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The anatomy and structure of the eye and olfactory organ were investigated by means of light and electron (scanning and transmission) microscopes in two marine teleost fishes living in different habitats, Carangoides bajad (Carangidae) and the Epinephelus chlorostigma (Serranidae).This study revealed that, the Carangoides bajad seemed to be a microsmatic fish where the olfactory organ is poorly developed, and it can classify to the eye-nose fish where it depends on the vision in its habitat and thus possesses a well-developed eye as demonstrated by various structural features of its retina and cornea: 1) various types of photoreceptors with square mosaic pattern to increase the visual sensitivity or give acute vision, 2) well-developed pigment epithelium with tapetal spheres (tapetum lucidum), which act to increase retinal sensitivity by reflecting light back through the photoreceptor layer, 3) presence of an iridescent layer of the cornea that allows the fish to focus objects at different distances, and thereby increasing the field of vision, 4) presence of the retinomotor movement phenomenon, which reflects high sensitivity in dark and light adaptation between the photopic and scotopic states and therefore enables the fish to find their prey under light and dark conditions. Accordingly, the eye in Carangoides bajad seems to play an important role in fish habitat. In contrast, the Epinephelus chlorostigma seemed to be a macrosmatic fish where the olfactory organ is well developed, and it can classify to the nose-eye fish where it depends mainly on olfaction in its habitation and thus possesses a well-developed olfactory organ as demonstrated by various ultrastructural features of its olfactory rosette: 1) the olfactory rosette has an elongated rosette containing numerous olfactory lamellae that show behavioral responses to olfactory stimulation and thus increase the olfaction area of the olfactory organ, 2) presence of three distinct receptor cell types, i.e. ciliated, microvillous and rod cells, probably gives a wide range for food selection and/or detection of the prey or enemy location, 3) presence of a huge number of ciliated non-sensory cells, which may help in increasing the flow of water through the olfactory chamber, and accordingly increase the efficiency of the olfactory organ to acclimatize at the deep distances in water. The present results probably are the first attempt to describe the ultrastructure of the eye and the olfactory organ of the Carangoides bajad and Epinephelus chlorostigma.

A nocturnal activity pattern is central to almost all hypotheses on the adaptive origins of primates. This enduring view has been challenged in recent years on the basis of variation in the opsin genes of nocturnal primates. A correspondence between the opsin genes and activity patterns of species in Euarchonta—the superordinal group that includes the orders Primates, Dermoptera (colugos), and Scandentia (treeshrews)—could prove instructive, yet the basic biology of the dermopteran visual system is practically unknown. Here we show that the eye of the Sunda colugo (Galeopterus variegatus) lacks a tapetum lucidum and has an avascular retina, and we report on the expression and spectral sensitivity of cone photopigments. We found that Sunda colugos have intact short wavelength sensitive (S-) and long wavelength sensitive (L-) opsin genes, and that both opsins are expressed in cone photoreceptors of the retina. The inferred peak spectral sensitivities are 451 and 562 nm, respectively. In line with adaptation to nocturnal vision, cone densities are low. Surprisingly, a majority of S-cones coexpress some L-opsin. We also show that the ratio of rates of nonsynonymous to synonymous substitutions of exon 1 of the S-opsin gene is indicative of purifying selection. Taken together, our results suggest that natural selection has favored a functional S-opsin in a nocturnal lineage for at least 45 million years. Accordingly, a nocturnal activity pattern remains the most likely ancestral character state of euprimates. Electronic supplementary material The online version of this article (doi:10.1007/s11692-013-9230-y) contains supplementary material, which is available to authorized users.

In the lycosoid spiders, the secondary eyes possess a grate-shaped tapetum lucidum that reflects light, causing eyeshine when these spiders are viewed with approximately coaxial illumination. This guanine-based reflective surface is thought to increase visual capabilities in low light. We explored the eyeshine of the posterior medial eye in eight taxa of pisaurid and lycosid spiders. The taxa included four pisaurids: Dolomedes tenebrosus Hentz 1844, D. triton (Walckenaer 1837), D. scriptus Hentz 1845 and D. vittatus Walckenaer 1837; and four lycosids: Gladicosa pulchra (Keyserling 1877), Hogna sp. (cf. Lycosa lenta (Hentz 1844) sensu Wallace 1942), Rabidosa punctulata (Hentz 1844) and Varacosa avara (Keyserling 1877). We found that there were significant family- and species-level differences in both the reflected spectra and the intensity of reflection. Although the peaks of the reflected spectra were in the green range for all spiders, the mean peak was further toward the blue end of the spectrum for the lycosids than for the pisaurids. Variation among species (about 54% of the total variation) was dominated by G. pulchra (Lycosidae) and D. vittatus (Pisauridae), both of whose spectra peaked near yellow, vs. V. avara (Lycosidae), whose spectra peaked to the blue side of green. The lycosid spiders showed overall brighter eyeshine. However, when corrected for their larger eyes, the lycosid spiders' reflections were dimmer for their eye size than were those of the pisaurid spiders. These results demonstrate that the reflective qualities of the tapeta, and perhaps the absorptive qualities of other tissues and media that the light must traverse, vary widely among lycosoid spiders. This variation may signal both functional differences in visual capabilities and interesting developmental or selective histories within this clade.

Weakly electric fish orient, hunt and communicate by emitting electrical pulses, enabling them to discriminate objects, conspecifics and prey. In addition to the electrosensory modality - although dominating in importance in these fishes - other modalities, like vision, play important roles for survival. The visual system of Gnathonemus petersii, a member of the family mormyridae living in West African blackwater streams shows remarkable specializations: Cone photoreceptors are grouped in bundles within a light reflecting tapetum lucidum, while the rods are also bundled but located at the back within a light-scattering guanine layer. Such an organization does not improve light sensitivity nor does it provide high spatial resolution. Thus, the function of the grouped retinal arrangement for the visual performance of the fish remains unclear. Here we investigated the contrast sensitivity of the temporal transfer properties of the visual system of Gnathonemus. To do so, we analyzed visual evoked potentials in the optic tectum and tested the critical flicker fusion frequency in a behavioral paradigm. Results obtained in Gnathonemus are compared to results obtained with goldfish (Carassius auratus), revealing differences in the filter characteristics of their visual systems: While goldfish responds best to low frequencies, Gnathonemus responds best at higher frequencies. The visual system of goldfish shows characteristics of a low-pass filter while the visual system of Gnathonemus has characteristics of a band-pass filter. Furthermore we show that the visual system of Gnathonemus is more robust towards contrast reduction as compared to the goldfish. The grouped retina might enable Gnathonemus to see large, fast moving objects even under low contrast conditions. Due to the fact that the electric sense is a modality of limited range, it is tempting to speculate that the retinal specialization of Gnathonemus petersii might be advantageous for predator avoidance even when brightness differences are small.

The relationship between serum vitamin A level and tapetum reflection was investigated using ultraviolet imaging. The tapetum was obtained from eyes of slaughtered cattle in which the serum vitamin A level was below 50 IU/dl. The tapetum reflection was examined in the tapetum lucidum, tapetum nigrum, and optic disc. For each part, the tapetum reflection decreased with increasing serum vitamin A level. A high tapetum reflection value might represent the bleached tapetum symptom that occurs because of the low serum vitamin A level.

