The structure and diversity of retinal ganglion cells in the masked greenling Hexagrammos octogrammus Pallas, 1814 (Pisces: Scorpaeniformes: Hexagrammidae).

The authors studied the structure and diversity of retinal ganglion cells (GC) in the masked greenling Hexagrammos octogrammus. In vivo labelling with horseradish peroxidase revealed GCs of various structures in retinal wholemounts. A total of 154 cells were camera lucida drawn, and their digital models were generated. Each cell was characterized by 17 structural and topological parameters. Using nine clustering algorithms, a variety of clusterings were obtained. The optimum clustering was found using silhouette analysis. It was based on a set of three variables associated with dendritic field size and dendrite stratification depth in the retina. A total of nine cell types were discovered. A number of non-parametric tests showed significant pair-wise between-cluster differences in at least four parameters with medium and large effect sizes. Three large-field types differed mainly in dendritic field size, total dendrite length, level of dendrite stratification in the retina and position of somata. Six medium- to small-field types differed mainly in the structural complexity of dendritic arbors and level of dendrite arborization. Cells similar and obviously homologous to types 1-4 were identified in many fish species, including teleosts. Potential homologues of type 5 cells were identified in fewer teleost species. Cells similar to types 6-9 in relative dendritic field size and dendrite arborization pattern were also described in several teleostean species. Nonetheless, their homology is more questionable as their stratification patterns do not match so well as they do in large types. Potential functional matches of the GC types were identified in a number of teleostean species. Type 1 and 2 cells probably match spontaneously active units with the large receptive field centre, so-called dimming and lightening detectors; type 4 may be a counterpart of changing contrast detectors with medium receptive field centre size preferring fast-moving stimuli. Type 3 (biplexiform) cells have no obvious functional matches. Probable functional matches of types 6, 8 and 9 belong to ON-centre elements with small receptive fields such as ON-type direction-selective cells, ON-type spot detectors or ON-type spontaneously active units. Type 5 and 7 cells may match ON-OFF type units, in particular, changing contrast detectors or orientation-selective units. Potential functional matches of GC types presently described are involved in a wide spectrum of visual reactions related to adaptation to gradual change in illumination, predator escape, prey detection and capture, habitat selection and social behaviour.

Photoreceptors, visual pigments and intraretinal variability in spectral sensitivity in two species of smelts (Pisces, Osmeridae).

The main goal of this study was to clarify whether the spectral properties of retinal photoreceptors reflect the features of behaviour of closely related fish species cohabiting shallow marine and fresh waters. The spectral sensitivity of photoreceptors was compared between two smelt species, Hypomesus japonicus and Japanese smelt Hypomesus nipponensis. The spectral absorption of the visual pigments was measured using microspectrophotometry. In H. japonicus, a mostly marine species, all photoreceptors contained visual pigments based on retinal and were distributed differently in specific retinal areas. The absorbance maxima (λmax ) of rods and long-wave-sensitive members of double cones throughout the retina amounted to 507 and 573 nm, respectively, but the λmax value of the short-wave-sensitive members of double cones and single cones in the temporal hemiretina showed a significant blue shift compared to the nasal hemiretina: 485 vs. 516 nm and 375 vs. 412 nm, respectively, thus enhancing the short-wave sensitivity of the temporal hemiretina. In H. nipponensis, an euryhaline species, the estimated λmax value of both rods and cones significantly varied between the groups caught in different localities (sea, river or estuary) because of the presence of rhodopsin/porphyropsin mixtures. The long-wavelength shift in rod and cone photoreceptors was observed because of changes in the chromophore complement in closely related but ecologically different species dwelling in freshened bodies of water. Considering the data available in the literature, several putative common opsin genes have been suggested for species under study.

Retinal ganglion cell topography and spatial resolution in the Japanese smelt Hypomesus nipponensis (McAllister, 1963).

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

Visual pigment genes and absorbance spectra in the Japanese sardine Sardinops melanostictus (Teleostei: Clupeiformes).

The spectral absorbance of photoreceptor visual pigments and the opsin gene class of the visual pigments was investigated in Sardinops melanostictus. Microspectrophotometric (MSP) measurements showed that the rod photoreceptors had peak absorbance spectra (λmax) at 502 nm. The spectral sensitivity of single cones was centered at 393 nm. Double cones had a λmax of 493/522 nm, but a few displayed a red-shifted absorbance of the long-wave member at 542 nm. The mRNAs of six different opsins were isolated from the retina, retrotranscribed, cloned, and sequenced. Three genes encoded opsins in the green-sensitive class (RH2), and three genes encoded opsins in the red-sensitive class (LWS), the ultraviolet (UV)-sensitive (SWS1) class, and the rod class (RH1). A Southern blot analysis showed that the blue-sensitive (SWS2) opsin gene is absent from this species, hence it was concluded that the λmax of 393 nm was generated from the SWS1 opsin. Phylogenetic analyses of S. melanostictus RH1, LWS, and SWS1 sequences placed them with orthologs from other species (e.g., the cyprinids Danio rerio and Carrasius auratus) in Otomorpha. However, unexpectedly, the RH2 sequences were more similar to orthologs in members of the Euteleosteomorpha (e.g., Oryzias latipes and Takifugu rubripes) than to cyprinid RH2 opsins.

