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During field surveys and culture procedures, large growth disparities in Anguilla japonica have been observed. However, the potential causes are unknown. This study explored differences in digestive ability, metabolic levels, and transcriptomic profiles of appetite-related genes between growth-retarded eel (GRE) and normal-growing eel (NGE) under the same rearing conditions. The results showed that growth hormone (gh) mRNA expression in GREs was considerably lower than NGEs. The levels of total protein (TP), total cholesterol (T-CHO), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), blood ammonia (BA), blood urea nitrogen (BUN), and alkaline phosphatase (ALP) in GREs were significantly lower than in NGEs. Conversely, levels of glucose (GLU), alanine aminotransferase (ALT), and aspartate transaminase (AST) were higher in GREs. The activities of SOD, CAT, and T-AOC levels were also significantly lower in GREs, as were the activities of glucose-related enzymes including hexokinase (HK), pyruvate kinase (PK), phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase (G6PASE). Additionally, orexigenic genes (npy and ghrelin) were dramatically downregulated, whereas anorexigenic genes (crh and pyy) were significantly upregulated in GREs. These findings suggested that variances in growth hormone, metabolic activities, and appetite level could be associated with the different growth rates of A. japonica. The present research not only revealed the characteristics of the growth, metabolism, and appetite of GREs but also offered new perspectives into the substantial growth discrepancies in A. japonica, providing novel ideas for enhancing fish growth. [ABSTRACT FROM AUTHOR]

Yellow-phase Japanese eel Anguilla japonica (Temminck & Schlegel, 1846) utilize a variety of anthropogenically impacted habitats. An extensive understanding of the trophic ecology and habitat use is necessary for the conservation of this endangered species. In this study, we analysed the trophic ecology and feeding habitat of Japanese eels in two small rivers, the Akugawa River and Takase River in Wakayama Prefecture, Japan, dominated by an urban landscape and agricultural land use, respectively. The main goals were to assess the dietary diversity and trophic niche width for eels of two size classes (small eels: ≤ 240 mm total length, large eels: > 240 mm total length) in both habitats. The suitability of both rivers as a feeding habitat for eels was also evaluated. The trophic ecology of eels was inferred based on analysis of carbon and nitrogen stable isotopes, as well as stomach contents. Our results indicated that both rivers are suitable habitats to accommodate this eel, but to varying degrees. The dietary diversity of small eels and trophic niche width differed between rivers, indicating limiting factors, such as insufficient food sources and unfavourable abiotic habitat requirements for small eels in urban rivers. Also, the body condition of eels was found to be lower in the urban Akugawa River than in the Takase River. [ABSTRACT FROM AUTHOR]

To understand the larval distribution, size variation, and stable isotope ratios of Japanese eel leptocephali in relation to the salinity front and their feeding ecology, larvae from 7 research cruises (2002–2013) in the North Equatorial Current (NEC) spawning area were examined. The smallest early‐stage larvae were distributed south of or near the salinity front, confirming that the salinity front is an important oceanic feature to understand spawning locations of the Japanese eel. Larger size larvae tended to distribute into higher latitudes. Transport to northern latitudes with their growth would facilitate transport into the Kuroshio region, but retention in the Subtropical Countercurrent (STCC) might be detrimental. Preleptocephalus isotope ratios reflected maternal ratios, but feeding‐stage leptocephali (8–56 mm) tended to have higher ∂15N values at lower latitudes typically in areas south of a salinity front. Feeding larvae quickly assimilate isotope ratios from the NEC after spawning and early growth. Large differences of ∂13C values of larvae between the NEC and STCC might vary with spatial baselines in the two currents. However, diel vertical migrations should be considered, because the isotope ratios in particulate organic matter distinctly depend on the depth. Comparisons among Japanese eel larvae and other taxa of leptocephali in the NEC illustrate the need for further studies on the trophic ecology of leptocephali. [ABSTRACT FROM AUTHOR]