Summary A Contribution to the Study of Abnormal Colouring of the Eye Fundus in Horses The incidence, etiology and significance of abnormal colouring of the eye fundus in horses is reviewed and critically surveyed. Findings and conclusions are based on the clinical picture observed in 6,000 horses of different ages, histological studies, and on the literature. Lesions were found in 130 instances, mainly in eyes without clinical changes. Limited changes in colour involve either tapetum, but mainly the tapetum nigrum, and differ in shape, size, type, and shade of colour. In the region of the tapetum lucidum the colour ranges from reddish to yellowish, in the tapetum nigrum there are green and red hues. The photographs Nos. 1, 2 and 3 show an extraordinary collection of morphological and topographical variations in the recorded changes. The abnormal colouring of some areas of the fundus are sequelae of a pigment defect in the choroid in the region of the tapetum lucidum as well as of a deficiency or complete lack of pigmentation in the stratum pigmenti of the retina and choroid in the region of the tapetum nigrum. The etiology is congenital or acquired, respectively degenerative and inflammatory, and ophthalmoscopic differentiation is impossible. The limited inflammatory processes in the choroid (choroiditis disseminata) pass without iridocyclitic symptoms, hence subsequently visible changes of colour in the fundus are not considered as sequelae of periodic ophthalmia from either the clinical or the forensic viewpoint. No impediment of sight was observed. The authors recommend the clinical designation albinismus fundi partialis for colour anomalies of the fundus. Zusammenfassung Beitrag zu den Untersuchungen von abnormalen Farbungen des Augenhintergrundes bei Pferden Die Ansichten uber das Vorkommen, die Atiologie und die Bedeutung der abnormen Farbung des Augenhintergrundes bei Pferden werden kritisch revidiert. Die Beobachtungen und Folgerungen entstammen den klinischen Untersuchungen von 6000 Pferden verschiedenen Alters, den histologischen Befunden und dem Schrifttum. Farben- Veranderungen wurden in 130 Fallen festgestellt, hauptsachlich an klinisch unveranderten Augen. Begrenzte Farbenveranderungen umfassen sowohl das Tapetum nigrum als auch das Tapetum lucidum, vor allem aber das erstere. Sie unterscheiden sich von einander in Form, Grose, sowie Art und Schattierung der Farbung. Im Bereich des Tapetum lucidum sind sie vorwiegend rotlich bis gelblich, im Gebiet des Tapetum nigrum sind die Verfarbungen grunlich oder rot. Die Abb. 1, 2 und 3 zeigen eine einzigartige Sammlung morphologischer und topographischer Variabilitaten in den registrierten Veranderungen. Die abnorm gefarbten Fundusstellen sind Folgen eines Pigmentmangels der Choroidea im Bereich des Tapetum lucidum sowie der mangelhaften oder ganzlich fehlenden Pigmentation des Stratum pigmenti der Retina und der Choroidea im Bereich des Tapetum nigrum. Die Atiologie ist kongenital oder erworben, beziehungsweise degenerativ und entzundlich; eine ophtalmologische Differentiation ist nicht moglich. Begrenzte Entzundungs-prozesse in der Choroidea (chorioiditis disseminata) vergehen ohne iridocyklitische Symptome, deshalb werden spatere offensichtliche Farbenveranderungen am Fundus weder vom klinischen noch vom forensischen Standpunkt als Folgen der Mondblindheit betrachtet. Sehstorungen wurden nicht festgestellt. Die Autoren empfehlen die klinische Diagnose von Albinismus fundi partialis bei Veranderungen der Farbe im Augenhintergrund. Resume Contribution a l'etude des colorations anormales du fond de l'oeil chez le chaval Les auteurs presentent une etude critique des opinions exprimees a propos de la frequence, de l'etiologie et de l'importance des colorations anormales du fond de loeil chez le cheval. Les observations reposent sur l'examen clinique de 6,000 chevaux d'âge variable et sur des analyses histologiques. Les conclusions tiennent compte des donnees de la litterature. On constata dans 130 cas des colorations anormales, le plus souvent sans autre lesion oculaire cliniquement decelable. Ces modifications locales de la coloration concernent aussi bien le Tapetum nigrum que le Tapetum lucidum, mais surtout le premier. Elles se distinguent par leur forme, leur etendue, ainsi que par la couleur ou la nuance. Dans la region du Tapetum lucidum elles sont generalement rougeâtres ou jaunâtres; dans la region du Tapetum nigrum, elles sont verdâtres ou rouges. Les figures 1, 2 et 3 presentent une collection unique de la variabilite morphologique et topographique des anomalies enregistrees. Ces discolorations proviennent d'une depigmentation de la choroide dans la region du Tapetum lucidum ainsi que d'une depigmentation partielle ou totale de la couche pigmentaire de la retine et de la choroide dans la region du Tapetum nigrum. L'etiologie est congenitale ou acquise, degenerative ou inflammatoire. Une differenciation ophthalmologique n'est pas possible. Des foyers inflammatoires localises de la choroide (choroidite disseminee) disparaissent sans avoir provoque de symptomes d'iri-docyclite. Aussi des modifications ulterieures manifestes du fond de l'oeil ne sont pas considerees tant du point de vue clinique que du point de vue de la medecine veterinaire legale comme une consequence de la fluxion periodique. Des troubles de la vision n'ont jamais ete constates. Les auteurs proposent d'appeler du nom d'Albinismus fundi partialis les modifications localisees de la coloration du fond de l'oeil. Resumen Una contribucion al estudio de la coloracion anormal del fondo del ojo en el caballo Se revisan criticamente los puntos de vista sobre la presencia, etiologia e importancia de la coloracion anormal del fondo del ojo en caballos. Las observaciones y deducciones proceden de examenes clinicos de 6.000 caballos de diversas edades, de los estudios histologicos y de la bibliografia. Alteraciones cromaticas se comprobaron en 130 casos, sobre todo en ojos clinicamente in-alterados. Alteraciones limitadas de color abarcan tanto el tapate negro como tambien el lucido, pero ante todo el primero. Se diferencian entre si en la forma, tamano, asi como en el tipo y en el matiz. En la zona del tapete lucido preponderan las coloraciones rojizas hasta amarillentas, mientras que en la region del tapete negro son verdosas o coloradas. Los grabados 1, 2 y 3 muestran una coleccion singular de variabilidades morfologicas y topograficas en las modificaciones registradas. Las partes anormalmente coloreadas del fondo son consecuencia de una carencia pigmentaria de la coroides en la parte del tapete lucido, asi como de una pigmentacion insuficiente o totalmente ausente de la capa pigmentaria de la retina y de la coroides en la parte del tapete negro. La etiologia es congenita o adquirida, o bien degenerativa e inflamatoria; no resulta posible la diferenciacion oftalmologica. Los procesos inflamatorios limitados en la coroides (coroiditis diseminada) transcurren sin sintomas iridocicliticos, por lo cual las marcadas alteraciones posteriores del color en el fondo no se consideran como consecuencia de la oftalmia periodica ni desde el punto de vista clinico ni desde el forense o legal. No se comprobaron trastornos visuales. Los autores recomiendan el diagnostico clinico de albinismo parcial del fondo en las variaciones de color en el fondo del ojo.

