, axial length), we estimated upper limits of spatial resolving power of 8 cycles/deg (temporal area, 1,800 cells/mm2), 7.7 cycles/deg (nasal area, 1,700 cells/mm2), and 4.2 cycles/deg (horizontal streak, 250 cells/mm2). Enhanced resolution of the temporal area toward the frontal visual field may facilitate grazing, while resolution of the horizontal streak and nasal area may help the discrimination of objects (predators, conspecifics) in the lateral and posterior visual fields, respectively. Given the presumed role of alpha cells to detect brisk transient stimuli, their similar distribution to the total ganglion cell population may facilitate the detection of approaching objects in equivalent portions of the visual field. Our finding of a nasal area in the river hippopotamus retina supports the notion that this specialization may enhance visual sampling in the posterior visual field to compensate for limited neck mobility as suggested for rhinoceroses and cetaceans. [ABSTRACT FROM AUTHOR]

ABSTRACT This study sought to determine whether the retinal organization of the white rhinoceros ( Ceratotherium simum), a large African herbivore with lips specialized for grazing in open savannahs, relates to its foraging ecology and habitat. Using stereology and retinal wholemounts, we estimated a total of 353,000 retinal ganglion cells. Their density distribution reveals an unusual topographic organization of a temporal (2,000 cells/mm2) and a nasal (1,800 cells/mm2) area embedded within a well-defined horizontal visual streak (800 cells/mm2), which is remarkably similar to the retinal organization in the black rhinoceros. Alpha ganglion cells comprise 3.5% (12,300) of the total population of ganglion cells and show a similar distribution pattern with maximum densities also occurring in the temporal (44 cells/mm2) and nasal (40 cells/mm2) areas. We found higher proportions of alpha cells in the dorsal and ventral retinas. Given their role in the detection of brisk transient stimuli, these higher proportions may facilitate the detection of approaching objects from the front and behind while grazing with the head at 45 °. Using ganglion cell peak density and eye size (29 mm, axial length), we estimated upper limits of spatial resolving power of 7 cycles/deg (temporal area), 6.6 cycles/deg (nasal area), and 4.4 cycles/deg (horizontal streak). The resolution of the temporal area potentially assists with grazing, while the resolution of the streak may be used for panoramic surveillance of the horizon. The nasal area may assist with detection of approaching objects from behind, potentially representing an adaptation compensating for limited neck and head mobility. J. Comp. Neurol., 525:2484-2498, 2017. © 2017 Wiley Periodicals, Inc. [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]

Sharks have long been described as having 'poor' vision. They are cone monochromats and anatomical estimates suggest they have low spatial resolution. However, there are no direct behavioural measurements of spatial resolution or contrast sensitivity. This study estimates contrast sensitivity and spatial resolution of two species of benthic sharks, the Port Jackson shark, Heterodontus portusjacksoni, and the brown-banded bamboo shark, Chiloscyllium punctatum, by recording eye movements in response to optokinetic stimuli. Both species tracked moving low spatial frequency gratings with weak but consistent eye movements. Eye movements ceased at 0.38 cycles per degree, even for high contrasts, suggesting low spatial resolution. However, at lower spatial frequencies, eye movements were elicited by low contrast gratings, 1.3% and 2.9% contrast in H. portusjacksoni and C. punctatum, respectively. Contrast sensitivity was higher than in other vertebrates with a similar spatial resolving power, which may reflect an adaptation to the relatively low contrast encountered in aquatic environments. Optokinetic gain was consistently low and neither species stabilised the gratings on their retina. To check whether restraining the animals affected their optokinetic responses, we also analysed eye movements in freeswimming C. punctatum. We found no eye movements that could compensate for body rotations, suggesting that vision may pass through phases of stabilisation and blur during swimming. As C. punctatum is a sedentary benthic species, gaze stabilisation during swimming may not be essential. Our results suggest that vision in sharks is not 'poor' as previously suggested, but optimised for contrast detection rather than spatial resolution. [ABSTRACT FROM AUTHOR]

