The Ultrastructure of the Nictitating Membrane of the Little Penguin (Eudyptula minor, Aves).

The ultrastructure of the nictitating membrane in the little penguin Eudyptula minor was studied using both scanning and transmission electron microscopy to improve our understanding of the function of ocular adnexa in diving birds. Following euthanasia, eyes were enucleated and immersion fixed in Karnovsky's fixative. The nictitating membrane and conjunctiva were embedded in araldite and semi- or ultra-thin sections were stained and photographed using compound and transmission electron microscopes, respectively. Ultrastructural dimensions were measured directly from digital photographs. Surface ultrastructure was examined using scanning electron microscopy. The transparent nictitating membrane consists of a dense stroma surrounded by epithelia on both the external (conjunctival) and internal (bulbar) surfaces. The conjunctival surface of the membrane near the leading edge is covered by microvilli, which transition to microplicae and finally to microridges in the periphery. Beneath the epithelial cells, there is a well-developed basement membrane. Scattered throughout this epithelium are a few goblet cells. The surface of the bulbar epithelium is covered by microvilli near the leading edge, which become denser peripherally. The stroma consists of densely-packed collagen fibrils, which are randomly oriented in bundles near the leading edge but are aligned in the same direction parallel with the epithelial and corneal surfaces and with the leading edge, when the membrane is extended. The ultrastructure of the nictitating membrane in the little penguin differs from other birds and its function is predominantly protective, while preserving clear vision in both water and air.

Sexual dimorphism of the electrosensory system: a quantitative analysis of nerve axons in the dorsal anterior lateral line nerve of the blue-spotted Fantail Stingray (Taeniura lymma).

Quantitative studies of sensory axons provide invaluable insights into the functional significance and relative importance of a particular sensory modality. Despite the important role electroreception plays in the behaviour of elasmobranchs, to date, there have been no studies that have assessed the number of electrosensory axons that project from the peripheral ampullae to the central nervous system (CNS). The complex arrangement and morphology of the peripheral electrosensory system has a significant influence on its function. However, it is not sufficient to base conclusions about function on the peripheral system alone. To fully appreciate the function of the electrosensory system, it is essential to also assess the neural network that connects the peripheral system to the CNS. Using stereological techniques, unbiased estimates of the total number of axons were obtained for both the electrosensory bundles exiting individual ampullary organs and those entering the CNS (via the dorsal root of the anterior lateral line nerve, ALLN) in males and females of different sizes. The dorsal root of the ALLN consists solely of myelinated electrosensory axons and shows both ontogenetic and sexual dimorphism. In particular, females exhibit a greater abundance of electrosensory axons, which may result in improved sensitivity of the electrosensory system and may facilitate mate identification for reproduction. Also presented are detailed morphological data on the peripheral electrosensory system to allow a complete interpretation of the functional significance of the sexual dimorphism found in the ALLN.

A comparison of behavioural (Landolt C) and anatomical estimates of visual acuity in archerfish (Toxotes chatareus).

Archerfish forage by shooting jets of water at insects above the water's surface. The challenge of detecting small prey items against a complex background suggests that they have good visual acuity, but to date this has never been tested, despite archerfish becoming an increasingly important model species for vertebrate vision. We used a modified Landolt C test to measure visual acuity behaviourally, and compared the results to their predicted minimum separable angle based on both photoreceptor and ganglion cell spacing in the retina. Both measures yielded similar estimates of visual acuity; between 3.23 and 3.57 cycles per degree (0.155-0.140° of visual arc). Such a close match between behavioural and anatomical estimates of visual acuity in fishes is unusual and may be due to our use of an ecologically relevant task that measured the resolving power of the part of the retina that has the highest photoreceptor density and that is used in aligning their spitting angle with potential targets.

Assessing the potential for post-copulatory sexual selection in elasmobranchs.