Retinal topographies of some cell types and distribution of the tapetum lucidum in the sheep's eye were investigated in this study. The tapetum was observed macroscopically in the fundus. The topographical distributions of retinal ganglion cells (RGCs), cones, and rods were simultaneously analyzed in retinal whole mounts stained with cresyl violet. Short-wavelength-sensitive (S) cones were immunocytochemically identified in retinal whole mounts. The tapetum was located dorsal to the optic disc, with the nasal part elongated horizontally and the temporal part expanded dorsally. RGCs were distributed densely in the area centralis, horizontal visual streak, and anakatabatic area. The highest density in the area centralis was approximately 18,000 RGCs/mm2. Cones showed high density in the horizontal area crossing the optic disc and dorsotemporal area, whereas rods showed high density in the horizontal area, which was greater in height than the horizontal area of high cone density. S cones showed high density in the dorsotemporal retina. The rod/cone ratios were high horizontally in the dorsal retina to the optic disc, with a mean value of 11:1. The cone/RGC and rod/RGC ratios were lower in the horizontal and dorsotemporal retina, and the rod/cone/RGC ratio was lowest in the area centralis (9:1:1). The retinal topographies and distribution of the tapetum were specialized in the horizontal and dorsotemporal fundus. This suggests that sheep have better visual acuity in horizontal and anteroinferior visual fields and that this specialization is related to the visual ecology of sheep. J. Comp. Neurol. 518:2305–2315, 2010. © 2010 Wiley-Liss, Inc.

The tapetum lucidum is a light-reflective device improving visual sensitivity in mesopic vision. There have been few studies on tapetal distribution and its relationship with degree of pigmentation in the retinal pigment epithelium (RPE). In the present study, sheep's eyes were used for macroscopical observation of the tapetum, and then histological sections of the posterior eyecups were made to analyze the distribution of tapetal thickness and degree of pigmentation in the RPE. Macroscopically, with available light, the tapetum was located in the dorsal eye fundus to the optic disc and showed an L-shape with the horizontally elongated nasal part and the dorsally expanded temporal part. In photographs with a flash, the tapetal area expanded and showed a more triangular shape. The tapetum histologically consisted of layers of dense collagen fibers and was thicker in the temporal part than in the nasal part. The maximum tapetal thickness was approximately 70 microm. The histological tapetal area was similar to the tapetal shape with a flash light. The pigmentation of the RPE was divided into three types, nonpigmented, transitional, and pigmented areas. The nonpigmented area was similar to the tapetal shape with available light. It is suggested that approximately 55% of the histological tapetal area is covered with the nonpigmented area and is functional under a natural light condition. The functional tapetal area was similar to the L-shaped high-density area of retinal ganglion cells.

Studies of visual receptors typically assume that only functionally similar structures are relevant to the evolution of complex eyes. This approach ignores growing evidence that different functional classes of organs often share structural and developmental patterns that pertain to biological sameness (deep homology). However, the potential relevance of non-receptor structures to eye evolution remains largely unexplored. An “ocular” feather color mechanism is described whose structural and optical features resemble those of chambered, image-forming eyes to a remarkable degree. These similarities include a laterally expanded, domed light receiving surface similar to that of an eye, an encapsulated spongy tissue mass whose coherent light scattering properties in the human-visible (destructive) and ultraviolet (constructive) wavelength ranges resemble those of cornea and lens, intervening spaces such as those with humors, and a laminar pigmented shelf whose structure and optics resemble a mirrored tapetum lucidum found behind many retinas. Fourier analysis and optical principles indicate that ocular structures adhere to the same light-handling properties regardless of higher function (receptor or signal). The extent to which chambered eyes and ocular feathers have evolved independently is surprisingly equivocal. On the one hand, broad differences in the location, composition, and development of chambered eyes and ocular feather signals suggest convergent evolution on an ocular organization. However, some level of evolutionary parallelism (generative homology) between chambered eyes and ocular feathers is implicated by similarities in constructional materials, tissue development, and signal transduction cascades. Structural, optical, and developmental similarities also occur between more primitive eyes and the colored dermal papillae responsible for avian skin ornamentation. Functional constraints on light-handling requirements, coupled with developmental constraints in high-stress environments on the body surface, may enhance the similar evolutionary outcomes in the different functional setting. Regardless of the mechanistic details, repeated evolution of eye-like structures in different functional settings reveals a biological potential to produce such organs that is much greater than would be inferred from a survey of receptor structures alone.

Evolutionary pathways to the larval eyes of insects. Higher Dipteran stemmata and the evolutionary development of Bolwig's organ 1. A cornrehensive morphological study of the photoreceptors in the so-called hemi — Institut fur Biologie I (Zoologie) der Albert-Ludwigs-Universitat, Albertstr. 21a, D-7800 Freiburg i. Brsg., FRGand acephalic lrvae of Brachycera was undertaken. In Brachyceran larvae the head casule originally is more or less retracted into the thorax. The larval hotoreceptor in Musca and Drosophila, here called Bolwig's organ, is situated on the outer side of the cephalopharyngeal apparatus, well below the surface. The aim of this study is to elucidate the homology of Bolwig's organ, i. e. whether Bolwig's organ originated from typical stemmata during the evolution of Cyclor-rhaphan larval organlsation or whether it represents a unique type of hotoreceptor. Larval photoreceptors and the anatomy of the head capsules of representative Erachycerans were examined by means of light and electron microscopy. In particular, the site and the structure of the photofecetors of Stratiomys (Stratiomyidae, Solva (Solvidae), Atherix ibis (Athericidae), Rhagio (Rhagionidae), Thereva (derevidae), Lonchoptera (Lonchoteridae), Epistrohe balteata, Volu-cella bombylans, Eristalis tenax (Syrphidae), Drosophila (f;rosophilidae), jannia and Musca domestica (Muscidae) were investigated. 2. Brachyceran stemmata are either situated on the ocular plates of the free remnant of the head (Stratiomyidae, Therevidae), or on the inner wall of an epidermal invaination, which includes epithelia of the head as well as of the thorax (Tabaniformia). In adfition, they can be found on the outer side of the tentorial phragmata (Cyclorrhapha). In all taxa studied, stemmata keep contact directly with the epithelium or through short processes of eithelial cells. 3. Ancestral Brachycera have fused stemmata that correspond with typical larval eyes of other holometabolan insects. 4. A cornea is found solely in Stratiomyidae and Xylophagidae, in contrast to all other taxa, where no dioptric apparatus is found. 5. In “Orthorrapha” and in “Aschiza”, rhabdomeric photoreceptors occur, forming a fused, star-shaped or Ktticed rhabdom. In Schizophoran larvae, retinular cells are distinguished by a different type of surface enlargement of the photoreceptor cell membrane. Here, the membrane forms flat ramella. The latter originate from transformed and/or rudimentary rhabdomeric microvilli. 6. In the primitive “Orthorrhapha”, pigment cups can be found, that are composed of retinular pirnent, whereas in derived Brachycera, pigment grains are absent. In Cyclorrhapha, tentorial piramata and their optic deressions operate as external and functional piment cups 7. In Therevidae and in Syrphidae, a tracheal tapetum lucidum can be founfi which la, been evolved independently in both families. The tapetum is always found together with inversely orientated retinular cells. 8. The homology of Brachceran stemmata is shown by a transformation series of stem-mata's site and fine structure. The homology is corroborated by the identical innervation in primitive and derived taxa, the identical site within Cyclorrhaha, and structural similarities of “Orthorrhaphan” and primitive Cylorrhaphan stemmata. Although transformations of the fine structure appear as a sequence or reductions, particular adaptations such as multilication of retinular celfs (Epistrophe), or deepening of the pigment cup (Musca) have evolved separately. Despite the fact that in Brachyceran larvae the head has been reorganized, the eyes still keep contact with the epithelium of the frontal sacks and/or tentorial phragmata. 9. The transformation series given shows that Bolwig's organ represents a highly modified stemma.

