Photoreceptors and cone patterns in the retina of the smelt Osmerus eperlanus (L.) (Osmeridae: Teleostei)

The outer retina of the smelt Osmerus eperlanus, a visually orientated plankton feeder, of Lake Hiidenvesi (Finland), was examined using both light and transmission electron microscopy. Apart from rods, six morphologically different cone photoreceptor types were identified: short single cones, long single cones, unequal/equal double cones and triple cones (triangular and linear variety). Additionally, in the dorsal region, multiple cone arrangements consisting of up to five members occur. Long single cones and triple cones were observed only sporadically throughout the retina. The incidence of short single cones as a regular element of the cone mosaic is restricted to the ventrotemporal area. The dominant pattern in the Osmerus retina is a pure or a twisted row pattern occurring in all regions. Ventrotemporally, however, square patterns were found as well. The highest cone densities occur in the peripheral ventrotemporal retina. These results indicate that the ventrotemporal region plays an important role in the vision of the smelt. The findings are discussed with respect to the photic habitat conditions and behavioural ecology of the smelt in Lake Hiidenvesi.     

The visual cells of the corsula mullet

The eye of Rhinomugil corsula has a duplex retina differentiated into dorsal and ventral halves, with the ventral retina 116·4 μm thicker than the dorsal retina. The rods of the ventral retina are relatively longer, with longer outer segments. The nuclei of the outer nuclear layer of the dorsal and ventral halves are in four and six to seven rows respectively. The rod outer segment bears a single incision. The mitochondria of cone and rod inner segments has a vitreal-scleral gradient. Single and double cones are present in both halves, with triple cones in the dorsal half only. The outer segments of double cones are equal and united. The single cones have two connecting cilia. The cone cells are arranged in a square mosaic with four double cones and five single cones to each unit in the dorsal half, and in a rhombic pattern in the ventral half.     

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.     

Outer retinal fine structure of the garfish Belone belone (L.) (Belonidae, Teleostei) during light and dark adaptation – photoreceptors, cone patterns and densities

In the present EM study, we investigate the retina of Belone belone , a visually-orientated marine predator living close to the water surface. In the duplex retina, four morphologically different cone types are observed: unequal and equal double cones, long single cones and triple cones. In the light-adapted state, five different cone patterns occur: row, twisted row, square, pentagonal and hexagonal patterns. High double cone densities are found ventro-nasally, ventro-temporally and dorso-temporally. Throughout the retina the double cone/single cone ratio is 2 : 1, in the ventral part, however, a 1 : 1 ratio occurs. In the vitreous body we found a curtain-like intraocular septum dividing the retina into two morphologically different regions. In most areas of the dark-adapted retina the cone patterns are absent at the ellipsoid level, with long single cones standing more vitreally in the light path than double cones. The mosaics are retained, however, in the outer nuclear layer. Typical dark adaptation, i.e. the retinomotor movements of the retinal pigment epithelium and photoreceptors in response to the dark adaptation (light change) is not present in the peripheral ventral and parts of the central ventral area. In both regions we found a twisted row pattern of cones having a vitreal position. The findings are discussed with respect to the photic habitat and feeding habits of this species.     

Intraretinal variability and specialization of cones in Japanese anchovy (Engraulis japonicus,Engraulidae)

