Temporal fluctuation in the abundance of alginate‐degrading bacteria in the gut of abalone Haliotis gigantea over 1 year

In this study, we identified and enumerated alginate‐degrading bacteria in the gut of abalone over 1‐year period. From a total of 360 colonies growing on agar medium enriched with alginate, 251 isolates (70%) had the ability to degrade alginate. In addition, a high number of viable alginate‐degrading bacteria were detected throughout the survey period. Alginate‐degrading bacteria were more abundant in the cold season relative to the summer season (10⁷ vs. 10⁴ CFU g^?1, respectively). Strong positive correlation was also observed between the number of alginate‐degrading bacteria and feed intake (R = 0.854; P < 0.01). The identified alginate‐degrading bacteria comprised of 35 species grouped into 11 genera including Algibacter, Formosa, Polarybacter, Tamlana, Tenacibaculum (CFB group), Roseobacter, Ruegeria, Silicibacter (α‐proteobacteria), Agarivorans, Shewanella and Vibrio (γ‐proteobacteria) respectively. More than 80% of the isolated alginate‐degrading bacteria belonged to the genus Vibrio, showing high homology to Vibrio cyclotorophicus, Vibrio splendidus, Vibrio halioticoli and Vibrio neonatus. Based on the results, it was suggested that algal‐polysaccharide (alginate) degrading bacteria (mainly Vibrio) commonly exist in the gut of abalone and may play an important role in the degradation and digestion of the host's feed.     

Establishment of a model for chemoattraction of <Emphasis Type="Italic">Symbiodinium</Emphasis> and characterization of chemotactic compounds in <Emphasis Type="Italic">Acropora tenuis</Emphasis>

Corals harbor symbiotic dinoflagellates, Symbiodinium spp., acquired from surrounding environments. Because Symbiodinium are present at low densities in the water column, corals may attract these symbionts using chemotactic compounds. To examine whether corals contain chemotactic compounds, we established an assay to measure the chemotactic activity for Symbiodinium using an extract of the coral Acropora tenuis, a major reef-building coral in Japan. Our assay revealed that Symbiodinium strain NBRC102920 (clade A), which is taken up by juvenile A. tenuis polyps, is attracted to crude A. tenuis extracts. We found that the chemotactic compounds are water-soluble, heat-labile macromolecules and that the chemotactic activity was inhibited by N-acetyl-d-glucosamine (GlcNAc). We separated the GlcNAc-binding fraction (Fr-ActL) and identified it as the most plausible candidate for the chemoattractant, since the chemotactic activity of the crude A. tenuis extract appeared to be mainly attributable to the activity of Fr-ActL and was also inhibited by the addition of GlcNAc. These results indicate that chemoattraction is mediated via the binding of Symbiodinium to Fr-ActL.     

Susceptibility of isogeneic ginbuna Carassius auratus langsdorfii Temminck et Schlegel to cyprinid herpesvirus‐2 (CyHV‐2) as a model species

Herpesviral haematopoietic necrosis (HVHN), caused by cyprinid herpesvirus‐2 (CyHV‐2), has affected the commercial production of the goldfish Carassius auratus and gibelio carp Carassius auratus gibelio. High water temperature treatments are reported to reduce the mortality rate of infected goldfish and elicit immunity in the survivors. To define the mechanism by which this intervention induces resistance, clonal ginbuna Carassius auratus langsdorfii, which is closely related to both species and has been used in fish immunology, may represent a promising model species. In this study, we investigated the susceptibility of clonal ginbuna strains to CyHV‐2 and the effect of high water temperature treatment on infected ginbuna and goldfish. Experimental intraperitoneal infection with CyHV‐2 at 25 °C caused 100% mortality in ginbuna strains, which was accompanied by histopathological changes typical of HVHN. Both infected ginbuna S3n strain and goldfish, exposed to high temperature for 6 days [shifting from 25 °C (permissive) to 34 °C (non‐permissive)], showed reduced mortalities after the 1st inoculation, and subsequent 2nd virus challenge to 0%, indicating induction of immunity. It was concluded that ginbuna showed a similar susceptibility and disease development in CyHV‐2 infection compared to goldfish, suggesting that ginbuna can be a useful fish model for the study of CyHV‐2 infection and immunity.     