A novel filamentous eel-leptocephalus pathogenic marine bacterium, designated strain EL160426T, was isolated from Japanese eel, Anguilla japonica, leptocephali reared at a laboratory in Mie, Japan. In experimental infection studies on eel larvae, the strain EL160426T caused massive larval mortality and was reisolated from moribund leptocephali. Characteristically, observations of infected larvae found that EL160426T forms columnar colonies on the cranial surface of larvae. The novel isolate exhibited growth at 15–30 °C, pH 7–9, and seawater concentrations of 60–150% (W/V). Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain EL160426T was most closely related to Aureispira maritima 59SAT with 97.7% sequence similarity. The whole genome sequence analysis of the strain EL160426T showed that the strain maintained a circular chromosome with a size of approximately 7.58 Mbp and the DNA G + C content was 36.2%. The major respiratory quinone was MK-7 and the predominant cellular fatty acids were 16:0, 20:4 w6c (arachidonic acid), 17:0 iso and 16:0 N alcohol. DNA relatedness between the closest phylogenetic neighbor strain EL160426T and A. maritima (JCM23207T) was less than 13%. On the basis of the polyphasic taxonomic data, the strain represents a novel species of the genus Aureispira, for which the name Aureispira anguillae sp. nov. is proposed. The type strain is EL160426T (= JCM 35024 T = TSD-286 T). [ABSTRACT FROM AUTHOR]

The biomass of the Japanese eel (Anguilla japonica) is generally determined by the recruitment of glass eels into freshwater habitats, but the behavioral biology of their inshore migration remains unknown. With the aid of an ocean prediction system, we elucidated a recruitment migration scenario that can quantitatively reproduce a regional difference in biomass in Japan, which was previously estimated by an environmental DNA sampling method. For their successfully reaching shores, it is necessary to incorporate behavioral changes of glass eels within their migration on the Kuroshio Current, such as shallower depth preferences and horizontal swimming toward lower salinity water. In particular, the latter is essential for encouraging recruitment into both the Seto Inland Sea, with a relatively high ratio (20%–30%) of the total recruitment to Japan and the coasts in the central part of the Pacific side of northern Japan (i.e., the northern limit of the habitable distribution), manifesting that glass eels actively swim toward freshwater near coastal regions. In the subsurface layer, where glass eels mainly conduct diel vertical migration, there is a bifurcation path connecting the Kuroshio Current to the second and third branches of the Tsushima Warm Current, restricting the recruitment of glass eels into the Sea of Japan side of the main inland in Japan. The simulated recruitment validated that the eDNA acts as a proxy indicator for estimating the relative biomass on the regional scale. The simulation supported that the inshore migration of glass eels is determined by active horizontal swimming. [ABSTRACT FROM AUTHOR]

Predictability of glass eel recruitment of Japanese eel (Anguilla japonica) was examined with a simple indicator constructed using coastal water temperature under the influence of the western boundary current, the Kuroshio. The 1971–2014 glass eel catch from an estuary, Lake Hamana, Japan, was compared with the mean water temperature difference (WTD) between two coastal sites, Shirahama and Lake Hamana, during the glass eel fishing season. The directions of interannual change (i.e., increase/decrease) in the glass eel catch and WTD fully synchronized for 12 consecutive years (1995–2006), and these variables were positively correlated (r = 0.68, p = 0.0150). The WTD was correlated with the southernmost latitude of the Kuroshio path (r = 0.40, p = 0.0068) in the fishing season of the study period (1971–2014); therefore, the WTD appeared to reflect the path fluctuations of the Kuroshio and the warm water intrusions from this western boundary current. The proportion of matching directions between the interannual changes in the glass eel catch and WTD was 0.71 for the non-meander (nearshore) path years (significantly higher than 0.5, the expected proportion of matching signs due to chance; χ 2 = 2.88 , p = 0.0448), whereas it was 0.46 for the meander (offshore) path years ( χ 2 = 0.15 , p = 0.6526). Thus, the WTD tends to interannually increase/decrease with the estuarine recruitment of A. japonica glass eels when the Kuroshio flows with non-meander paths. This environment-recruitment relationship (western boundary current-recruitment link) in the Japanese eel demonstrates the importance of continuous environmental monitoring of coastal waters. [ABSTRACT FROM AUTHOR]