The retina of anchovies is characterized by an unusual arrangement and ultrastructure of cones. In the retina of Japanese anchovies, Engraulis japonicus, three types of cones are distributed into rows. The nasal, central, temporal, and ventro‐temporal regions of the retina were occupied exclusively by the long and short cones. Triple cones, made up of two lateral components and one smaller central component, were found only in the dorsal and ventro‐nasal retinal regions. In the outer segments of all short and long cones from the ventro‐temporal region, the lamellae were oriented along the cell axis and were perpendicular to the lamellae in the long cones, providing a morphological basis for the detection of polarization. This lamellar orientation is unique to all vertebrates. The cones were examined with respect to regional differentiation in their size and spectral properties via light microscopy, transmission electron microscopy, and microspectrophotometry. Various dimensions of cones were measured in preparations of isolated cells. The cones from the ventro‐temporal region had different dimensions than cones of the same type located in other retinal regions. Triple cones from the dorsal region were significantly larger than triple cones from the ventro‐nasal region. The spectral absorbance of the lateral components of triple cones in the ventro‐nasal retina was identical to the absorbance of all long and short cones from the ventro‐temporal region. These are shifted to shorter wavelengths relative to the absorbance of the lateral components of the triple cones located in the dorsal retina. Thus, the retina of the Japanese anchovy shows some features of regional specialization common in other fishes that improves spatial resolution for the upwards and forwards visual axis and provides spectral tuning in downwelling light environment. That results from the differentiation of cone types by size and by different spectral sensitivity of various retinal areas. J. Morphol. 277:472–481, 2016. © 2016 Wiley Periodicals, Inc.     

Photoreceptor topography in the duplex retina of the paddlefish (Polyodon spathula)

Retinal whole-mount preparations from the eyes of the North American paddlefish, Polyodon spathula, were examined with a combination of bright field and differential interference contrast microscopy. The entire retina was mapped and population counts of rod and cone photoreceptors were made at regular intervals throughout the retina. The retina is dominated by rods, but a significant percentage (ca. 38%) of the photoreceptors are cones. Mean cone packing density for the entire retina is 6,402+/-1,216 cones/mm2. There is a small (16%) but statistically significant difference between cone packing density in the dorsal retina (6,674+/-1,168 cones/mm2) and the ventral retina (5,745+/-1,076 cones/mm2). There is no region of unusually high cone concentration that might be construed as a fovea or a visual streak. Mean rod packing density for the entire retina is 10,271+/-1,205 rods/mm2. Except in the far periphery, where rods are less numerous, the density of rods is fairly uniform throughout the retina. The data are discussed with regard to paddlefish habitat and behavior.     

Retinal cytoarchitecture in Some Mountain-stream Teleosts of India

In this study, the cytoarchitecture of retina in 10 mountain-stream teleosts (seven cyprinid and three loach species) of India was examined by conventional light microscopy. The mountain-streams are shallow, cold water bodies having high turbidity in the Monsoon and clear water in the colder months. The incumbent fishes are, thus, periodically exposed to low and bright light of the streams, respectively. It was of particular interest to see whether their retina would reflect adaptations to the changing photic environments of the mountain streams. The retina of the cyprinids possess multiple cone types, a moderate cone density, high convergence ratios and prominent retinomotor responses. Triple cones occur only in the danio, Danio aequipinnatus; other cyprinids possess miniature, short and long single cones, and double or twin cones. In general, the cones are bulky in appearance. Cone droplets (ellipsosomes) are found in Garra lamta and G. gotyla gotyla. All cone types, including the miniature single cones, undergo elongation during dark adaptation. The common retinal features in the loaches are the bulky cones, ellipsosomes, low rod density and low convergence ratios. There is no area-like specialization of the retina in any of the species examined. No relationship is obvious between cone pattern and ambient light or food habit in any of those teleosts studied; row pattern is present throughout the retina of the surface-dwelling, total insectivores (cyprinids: Danio aequipinnatus and Barilius spp.), whereas square mosaic pattern is present in the bottom-dwelling total herbiovores (Garra spp.) as well as in the insectivores (loaches: Nemacheilus spp.). The retinal features have suggested that the cyprinids are well-adapted to both clear and turbid water (ellipsosomes, multiple cone types, bulky cones, high rod count and high convergence ratios); the loaches, on the other hand, are better adapted to clear water of the mountain streams.     

