Taeko Miyazaki

Feeding behaviour of hatchery-reared juveniles of the Japanese flounder following a period of starvation.

Abstract The effect of starvation on feeding behaviour was studied in hatchery-reared juveniles of the Japanese flounder in the context of improving stock enhancement strategies. Fish (54.0±5.9 mm in total length) were deprived of food (mysids) for 1, 2, or 4 days, and their subsequent feeding behaviour was observed using a video camera. The phase of feeding action, the foraging pattern, the off-bottom duration, and the swimming speed were defined and analyzed. The feeding action was divided into four phases: aim, creep, attack, and return. The foraging patterns were classified into four types. The typical foraging pattern, with a single attack and a quick return to the initial position (type A), became less common as the fish were starved, while there was a greater incidence of slower returns (type B), returns to different positions (type C), and multiple feeding (type D). The frequency of the feeding action and the duration of off-bottom swimming increased as the food deprivation period increased. The swimming speed for creeping to the mysid prey increased, but that of the return decreased as the fish were starved. These changes in feeding behaviour under starvation may prolong the time flounder juveniles spend in the water column and may result in a higher risk of predation. In the stock enhancement program of Japanese flounder, starvation should be avoided to reduce predation and improve survival.

Three cone opsin genes and cone cell arrangement in retina of juvenile Pacific bluefin tuna Thunnus orientalis

In bluefin tuna aquaculture, collision of juveniles with the tank or net walls is a major cause of high mortality. This problem may be related to color sensibility of the visual mechanisms of this species. As a first step in understanding of color vision of Pacific bluefin tuna Thunnus orientalis, we applied a molecular technique and histology to study cone cell distribution in the retina of juvenile fish. We isolated three cone opsin genes encoding one blue-sensitive (SWS2) and two green-sensitive (RH2) visual pigments. In situ hybridization revealed that SWS2 mRNA is localized in the single-cone photoreceptors. The localization of the two RH2 signals in distinct cone cells was not determined, probably because of the high homology between the two RH2 genes. Single-cone photoreceptors appeared frequently in the ventral-temporal retina in approximately 80-mm fish and in the temporal retina in approximately 230-mm fish. These cone distributions may define a visual field with best color contrast vision in front and above the fish with a short wavelength (blue) reflecting target (sensed by single cones), and may be enhanced against the longer wavelength (green) background when fish see a target below them (sensed by double cones).

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

SUMMARY 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.

Retinal topography of ganglion cells and putative UV-sensitive cones in two Antarctic fishes: Pagothenia borchgrevinki and Trematomus bernacchii (Nototheniidae).

Abstract Accessory corner cones (ACC) have recently been suggested to be UV-sensitive photoreceptor cells. With a view toward explaining prey detection, we examined the topography of retinal ganglion cells and ACCs in two Antarctic nototheniids occupying different ecological niches: the cryopelagic Pagothenia borchgrevinki and the benthic Trematomus bernacchii. Isodensity maps of retinal ganglion cells showed that the main visual axis, coincident with the feeding vector, was in a forward direction in both species. Visual acuity was determined as 3.64 and 4.77 cycles/degree for the respective species. In P. borchgrevinki the highest density of ACCs was associated with the eyes main visual axis. This suggested that this species uses UV-vision during forward-swims and probably in encounters with prey. On the other hand, T. bernacchii possessed two horizontal band-shaped high-density areas of ACCs, which stretched from temporal to nasal and ventral to peripheral retinal regions. Therefore, this species appears to use UV-vision to watch prey across the entire circumference of the lateral area and in the water column above its head.

Putative ultraviolet-photosensitivity in the retina of 1-year-old nibbler Girella punctata: Based on molecular and histological evidences

Ultraviolet (UV) photosensitivity in 1-year-old nibbler Girella punctata, which inhabits the coastal reefs of Japan, was investigated, based on retinal histology and visual pigment cDNA cloning. The retinal cone mosaic of the nibbler showed a square-shaped arrangement of double cones with a central single cone and accessory corner cones, which are representative putative UV-photoreceptors, at each corner of the square. Six cDNA fragments encoding putative visual pigment of nibbler were isolated and sequenced. Its deduced amino acid sequences were classified into five classes of opsin: UV, blue, green (one gene each), red (two genes), and rod (one gene) by comparison with other teleost opsins and phylogenetic analysis. Possible contributions of UV-sensitivity in nibbler was discussed regarding homing, feeding, and schooling.