Frequent changes in the tide levels in estuaries cause constant changes in the hydraulics of fish passage systems, with important effects on successful fish passage and swimming behavior. In most cases, Japanese eels often have low passage rates in engineered fishways because of their special habits. In this study, we established a 1:4 scaled-down weir-hole combination bulkhead fishway, studied the effects of different tidal differences and water depths on the passage rates and swimming behavior of yellow-phase Japanese eels, and analyzed the response of the Japanese eels to the hydraulic factors by superimposing their swimming trajectories and the flow field simulation results. We found that the passage rate of the eels decreased from 68.18% to 50.00% and 45.45% under extreme high tide differences and extreme low tide differences, respectively. The eels tended to use the low-velocity area to climb up the wall, and when crossing the mainstream, the yellow-phase Japanese eels preferred the area with a flow velocity of 0.1~0.36 m/s and a turbulent kinetic energy range of 0.001~0.007 m2/s2. Their upstream swimming speed was maintained at a range of 0.1~0.3 m/s. [ABSTRACT FROM AUTHOR]

Background: The Japanese eel Anguilla japonica has a single spawning area on the west coast of the Mariana Islands, and the larvae disperse widely via the North Equatorial Current and the Kuroshio Current to freshwater and coastal areas of East Asia for nursery and growth habitats. Spawning adults reared in their respective habitats begin their migration to spawn from September to February, but little is known about regional differences in migration routes, their contribution to spawning populations, or their migratory behavior. To better understand the spawning migrations of Japanese eels, we tracked adult migratory stage eels (silver eels) captured in coastal areas of three regions (Tokai region, Sea of Japan coastline, and Tohoku region) from October 2019 to February 2020 using pop-up satellite archival tags. Results: Twenty-three tagged eels were released in each coastal area, and data were collected on 15 eels for a maximum of 59 days. Eels released in the Tokai region migrated southeast along the Kuroshio Current. On the other hand, eels on the Sea of Japan coastline and in the Tohoku region tended to migrate westward and southward against the ocean current, respectively, but no eels from either region reached waters near the Kuroshio Current. In addition, the present study found region-specific vertical movement patterns among the eels. Eels from the Tokai region showed typical diurnal vertical movements, while an eel from the Sea of Japan coastline repeated non-diurnal vertical movements, which was attributed to regional differences in bathymetry and water mass structure. Except for one eel suspected of being preyed upon, all other eels did not venture into depth zones with temperatures below 4 ∘ C , thus suggesting that the maximum depth of vertical movement is limited by water temperature. Conclusions: This study provided new insights into the migratory behavior of Japanese silver eel from the Sea of Japan coastline and the Tohoku region, but further long-term tracking is required to reveal the fate of the spawning adults from the coast of Japan. [ABSTRACT FROM AUTHOR]

Various aquatic organisms are suffering from alterations of estuarine environments, where human activities are generally concentrated. The commercially important but endangered Japanese eel (Anguilla japonica) is one of the species most affected by estuarine habitat loss, because it mainly uses estuarine benthic environments for growth before long migrations to an offshore spawning area. To enhance their estuarine habitats, this study aimed to explore what type of shelter structures promote medium- to long-term settlement of this species and verify whether such shelters improve their growth in a riverine estuary in Japan whose channel was heavily lined with concrete revetment. A survey of the recapture rate of elastomer-tagged eels from the fishing gear "Ishikura net (IKN)," which consists of dozens to hundreds of stones, showed that the rate was higher when their first captures were from IKNs with fine pore structures formed by approximately 10-cm-sized stones than from those with coarser pores. Furthermore, the girth length (nutritional condition index) of recaptured individuals increased during their settlement in the IKNs. The use of the IKNs may have reduced energetic costs of behaviors such as predation avoidance and feeding, considering that a large amount of prey animals (e.g., crabs) were found, especially in the IKNs with fine pores. This study showed that the establishment of artificial shelters with fine pore structures is effective not only for the temporary residence of eel individuals but also for improvement of their body conditions through promoting their settlement and perhaps gathering their prey species. [ABSTRACT FROM AUTHOR]

Under aquaculture conditions, Japanese eels (Anguilla japonica) produce a high percentage of males. However, females gain higher body weight and have better commercial value than males, and, therefore, a high female ratio is required in eel aquaculture. In this study, we examined the effects of isoflavones, genistein, and daidzein on sex differentiation and sex-specific genes of eels. To investigate the effects of these phytoestrogens on the gonadal sex, we explored the feminizing effects of soy isoflavones, genistein, and daidzein in a dose-dependent manner. The results showed that genistein induced feminization more efficiently than daidzein. To identify the molecular mechanisms of sex-specific genes, we performed a comprehensive expression analysis by quantitative real-time PCR and RNA sequencing. Phenotypic males and females were produced by feeding elvers a normal diet or an estradiol-17β- or genistein-treated diet for 45 days. The results showed that female-specific genes were up-regulated and male-specific genes were down-regulated in the gonads, suggesting that genistein induces feminization by altering the molecular pathways responsible for eel sex differentiation. [ABSTRACT FROM AUTHOR]