This review highlights the potential role that post-copulatory sexual selection plays in elasmobranch reproductive systems and the utility of this group to further understanding of evolutionary responses to the post-copulatory processes of sperm competition and cryptic female choice. The growing genetic evidence for female multiple mating (polyandry) in elasmobranchs is summarized. While polyandry appears to be common in this group, rates of multiple paternity are highly variable between species suggesting that there is large variance in the strength of post-copulatory sexual selection among elasmobranchs. Possible adaptations of traits important for post-copulatory sexual selection are then considered. Particular emphasis is devoted to explore the potential for sperm competition and cryptic female choice to influence the evolution of testes size, sperm morphology, genital morphology and sperm storage organs. Finally, it is argued that future work should take advantage of the wealth of information on these reproductive traits already available in elasmobranchs to gain a better understanding of how post-copulatory sexual selection operates in this group.

Vision in elasmobranchs and their relatives: 21st century advances.

This review identifies a number of exciting new developments in the understanding of vision in cartilaginous fishes that have been made since the turn of the century. These include the results of studies on various aspects of the visual system including eye size, visual fields, eye design and the optical system, retinal topography and spatial resolving power, visual pigments, spectral sensitivity and the potential for colour vision. A number of these studies have covered a broad range of species, thereby providing valuable information on how the visual systems of these fishes are adapted to different environmental conditions. For example, oceanic and deep-sea sharks have the largest eyes amongst elasmobranchs and presumably rely more heavily on vision than coastal and benthic species, while interspecific variation in the ratio of rod and cone photoreceptors, the topographic distribution of the photoreceptors and retinal ganglion cells in the retina and the spatial resolving power of the eye all appear to be closely related to differences in habitat and lifestyle. Multiple, spectrally distinct cone photoreceptor visual pigments have been found in some batoid species, raising the possibility that at least some elasmobranchs are capable of seeing colour, and there is some evidence that multiple cone visual pigments may also be present in holocephalans. In contrast, sharks appear to have only one cone visual pigment. There is evidence that ontogenetic changes in the visual system, such as changes in the spectral transmission properties of the lens, lens shape, focal ratio, visual pigments and spatial resolving power, allow elasmobranchs to adapt to environmental changes imposed by habitat shifts and niche expansion. There are, however, many aspects of vision in these fishes that are not well understood, particularly in the holocephalans. Therefore, this review also serves to highlight and stimulate new research in areas that still require significant attention.

Phylogenetic and ecological factors influencing the number and distribution of electroreceptors in elasmobranchs.

Electroreception is found throughout the animal kingdom from invertebrates to mammals and has been shown to play an important role in prey detection, facilitating social behaviours, the detection of predators and orientation to the earth's magnetic field for navigation. Electroreceptors in elasmobranchs, the ampullae of Lorenzini, detect minute electric fields and independently process these stimuli, thereby providing spatial information to the central nervous system on the location of a source, often potential prey. The ampullae of Lorenzini are individually connected to a single somatic pore on the surface of the skin, with the spatial separation of each pore directly influencing how electrical stimuli are detected and processed. Pore abundance varies across taxonomic groups resulting in unique species-specific differences. The intricate distribution patterns created by the specific positioning of somatic pores on the head are, however, consistent within families, resulting in patterns that are identifiable at higher taxonomic levels. As elasmobranchs evolved, the electrosensory system became more complex and highly specialized, which is evident by a general trend of increasing pore abundance over time. The elasmobranch electrosensory system has evolved to operate efficiently under the environmental conditions of the particular habitat in which a species lives. For example, reduced pore abundance is evident in oceanic pelagic elasmobranchs, for whom visual cues are thought to be of great importance. Pore abundance and spatial distribution may be influenced by multiple factors including head morphology, phylogeny, feeding behaviour and habitat.

Sensory systems in sawfishes. 1. The ampullae of Lorenzini.