The spectral sensitivity of the retinal photoreceptors of the Asiatic smelt <Emphasis Type="Italic">Osmerus dentex</Emphasis> (Steindachner et Kner, 1870)

The spectral sensitivity and complement of the retinal photoreceptors of the Asiatic smelt from the Sea of Japan were studied by microspectrophotometry and light microscopy. Apart from rods, one type of single cones and one type of unequal double cones were found in major parts of the retina. The dominant type of the cone pattern (mosaic) is a row pattern consisting of various linear arrangements of separate single and double cones. The absorbance maxima of rods and a majority of singe cones and double cones equaled 516, 425 and 514/565 nm, respectively. It has been established that all of the pigments are based on retinal. The findings are compared with data on the osmerid retina from the literature and discussed with respect to the adaptations to light conditions, peculiarities of behavior, and seasonal migrations of smelts.     

Ontogeny of Double Cones in the Retina of Perch Fry (Perca fluviatilis,Teleostei)

The formation of double cones in the retina of fry of Perca fluviatilis has been investigated by light and electron microscopy. The retina of newly hatched fry is provided with single cones and rods, single cones being the predominant receptor type. Double cones are seen for the first time 22 days after hatching. Mitoses are observed in the periphery of the retina, but are also seen in more central parts of the retina containing differentiated receptors and a cone mosaic. The fate of the cells resulting from the centrally located mitoses is not known. No signs of longitudinal fission of differentiated single cones are seen. It is suggested that double cones in the retina of perch fry arise by fusion of single cones which associate closely and develop subsurface cisterns coextensive with the region of intimate contact in the ellipsoid. During the first few weeks after hatching, there is a gradual shift in arrangement of the cones. In the newly hatched fry, the single cones are arranged in rows. When double cones are first seen, square-pattern units appear, built up from four double cones and a single cone.     

Photoreceptors and visual pigments in the retina of the fully anadromous green sturgeon (<Emphasis Type="Italic">Acipenser medirostrus</Emphasis>) and the potamodromous pallid sturgeon (<Emphasis Type="Italic">Scaphirhynchus albus</Emphasis>)

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).     

Duplex Retina in the Mesopelagic Deep-Sea Teleost Lestidiops affinis (Ege, 1930)

The eye of the deep-sea teleost Lestidiops affinis has been examined primarily by light microscopy and found to possess a duplex retina consisting of two main divisions, a pure-cone and a pure-rod region, with a narrow zone of transition, possessing both cones and rods, joining the two. The pure-cone region is located in the temporal (caudal) part of the retina subserving binocular vision in the rostral direction. It has an area temporalis retinae with particularly long and densely packed single cones arranged in a regular hexagonal mosaic. Joined (double or twin) cones have not been recognized with certainty in the pure-cone region. The pure-rod region, comprising the larger part of the retina, contains rods grouped in bundles separated by retinal pigment epithelium (RPE) processes with pigmented cores. The synaptic endings of the rods are arranged in separate clusters in the outer plexiform layer, there being apparently a separate rod pedicle cluster beneath (vitread to) each rod bundle. Structural comparisons with certain other deep-sea teleosts suggest the likely presence of a retinal tapetum in L. affinis, i.e. each single cone or rod bundle is situated in a reflecting pit formed by the RPE, with a discrete reflector apposed to the tip of each cone outer segment and the tips of the outer segments of each square-cut rod bundle.     

Note on the Rod-like Photoreceptors in the Retina of the Snake Telescopus fallax (Fleischmann, 1831)

Abstract A previous light microscopic study of the retina of Telescopus fallax has showed the presence of three types of photoreceptors: large single cones (type A), large double cones (type B), and small single rods (type D). The present electron microscopic study has demonstrated that the small single rod is a morphological cone characterized by possessing a slightly tapering outer segment and a very extensive zone of continuity between the discs and the plasma membrane of the outer segment.     