The dual rod system of amphibians supports colour discrimination at the absolute visual threshold.

The presence of two spectrally different kinds of rod photoreceptors in amphibians has been hypothesized to enable purely rod-based colour vision at very low light levels. The hypothesis has never been properly tested, so we performed three behavioural experiments at different light intensities with toads (Bufo) and frogs (Rana) to determine the thresholds for colour discrimination. The thresholds of toads were different in mate choice and prey-catching tasks, suggesting that the differential sensitivities of different spectral cone types as well as task-specific factors set limits for the use of colour in these behavioural contexts. In neither task was there any indication of rod-based colour discrimination. By contrast, frogs performing phototactic jumping were able to distinguish blue from green light down to the absolute visual threshold, where vision relies only on rod signals. The remarkable sensitivity of this mechanism comparing signals from the two spectrally different rod types approaches theoretical limits set by photon fluctuations and intrinsic noise. Together, the results indicate that different pathways are involved in processing colour cues depending on the ecological relevance of this information for each task.This article is part of the themed issue 'Vision in dim light'.

Three cone opsin genes determine the properties of the visual spectra in the Japanese anchovy, Engraulis japonicus (Engraulidae, Teleostei).

A complement of cone visual pigments was identified in the Japanese anchovy Engraulis japonicus, one of the engraulid fish species that has a retina specialized for polarization and color vision. The nature of the chromophore bound to opsin proteins was investigated using high performance liquid chromatography. The opsin genes were then cloned and sequenced, and the absorption spectra of different types of cones were obtained by microspectrophotometry. Two green (EJ-RH2-1, EJ-RH2-2) and one red (EJ-LWS) cone opsin genes were identified and are presumably related to the vitamin A1-based visual pigments (i.e. rhodopsins) with λmax values of 492, 474 and 512 nm, respectively. The long and short cones from the ventro-temporal retinal zone consisted of a pure population of RH2 class gene-based pigments (λmax=492 nm). The long and short cones from other retinal areas and the lateral components of the triple cones possessed a mixture of RH2 and LWS class gene-based pigments that exhibited a λmax of ~502 nm. The central component of the triple cones contained only RH2 class gene-based pigments (λmax=474 nm). Thus, E. japonicus possesses a middle-wave range of spectral sensitivity and acquires different color vision systems in distinct visual fields.

Structure and spectral sensitivity of photoreceptors of two anchovy species: Engraulis japonicus and Engraulis encrasicolus.

The morphology, fine structure and spectral sensitivity of retinal photoreceptors of two anchovy species were investigated using light and electron microscopy and microspectrophotometry. Distinct regional specialisation of cones was observed. Long and short (bilobed) cones were observed in the horizontal retinal belt, including the nasal and temporal retinal zones. Only triple cones with two long lateral components, one small central component were observed in the dorsal and ventro-nasal retinal regions. The long cones presented various lamellar organisation patterns: (1) in parallel along the cell axis in the central retina, (2) oriented transversely at the base of the outer segment, and (3) tilted longitudinally while extending to the tip of the cone in the retinal periphery. In the short cones, the lamellae were always oriented along the cell axis, and their planes were perpendicular to the lamellae in the long cones, providing a structural basis for the detection of polarisation of incident light. The lamellae in all the outer segments of the triple cones are arranged perpendicular to the long cell axis. In both species, the long and short cones from the ventro-temporal retina were slender and more densely packed, and the outer segments of the long cones lay far more sclerad compared with the outer segments of the bifid cones. Microspectrophotometry revealed that in both species the lateral components of the triple cones displayed a maximum absorbance wavelength (λ(max)) of approximately 502nm, while the short central components were more shortwave sensitive (λ(max)=475nm). The λ(max) of all long and short cones in the ventro-temporal zone was 492nm, compared to 502nm in other retinal regions. Anchovies are unique among vertebrates in that they contain clear structural basis for both colour and polarisation vision in the same retina.

Long-wave sensitivity in the masked greenling (Hexagrammos octogrammus), a shallow-water marine fish.

Microspectrophotometry (MSP) revealed that surprisingly for a "fully marine" species, in summer, photoreceptors of the nearshore scorpaeniform fish known as the masked greenling, Hexagrammos octogrammus, contained exclusively, or presumably, porphyropsin with a small admixture of rhodopsin. As a result of this, the lambda(max) of the spectral sensitivity of the photoreceptors were significantly shifted to longer wavelengths as compared to the lambda(max) typical of marine shallow-water fishes, showing about 530 nm for rods and single cones, and 570/625 nm for double-cone members. These unique spectral shifts would permit a cone-driven wavelength discrimination in spite of high-density orange corneal filters which block light at lower wavelengths.