A preliminary experiment regarding the natural induction of gonadal development in female Japanese eels without hormone treatment

To test whether gonadal development of female eels could be promoted without any exogenous hormone treatments, we observed the effect of water temperature manipulation. After 3–5 months of water temperature treatments, three silver eels showed higher gonadosomatic indices (GSI). In particular, one eel in the 5–15°C fluctuating temperature treatment group (5°C daytime and 15°C night‐time) had the highest GSI of 8.5 with secondary yolk globule stage oocytes of large diameter (OD; 412 μm) after 3 months, which indicated definite gonadal development compared with those in the initial states (mean GSI, 2.4; OD, 226.7 μm). The 5–15°C fluctuating temperature and constant 5°C groups had low oocyte breakdown (atretic) rates. Because daily temperature fluctuations and cold daytime water are experienced by vertically migrating silver eels in the ocean, these temperature conditions may be key maturation process components that could be useful for hormone‐free artificial maturation.     

Isolation and characterization of bacteria causing mortality in early stage larvae of captive‐bred Japanese eels (Anguilla japonica Temminck & Schlegel)

Bacteria with lethal effect on eel larvae were isolated from moribund captive‐bred eel larvae and their 16S rRNA gene sequences were analysed. Nucleotide sequence of 10 isolates showed highest similarity with Lacinutrix algicola, Crocinitomix catalasitica and Pseudoalteromonas rubra. Age‐dependent changes in the susceptibility of eel larvae were observed in response to challenge by a highly lethal isolate. Compared with 10 and 11 days after hatching (DAH), larval susceptibility to the highly lethal isolate was lower at 18 DAH. We found that the bacterial isolates have lethal effect on the captive‐bred eel larvae, especially at the early developmental stage. These results will be useful to establish appropriate culture practices for eel larvae that will improve the success of mass production of glass eels for aquaculture.     

Assessing the potential impacts of climate change on the alpine habitat suitability of Japanese stone pine (Pinus pumila)

To assess the potential distribution of Pinus pumila, a dominant species of the Japanese alpine zone, and areas of its habitats vulnerable to global warming, we predicted potential habitats under the current climate and two climate change scenarios (RCM20 and MIROC) for 2081–2100 using the classification tree (CT) model. The presence/absence records of P. pumila were extracted from the Phytosociological Relevé Database as response variables, and five climatic variables (warmth index, WI; minimum temperature for the coldest month, TMC; summer precipitation, PRS; maximum snow water equivalent, MSW; winter rainfall, WR) were used as predictor variables. Prediction accuracy of the CT evaluated by ROC analysis showed an AUC value of 0.97, being categorized as “excellent”. We designated Third Mesh cells with an occurrence probability of 0.01 or greater as potential habitats and further divided them into suitable and marginal habitats based on the optimum threshold probability value (0.06) in ROC analysis. Deviance weighted scores revealed that WI was the largest contributing factor followed by MSW. Changes in habitat types from the current climate to the two scenarios were depicted within an observed distribution (Hayashi’s distribution data). The area of suitable habitats under the current climate decreased to 25.0% and to 14.7% under the RCM20 and MIROC scenarios, respectively. Suitable habitats were predicted to remain on high mountains of two unconnected regions, central Honshu and Hokkaido, while they were predicted to vanish in Tohoku and southwestern Hokkaido. Thus P. pumila populations in these regions are vulnerable to climate change.     

Preparation of β-1,3-glucanase from scallop mid-gut gland drips and its use for production of novel heterooligosaccharides

The mid-gut gland of scallop Patinopecten yessoensis has been discarded in scallop processing factories as a fishery waste and various attempts have been made to turn the waste into valuable resources. In the present study, we tried to use mid-gut gland drips from scallop as a source of β-1,3-glucanase. The mid-gut gland drips were collected in a local fishery factory in Yubetsu-cho, Hokkaido Prefecture. β-1,3-Glucanase was purified from the mid-gut gland drips by ammonium sulfate fractionation followed by successive chromatography on Toyopearl Phenyl-650M and Toyopearl DEAE-650M. The scallop β-1,3-glucanase, named PyLam38 in the present study, showed a molecular mass of approximately 38 kDa by sodium dodecylsulfate-polyacrylamide gel electrophoresis, and hydrolyzed laminarin, a β-1,3-glucan from Laminaria sp., producing laminaribiose and glucose with an optimal pH and temperature of 6.0 and 45°C, respectively. PyLam38 exhibited high transglycosylation activity toward various accepter substrates such as monosaccharides, alcohols and xylooligosaccharides. Thus, PyLam38 was found to be useful for the production of various novel heterooligosaccharides consisting of laminarioligosaccharides and various accepters.