The fine structure of the pigment epithelium and photoreceptor cells of the newt,Triturus viridescens dorsalis (Rafinesque)

The morphology of the retinal pigment epithelium and photoreceptor cells has been studied in the common newt Triturus viridescens dorsalis by light, conventional transmission and scanning electron microscopy. The pigment epithelium is formed by a single layer of low rectangular cells, separated by a multilayered membrane (Bruch's membrane) from the vessels of the choriocapillaris. The scleral border of the pigment epithelium is highly infolded and each epithelial cell contains smooth endoplasmic reticulum, myeloid bodies, mitochondria, lysosomes, phagosomes and an oval nucleus. Inner, pigment laden, epithelial processes surround the photoreceptor outer and inner segments. The three retinal photoreceptor types, rods, single cones and double cones, differ in both external and internal appearance. The newt, rod, outer segments appear denser than the cones in both light and electron micrographs, due to a greater number of rod lamellae per unit distance of outer segment and to the presence of electron dense intralamellar bands. The rod outer segments possess deep incisures in the lamellae while the cone lamellae lack incisures. Both rod and cone outer segments are supported by a peripheral array of dendritic processes containing longitudinal filaments which originate in the inner segment. The inner segment mitochondria, forming the rod ellipsoid, arelong and narrow while those in the cone are spherical to oval in shape. The inner segments of all three receptor cell types also contain a glycogen-filled paraboloid and a myoid region, just outside the nucleus, rich in both rough and smooth endoplasmic reticulum. The elongate, cylindrical nuclei differ in density. The rod nuclei are denser than those of the cones, contain clumped chromatin and usually extend further vitreally. Similarly, the cytoplasm of the rod synaptic terminal is denser than its cone counterpart and contains synaptic vesicles almost twice as large as those of the cones. Photoreceptor synapses in rods and cones are established by both superficial and invaginated contacts with bipolar or horizontal cells.     

Fine structure of the retina of black bass, Micropterus salmoides (Centrarchidae, Teleostei).

The structure of light- and dark-adapted retina of the black bass, Micropterus salmoides has been studied by light and electron microscopy. This retina lacks blood vessels at all levels. The optic fiber layer is divided into fascicles by the processes of Müller cells and the ganglion cell layer is represented by a single row of voluminous cells. The inner nuclear layer consists of two layers of horizontal cells and bipolar, amacrine and interplexiform cells. In the outer plexiform layer we observed the synaptic terminals of photoreceptor cells, rod spherules and cone pedicles and terminal processes of bipolar and horizontal cells. The spherules have a single synaptic ribbon and the pedicles possess multiple synaptic ribbons. Morphologically, we have identified three types of photoreceptors: rods, single cones and equal double cones which undergo retinomotor movements in response to changes in light conditions. The cones are arranged in a square mosaic whereas the rods are dispersed between the cones.     

The structure of anchovy outer retinae (Engraulididae, Clupeiformes) - a comparative light- and electron-microscopic study using museum-stored material

The outer retinal architecture of Engraulididae is uncommon among vertebrates. In some anchovies, e.g., Anchoa, two cone types are arranged alternating in long photoreceptor chains, i.e., polycones. The cones have radially oriented outer segment lamellae in close contact with a complex guanine tapetum, most probably subserving polarization contrast vision. To clarify the distribution of the aberrant polycone architecture within the Engraulididae and to provide indications about polycone evolution, the outer retina morphology of 16 clupeoid species was investigated by light and electron microscopy, predominantly using museum-stored material. The outgroup representatives of four clupeid subfamilies (Clupeonella cultriventris, Dorosoma cepedianum, Ethmalosa fimbriata, Pellonula leonensis) show a row pattern of double cones, partially with single cones at defined positions and a pigment epithelium with lobopodial protrusions containing melanin. The pristigasterid Ilisha africana has double rows of single cones lying between linear curtains of pigment epithelium processes filled with minute crystallites and melanin concentrated near their vitreal tips. Within the Engraulididae, two main architectures are found: Coilia nasus and Thryssa setirostris have linear multiple cones or polycones separated by long pigment epithelium barriers containing tapetal crystallites and melanin in the tips (also found in Setipinna taty), whereas Anchoviella alleni, Encrasicholina heteroloba, Engraulis encrasicolus, Engraulis mordax, Lycengraulis batesii, and Stolephorus indicus exhibit the typical polycone architecture. Cetengraulis mysticetus and Lycothrissa crocodilus show cone patterns and pigment epithelium morphology differing from the other anchovy species. The sets of characters are compared, corroborated with the previous knowledge on clupeoid retinae and discussed in terms of functional morphology and visual ecology. A scenario on polycone evolution is developed that may serve as an aid for the reconstruction of engraulidid phylogeny. Furthermore, this study demonstrates the suitability of museum material for morphological studies, even at the electron microscopic level.     