• The transcripts of cyp19a1 , sf-1 , foxl2s , and esrs in the midbrain/pituitary were significantly increased during testicular differentiation. • The expressions of gths and gh transcripts in the midbrain/pituitary were significantly increased during testicular differentiation. • The increased aromatase IHC staining was detected in the parvocellular preoptic nucleus of forebrain in the male differentiating elvers. • No significant changes of the genes examined in the forebrain and midbrain/pituitary in the E2-feminized Japanese eels. Eels are gonochoristic species whose gonadal differentiation initiates at the yellow eel stage and is influenced by environmental factors. We revealed some sex-related genes were sex dimorphically expressed in gonads during gonadal sex differentiation of Japanese eel (Anguilla japonica); however, the expression of sex-related genes in the brain-pituitary during gonadal sex differentiation in eels is still unclear. This study aimed to investigate the sex-related gene expressions in the brain-pituitary and tried to clarify their roles in the brain and gonads during gonadal sex differentiation. Based on our previous histological study, the control eels developed as males, and estradiol-17β (E2) was used for feminization. Our results showed that during testicular differentiation, the brain cyp19a1 transcripts and aromatase proteins were increased significantly; moreover, the cyp19a1 , sf-1 , foxl2s , and esrs (except gperb) transcripts in the midbrain/pituitary also were increased significantly. Forebrain gnrh1 transcripts increased slightly during gonadal differentiation of both sexes, but the gnrhr1b and gnrhr2 transcripts in the midbrain/pituitary were stable during gonadal differentiation. The expression levels of gths and gh in the midbrain/pituitary were significantly increased during testicular differentiation and were much higher in males than in E2-feminized females. These results implied that endogenous estrogens might play essential roles in the brain/pituitary during testicular differentiation, sf-1 , foxl2s , and esrs may have roles in cyp19a1 regulation in the midbrain/pituitary of Japanese eels. For the GnRH-GTH axis, gths , especially fshb , may be regulated by esrs and involved in regulating testicular differentiation and development in Japanese eels. [ABSTRACT FROM AUTHOR]

Farmed anguillid eels are frequently stocked into natural fresh waters to enhance eel resources, but little is known about what happens to these eels or their interactions with wild eels after stocking. A recent study observed a depressed survival and growth rate of farmed Japanese eels when they were reared with wild eels, which indicated that wild eels might interfere with the survival and growth of farmed‐and‐stocked eels through intraspecific competition. To contribute to improving eel stocking efficiency, the growth of farmed‐and‐stocked Japanese eels was compared among four rivers with different wild eel densities using mark‐and‐recapture studies. Based on the 2‐year recapture survey after stocking, it was found that the density of the farmed‐and‐stocked eels was not significantly different among rivers. The daily growth rates of farmed‐and‐stocked eels in the rivers with lower wild eel density were significantly higher than those of the eels stocked into the rivers with higher wild eel density. The farmed‐and‐stocked eels moved significantly greater distances downstream than wild eels that showed sedentary behaviour. This and previous studies indicate that significant questions remain about the effectiveness of stocking farmed eels into water bodies where naturally recruited wild eels are present. [ABSTRACT FROM AUTHOR]