potto (Perodicticus potto) is an arboreal strepsirhine found in the rainforests of central Africa. In contrast to most primates, the potto shows slow-moving locomotion over the upper surface of branches, where it forages for exudates and crawling invertebrates with its head held very close to the substrate. Here, we asked whether the retina of the potto displays topographic specializations in neuronal density that correlate with its unusual lifestyle. Using stereology and retinal wholemounts, we measured the total number and topographic distribution of retinal ganglion cells (total and presumed parasol), as well as estimating the upper limits of the spatial resolution of the potto eye. We estimated ~ 210,000 retinal ganglion cells, of which ~ 7% (~ 14,000) comprise presumed parasol ganglion cells. The topographic distribution of both total and parasol ganglion cells reveals a concentric centroperipheral organization with a nasoventral asymmetry. Combined with the upwardly shifted orbits of the potto, this nasoventral increase in parasol ganglion cell density enhances contrast sensitivity and motion detection skywards, which potentially assists with the detection of predators in the high canopy. The central area of the potto occurs ~ 2.5 mm temporal to the optic disc and contains a maximum ganglion cell density of ~ 4,300 cells/mm 2 . We found no anatomical evidence of a fovea within this region. Using maximum ganglion cell density and eye size (~ 14 mm), we estimated upper limits of spatial resolving power between 4.1 and 4.4 cycles/degree. Despite their reported reliance on olfaction to detect exudates, this level of spatial resolution potentially assists pottos with foraging for small invertebrates and in the detection of predators. [ABSTRACT FROM AUTHOR]

The family Herpestidae (cusimanses and mongooses) is a monophyletic radiation of carnivores with remarkable variation in microhabitat occupation and diel activity, but virtually nothing is known about how they use vision in the context of their behavioral ecology. In this paper, we measured the number and topographic distribution of neurons (rods, cones and retinal ganglion cells) and estimated the spatial resolving power of the eye of the diurnal, forest-dwelling Ansorge's cusimanse (Crossarchus ansorgei). Using retinal wholemounts and stereology, we found that rods are more numerous (42,500,000; 92%) than cones (3,900,000; 8%). Rod densities form a concentric and dorsotemporally asymmetric plateau that matches the location and shape of a bright yellow tapetum lucidum located within the dorsal aspect of the eye. Maximum rod density (340,300 cells/mm2) occurs within an elongated plateau below the optic disc that corresponds to a transitional region between the tapetum lucidum and the pigmented choroid. Cone densities form a temporal area with a peak density of 44,500 cells/mm2 embedded in a weak horizontal streak that matches the topographic distribution of retinal ganglion cells. Convergence ratios of cones to retinal ganglion cells vary from 50:1 in the far periphery to 3:1 in the temporal area. With a ganglion cell peak density of 13,400 cells/mm2 and an eye size of 11 mm in axial length, we estimated upper limits of spatial resolution of 7.5-8 cycles/degree, which is comparable to other carnivores such as hyenas. In conclusion, we suggest that the topographic retinal traits described for Ansorge's cusimanse conform to a presumed carnivore retinal blueprint but also show variations that reflect its specific ecological needs. © 2015 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

The eyes of five ray species (Taeniura lymma, Neotrygon kuhlii, Pastinachus atrus, Himantura uarnak and Urogymnus asperrimus) from the same taxonomic family (Dasyatidae) and the same geographic region (Ningaloo Reef, Western Australia) were studied to identify differences in retinal specializations that may reflect niche specialization. The topographic distributions of photoreceptors (rods and all cones) and ganglion cells were assessed and used to identify localized peaks in cell densities that indicate specializations for acute vision. These data were also used to calculate summation ratios of photoreceptors to ganglion cells in each species and estimate the anatomical spatial resolving power of the eye. Subtle differences in the distribution of retinal neurons appear to be related to the ecology of these closely related species of stingrays. The main specialization in the retinal cell density distribution is the dorsal streak that allows these animals to scan the substrate for potential prey. The blue-spotted fantail ray, T. lymma, showed the highest peak density of rods (86,700 rods mm-2) suggesting a specialization for scotopic vision. The highest peak density of cones (9,970 cones mm-2) was found in H. uarnak, and the highest peak density of ganglion cells (4,500 cells mm-2) was found in P. atrus. The proportion of rods to cones in the dorsal streak was higher in the two smaller species (12.5-14:1 in T. lymma and N. kuhlii) than the larger stingrays (6-8:1 in P. atrus, H. uarnak and U. asperrimus). Visual specializations in different sympatric species are subtle but may reflect specializations to specific ecological niches. © 2015 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