Alpha transducin is present in blue-, green-, and red-sensitive cone photoreceptors in the human retina

Phototransduction in vertebrate rod and cone photoreceptor cells involves G protein-mediated light stimulation of cGMP hydrolysis. Enzymes of the cGMP hydrolysis cascades of rods and cones are products of different genes. Three different classes of cones in the human retina are maximally sensitive to either blue, green, or red light. Distinct opsin genes are expressed in each type of cone. The distribution of cone types in human retina was determined using anti-peptide antibodies that recognize specific amino acid sequences in green/red opsin and blue opsin. These antibodies together with an anti-peptide antibody against Tc alpha were used in double labeling experiments to demonstrate the presence of the Tc alpha peptide in all types of cones. cDNA clones corresponding to human rod and cone transducin alpha subunit (Tr alpha and Tc alpha) genes were isolated. Southern blot analyses of human genomic DNA suggest that there is only one rod T alpha gene but more than one cone T alpha gene. The multiple Tc alpha genes could be closely related genes or different Tc alpha alleles, or one could be a pseudogene.     

Photoreceptors and photopigments in a subterranean rodent,the pocket gopher (<Emphasis Type="Italic">Thomomys bottae</Emphasis>)

Pocket gophers (Thomomys bottae) are rodents that spend much of their lives in near-lightless subterranean burrows. The visual adaptations associated with this extreme environment were investigated by making anatomical observations of retinal organization and by recording retinal responses to photic stimulation. The size of the eye is within the normal range for rodents, the lens transmits light well down into the ultraviolet, and the retina conforms to the normal mammalian plan. Electroretinogram recording revealed the presence of three types of photopigments, a rod pigment with a spectral peak of about 495 nm and two types of cone pigment with respective peak values of about 367 nm (UV) and 505 nm (medium-wavelength sensitive). Both in terms of responsivity to lights varying in temporal frequency and in response recovery following intense light adaptation, the cone responses of the pocket gopher are similar to those of other rodents. Labeling experiments indicate an abundance of cones that reach densities in excess of 30,000 mm^–2. Cones containing UV opsin are found throughout the retina, but those containing medium-wavelength sensitive opsin are mostly restricted to the dorsal retina where coexpression of the two photopigments is apparently the rule. Rod densities are lower than those typical for nocturnal mammals.     

Number and Distribution of Mouse Retinal Cone Photoreceptors: Differences between an Albino (Swiss) and a Pigmented (C57/BL6) Strain

We purpose here to analyze and compare the population and topography of cone photoreceptors in two mouse strains using automated routines, and to design a method of retinal sampling for their accurate manual quantification. In whole-mounted retinas from pigmented C57/BL6 and albino Swiss mice, the longwave-sensitive (L) and the shortwave-sensitive (S) opsins were immunodetected to analyze the population of each cone type. In another group of retinas both opsins were detected with the same fluorophore to quantify all cones. In a third set of retinas, L-opsin and Brn3a were immunodetected to determine whether L-opsin⁺cones and retinal ganglion cells (RGCs) have a parallel distribution. Cones and RGCs were automatically quantified and their topography illustrated with isodensity maps. Our results show that pigmented mice have a significantly higher number of total cones (all-cones) and of L-opsin⁺cones than albinos which, in turn, have a higher population of S-opsin⁺cones. In pigmented animals 40% of cones are dual (cones that express both opsins), 34% genuine-L (cones that only express the L-opsin), and 26% genuine-S (cones that only express the S-opsin). In albinos, 23% of cones are genuine-S and the proportion of dual cones increases to 76% at the expense of genuine-L cones. In both strains, L-opsin⁺cones are denser in the central than peripheral retina, and all-cones density increases dorso-ventrally. In pigmented animals S-opsin⁺cones are scarce in the dorsal retina and very numerous in the ventral retina, being densest in its nasal aspect. In albinos, S-opsin⁺cones are abundant in the dorsal retina, although their highest densities are also ventral. Based on the densities of each cone population, we propose a sampling method to manually quantify and infer their total population. In conclusion, these data provide the basis to study cone degeneration and its prevention in pathologic conditions.     