To gain knowledge of visual specializations influencing the behavior of white-tailed deer ( Odocoileus virginianus (Zimmermann, 1780)), we examined gross eye characteristics, structural organization of the retina, and the density and distribution of cone photoreceptors. White-tailed deer possess ocular features similar to other ungulates including a horizontal slit pupil, reflective tapetum lucidum, typical retinal structure, and medium wavelength sensitive cone photoreceptors concentrated in a horizontal visual streak. The tapetum was found to cover the superior portion of the eye and overlapped the horizontal visual streak. Comparisons between fawns and adults did not reveal any differences in retinal thickness, retinal nuclei counts, or cone photoreceptor counts. While M-cones had increased density in the visual streak, S-cones were distributed evenly across the entire retina. Schematic eye calculations of a 0.5-year-old deer indicated a hyperopic eye (+7.96) with a F/# ranging from 5.55 to 1.39 for pupil diameters of 3 to 12 mm. As expected for a crepuscularly active prey species, the visual system of white-tailed deer is specialized for sensitivity in low-light conditions and detection of predators.

Assessing the impact of human development on animals is complicated by the fact that overt effects may have covert causes. Cryptic impacts (sensu Raiter et al. 2014) can arise where sensory stimuli to which species respond fall outside the human sensory range. Ultraviolet (UV) light, which is detected by a range of nonprimate mammals, is a potential example. We review evidence that dark-adapted eyes of reindeer–caribou Rangifer tarandus can detect light at 330–410 nm emitted by electrical corona on high-voltage power lines, which is necessarily barely visible to humans. Based on this, we suggest that the superior ability of Rangifer to detect corona UV light may partly account for the tendency of the animals to avoid power lines. Rangifer has UV-permissive ocular media that transmit approximately 15 times more corona light than human eyes. Retinal irradiance under full dilation is in the order of 7 times greater in Rangifer compared with humans. Seasonal transformation of the tapetum lucidum substantially increases retinal sensitivity in this species in winter. Threshold distances of detection of corona by Rangifer are in the order of hundreds of meters. Displays of corona may catch the animals' attention, and plume coronas, in particular, may induce the illusion of motion (the phi phenomenon), thereby falsely signaling the presence of potential predators. Both features are likely to increase wariness and cause animals to withdraw from the source of the stimulus. We suggest that spatial and temporal variability of corona contributes to substantial variation observed in the strength and persistence of avoidance responses in Rangifer at these structures. © 2016 The Authors. Wildlife Society Bulletin published by Wiley Periodicals, Inc. on behalf of The Wildlife Society.

The Australian lungfish Neoceratodus forsteri (Dipnoi) is an ancient fish that has a unique phylogenetic relationship among the basal Sarcopterygii. Here we examine the ultrastructure, histochemistry, and distribution of the retinal photoreceptors using a combination of light and electron microscopy in order to determine the characteristics of the photoreceptor layer in this living fossil. Similar proportions of rods (53%) and cones (47%) reveal that N. forsteri optimizes both scotopic and photopic sensitivity according to its visual demands. Scotopic sensitivity is optimized by a tapetum lucidum and extremely large rods (18.62 +/- 2.68 microm ellipsoid diameter). Photopic sensitivity is optimized with a theoretical spatial resolving power of 3.28 +/- 0.66 cycles degree(-1), which is based on the spacing of at least three different cone types: a red cone containing a red oil droplet, a yellow cone containing a yellow ellipsoidal pigment, and a colorless cone containing multiple clear oil droplets. Topographic analysis reveals a heterogeneous distribution of all photoreceptor types, with peak cone densities predominantly found in temporal retina (6,020 rods mm(-2), 4,670 red cones mm(-2), 900 yellow cones mm(-2), and 320 colorless cones mm(-2)), but ontogenetic changes in distribution are revealed. Spatial resolving power and the diameter of all photoreceptor types (except yellow cones) increases linearly with growth. The presence of at least three morphological types of cones provides the potential for color vision, which could play a role in the clearer waters of its freshwater environment.

The present work attempted to study the importance of vision in Centroscymnuscoelolepis , the most abundant shark in the Mediterranean beyond a depthof 1000 m, by using anatomical and histological data. C.coelolepis exhibited large lateral eyes with a large pupil, spherical lens and a tapetum lucidum that gave the eye a strong greenish-golden “eye shine”. In the outer retinal layer, a uniform population of rod-like photoreceptors was observed while in the vitreal retina a thick inner plexiform layer comprised up to 30% of the whole retinal thickness. The cell distribution of the ganglion cell layer formed a thin elongated visual streak in the central plane of the eye that provided a horizontal panoramic field of view. A specialised area of higher visual acuity was located caudally at 32-44o from the geometric centre of the retina and 5-10o above the horizontal plane of the eye. This position indicated that the visual axis pointed in a slightly outward-forward direction with respect to the fish body axis. A non-uniform distribution of large ganglion cells was also found in the horizontal plane of the retina that practically coincided with the distribution of the total cell population in the ganglion cell layer. This is the first time that this type of retinal specialisation has been observed in the elasmobranchs. These characteristics indicate that the retina of C.coelolepis is designed not only to increase sensitivity in the horizontal field of view, as was also observed in other sharks, but also to improve motion detection in the same plane. The visual capacities evolved by C.coelolepis make this species adapted for discriminating the horizontal gradation of light that exists in the mesopelagic environment. Similarly, the large ganglion cell distribution observed in its retina seems to be related to its predatory behaviour, since it allows this shark to perceive the movement of bioluminescent prey against a totally dark background.

Morphology of the photoreceptor cells and tapetum of the Japanese anchovy Engraulis japonicus was studied by histologically and by chemical analysis. The Japanese anchovy has a duplex retina. The cones form parallel rows consisting of alternately placed long cones and bifid cones. Both types of cones are intimately associated and form triple units that are regularly spaced along the row of cones. The rods are grouped and stacked. This fish has a retinal tapetum lucidum composed of guanine and hypoxanthine. Three structures of the tapetum lucidum were recognized: platelet, diamond and rod types. Photomechanical changes include movements of the photoreceptor cells and the retinal tapetum. The retina of the Japanese anchovy is thought to be highly sensitive and well adapted to a dim light environment.

A study of the morphogenesis of the grenadier anchovy retina was undertaken using light and electron microscopy. Five developmental stages from prelarvae 3 days after fertilization to adult fish were studied. In addition to the general morphology of the eye and retina, special emphasis was given to the development of the photoreceptors and pigment epithelium (PE). The earliest retinae showing structural features indicative of a functioning eye are pure cone retinae composed of rows of alternating long and short cones forming a transient, tesselated pattern. At this stage there is a conventional PE containing melanin. In older stages cone rows are separated by the newly formed rods and by PE wedges filled with diffusely reflecting guanine crystallites. The findings are compared with the retinae of other engraulidids and with the development of teleost retinae in general. Moreover, the observed structural changes are discussed with respect to the photic habitat conditions of these anadromous fish that move between coastal waters, estuary, and river. J. Morphol. 248:41–55, 2001. © 2001 Wiley-Liss, Inc.

The tapetum surrounds the developing sporogenous tissue, and therefore is suggested to have an important role during sporogerlesis. In pteridophytes details about tapetum development and function during sporangium development are poorly known. The current article analyses the leptospor-angiate development of the pteridophyte Cyrtomium falcatum in terms of functional compartmentalization related to tapetum function. Morphology, development and localization of different compounds during sporangium development are studied. Dur ing C. falcatum leptosporangiate development phases of initia tion, division, differentiation, and maturation are identified. Functional compartments are distinguished by the presence of a callose culture sac during the division and differentiation phase, and a callose spore sac during the differentiation and ma turation phase.