The distribution and density of the ampullary electroreceptors in the skin of elasmobranchs are influenced by the phylogeny and ecology of a species. Sensory maps were created for 4 species of pristid sawfish. Their ampullary pores were separated into pore fields based on their innervation and cluster formation. Ventrally, ampullary pores are located in 6 areas (5 in Pristis microdon), covering the rostrum and head to the gills. Dorsally, pores are located in 4 areas (3 in P. microdon), which cover the rostrum, head and may extend slightly onto the pectoral fins. In all species, the highest number of pores is found on the dorsal and ventral sides of the rostrum. The high densities of pores along the rostrum combined with the low densities around the mouth could indicate that sawfish use their rostrum to stun their prey before ingesting it, but this hypothesis remains to be tested. The directions of ampullary canals on the ventral side of the rostrum are species specific. P. microdon possesses the highest number of ampullary pores, which indicates that amongst the study species this species is an electroreception specialist. As such, juvenile P. microdon inhabit low-visibility freshwater habitats.

Sensory systems in sawfishes. 2. The lateral line.

The lateral line system allows elasmobranchs to detect hydrodynamic movements in their close surroundings. We examined the distribution of pit organs and lateral line canals in 4 species of sawfish (Anoxypristis cuspidata, Pristis microdon, P. clavata and P. zijsron). Pit organs could only be located in A. cuspidata, which possesses elongated pits that are lined by dermal denticles. In all 4 pristid species, the lateral line canals are well developed and were separated into regions of pored and non-pored canals. In all species the tubules that extend from pored canals form extensive networks. In A. cuspidata, P. microdon and P. clavata, the lateral line canals on both the dorsal and ventral surfaces of the rostrum possess extensively branched and pored tubules. Based on this morphological observation, we hypothesized that these 3 species do not use their rostrum to search in the substrate for prey as previously assumed. Other batoids that possess lateral line canals adapted to perceive stimuli produced by infaunal prey possess non-pored lateral line canals, which also prevent the intrusion of substrate particles. However, this hypothesis remains to be tested behaviourally in pristids. Lateral line canals located between the mouth and the nostrils are non-pored in all 4 species of sawfish. Thus this region is hypothesized to perceive stimuli caused by direct contact with prey before ingestion. Lateral line canals that contain neuromasts are longest in P. microdon, but canals containing neuromasts along the rostrum are longest in A. cuspidata.

Early evolution of multifocal optics for well-focused colour vision in vertebrates.

Jawless fishes (Agnatha; lampreys and hagfishes) most closely resemble the earliest stage in vertebrate evolution and lamprey-like animals already existed in the Lower Cambrian [about 540 million years ago (MYA)]. Agnathans are thought to have separated from the main vertebrate lineage at least 500 MYA. Hagfishes have primitive eyes, but the eyes of adult lampreys are well-developed. The southern hemisphere lamprey, Geotria australis, possesses five types of opsin genes, three of which are clearly orthologous to the opsin genes of jawed vertebrates. This suggests that the last common ancestor of all vertebrate lineages possessed a complex colour vision system. In the eyes of many bony fishes and tetrapods, well-focused colour images are created by multifocal crystalline lenses that compensate for longitudinal chromatic aberration. To trace the evolutionary origins of multifocal lenses, we studied the optical properties of the lenses in four species of lamprey (Geotria australis, Mordacia praecox, Lampetra fluviatilis and Petromyzon marinus), with representatives from all three of the extant lamprey families. Multifocal lenses are present in all lampreys studied. This suggests that the ability to create well-focused colour images with multifocal optical systems also evolved very early.

The corneal surface of aquatic vertebrates: microstructures with optical and nutritional function?