Ontogeny of the photoreceptors in the embryonic retina of the viviparous guppy,Poecilia reticulata P. (Teleostei)

Summary Ultrastructural analyses of retinal development in the guppy embryo show that at midgestation all types of photoreceptors are differntiated in the fundus, and at birth differentiation extends over the whole retina. Formation of discs of outer segments is more rapid in rods than in cones. Double cones differentiate simultaneously with long single cones and are formed by the adhesion of two primordial inner segments; short single cones develop last. Wherever cones are differentiated, they are arranged in an adult-type square mosaic. The rods in the embryo, as opposed to the adult, are likewise regularly arranged within the mosaic unit.These results are at variance with the generally held opinion that adult teleosts which possess duplex retinae have larvae with pure cone retinae, and that rods, double cones and mosaics appear in late larval life or only at metamorphosis.In the double cones of the guppy embryo subsurface cisternae develop along the adjoining primordial inner segments. Additionally, regularly distributed subsurface cisternae are formed in the regions of intimate contact of long single cones with double cones and rods.We suggest that the early development of rods and double cones, and a square-mosaic with regular distribution of rods and subsurface cisternae, provide the newly born with a fully functional optical apparatus, especially suited to perception of movements. This is necessary for its survival against predatory, especially maternal, attacks.This work was in part subsidized by a grant from the Medical Research Council of Ireland to Y.W.K.     

Retinal Cone Photoreceptors of the Deer Mouse Peromyscus maniculatus: Development,Topography, Opsin Expression and Spectral Tuning

A quantitative analysis of photoreceptor properties was performed in the retina of the nocturnal deer mouse, Peromyscus maniculatus, using pigmented (wildtype) and albino animals. The aim was to establish whether the deer mouse is a more suitable model species than the house mouse for photoreceptor studies, and whether oculocutaneous albinism affects its photoreceptor properties. In retinal flatmounts, cone photoreceptors were identified by opsin immunostaining, and their numbers, spectral types, and distributions across the retina were determined. Rod photoreceptors were counted using differential interference contrast microscopy. Pigmented P. maniculatus have a rod-dominated retina with rod densities of about 450.000/mm² and cone densities of 3000 - 6500/mm². Two cone opsins, shortwave sensitive (S) and middle-to-longwave sensitive (M), are present and expressed in distinct cone types. Partial sequencing of the S opsin gene strongly supports UV sensitivity of the S cone visual pigment. The S cones constitute a 5-15% minority of the cones. Different from house mouse, S and M cone distributions do not have dorsoventral gradients, and coexpression of both opsins in single cones is exceptional (<2% of the cones). In albino P. maniculatus, rod densities are reduced by approximately 40% (270.000/mm²). Overall, cone density and the density of cones exclusively expressing S opsin are not significantly different from pigmented P. maniculatus. However, in albino retinas S opsin is coexpressed with M opsin in 60-90% of the cones and therefore the population of cones expressing only M opsin is significantly reduced to 5-25%. In conclusion, deer mouse cone properties largely conform to the general mammalian pattern, hence the deer mouse may be better suited than the house mouse for the study of certain basic cone properties, including the effects of albinism on cone opsin expression.