The tapetum lucidum is a light-reflective tissue in the eyes of many animals. Many ungulates have a fibrous tapetum. The horse has one of the largest eyes of any living animal and also has excellent vision in low-light environments. This study aimed to clarify the macroscopic tapetal shape, relationship between the tapetal thickness and the degree of pigmentation of the retinal pigment epithelium (RPE), spatial relationship between the visual streak and the tapetum, and wavelength of the light reflected from the tapetum in the horse. Macroscopically, weak light revealed the tapetum as a horizontal band located dorsal to and away from the optic disc. The tapetum expanded dorsally as the illumination increased. The tapetal tissue consisted of lamellae of collagen fibrils running parallel to the retinal surface; these spread over almost the entire ocular fundus and were thicker in the horizontal band dorsal to the disc. Only the horizontal band of the tapetum was covered by unpigmented RPE, suggesting that this band reflects light and is responsible for mesopic and scotopic vision. The visual streak was located in the ventral part of the horizontal band, ventral to the thickest part of the tapetum. The wavelength of the light reflected from the horizontal band of the tapetum was estimated from the diameter and interfibrous distance of the collagen fibrils to be approximately 468 nm. Therefore, the light reflected from the tapetum should be more effectively absorbed by rods than by cones, and should not interfere with photopic vision.

The fine structure of the four ocelli ofAnoplodactylus petiolatus was examined using serial longitudinal and transversal sections of the eye hill. Each pigment cup ocellus is composed of a (planconvex) cuticular lens, lens forming hypodermal cells, inverse retinula cells with latticed rhabdom and surrounding tapetum and pigment layers. Within the retinula cells a distal “vitreous” zone, a nucleus zone and a proximal rhabdomeric zone can be distinguished. Retinula cell axons originate proximally. The tapetum cells contain several layers of reflecting crystals. Distally, they have a common microvillous region. The intraretinal “vitreous” zone contains glycogen-like particles in the centre and rough ER in the periphery. Contrary to other Pantopoda vitreous cells, a praeretinal membrane and a vertical lens groove have not been observed inAnoplodactylus. While the presence of four (median) ocelli appears to be a primitive characteristic, the inverse orientation of the retinula cells in combination with a tapetum lucidum represents a highly derived characteristic among arthropod median eyes.

A new explanation for the origin of the primate suborder Anthropoidea is presented. Functional analyses of the "forward"-facing orbits, postorbital septum and retinal fovea are used to reconstruct the morphological and ecological contexts in which these features are most likely to have evolved. The postorbital septum is argued to have evolved as an adaptation to protect the orbital contents from encroaching fibers of anterior temporalis. This encroachment resulted from increasing convergence and frontation of the orbital margins in a lineage of small-bodied animals with relatively large eyes. Increasing orbital convergence is hypothesized to have resulted from reduction in relative orbit diameter associated with a shift to diurnality at small body size (

Objective To describe the gross, histopathological, and ultrastructural findings in a dog with bilateral tapetal dysplasia. Procedures The globes of a 15-year-old neutered male Swedish Vallhund dog with a ventrally displaced tapetum in both eyes were fixed in 10% formalin and submitted to the Comparative Ocular Pathology Laboratory of Wisconsin for histological evaluation. Sections were stained with hematoxylin and eosin, Masson's trichrome, and Melan-A immunohistochemistry (IHC), and tissues were subsequently processed for transmission electron microscopy. Results Bilateral fundic and gross examination revealed a tapetal fundus inferior to the optic nerve head (ONH) and a nontapetal fundus with mild scattering of tapetal tissue superior to the ONH. Histologically, there was decreased pigmentation of the retinal pigment epithelium with only a few melanin granules in the peripheral retina. The affected tapetum was relatively acellular and fibrous with occasional tapetal cells scattered throughout the inner choroid or displaced into the vascular outer choroid. Special stains revealed that the tapetum was mostly composed of collagen (Masson's trichrome) and failed to express Melan-A (IHC) unlike a normal canine control tapetum. Ultrastructurally, the tapetum was markedly dysplastic both superior and inferior to the ONH with no uniformly arranged tapetal cells. The few cells identified within the tapetum contained irregularly arranged and disorganized electron-dense structures within their cytoplasm, which were interpreted as dysplastic tapetal rodlets. Conclusions Based on microscopic and ultrastructural findings, this is the first report of tapetal dysplasia in a dog.

The ultrastructure and chemical composition of reflective organelles in the anterior pigment epithelium of the iris of the European starling Sturnus vulgaris were examined. The reflective organelles produced a diffuse white reflectance at the iris mid-section which was visible only when the stroma was removed. The pigment granules were clear, angular, and birefringent under the light microscope. In electron micrographs the granules were irregular in shape and density, sometimes crystalline in appearance, but more often they were lost during sectioning or staining. Guanine was abundant in the modified pigment epithelium of the starling, but not in the pigment epithelia of other birds that lacked birefringent granules. Pteridines, such as xanthopterin and leucopterin, were present in small amounts. Pteridines were also present in the iris stroma which had no reflective organelles. The reflective organelles in the starling pigment epithelium resemble both the reflecting platelets of lower vertebrate chromatophores and the reflective granules in the tapeta of various vertebrates. Possible derivation of the organelles from these sources is discussed.

The structure of the retina, the distribution of ganglion cells, and the extent of the tapetum lucidum were studied in Dall's porpoise (Phocoenoides dalli) with the aim of understanding the role vision plays in this species of cetacean. The basic organization of the retina was similar to that of other vertebrates. Average ganglion cell size was 21.5 μm. The distribution of the ganglion cells in the retina was not even and there were two high-density areas, one in the temporal and one in the rostral part of the retina. Retinal resolving power was estimated using a terrestrial animal model incorporating the density of ganglion cells and other morphological data. The resolving power in the right eyes of two individuals was 2.60 and 2.64 cycles per degree. These values were close to those of other oceanic cetaceans, but inferior to those of terrestrial mammals. On the basis of amino acid analyses, it was shown that the choroid tapetum lucidum in Dali's porpoise contained collagen. The tapetum lucidum was thick at the fundal but thin at the peripheral part of the choroid.

The colored oil droplets in the retina of 30 avian species with different ecological habits and different taxonomic positions were examined by microspectrophotometry and histology, and their role or significance was conjectured. In diurnal birds such as the Night Heron, Jungle Crow, and Japanese Lesser Sparrow Hawk, four kinds of colored oil droplets, red, yellow, orange, and pale green, were discriminated ; whereas in nocturnal birds, only pale green oil droplets were found. Cluster analysis of the distribution of droplets showed that the color of the oil droplets in the 13 species of wild birds correlates with their ecological behavior. The absorption spectra of the red, yellow, and orange oil droplets displayed only minor interspecific differences. The spectra of the pale green droplets differed significantly between the Eastern House Swallow and the Japanese Jungle Nightjar. In the King Penguin, the retina contained only pale green droplets, whose spectrum was completely different from that of other diurnal birds. The upper half of the retina of the Japanese Jungle Nightjar was lined characteristically with the tapetum lucidum of nocturnal birds ; and the under half, with the diurnal birds' pigmented epithelium. Our findings suggest that each colored oil droplet has specific advantage for visual perception and that each bird has the retina that best matches its natural environment and feeding behavior in each habitat.

