Topographic Organization of the Ganglion Cell Layer and Intraocular Vascularization in the Retinae of Two Reef Teleosts

The retinae of two species of teleosts, the rippled blenny Istiblennius edentulus and the blue tusk fish Choerodon albigena are examined in wholemount. The retinal topography of Nissl-stained cells within the ganglion cell layer in each species reveals a temporal (4.32 X 100 000 cells per mm2) and a nasal (3.83 X 100 000 cells per mm2) area centralis in the rippled blenny and two temporal areae centrales (8.30 X 100 000 and 8.00 X 100 000 cells per mm2) and a horizontal streak (5.00 X 100 000 cells per mm2) in the tusk fish. These areas are thought to subserve higher spatial resolution. Transcardial perfusions of indian ink reveal an extensive network of vitreal blood vessels which are supplied by the hyaloid artery and overlie the retina in each species. This rich network of vitreal vessels supplies areas of increased ganglion cell density although areas of maximum cell density are devoid of vessels to preserve the high spatial resolving power of the eye within this region. Unique blood vessel plexuses overlying the optic disc and falciform process in the tusk fish are also described. The diameter of the overlying vitreal vessels is compared to the soma sizes of cells within the ganglion cell layer

A Comparative Study of the Corneal Endothelium in Vertebrates

In vertebrates, a corneal endothelium is essential for the maintenance of corneal transparency in a variety of environments, including aerial, terrestrial and aquatic. Knowledge of the surface structure of the corneal endothelium may assist our understanding of this unique tissue and its evolutionary development. Except for humans and some mammals, there have been few studies of other vertebrates, particularly the unique Australian species. The field emission scanning electron microscope was used to study the corneal endothelium in representatives of four vertebrate classes: Teleostei (five species), Reptilia (two species) Aves (four species) and Mammalia (three species), including Marsupilia (two species). Endothelial cell densities were calculated from micrographs using computer-based image analysis. The cell densities varied considerably from 1.900 +- 197 cells per mm2 for the bream to 11.734 +- 1.687 cells per mm2 for the emu. Most of the corneal endothelia were similar to those reported for mammals. However, in some species such as the koala, the pattern was irregular. Some endothelial cells in birds possessed cilia. The shape of the corneal endothelial cells of vertebrates is typically a mixture of hexagonal and pentagonal cells, in which the cell borders are irregular and interdigitating. An exception is the koala, in which the cells were markedly irregular. Many of the cells have microvilli but only the birds are cilia found in the centre of many endothilial cells. In spite of the range of corneal environments, there are no systematic differences in the cell densities of the various classes and species

The Head and Eye of the Sandlance, Limnichtyes fasciatus (Creediidae): A Field Emission Scanning Electron Microscopy Study

In humans and non-aquatic vertebrates, the anterior surface of the cornea provides the major refractive component of the eye. However, without a smooth optical surface provided by an adequate tear film a sharp retinal image cannot be formed. Stability of the tear film is aided by the presence of microvilli and/or microplicae. These increase the surface area of the superficial epithelial cells permitting the adsorption of more tear mucin onto the corneal cell surface. There have been very few scanning electron microscopy (SEM) studies of the cornea of teleosts and none using field emission SEM. Some teleosts have an intricate pattern of microplicae on the surface of the cornea. The sandlance or tommy fish, Limnichthyes fasciatus, is a small fish 15 to 30mm long, found in the coastal waters of Australia. The suface cells of the cornea are covered with a distinct complex pattern of microplicae'. A similar patttern of slightly thicker ridges covers the conjunctiva. However, this study was primarily a transmission electron microscopic investigation of the structure of the cornea with only a minimal description of the ocular surface. In order to understand better the nature and role of the surface microplicae in teleost vision, we investigated and compared the surface morphology of the cornea, the conjunctivea and the facial tissues of the sandlanc

Observations On The Shape Of The Lens In The Eye Of the Silver Lamprey, Ichthyomyzon Unicuspis