The anterior surface of the mammalian cornea plays an important role in maintaining a smooth optical interface and consequently a sharp retinal image. The smooth surface is produced by a tear film, which adheres to a variety of microprojections, which increase the cell surface area, improve the absorbance of oxygen and nutrients and aid in the movement of metabolic products across the outer cell membrane. However, little is known of the structural adaptations and tear film support provided in other vertebrates from different environments. Using field emission scanning electron microscopy; this study examines the density and surface structure of corneal epithelial cells in representative species of the classes Cephalaspidomorphi, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia, including some Marsupialia. Variations in cell density and the structure and occurrence of microholes, microridges, microplicae and microvilli are described with respect to the demands placed upon the cornea in different aquatic environments such as marine and freshwater. A progressive decrease in epithelial cell density occurs from marine (e.g. 29348 cells mm(-2) in the Dover sole Microstomius pacficus) to estuarine or freshwater (e.g. 5999 cells mm(-2) in the black bream Acanthopagrus butcheri) to terrestrial (e.g. 2126 cells mm(-2) in the Australian koala Phascolarctos cinereus) vertebrates, indicating the reduction in osmotic stress across the corneal surface. The microholes found in the Southern Hemisphere lampreys, namely the pouched lamprey (Geotria australis) and the shorthead lamprey (Mordacia mordax) represent openings for the release of mucus, which may protect the cornea from abrasion during their burrowing phase. Characteristic of marine teleosts, fingerprint-like patterns of corneal microridges are a ubiquitous feature, covering many types of sensory epithelia (including the olfactory epithelium and the oral mucosa). Like microplicae and microvilli, microridges stabilize the tear film to maintain a smooth optical surface and increase the surface area of the epithelium, assisting in diffusion and active transport. The clear interspecific differences in corneal surface structure suggest an adaptive plasticity in the composition and stabilization of the corneal tear film in various aquatic environments.

The relationship between the position of the retinal area centralis and feeding behaviour in juvenile black bream Acanthopagrus butcheri (Sparidae: Teleostei).

The topography of the neurons in the retinal ganglion cell layer of juvenile black bream Acanthopagrus butcheri changes during development. The region of high cell density the area centralis (AC), relocates from a temporal (central) to a dorsal (peripheral) position within the dorso-temporal retinal quadrant. To ascertain whether the differences in the position of the AC during development are related to feeding behaviour, we monitored fishes that were given a choice of food. A range of feeding behaviour patterns was recorded in individual fishes. The smallest fishes (8-15 mm standard length (SL)) took live food from the water column. Following weaning onto pellets, fishes exhibited a preference for taking food from either the substrate or the surface (but not both). When greater than 20 mm SL, a number of individuals then divided their time between surface and substrate feeding before all fishes became exclusive benthic feeders at a stage between 50 and 80 mm SL. Three individual fishes, for which behaviour patterns were categorized, were killed and the topography of the retinal ganglion cell layer analysed. A range of positions for the AC was found with the smallest fish (12 mm SL) possessing a region of high cell density in the temporal retina. In a larger fish (70 mm SL), feeding from both the substrate and the surface, the AC was found in an intermediate dorso-temporal position. The AC of a fish (51 mm SL) preferentially taking food from the substrate was located in a dorsal position.

Foveate vision in deep-sea teleosts: a comparison of primary visual and olfactory inputs.

The relative importance of vision in a foveate group of alepocephalid teleosts is examined in the context of a deep-sea habitat beyond the penetration limits of sunlight. The large eyes of Conocara spp. possess deep convexiclivate foveae lined with Müller cells comprising radial shafts of intermediate filaments and horizontal processes. Photoreceptor cell (171.8 x 10(3) rods mm(-2)) and retinal ganglion cell (11.9 x 10(3) cells mm(-2)) densities peak within the foveal clivus and the perifloveal slopes, respectively, with a centro-peripheral gradient between 3:1 (photoreceptors) and over 20:1 (ganglion cells). The marked increase in retinal sampling localized in temporal retina, coupled with a high summation ratio (13:1), suggest that foveal vision optimizes both spatial resolving power and sensitivity in the binocular frontal visual field. The elongated optic nerve head is comprised of over 500 optic papillae, which join at the embryonic fissure to form a thin nervous sheet behind the eye. The optic nerve is divided into two axonal bundles; one receiving input from the fovea (only unmyelinated axons) and the other from non-specialized retinal regions (25% of axons are myelinated), both of which appear to be separated as they reach the visual centres of the central nervous system. Comparison of the number of primary (first-order) axonal pathways for the visual (a total of 63.4 x 10(6) rod photoreceptors) and olfactory (a total of 15.24 x 10(3) olfactory nerve axons) inputs shows a marked visual bias (ratio of 41:1). Coupled with the relative size of the optic tecta (44.0 mm3) and olfactory bulbs (0.9 mm3), vision appears to play a major role in the survival of these deep-sea teleosts and emphasizes that ecological and behavioural strategies account for significant variation in sensory brain structure.