Neurokinin B (NKB), a member of the tachykinin (TAC) family, plays important roles in mammalian neuropeptide secretion in related to reproduction. However, its potential role in spawning migration teleost is less clear. In the present study, Japanese eel (Anguilla japonica) was employed to study the performance of NKB in regulating reproduction. Results showed that two tac3 and one tacr3 genes were identified in Japanese eel. Sequence analysis showed that two tac3 transcripts, tac3a and tac3b, encode four NKBs: NKBa-13, NKBa-10, NKBb-13, and NKBb-10. However, compared with other species, a mutation caused early termination of TACR3 protein was confirmed, leading to the loss of the 35 amino acid (aa) C-terminal of the receptor. Expression analysis in different tissues showed that both tac3a and tac3b mRNAs were highly expressed in the brain. In situ hybridization localized both tac3a and tac3b mRNAs to several brain regions, mainly in the telencephalon and hypothalamus. Because of the mutation in TACR3 of Japanese eel, we further analyzed whether it could activate the downstream signaling pathway. Luciferase assay results showed the negative regulation of cAMP Response Element (CRE) and Sterol Response Element (SRE) signal pathways by Japanese eel NKBs. Intraperitoneal injection of four different NKB mature peptides at 100 ng/g had negative effect on either gnrh or gth gene expression. However, the high concentration of NKBa-10 and NKBb-13 (1,000 ng/g) upregulated mgnrh and fshb or lhb expression level significantly, which may be mediated by other receptors. In general, the NKBs/NK3Rs system has important functions in regulating eel puberty onset. [ABSTRACT FROM AUTHOR]

To contribute to improving eel stocking procedures, the survival, growth and behaviour of farmed Japanese eels reared together with wild individuals were evaluated to learn about possible encounters of farmed‐ and stocked eels with wild eels. Two experiments were conducted to evaluate the (a) effect of the presence of wild eels on survival and growth of farmed eels in experimental ponds and (b) behavioural differences between wild and farmed eels placed in a small tank. In the survival and growth experiment, significantly lower survival and growth rates of farmed eels reared with wild eels were observed compared with farmed eels reared without wild eels (control). In the behavioural observations, the authors observed significantly higher occupation rates of a refuge and more frequent biting for wild eels, indicating a dominance of wild eels over farmed eels. Because wild and farmed eels used in the current experiments belong to the same genetic population, these differences resulted within one generation through the aquaculture rearing process. [ABSTRACT FROM AUTHOR]

Most cultured Japanese eels (Anguilla japonica) show male sex differentiation; however, natural gonadal sex differentiation has not been evaluated. In this study, this process was characterized in wild eels. Differentiated ovaries and testes were observed after the eels grew to 320 and 300 mm in total length, respectively. The youngest ovary and testis appeared at 3 and 4 years old, respectively; however, undifferentiated gonads were found up to 7 years, suggesting that sex differentiation was triggered by growth rather than aging. gsdf, amh, foxl2b and foxl3b were highly expressed in the testes, whereas figla, sox3, foxn5, zar1, and zp3 were highly expressed in the ovaries. The expression of cyp19a1a and foxl2a did not differ significantly between the testis and ovary. In the ovaries, the cyp19a1a and foxl2a levels were highest in the early stages, suggesting that their function is limited to early ovarian differentiation. The foxn5, zar1 and zp3 levels tended to increase in the later stages, suggesting that they function after the initiation of ovarian differentiation. In undifferentiated gonads, dimorphic gene expression was not observed, suggesting that the molecular sex differentiation phase is short and difficult to detect. These findings provide the first demonstration of the whole course of natural gonadal sex differentiation in eels at molecular and morphological levels. [ABSTRACT FROM AUTHOR]

The late-summer diel activity patterns and space utilization of five Japanese eels, Anguilla japonica, were investigated using an acoustic telemetry positioning system in a shallow brackish lake in western Japan. Eels were highly nocturnal and crepuscular, with 98.1% of positions detected between sunset and sunrise, and were more active during dusk and dawn compared with nighttime, and during full moons compared with other lunar periods. Most diurnal activity occurred during darker conditions, and two individuals extended their activity into the morning, suggesting that overcast skies reduced light intensity and prolonged nocturnal activity into daytime. Individual eels had different home-range patterns, but appeared to rest nearshore during the day, with space utilization expanding offshore at dusk and night, followed by returning to nearshore at dawn. Their home ranges and core activity areas were bordered by the shoreline, with eels tending to exhibit small activity spaces and strong site fidelity. This study provides valuable ecological knowledge about the individual behavior of eels inhabiting a small brackish lake at the northern limit of the natural distributional range of the species along the Sea of Japan, which can contribute to management and conservation of the species. [ABSTRACT FROM AUTHOR]