The shape of the lens in the eye of the silver lamprey, ichthyomyzon unicuspis was examined in live, frozen, and fixed material. Contrary to other reports, the lens was found to be nonspherical with a cone-shaped posterior. The egg-shaped lens, which contains horizontal sutures on both the anterior and posterior surfaces, is also asymmetric in the nasotemporal axis. Its equatorial diameter exceeds its axail diameter (thickness) and the radius of curvature of the lens in the dorsoventral axis is greater than the radius of curvature in the anterioposterior axis. The lens is surrounded by a thick basement membrane with the anterior lens surface covered by a single layer of cuboidal epithelial cells. Juxtaposed to the lens capsule is a dense layer of lens fibres, which stain more darkly and surround an ill-defined lens nucleus. The shape of the lens is discussed in relation to that in aquatic gnathostomes and compared with the putative multifocal lenses of some mesopelagic teleosts. It is also hypothesized that the previously reported active focussing ability of the lamprey eye may have been misinterpreted, owing to failure to take into account the nonspherical lens shape, and may reflect measurements taken of the eye and lens at different angles

Ontogenetic Shifts in the Number of Axons in the Olfactory Tract and Optic Nerve in Two Species of Deep-Sea Grenadier Fish (Gadiformes: Macrouridae: Coryphaenoides)

Neuroanatomical studies of the peripheral sense organs and brains of deep-sea fishes are particularly useful for predicting their sensory capabilities and ultimately their behavior. Over the abyssal plane (between 2,000 and 6,000 m), communities of grenadiers (Gadiformes: Macrouridae) play an important ecological role as predator-scavengers. Previous studies suggest that these fishes rely heavily on chemosensation, especially olfaction. Furthermore, at least one species, Coryphaenoides armatus, undergoes an ontogenetic shift in the relative size of the optic tectum and the olfactory bulbs, suggesting. a shift from a reliance on vision to olfaction during ontogeny, apparently in association with a shift to a more scavenging lifestyle. Here, we compared the olfactory and visual sensory inputs to the brain in C. armatus, and in a second, closely-related species, Coryphaenoides profundicolus, by assessing the total number of axons (myelinated and unmyelinated) in the olfactory tract and optic nerve in a range of individuals from both species. In C. armatus, the numbers of axons in both tract and nerve increased with body size, with the total number of axons in the olfactory tract being far greater than the number of axons in the optic nerve. These differences became more pronounced in larger animals. In the two smaller C. profundicolus individuals (≤ 315 mm SL), there were more axons in the optic nerve than in the olfactory tract, but the opposite situation was found in larger individuals. As in C. armatus, the number of olfactory tract axons also increased with body size in C. profundicolus, but in contrast, the number of optic nerve axons decreased in this species. These results suggest that both C. armatus and C. profundicolus undergo an ontogenetic shift in sensory orientation, with olfaction becoming relatively more important than vision in larger animals. The differences in the ratio of olfactory tract to optic nerve axons in C. armatus indicate that olfaction is of particular importance to larger individuals of this species. In both species, the percentage of myelinated axons in the olfactory tract was relatively low, but we found evidence for interspecific and ontogenetic variation in the percentages of myelinated axons in the optic nerve

Morphology and Spectral Absorption Characteristics of Retinal Photoreceptors in the Southern Hemisphere Lamprey (Geotria australis)

The morphology and spectral absorption characteristics of the retinal photoreceptors in the southern hemisphere lamprey Geotria australis (Agnatha) were studied using light and electron microscopy and microspectrophotometry. The retinae of both downstream and upstream migrants of Geotria contained two types of cone photoreceptor and one type of rod photoreceptor. Visual pigments contained in the outer segments of these three photoreceptor types had absorbance spectra typical of porphyropsins and with wavelengths of maximum absorbance (downstream/upstream) at 610/616 nm (long-wavelength-sensitive cone,LWS),515/515nm(medium-wavelength-sensitive cone, MWS), and 506/500 nm (medium-wavelength-sensitive rod). A "yellow" photostable pigment was present in the myoid region of all three types of photoreceptor in the downstream migrant. The same short-wavelength-absorbing pigment, which prevents photostimulation of the beta band of the visual pigment in the outer segment, was present in the rods and LWS cones of the upstream migrant, but was replaced by a large transparent ellipsosome in the MWS cones. Using microspectrophotometric and anatomical data, the quantal spectral sensitivity of each photoreceptor type was calculated. Our results provide the first evidence of a jawless vertebrate, represented today solely by the lampreys and hagfishes, with two morphologically and physiologically distinct types of cone photoreceptors, in addition to a rod-like photoreceptor containing a colored filter (a cone-like characteristic). In contrast, all other lampreys studied thus far have either (1) one type of cone and one type of rod, or (2) a single type of rod-like photoreceptor. The evolution or retention of a second type of cone in adult Geotria is presumably an adaptation to life in the brightly lit surface waters of the Southern Ocean, where this species lives during the marine phase of its life cycle. The functional significance of the unique visual system of Geotria is discussed in relation to its life cycle and the potential for color vision