Variability in the location of the retinal ganglion cell area centralis is correlated with ontogenetic changes in feeding behavior in the black bream, Acanthopagrus butcheri (Sparidae, teleostei).

The development of neural cell topography in the retinal ganglion cell layer was examined in a teleost, the black bream (Acanthopagrus butcheri). From Nissl-stained wholemounts, it was established that fish between 10 and 15 mm standard body length (SL) possess high cell densities throughout the dorso-temporal retinal quadrant, with peak cell densities located in temporal regions of the retina. However, in fish between 15 and 80 mm SL, a wide variation in the position of the peak cell density is revealed with the locations of the areae centrales (AC) ranging from exclusively temporal to periphero-dorsal retina. Fish larger than 80 mm SL always possess an AC located in the dorsal region of the dorso-temporal retinal quadrant. The topography of ganglion cells within the ganglion cell layer was determined by comparing the numbers of ganglion cells retrogradely-labeled from the optic nerve with the total population of Nissl-stained neurons (ganglion plus displaced amacrine cells) in a range of different-sized individuals. Ganglion cell topography was the same as that recorded for all Nissl-stained neurons. The feeding behavior of juveniles from metamorphosis to 80 mm SL was observed, where fish were given the choice of feeding on live food in mid-water (until 15 mm SL) or obtaining pellets from the surface or the bottom. A range of feeding patterns was recorded, with the smallest fish taking food from mid-water but individuals between 15 and 80 mm SL taking food either from the surface or the bottom or both. A correlation between the preferred mode of feeding and the position of the AC was found, such that those individuals feeding in mid-water or at the surface possess a temporal or intermediate (dorso- temporal) AC, whereas those predominantly feeding from the bottom possess a dorsal AC.

Retinal ganglion cell topography in elasmobranchs.

Retinal wholemounts are used to examine the topographic distribution of retinal cells within the ganglion cell layer in a range of elasmobranchs from different depths. The retina is examined for regional specializations for acute vision in six species of selachians, Galeocerdo cuvieri, Hemiscyllium ocellatum, Scyliorhinus canicula, Galeus melastomus, Etmopterus spinax, Isistius brasiliensis, one species of batoid, Raja bigelowi and one species of chimaera, Hydrolagus mirabilis. These species represent a range of lifestyles including pelagic, mesopelagic and benthic habitats, living from shallow water to the sea bottom at a depth of more than 3000 m. The topography of cells within the ganglion cell layer is non-uniform and changes markedly across the retina. Most species possess an increased density of cells across the horizontal (dorsal) meridian or visual streak, with a density range of 500 to 2,500 cells per mm(2) with one or more regional increases in density lying within this specialized horizontal area. It is proposed that the higher spatial resolving power provided by the horizontal streak in these species mediates panoramic vision in the lower frontal visual field. Only I. brasiliensis possesses a concentric arrangement of retinal iso-density contours in temporal retina or an area centralis, thereby increasing spatial resolving power in a more specialized part of the visual field, an adaptation for its unusual feeding behavior. In Nissl-stained material, amacrine and ganglion cell populations could be distinguished on the criteria of soma size, soma shape and nuclear staining. Quantitative analyses show that the proportion of amacrine cells lying within the ganglion cell layer is non-uniform and ranges between 0.4 and 12.3% in specialized retinal areas and between 8.2 and 48.1% in the peripheral non-specialized regions. Analyses of soma area of the total population of cells in the ganglion cell layer also show that the pelagic species possess significantly smaller soma (9-186 micrometer(2)) than benthic and/or deep-sea species (16-338 micrometer(2)), and that a number of different morphological classes of cells are present including a small population of giant ganglion cells.