The tapetum lucidum of the paca or spotted cavy (Cuniculus paca) has been studied by light and electron microscopy. The reflective layer in this species is a tapetum cellulosum situated in the choroid of the superior fundus. Posteriorly, the tapetum is composed of several layers of ovoid to flattened cells while peripherally the tapetum gradually thins down and disappears. Within the tapetal cells most cell organelles are located near the nucleus. The dominant feature of these cells is, however, a large accumulation of electron-dense rodlets which show a variety of sizes, shapes and orientations. These rodlets are the reflective material of the tapetum, and energy dispersive studies indicate that they are rich in sulphur. The diameter and spacing of these irregular rodlets is too varied to be consistent with the principles of constructive interference, and the tapetum in the paca is probably capable of only diffuse reflectance. The retinal epithelium over the tapetum is nonpigmented while in nontapetal locations it is normally pigmented.

SUMMARY Elasmobranch fishes utilise their vision as an important source of sensory information, and a range of visual adaptations have been shown to reflect the ecological diversity of this vertebrate group. This study investigates the hypotheses that visual optics can predict differences in habitat and behaviour and that visual optics change with ontogenetic growth of the eye to maintain optical performance. The study examines eye structure, pupillary movement,transmission properties of the ocular media, focal properties of the lens,tapetum structure and variations in optical performance with ontogenetic growth in two elasmobranch species: the carcharhinid sandbar shark,Carcharhinus plumbeus, inhabiting nearshore coastal waters, and the squalid shortspine spurdog, Squalus mitsukurii, inhabiting deeper waters of the continental shelf and slope. The optical properties appear to be well tuned for the visual needs of each species. Eyes continue to grow throughout life,resulting in an ontogenetic shift in the focal ratio of the eye. The eyes of C. plumbeus are optimised for vision under variable light conditions, which change during development as the animal probes new light environments in its search for food and mates. By contrast, the eyes of S. mitsukurii are specifically adapted to enhance retinal illumination within a dim light environment, and the detection of bioluminescent prey may be optimised with the use of lenticular short-wavelength-absorbing filters. Our findings suggest that the light environment strongly influences optical features in this class of vertebrates and that optical properties of the eye may be useful predictors of habitat and behaviour for lesser-known species of this vertebrate group.

We report for the first time the existence of a structural mechanism of feathers different from iridescence that makes plumage conspicuous. By using electron and light microscopy, we show that the mechanism consists of special lengthened and twisted distal barbules that are very susceptible to damage. The dorsal side of these barbules is translucent, which creates a distinctive sheen colouration to feathers that otherwise would be dark. When distal sheen barbules are broken, the black proximal barbules are exposed, thus generating a conspicuous difference between abraded and non-abraded areas. Total and ultraviolet reflectance of sheen (non-abraded) areas are strikingly higher than in abraded areas. We propose that this mechanism represents a case of convergent evolution in species that are limited in developing colourful plumage patches. Future studies should explore the potential of this colour mechanism to act as a signal of individual quality or identity., Financial support for I. G. was obtained from the project CGL2007-61251 and a FPI grant from the Spanish Ministry of Science and Innovation (formerly Ministry of Education and Science). J. J. N. was supported by project CGL2006-07481 on the mechanisms of colour evolution and funded by the Spanish Ministry of Science and Innovation. G. R. B. was supported by a grant from the Natural Sciences and Engineering Research Council of Canada, and the Stuart and Mary Houston Prof. in Ornithology. A. M. was supported by Ministerio de Medio Ambiente, Medio Rural y Marino and Departament de Medi Ambient i Habitatge of Generalitat de Catalunya.

We conducted laboratory experiments to determine relationships between water turbidity, piscivory, fork length, and development of the retina in juvenile walleyes Stizostedion vitreum. Walleyes 85 mm and longer ate a greater weight and number of fathead minnows Pimephales promelas in turbid water than in clear water. Feeding was inhibited at the highest turbidities (100 and 161 nephelometric turbidity units, NTU) in 1-h feeding trials but not at the highest turbidity (121 NTU) in 4-h trials. In contrast, walleyes in the shorter length-group (≤75 mm) fed at approximately the same rate in clear and turbid trials, but consumption was inhibited at the two highest turbidities (100 and 161 NTU) used in 1-h trials. The tapetum lucidum was present in the retina of walleyes used in all the experiments. Aggregation of photoreceptor cells in the retina into groups of 20 to 30 (to form macroreceptors) begins when walleyes are approximately 60 mm long. Macroreceptors are believed to increase acuity in dim lig...

The superposition eyes and simple eyes of many arthropods have apertures (A) with a diameter bigger than, or about the same size as, the focal length of the eye (f). That is, these eyes have low F-numbers (f/A). Many of the light rays focussed onto a photoreceptor will not be trapped by total internal reflection in the photoreceptor and will therefore pass through and be absorbed in photoreceptors other than that for which the light was intended. This spread of light in the retina leads to a broadening of the angular-sensitivity function and a consequent degrading of the image at the retinal level. A number of solutions to this problem are found in nature, with the most effective that of isolating the photoreceptors with a sheath of either light-absorbing pigment or reflecting tapetum. A ray-tracing model was used to assess the relative merits of the tapetal and pigment sheath designs in low F-number superposition eyes, and also to investigate the effect of changing the refractive index and absorption coefficient of the rhabdom. Which sheathing solution is best depends on the quality of the image on the retina, on the spacing of the rhabdoms in the retina and on the intensity of light normally experienced by the eye. In a retina with closely packed rhabdoms, the model predicts full sheathing to be the optimal solution if the image is well-focussed, partial sheathing if poorly focussed and no sheathing if moderately well-focussed. In a retina with rhabdoms spaced apart and a well-focussed image, the model predicts partial sheathing to be optimal. A pigment sheath is predicted to be useful in eyes which experience bright light and have no need for high sensitivity. A tapetal sheath is predicted to be useful at any intensity. A survey of arthropod eyes with low F-number supports the predictions of the model.

The African weakly electric fish Gnathonemus petersii is well known for its electrosensory capabilities. These animals can detect and distinguish objects through active electrolocation in complete darkness. Because of their nocturnal lifestyle, a low contribution of vision for orientation and object detection has been expected. However, as we show in this review, the retina of G. petersii is highly specialized with hundreds of rods and tens of cones grouped together in bundles in a complex way, ensheathed by a tapetum lucidum. The structure of the bundles goes beyond what would be expected if only photon catch was supposed to be increased. During daytime, the structure of these "macro-receptors" changes dramatically depending on retinomotor movements. During the day, the rods and cones are located in different compartments of the bundle, separated by a narrow canal in the form of a "bottle neck". Investigations on cell structure and neurochemistry in the retina indicate a general organization that is simpler in terms of bipolar and ganglion cell diversity than in tetrachromatic species such as goldfish, yet similar in terms of neurochemical differentiation of amacrine cells. In both respects, the inner retina of the elephantnose fish bears the greatest similarity to catfish and some deep-sea fish retinae. Neuronal circuits and bundle structure give hints of possible adaptations for contrast and/or movement detection. Behavioral experiments suggest that, in contrast to the vision specialists Lepomis gibbosus, pattern detection of G. petersii is not affected by higher spatial frequencies. A pattern of low spatial frequencies, however, was equally well detected by G. petersii and L. gibbosus. Optomotor response experiments indicate that motion vision is important for Gnathonemus, narrowing down the search for the functional specialization of the Gnathonemus retina and providing a starting point for work on multisensory integration in these fish.