Multiple Areas Of Acute Vision In Two Freshwater Teleosts, The Creek Chub, Semotilus Atromaculatus (Mitchill) And The Cutlips Minnow, Exoglossum Maxillingua (Lesueur)

The topography of Nissl-stained cells within the retinal gangalion cell layer is examined in two closely related freshwater teleosts from the family Cyprinidae. Regardless of the close phylogenetic relationship and the sympatric habitats of the two species, pronounced differences in the number and position of areas of increased cell density are observed in their retinae. In the creek chub, Semotilus atromaculatus, a midwater crepuscular feeder, three retinal specializations or areae centrales are identified in the dorsonasal, nasal, and temporal regions of the retina. In the cutlips minnow, Exoglossum maxillingua, a benthic diurnal feeder, two areae centrales are identified in temporal and nasal retina. The upper limits of the spatial resolving power of each species are calculated from the spacing of cells within the ganglion cell layer. Differences in the arrangement of isodensity contours appear to reflect the symmetry of each species' visual environment. The development and significance of up to three visually acute zones are discussed

Multiple Cone Visual Pigments and the Potential for Trichromatic Colour Vision in Two Species of Elasmobranch

Elasmobranchs (sharks, skates and rays) are the modern descendents of the first jawed vertebrates and, as apex predators, often occupy the highest trophic levels of aquatic (predominantly marine) ecosystems. However, despite their crucial role in the structure of marine communities, their importance both to commercial and to recreational fisheries, and the inherent interest in their role in vertebrate evolution, very little is known about their visual capabilities, especially with regard to whether or not they have the potential for colour vision. Using microspectrophotometry, we show that the retinae of the giant shovelnose ray (Rhinobatos typus) and the eastern shovelnose ray (Aptychotrema rostrata) contain three spectrally distinct cone visual pigments with wavelengths of maximum absorbance (lamda_max) at 477, 502 and 561 nm and at 459, 492 and 553 nm, respectively. The retinae of R. typus and A. rostrata also contain a single type of rod visual pigment with lamda_max at 504 and 498 nm, respectively. R. typus, living in the same estuarine waters as A. rostrata, were found to have identical visual pigments to R. typus inhabiting coral reef flats, despite a considerable difference in habitat spectral radiance. This is the first time that multiple cone visual pigments have been measured directly in an elasmobranch. The finding raises the possibility that some species are able to discriminate colour - a visual ability traditionally thought to be lacking in this vertebrate class - and it is evident that the visual ecology of elasmobranchs is far more complex than once thought

Visual pigments in a living fossil, the Australian lungfish Neoceratodus forsteri

Background. One of the greatest challenges facing the early land vertebrates was the need to effectively interpret a terrestrial environment. Interpretation was based on ocular adaptations evolved for an aquatic environment millions of years earlier. The Australian lungfish Neoceratodus forsteri is thought to be the closest living relative to the first terrestrial vertebrate, and yet nothing is known about the visual pigments present in lungfish or the early tetrapods. Results. Here we identify and characterise five visual pigments (rh1, rh2, lws, sws1 and sws2) expressed in the retina of N. forsteri. Phylogenetic analysis of the molecular evolution of lungfish and other vertebrate visual pigment genes indicates a closer relationship between lungfish and amphibian pigments than to pigments in teleost fishes. However, the relationship between lungfish, the coelacanth and tetrapods could not be absolutely determined from opsin phylogeny, supporting an unresolved trichotomy between the three groups. Conclusion. The presence of four cone pigments in Australian lungfish suggests that the earliest tetrapods would have had a colorful view of their terrestrial environment