The ocular morphology of the southern hemisphere lamprey Mordacia mordax Richardson with special reference to a single class of photoreceptor and a retinal tapetum.

A description is given of the morphological and ultrastructural characteristics of the eye of Mordacia mordax, one of the two parasitic species of the southern hemisphere lamprey family Mordaciidae. The avascular retina is well-differentiated, containing two ganglion cell sublaminae, two layers of horizontal cells and a large population of displaced bipolar cells. Intermediate filaments, which stain positively for glial fibrillary acidic protein (GFAP), lie in perpendicularly-arranged bundles between the two layers of horizontal cells. M. mordax contains only a single type of rod-like photoreceptor, in contrast to representatives in the other two lamprey families Geotriidae and Petromyzontidae, which possess three (two cone-like and one rod-like) and two (one cone and one rod) types of photoreceptor, respectively. The large photoreceptors of M. mordax possess a large mitochondrial ellipsosome, a cylindrical outer segment with incisures, and a nucleus immediately abutting the outer limiting membrane. The photoreceptors are arranged in an hexagonal array with their outer segments surrounded by a retinal tapetum which elicits a yellow fundal eyeshine. The tapetum contains both diffuse (vitread spheroidal granules) and specular (sclerad needles) reflectors. The characteristics of the eye of M. mordax are related to the diurnal burrowing of this species during its upstream migration, a feature not found in the other two extant families of lampreys. The absence of cone-like photoreceptors and the presence of a tapetum suggest that although photopic vision is probably greatly diminished, characteristics have been evolved by the eye to maximize light capture and increase sensitivity in the low light intensities encountered during this species' nocturnal migration.

Prey capture and accommodation in the sandlance, Limnichthyes fasciatus (Creediidae; Teleostei).

The eyes of the sandlance, Limnichthyes fas ciatus (Creediidae, Teleostei) move independently and possess a refractive cornea, a convexiclivate fovea and a non-spherical lens giving rise to a wide separation of the nodal point from the axis of rotation of the eye much like that of a chameleon. To investigate this apparent convergence of the visual optics in these phylogenetically disparate species, we examine feeding behaviour and accommodation in the sandlance with special reference to the possibility that sandlances use accommodation as a depth cue to judge strike length. Frame-by-frame analysis of over 2000 strikes show a 100% success rate. Explosive strikes are completed in 50 ms over prey distances of four body lengths. Close-up video confirms that successful strikes can be initiated monocularly (both normally and after monocular occlusion) showing that binocular cues are not necessary to judge the length of a strike. Additional means of judging prey distance may also be derived from partallax information generated by rotation of the eye as suggested for chameleons. Using photorefraction on anaesthetised sandlances, accommodative changes were induced with acetylcholine and found to range between 120 D and 180 D at a speed of 600-720 D s(-1). The large range of accommodation (25% of the total power) is also thought to be mediated by corneal accommodation where the contraction of a unique cornealis muscle acts to change the corneal curvatures.

A comparative SEM study of the vertebrate corneal epithelium.

PURPOSE: The anterior surface of the cornea of mammals, including humans, has numerous folds in the anterior epithelial cell membranes in the form of microvilli and microplicae. The role of these surface irregularities may be to increase cell-surface area and therefore aid in intra- and extracellular movement of nutritional and waste products across the cell membranes in addition to stabilizing the corneal tear film. The aim of this study was to investigate and compare the nature of these corneal-surface features in various vertebrate classes residing in different environments. METHODS: The anterior corneal surfaces of various vertebrates were investigated by using field emission scanning electron microscopy. Cell areas were analyzed by using image-analysis software. RESULTS: Representative species were examined from all the vertebrate classes, with the exception of the Cephalaspidomorphi. The mean epithelial cell density of aquatic vertebrates (17,602 +/- 9,604 cells/mm2) is greater (p = 0.000018) than that of aerial and terrestrial vertebrate species, including amphibians (3,755 +/- 2,067 cells/ mm2). Similarly, the mean epithelial cell density for the marine vertebrates (22,553 +/- 8,878 cells/mm2) is greater (p = 0.0015) than that of the freshwater and estuarine species (10,529 +/- 5,341 cells/mm2). The anterior corneal surfaces of all species examined were found to show a variety of cell-surface structures. Microvilli are predominant in reptiles, birds, and mammals; microridges appear to be characteristic of the Osteichthyes; and microholes were observed only in the Chondrichthyes. CONCLUSION: The function of these morphologic variations in surface structure appear to be correlated with the range of ecologic environments (marine, aerial, and terrestrial) occupied by each species, corneal phylogeny, and the demands placed on the cornea to ensure clear vision.