ABSTRACT The avian retina possesses one of the most diverse complements of photoreceptor types among vertebrates but little is known about their spatial distribution. Here we used retinal wholemounts and stereological methods to present the first complete maps of the topographic distribution of rods and cones in four species of Australian passerines with diverse trophic specializations. All species studied have one central and one temporal rod-free zone. In the insectivorous yellow-rumped thornbill, the central rod-free zone is unusually large, occupying ∼17% (56°) of the retinal area (angular subtense), whereas in nectarivorous and frugivorous species it represents only ∼0.1% (5-7°) to 0.3% (10°) of the retinal area (angular subtense). In contrast, the temporal rod-free zone varies little between species (∼0.02-0.4%; 2-10°). In all species, rods follow a pronounced dorsoventral gradient with highest densities in the ventral retina. The topographic distribution of cones is concentric and reveals a central fovea and a temporal area. In the yellow-rumped thornbill, cone densities form an extended plateau surrounding the fovea, beyond which densities fall rapidly towards the retinal periphery. For the other species, cone densities decline gradually along a foveal to peripheral gradient. Estimates of spatial resolving power calculated using cone peak densities are higher in the central fovea (19-41 cycles/degree) than in the temporal area (9-15 cycles/degree). In conclusion, we suggest that the unusual organization of the rod-free zone and the distinct topographic distribution of rods and cones correlate with specific ecological needs for enhanced visual sensitivity and spatial resolution in these birds. J. Comp. Neurol. 523:1073-1094, 2015. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Topographic analyses of retinal ganglion cell density are very useful in providing information about the visual ecology of a species by identifying areas of acute vision within the visual field (i.e. areas of high cell density). In this study, we investigated the neural cell distribution in the ganglion cell layer of a range of lanternfish species belonging to 10 genera. Analyses were performed on wholemounted retinas using stereology. Topographic maps were constructed of the distribution of all neurons and both ganglion and amacrine cell populations in 5 different species from Nissl-stained retinas using cytological criteria. Amacrine cell distribution was also examined immunohistochemically in 2 of the 5 species using anti-parvalbumin antibody. The distributions of both the total neuron and the amacrine cell populations were aligned in all of the species examined, showing a general increase in cell density toward the retinal periphery. However, when the ganglion cell population was topographically isolated from the amacrine cell population, which comprised up to 80% of the total neurons within the ganglion cell layer, a different distribution was revealed. Topographic maps of the true ganglion cell distribution in 18 species of lanternfishes revealed well-defined specializations in different regions of the retina. Different species possessed distinct areas of high ganglion cell density with respect to both peak density and the location and/or shape of the specialized acute zone (i.e. elongated areae ventro-temporales, areae temporales and large areae centrales). The spatial resolving power was calculated to be relatively low (varying from 1.6 to 4.4 cycles per degree), indicating that myctophids may constitute one of the less visually acute groups of deep-sea teleosts. The diversity in retinal specializations and spatial resolving power within the family is assessed in terms of possible ecological functions and evolutionary history. © 2014 S. Karger AG, Basel [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]

Penguins are a group of flightless seabirds that exhibit numerous morphological, behavioral and ecological adaptations to their amphibious lifestyle, but little is known about the topographic organization of neurons in their retinas. In this study, we used retinal wholemounts and stereological methods to estimate the total number and topographic distribution of retinal ganglion cells in addition to an anatomical estimate of spatial resolving power in two species of penguins: the little penguin, Eudyptula minor, and the king penguin, Aptenodytes patagonicus. The total number of ganglion cells per retina was approximately 1,200,000 in the little penguin and 1,110,000 in the king penguin. The topographic distribution of retinal ganglion cells in both species revealed the presence of a prominent horizontal visual streak with steeper gradients in the little penguin. The little penguin retinas showed ganglion cell density peaks of 21,867 cells/mm2, affording spatial resolution in water of 17.07-17.46 cycles/degree (12.81-13.09 cycles/degree in air). In contrast, the king penguin showed a relatively lower peak density of ganglion cells of 14,222 cells/mm2, but - due to its larger eye - slightly higher spatial resolution in water of 20.40 cycles/degree (15.30 cycles/degree in air). In addition, we mapped the distribution of giant ganglion cells in both penguin species using Nissl-stained wholemounts. In both species, topographic mapping of this cell type revealed the presence of an area gigantocellularis with a concentric organization of isodensity contours showing a peak in the far temporal retina of approximately 70 cells/mm2 in the little penguin and 39 cells/mm2 in the king penguin. Giant ganglion cell densities gradually fall towards the outermost isodensity contours revealing the presence of a vertically organized streak. In the little penguin, we confirmed our cytological characterization of giant ganglion cells using immunohistochemistry for microtubule-associated protein 2. This suite of retinal specializations, which is also observed in the closely related procellariiform seabirds, affords the eyes of the little and king penguins panoramic surveillance of the horizon and motion detection in the frontal visual field. Copyright © 2012 S. Karger AG, Basel [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]