Summary The morphology of the tapetum lucidum of the domestic cat (Felis catus) was studied by electron microscopy. The tapetum is a tapetum cellulosum, localized in the choroid of the dorsal fundus. The cells comprising this structure are described.

The morphology of the retinal epithelium (RPE) and closely associated choriocapillaris and Bruch's membrane (complexus basalis) has been investigated in the eye of the domestic cat (felis catus) by light and electron microscopy. The RPE consists of a single layer of cuboidal cells joined laterally by apically-located junctional complexes. Basally (sclerally) these cells display numerous infoldings while apically (vitreally) two distinct types of processes enclose photoreceptor outer segments. Internally the large vesicular nucleus is centrally located. Smooth endoplasmic reticulum, mitochondria, polysomes and lysosome-like bodies are abundant. Rough endoplasmic reticulum while present is not plentiful. Over the tapetum lucidum, melanosomes are absent from the RPE cells while in non-tapetal locations melanosomes are large and plentiful. The endothelium of the choriocapillaris is highly fenestrated and these capillaries are deeply indented into the epithelial layer over the tapetum. Bruch's membrane (complexus basalis) is non-tapetal regions is the typical pentalaminate structure noted for most mammals. Over the tapetum however it is reduced to a trilaminate structure and when associated with the indented capillary profiles is further reduced to a single thickened basal lamina.

The ocular tapetum lucidum has evolved in the elasmobranchs as a means of controlling photon capture by the retina. Tapeta may be of two kinds, fixed and modifiable. In the latter, light reflection from the tapetum is modulated by the movement of screening pigment. In this paper I review past studies of the mechanisms by which such pigment movements are controlled. Evidence is presented that indicates that control is located within the eye itself and is most probably a function of the retina. I also suggest that a complete understanding of ocular tapetum lucidum structure and function awaits the results of studies by workers in various fields of elasmobranch biology, making this subject ideal for an interdisciplinary approach.

Has this happened to you? In the darkness, on a lonely country road, your headlights play across a pair of bright green, almost iridescent, spots of light; they seem to float eerily across the road without support. These furtive spots seem to move and blink, clearly alive. Eyeshine. It is all you see of the creature, as the rest of the body seems to disappear into the darkness that surrounds it. But, what exactly is eyeshine, and why did it evolve? The tapetum lucidum (Latin, carpet shining) is a reflective structure found in the eyes of many diverse creatures and represents convergent ocular evolution solely for maximising photon capture. Surprisingly, the techniques for the production of these reflective mechanisms are variable, and much like the crystalline lens, seem to be drawn from whatever materials the evolutionary process found at hand. Guanine crystals, for example, provide biological reflection and support camouflage by making a fish glisten and gleam, and hence appear invisible or at least present confusing reflections to a predator. Although we can never know for sure, this coating probably appeared very …

As previously reported in the literature, the electroretinogram (ERG) of the Beagle dog includes a large post b-wave negativity, the origin of which is not yet established. In the course of our investigations on the electroretinogram in dogs, we examined two Beagle dogs (2 years apart) who had one eye devoid of a Tapetum Lucidum (TL). Photopic (cone-mediated) and scotopic (rod-mediated) ERGs were obtained according to the guidelines for clinical electroretinography in dog. In both dogs the short-latency ERG components (i.e. a- and b-waves) were found to be within the normal range in amplitude, peak time and morphology O.U. However, the large negative component that, in Beagle dogs, normally follow the b-wave was absent from the photopic as well as the scotopic signals obtained from the TL-free eye. Our results thus suggest a possible contribution of the TL to the ERG of Beagle dogs.

Objectives The phenomenon of ‘eye-shine’ is seen in a variety of animal species, and is generally thought to be related to the presence of an intraocular reflecting structure, the tapetum lucidum. The tapetum lucidum is a biologic reflector system that is a common feature in the eyes of vertebrates. It normally functions to provide the light-sensitive retinal cells with a second opportunity for photon-photoreceptor stimulation, thereby enhancing visual sensitivity at low light levels. The tapetum lucidum is presented here according to a classification based on the location, as well as the composition, of this reflective layer. Finally, the physical and chemical properties, as well as the origins of the different tapeta lucida, are discussed and compared. Methods The anatomic and biochemical aspects of the tapetum lucidum in various vertebrates are examined. Morphologic observations were made from paraffin and plastic embedded specimens. Specimens were treated with traditional stains and observed by light and transmission electron microscopy. Results Some species (primates, squirrels, birds, red kangaroo and pig) do not have this structure and they usually are diurnal animals. In vertebrates, the tapetum lucidum exhibits diverse structure, organization and composition. Therefore, the retinal tapetum (teleosts, crocodilians, marsupials, fruit bat), the choroidal guanine tapetum (elasmobranchs), the choroidal tapetum cellulosum (carnivores, rodents, cetacea), and the choroidal tapetum fibrosum (cow, sheep, goat, horse) are described. Conclusions The tapetum lucidum represents a remarkable example of neural cell and tissue specialization as an adaptation to a dim light environment and, despite these differences, all tapetal variants act to increase retinal sensitivity by reflecting light back through the photoreceptor layer. These variations regarding both its location and structure, as well as the choice of reflective material, may represent selective visual adaptations associated with their feeding behavior, in response to the use of specific wavelengths and amount of reflectance required.

The tapetum is a specialized cellular layer in the posterior ocular segment located between the choriocapillaris and the pigment epithelium of the retina. The tapetal layer is located only in the superior aspect of the fundus. The tapetum may be present in all domestic animals except the pig. The tapetal layer in the the dog and cat is called the tapetum cellulosum and in ungulates and the horse the tapetum fibrosum. The tapetum may be absent from some animals especially color dilute dogs or animals with incomplete melanin pigment formation such as the Siamese cat. The tapetum lucidum is composed of 5–10 layers of non pigmented reflective cells that containa high content of zinc. The zinc content of the tapetum in the dog has been determined to be 64.9 +/- 18.4 mg/g tissue.4,5,12

To assess the potential adverse effects in people of the antipsychotic agent 1192U90, we dosed mice, rats, beagles, and cynomolgus monkeys for up to 3 mo. In dogs, but not the other species, 1192U90 caused ocular changes detectable ophthalmoscopically as loss of tapetal reflectivity, altered tapetal color, and the appearance of black pigmentation on the tapetal fundus. Eyes from affected dogs had atrophic tapeta lucidum due to cell loss. Rodlets in remaining tapetal cells were separated by electron-lucent spaces or finely granular material, varied in size and shape, and often contained irregularly shaped electron-dense inclusions. Nontapetal ocular structures were unaffected. Because 1192U90 caused no ocular changes in nontapetal species, we hypothesized that it targeted only tapetum lucidum and spared other ocular structures. We tried to test this hypothesis by dosing congenitally atapetal dogs; however, although these dogs were ophthalmoscopically "atapetal," they had scattered tapetal cells visible by electron microscopy, and these tapetal cells had ultrastructural changes indistinguishable from those that occurred in treated normal-eyed dogs. Tapetal degeneration caused by 1192U90 resembled that described in hereditary tapetal degeneration in beagles. That 1192U90 caused no ocular changes in nontapetal species suggests that the ocular changes in dogs do not imply a risk for humans, whose eyes also lack a tapetum lucidum.