Morphological characteristics of eyes and retinas of two sardines (Sardinops melanostictus and Etrumeus sadina, Clupeidae) and an anchovy (Engraulis japonicus, Engraulididae)

The morphology of the eyes and distribution of retinal ganglion cells in two sardine species (Sardinops melanostictus and Etrumeus sadina, Clupeidae) and the Japanese anchovy (Engraulis japonicus, Engraulididae) were investigated anatomically and histologically. The eyes of the sardines faced a slightly dorsolateral direction with the visual field extended obliquely upward. In contrast, the eyes in the anchovy were almost laterally directed. It was hypothesized that the sardines may have an advantage in receiving more downward irradiance compared with the anchovy. The lens muscle was larger in these three species than in many other teleosts, and its surface was entirely melanin‐pigmented. Also, the lens muscle directly and tightly adhered to the backside surface of the iris. The relative area of the lens muscle to the area of the lens, a referential value of the relative power of visual accommodation were notably larger in the species studied than in other teleost values that have been previously reported. A higher M/L% value of these clupeid fishes could facilitate fast and wide ranging visual accommodation and was considered to be associated with maintaining and/or re‐establishing school formations quickly. Analysis of topographical distributions of cells in the ganglion cell layer showed that cell density was highest in the ventrotemporal quadrant of the retina (temporal of the optic cleft) in all three species. Another potentially important role for the black‐pigmented lens muscle may be to block the specialized retinal area from intense sunlight that scatters and irradiates upward or laterally in the surface waters that they inhabit. Thus, the sardine and anchovy may take advantage of efficient detection of visual signals in the frontal‐upward direction and further improve visibility of the target in this direction. J. Morphol. 276:415–424, 2015.

Retinal ganglion cell topography in juvenile Pacific bluefin tuna Thunnus orientalis (Temminck and Schlegel)

The retinal ganglion cell distribution, which is known to reflect fish feeding behavior, was investigated in juvenile Pacific bluefin tuna Thunnus orientalis. During the course of examination, regularly arrayed cells with a distinctive larger soma, which may be regarded as motion-sensitive cells, were found. The topographical distribution of ordinary-sized ganglion cells, which is usually utilized to estimate fish visual axis and/or visual field characteristics, showed that the highest-density area, termed the area centralis, was localized in the ventral-temporal retina. The retinal topography of ordinary-sized ganglion cells seems to reflect the bluefin tuna’s foraging behavior; while cruising, cells in the area centralis may signal potential prey, such as small schooling pelagic fishes or squids, that are present in the upward-forward direction. Judging from morphological characteristics, the large ganglion cells localized in the small temporal retinal area seem to be equivalent to physiologically categorized off-center Y-cells of cat, which are stimulated by a transient dark spot in a bright visual field. It was inferred that presumed large off-center cells in the temporal retina detect movements of agile prey animals escaping from bluefin tuna as a silhouette against environmental light.

Sequence and localization of an ultraviolet (sws1) opsin in the retina of the Japanese sardine Sardinops melanostictus (Teleostei: Clupeiformes)

A full-length complementary (c)DNA encoding ultraviolet (UV)-sensitive opsin (sws1) was isolated from the retina of the Japanese sardine Sardinops melanostictus. The sws1 phylogenetic tree showed a sister group relationship with the Cypriniformes, following the ray-finned fish phylogeny. By expressing reconstituted opsin in vitro, it was determined that the maximum absorbance spectrum (λmax ) of sws1 is around 382 nm, being intermediate in position between two subtypes of sws1 pigment that are UV sensitive (λmax  = 355-380 nm) and violet sensitive (λmax  = 388-455 nm), which have been reported to date. The ocular media transmitted >20% transmittance of light in the range of 360-600 nm. In situ hybridization analyses revealed that sws1 messenger (m)RNA is localized in a central single cone surrounded by four double cones in a square mosaic. The square mosaic occupies the ventro-temporal quadrant of the retina and the in situ hybridization signals were dominant in this area suggesting that the fish may use UV vision when looking upward. Based on these results, considerable significances of potential UV sensitivity, in relation to characteristic habits of S. melanostictus, are discussed.

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.