Several visual traits have previously been assessed in elasmobranchs; however, few studies have examined and compared multiple visual attributes within a particular genus. The primary advantage of studying closely related species is that any differences between them are more likely to reflect functional ecological adaptations rather than the effects of phylogenetic separation. In this study, the visual capabilities of 4 wobbegong shark species, which vary in life-history and/or habitat, were examined: the western wobbegong (Orectolobus hutchinsi), the spotted wobbegong (O. maculatus), the ornate wobbegong (O. ornatus) and the dwarf spotted wobbegong (O. parvimaculatus). The retinae of all 4 wobbegong species are duplex; rod and cone photoreceptors can be distinguished easily on the basis of morphology. Some variation in relative eye size exists, with O. parvimaculatus possessing the largest eyes. The topographic distribution of cells within the ganglion cell layer of O. hutchinsi reveals a weakly elongated central visual streak of increased cell density, mediating a higher spatial resolving power of 2.06 cycles deg-1 in the frontal visual field. Retinal topography of O. maculatus and O. parvimaculatus is similar, with both possessing a dorsal horizontal streak facilitating an increased spatial resolving power of 3.51 cycles deg-1 and 3.91 cycles deg-1, respectively, in the lower visual field. O. parvimaculatus also possesses an area of increased cell density in the naso-ventral region of the retina, mediating acute vision in the upper caudal region of the visual field. While all 4 species have visual systems optimised for increased visual sensitivity, O. maculatus and O. parvimaculatus appear to be particularly well suited to activity under low light conditions. Copyright © 2010 S. Karger AG, Basel [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]

The total number, distribution and peak density of presumed retinal ganglion cells was assessed in 10 species of elasmobranch (nine species of shark and one species of batoid) using counts of Nissl-stained cells in retinal wholemounts. The species sampled include a number of active, predatory benthopelagic and pelagic sharks that are found in a variety of coastal and oceanic habitats and represent elasmobranch groups for which information of this nature is currently lacking. The topographic distribution of cells was heterogeneous in all species. Two benthic species, the shark Chiloscyllium punctatum and the batoid Taeniura lymma, have a dorsal or dorso-central horizontal streak of increased cell density, whereas the majority of the benthopelagic and pelagic sharks examined exhibit a more concentric pattern of increasing cell density, culminating in a central area centralis of higher cell density located close to the optic nerve head. The exception is the shark Alopias superciliosus, which possesses a ventral horizontal streak. Variation in retinal ganglion cell topography appears to be related to the visual demands of different habitats and lifestyles, as well as the positioning of the eyes in the head. The upper limits of spatial resolving power were calculated for all 10 species, using peak ganglion cell densities and estimates of focal length taken from cryo-sectioned eyes in combination with information from the literature. Spatial resolving power ranged from 2.02 to 10.56 cycles deg–1, which is in accordance with previous studies. Species with a lower spatial resolving power tend to be benthic and/or coastal species that feed on benthic invertebrates and fishes. Active, benthopelagic and pelagic species from more oceanic habitats which feed on larger, more active prey, possess a higher resolving power. Additionally, ganglion cells in a juvenile of C. punctatum, were retrogradely-labeled from the optic nerve with biotinylated dextran amine (BDA). A comparison of the BDA- labeled material and tissue stained for Nissl substance indicates that 76% of the cells in the retinal ganglion cell and inner plexiform layers of the central retina in this species are non-ganglion cells. Copyright © 2008 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

In addition to compound eyes, insects possess simple eyes known as ocelli. Input from the ocelli modulates optomotor responses, flight-time initiation, and phototactic responses - behaviours that are mediated predominantly by the compound eyes. In this study, using pattern electroretinography (pERG), we investigated the contribution of the compound eyes to ocellar spatial vision in the diurnal Australian bull ant Myrmecia tarsata by measuring the contrast sensitivity and spatial resolving power of the ocellar second-order neurons under various occlusion conditions. Furthermore, in four species of Myrmecia ants active at different times of the day, and in European honeybee Apis mellifera, we characterized the ocellar visual properties when both visual systems were available. Among the ants, we found that the time of activity had no significant effect on ocellar spatial vision. Comparing day-active ants and the honeybee, we did not find any significant effect of locomotion on ocellar spatial vision. In M. tarsata, when the compound eyes were occluded, the amplitude of the pERG signal from the ocelli was reduced 3 times compared with conditions when the compound eyes were available. The signal from the compound eyes maintained the maximum contrast sensitivity of the ocelli as 13 (7.7%), and the spatial resolving power as 0.29 cycles deg-1. We conclude that ocellar spatial vison improves significantly with input from the compound eyes, with a noticeably larger improvement in contrast sensitivity than in spatial resolving power., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2021. Published by The Company of Biologists Ltd.)