The structure of the retinal epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) has been studied by light and electron microscopy in the Port Jackson shark (Heterodontus phillipi). In this elasmobranch the RPE consists of a single layer of low cuboidal cells which show basal (scleral) infoldings and apical (vitreal) processes that enclose photoreceptor outer segments. Laterally these epithelial cells are joined by a series of apically located tight junctions. The RPE cells display a large vesicular nucleus, abundant smooth endoplasmic reticulum as well as numerous polysomes and mitochondria. Phagosomes are present, rough endoplasmic reticulum is scarce and myeloid bodies were not observed. Melanosomes are absent over the choroidally located tapetum lucidum, but are not abundant even in extratapetal areas. This paucity of melanosomes probably makes retinomotor movements unimportant. Bruch's membrane or complexus basalis is a pentalaminate structure. The endothelium of the choriocapillaris is thin but minimally fenestrated.

The tapetum lucidum of the short-tailed stingray (Dasatis brevicaudata) is located in the choroid of the superior fundus immediately external to the choriocapillaris. In this species the tapetum consists of a single layer of overlapping cells oriented at an angle of about 30" to the incoming light. These tapetal cells alternate with and are separated from one another by melanocytes which extend beyond the tapetal cells to intervene between the tapetal cells and the choriocapillaris. The tapetal cells and the melanocytes are flattened plate-like cells with their widest dimension facing the retina. Internally the tapetal cells display a peripherally located vesicular nucleus with most organelles in a paranuclear location. The bulk of the cell is packed with regularly spaced crystals reported to be guanine. The size and spacing of these reflective crystals is commensurate with constructive interference. In lightadaptation, the melanosomes of the intervening melanocytes are widely dispersed and for the most part block the passage of light to the tapetal cells. Although dark-adapted specimens were not examined, it seems reasonable to assume that in dark-adaptation, the melanosomes will retreat to unmask the tapetum and allow it to function as a reflective layer.

Tapetal Effect of an Azalide Antibiotic Following Oral Administration in Beagle Dogs. Fortner, J. H., Milisen, W. B., Lundeen, G. R., Jakowski, A. B., and Marsh, P.M. (1993). Fundam. Appl. Toxicol. 21, 164-173. An azalide antibiotic (CP-62,993) was administered at 100 mg/kg by oral gavage once daily for 35 consecutive days to 3 normal Beagle dogs (tapetal) and 3 Beagle dogs lacking a clinically apparent ocular tapetum (atapetal). The total dose delivered was approximately 100-fold the recommended clinical dose. Bilateral ophthalmoscopic changes were observed in the treated tapetal dogs on Day 36, consisting of mild to moderate tapetal decoloration with loss of the normal color change at the junction with the nontapetal fundus and muting of reflectivity of the normally highly reflective tapetum: treated atapetal and all control tapetal and atapetal dogs had no ophthalmoscopic changes. Microscopic examination of ocular tissue revealed rudimentary tapetal cell layers in the correct location in untreated, clinically atapetal eyes. Tapetal cells from treated tapetal and atapetal dogs were swollen and vacuolated, and contained intracytoplasmic, electron-dense debris but no recognizable tapetal rodlets. Lysosomal lamellar bodies were observed in the retinal ganglion cells of both treated groups and were neither enhanced nor reduced by the presence of a functional tapetum. Necrosis and inflammation were not observed in any ocular tissue. The altered ophthalmoscopic appearance of treated tapetal dogs was not influenced by the retinal changes because any effect on retinal transparency would have been seen in treated atapetal dogs. The decoloration and muting of reflectivity observed clinically in the tapetal fundus of dogs following prolonged exposure to high levels of CP-62,993 result from unique changes within the ocular tapetum itself and cannot be interpreted to be of consequence to nontapetal species including humans.

Homeotherms are generally considered to lack classical active dermal pigment cells (chromatophores) in their integument, attributable to the development of an outer covering coat of hair or feathers. However, bright colored dermal pigment cells, comparable to chromatophores of lower vertebrates, are found in the irides of many birds. We propose that, because of its exposed location, the iris is an area in which color from pigment cells has sustained a selective advantage and appears to have evolved independently of the general integument. In birds, the iris appears to have retained the potential for the complete expression of all dermal chromatophore types. Differences in cell morphology and the presence of unusual pigments in birds are suggested to be the result of evolutionary changes that followed the divergence of birds from reptiles. By comparison, mammals appear to have lost the potential for producing iridophores, xanthophores, or erythrophores comparable to those of lower vertebrates, even though some species possess brightly colored irides. It is proposed that at least one species of mammal (the domestic cat) has recruited a novel iridial reflecting pigment organelle originally developed in the choroidal tapetum lucidum. The potential presence of classical chromatophores in mammals remains open, as few species with bright irides have been examined.

The choroidally located tapetum lucidum of the southern fiddler ray (Trygonorhina fasciata) has been examined by light and electron microscopy in both light- and dark-adaptation. In this species. the tapetum consists of a single layer of overlapping cells oriented at an angle of about 30" to the incoming light. These are situated immediately external to the choriocapillaris. These tapetal cells alternate with and are separated from one another by melanocytes which have an inner extension that curves and intervenes between the tapetal cells and the choriocapillaris. The tapetal cells and the melanocytes are flattened cells with their widest dimension facing the retina. Internally the tapetal cells display a peripherally-located, vesicular nucleus with most of the cell organelles in a paranuclear location. The bulk of the cell is packed with regularly-spaced crystals reported to be guanine. The size and spacing of these reflective crystals is commensurate with constructive interference. In light-adaptation the small melanosomes of the melanocytes are widely dispersed and fill the portion of the cell intervening between the tapetal cells and the incoming light. This effectively occludes the tapetum as light is unable to reach the reflective material. In dark-adaptation the melanosomes withdraw from this location, exposing the tapetum to light and allowing it to act as a reflective layer. The retinal epithelium overlying the tapetal area is totally unpigmented so as not to interfere with the passage of light.

The distribution and morphology of choroidal melanocytes in dogs and cats which have a tapetum were compared with those of humans who do not. In dogs or cats, tapetal cell-like melanocytes were arranged in layers on the scleral side of the tapetum and underneath the choriocapillaris in the non-tapetal area. Although the tapetum of the dog occupied a smaller area than that of the cat, the tapetum plus the area of tapetal cell-like multilayered melanocytes occupied most of the fundus in the dog in almost the same way as in the cat. These multilayered melanocytes contained few intracytoplasmic organelles except for melanin granules, and some had regularly arranged melanin granules. In human eyes tapetal cell-like melanocytes were not found anywhere. It was concluded that the morphology and structural architecture of choroidal melanocytes of dogs or cats are different from those of human eyes and closely correspond to the tapetum.

Many mammals including dogs, cats, deer, and dolphins, but not humans, have a reflective membrane at the back of the eye. This tapetum lucidum affords the light‐sensitive retinal cells a second chance for photon‐photoreceptor stimulation, enhancing low light visual sensitivity. Dissection of preserved specimens and computed tomography (CT) scans of live dolphins have shown prominent air cavities around the ear, especially medial and posterior. Histologic evaluation of peri‐otic tissue revealed epithelial lined air sinuses and erectile tissue containing nerves, ganglia, smooth muscle bands, vascular spaces, and periarterial venous networks. Two dolphins were trained to periodically recognize and respond to a 1.5‐s, 70‐kHz target tone. In three experiments, 20 target tones were presented through a jaw phone after injection of 18 Fluro‐deoxy‐d‐glucose (FDG). Subsequent positron emission tomography (PET) scans revealed that the tone, to which the dolphin had responded and been rewarded numerous times in the p...