Jun Kohbara

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).

Self-feeding behavior of yellowtail, Seriola quinqueradiata, in net cages: diel and seasonal patterns and influences of environmental factors

Abstract The self-feeding pattern of yellowtail, Seriola quinqueradiata , maintained in floating net cages was examined throughout a year, and the influence of seasonal changes of the photoperiod and water temperature on the feeding pattern was investigated. Two groups of 50 yellowtail (initial mean body weight about 50 and 80 g, respectively) were kept in two experimental net cages (2×3×4 m deep), with a self-feeding device installed on each. It was possible to maintain yellowtail in net cages using a self-feeder throughout almost a year. The number of daily food demands was greatly affected by the seasonal changes in water temperature. Yellowtail showed high self-feeding activity, above 18 °C; depressing water temperatures did not influence the activity when the water temperature was over 18 °C. However, the activity decreased when the water temperature fell below 18 °C. The feeding pattern in a 24-h period was greatly affected by the temporal changes in light intensity. Annual observations revealed that yellowtail showed generally crepuscular plus nocturnal feeding behavior and had two peaks of feeding activity a day. These peaks appeared at dawn and dusk; moreover, a clear association between the most active time of self-feeding and the period of the greatest change in light intensity was observed. The results suggest that a change in light intensity might stimulate the appetite of yellowtail or that there is a light level at which yellowtail prefer to eat.

Dietary utility of enzyme-treated fish meal for juvenile Pacific bluefin tuna Thunnus orientalis

In order to develop an artificial diet, the dietary utility of enzyme-treated fish meal was investigated for juvenile Pacific bluefin tuna Thunnus orientalis (PBT). Diets containing each 63% of Chilean fish meal (FM), enzyme-treated chilean fish meal (EC) and enzyme-treated Peruvian fish meal (EP), with 10% bonito oil and raw sand lance Ammodytes personatus (SL) were fed to juvenile tuna six times per day for one week. In a different trial, diets EC and SL were fed to tuna six times per day for 2 weeks. Only diet EC sustained similar growth or caused lower survival and higher feed efficiency, hepato- and enterosomatic indices and final carcass lipid content as compared to those of SL. Diets FM and EP led to lower specific growth rate (SGR) but similar feed efficiency, survival and hepatosomatic index, yet higher enterosomatic index. Moreover, PBT fed diet EC for 2 weeks led to similar growth performance but higher final carcass and hepatic lipid contents, and plasma cholesterol and phospholipid levels than those fed SL. Carcass fatty acid composition of diet EC group had lower 20∶5 n-3 and 22∶6 n-3 levels than the SL group. These results revealed that EC, as a suitable dietary protein source, could sustain growth of PBT, while dietary bonito oil led to higher carcass lipid but lower accumulation of n-3 highly unsaturated fatty acids.

Amino acid receptor sites in the facial taste system of the sea catfish Arius felis

Abstract1.The amino acid sensitivity and specificity of the facial taste system of the marine catfish, Arius felis, is characterized electrophysiologically.2.The facial taste system of Arius felis responded to all 28 amino acids tested, but was highly sensitive to only a few. In general, acidic amino acids and neutral amino acids with short side chains were more effective than imino, basic and neutral amino acids with long side chains.3.A reciprocal cross-adaptation protocol used to characterize the receptor sites identified at least some relatively independent receptor sites for L-arginine, L-histidine, L-proline, L-alanine, glycine, D-alanine and L-glutamate.4.Of the 7 amino acids that were indicated to have relatively independent receptor sites, the median electrophysiological threshold for L-alanine, the most stimulatory, and L-proline, the least stimulatory compounds, were 10 nM and 10,000 nM, respectively. The integrated facial taste response did not saturate at test amino acid concentrations up to 10 mM.5.The generalized depression in responsiveness to test stimuli observed during amino acid adaptation is proposed to be a result of the co-distribution of sensitivity at the level of single taste cells rather than high cross-reactivity of the respective amino acid receptor sites for the test stimuli.

Identification of feeding stimulants from a jack mackerel muscle extract for young yellowtail Seriola quinqueradiata

The feeding-stimulatory components of an aqueous extract of jack mackerel white muscle for young yellowtail were identified. The extract was fractionated using anion-exchange chromatography, and the feeding-stimulatory effectiveness of fractionated components examined by adding them to starch pellets and feeding to yellowtail. The filtrate (FE) of the muscle extract filtered through a membrane having a mol. wt. cut-off of 10,000 Da was found to have a high feeding-stimulatory effectiveness on the yellowtail. FE was fractionated using DEAE-Sephadex A-25 at pH 5.5. The DEAE-Sephadex adsorbate (AA) showed an effectiveness close to but slightly weaker than that of FE. The DEAE-Sephadex non-adsorbate (NA) had no appreciable effect. Subdividing AA components by stepwise elutions with NaCl solutions suggested that inosine-5′-monophosphate and lactic acid were largely responsible for the stimulatory effectiveness of AA. Some components in NA might also be synergistic with AA components to elicit the full effectiveness of FE.

Gustatory and olfactory sensitivity to extracts of jack mackerel muscle in young yellowtail Seriola quinqueradiata.

Abstract The stimulatory effects of extract fractions of jack mackerel muscle, obtained using anion exchange chromatography, and their components on the gustatory and olfactory receptors of young yellowtail were investigated by recording electrical responses from the palatal taste nerve and the olfactory bulb. Ultrafiltrate (mol. wt. cut-off, 10,000 Da) of the muscle extract (FE) stimulated both chemosensory systems. FE was treated with a DEAE-Sephadex A-25 column (pH 5.5). The olfactory receptors showed a high sensitivity to both the adsorbate and non-adsorbate from the DEAE-Sephadex column. Their thresholds in the olfactory bulb response were around 10−5 and 10−8 of the original concentration, respectively. Those in the gustatory nerve response were 10−4 and 10−2. Amino acids were highly effective for the olfactory receptors. The threshold of l -glutamine was around 10−8 M. 5′-Inosinic acid and l -lactic acid were more effective for the gustatory receptors than for the olfactory receptors; their thresholds for the former were 10−4 M and 10−6 M, while those for the latter were 10−3 M and above 10−3 M. These findings suggest that the gustatory receptors are involved in discriminating food items during feeding and that the olfactory receptors may detect foods at some distance.

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.

L-Lactate Sensitivity of Fish Taste Receptors

Organic acids are abundant in the tissue extracts of fish and other aquatic organisms. However, our knowledge of their stimulatory effect on feeding and their stimulatory effect on the taste receptors of fish is still fragmentary. In a previous study [1], the palatal taste receptors of the yellowtail Seriola quinqueradiata were found to be sensitive to some organic acids, especially to L-lactic acid. This suggests the biological importance of these acids associated with the gustatory sense of fish, since the tissues of potential food organisms contain these acids at varying concentrations as cell metabolites. Moreover, it seems interesting to determine whether lactic acid stimulates the receptor sites for amino acids, since its chemical structure has some resemblance to alanine.