The corneal endothelium in the blowfish (Torquigener pleurogramma).

PURPOSE: In vertebrates, a corneal endothelium is essential for the maintenance of corneal transparency in a variety of environments. Knowledge of the surface structure of the corneal endothelium may assist our understanding of this unique tissue and its evolutionary development. Although there have been many studies of the corneal endothelium of humans and some mammals, there have been few in other vertebrates. METHODS: The field emission scanning electron microscope was used to study the surface structure of the corneal endothelium in the blowfish, Torquigener pleurogramma (Tetraodontidae, Teleostei), and to examine cell density. Cell areas were measured by using image-analysis software. RESULTS: The endothelium is composed of a sheet of interdigitating hexagonal and pentagonal cells with a mean area of 154 microm2 and a density of 6,486 cells/mm2. Two types of surface features are identified; primary cilia and microvilli. The cilia are cylindrical, protrude from either a pore or circular indentation in the cell center, and possess a knob-like ending. The microvilli are button-like protrusions with a density of -3.5 x 105 microvilli/mm2 or 54 microvilli/cell in central cornea. CONCLUSION: The results show that the surface structure of teleost endothelial cells is similar to those described for other vertebrates and indicate that cell density varies across classes, with the presence of cilia a more widespread occurrence than previously believed.

The ocular morphology of the southern hemisphere lamprey geotria australis gray, with special reference to optical specialisations and the characterisation and phylogeny of photoreceptor types.

This paper describes the ocular morphology of young adults of the southern hemisphere lamprey Geotria australis, the sole representative of the Geotriidae, and makes comparisons with those of holarctic lampreys (Petromyzontidae). As previously reported for the holarctic lamprey Ichthyomyzon unicuspis [Collin and Fritzsch, 1993], the lens of G. australis is non-spherical and possesses a cone-shaped posterior that may be capable of mediating variable focus. The avascular retina of G. australis is well differentiated, containing three retinal ganglion cell populations, three layers of horizontal cells and three photoreceptor types. In contrast to petromyzontids that contain only two photoreceptor types (short and long), G. australis possesses one rod-like (R1) and two cone-like (C1 and C2) photoreceptors. Although the rod-like receptor in G. australis may be homologous with the short receptors of holarctic lampreys, the two cone-like receptors have morphological characteristics that differ markedly from those of the long receptors of their holarctic counterparts. The features which distinguish the two cone-like receptors from those of the long receptor type in holarctic lampreys are the characteristics of the mitochondria and the presence of large amounts of two different types of stored secretory material in the endoplasmic reticulum of the myoid (refractile bodies). The endoplasmic reticulum of each receptor type has a different shape and staining profile and is polymorphic, each showing a continuum of distension. It is proposed that the presence of two cone-like photoreceptors with different characteristics would increase the spectral range of G. australis and thus be of value during the parasitic phase, when this lamprey lives in the surface marine waters. The irideal flap, present in G. australis but not petromyzontids, would assist in reducing intraocular flare during life in surface waters. The results of this study, which are discussed in the context of the proposed evolution of lampreys, emphasise that it is important to take into account the characteristics of the eyes of southern hemisphere lampreys when making generalizations about the eyes of lampreys as a whole.