Objective. According to reported spawning characteristics of Japanese eel, Anguilla japonica, which exhibit spawning and migration patterns that are synchronized with lunar cycles and photoperiod, we hypothesized that a close association exists between specific photic signals (daylight, daylength, and moonlight) and endocrinological regulation. Given the photic control in melatonin secretion, this hypothesis was tested by investigating whether melatonin signals act as mediators relaying photic signals during testis development in the eel. Methods. We examined changes in melatonin-secretion patterns using time-resolved fluorescence immunoassays in sexually immature and mature male Japanese eels under the condition of a new moon (NM) and a full moon (FM). Results. The eye and plasma melatonin levels exhibited a nocturnal pattern under a 12-h light: dark cycle (12L12D) or under constant darkness (DD), but not with constant light (LL). Eye melatonin levels were similar under the 12L12D and short-day (9L15D) conditions. In the long-day condition (15L9D), secreted plasma melatonin levels were stable, whereas short-day melatonin secretion began when darkness commenced. Sexual maturation began at 8 weeks following intraperitoneal injection of human chorionic gonadotropin (hCG), and NM exposure led to significantly higher eye and plasma melatonin levels compared with those detected under FM exposure. [ABSTRACT FROM AUTHOR]

During the induction of Japanese eel maturation, administering maturation-inducing steroids (MIS) or their precursors at an inappropriate maturational status is a major cause of poor egg quality. In this study, we investigated the feasibility of controlling oocyte maturation progress by rearing in cold and warm water to manipulate the timing of MIS administration. Mature females with oocytes at the migratory nucleus stage were reared for two terms (3 days to 1 day and 1 day to 0 days before MIS administration) at 20/20 °C, 20/15 °C, 15/20 °C, or 15/15 °C, and the maturational status was monitored based on their lipid droplet morphology and oocyte diameter. Oocytes matured faster at 20 °C than at 15 °C in either term. Next, the mature females were reared at 15 or 20 °C depending on the maturational status of each female 3 days and 1 day before MIS administration; the immature females were reared at 20 °C to accelerate their maturation. Consequently, the maturational status of most females was similar at MIS administration. After improvement, this method would lead most females to the optimum maturational status at MIS administration by properly rearing in cold and warm water. [ABSTRACT FROM AUTHOR]

Two DNA viruses, Japanese eel endothelial cell-infecting virus (JEECV) and Anguillid herpesvirus 1 (AngHV-1), cause serious diseases in eels that can negatively affect the economy of eel aquaculture industries in South Korea. Efficient virus detection is crucial for disease management; however, the practical application of current methodologies is challenging, in part owing to extensive time and specialized equipment requirements. The aim of this study was to develop a rapid real-time quantitative loop-mediated isothermal amplification (qLAMP) technique for the detection of JEECV and AngHV-1. Six different primers were designed to target eight different regions of the large T-antigen gene for JEECV detection, and the DNA polymerase catalytic subunit gene for AngHV-1 detection. Both sets of primers worked optimally at 65 °C. The qLAMP assay exhibited a minimum detection limit of 2.39 × 104 copies/μL for JEECV and 2.37 × 104 copies/μL for AngHV-1. Moreover, the primers demonstrated excellent specificity, showing no reaction with two major viruses, namely ostreid herpesvirus-1 microvariant (OsHV-1 μvar) and red sea bream iridovirus (RSIV). In addition, JEECV primers did not react with AngHV-1, and vice versa. Based on the standard curve and coefficient of determination values, both viruses could be quantified using this method. The qLAMP assay was validated with 104 clinical samples, including 72 infected and 32 healthy eels, and the assay findings were compared with previously reported duplex qPCR results that reported high sensitivity and specificity. The qLAMP results concorded well with the previous results. Evaluation of the threshold time (Tt) of the qLAMP results revealed that the samples could be analyzed in less than 15 min. The clinical diagnostic sensitivity (dSe) and diagnostic specificity (dSp) were assessed based on the confusion matrix; dSe and dSp for JEECV were 100% and 94.1%, respectively, and dSe and dSp for AngHV-1 were 96.8% and 77.8%, respectively. The Kappa values, which were used to represent the level of agreement between qLAMP and duplex qPCR results, for JEECV and AngHV-1 were 0.956 and 0.755, respectively, indicating that the qLAMP assay is a promising detection method for those viruses. Thus, the qLAMP assay developed herein could be applied as a rapid and sensitive tool to diagnose diseases caused by JEECV and AngHV-1 in the eel aquaculture industry. • A real-time quantitative loop-mediated isothermal amplification assay was developed. • The assay could detect JEECV and AngHV-1, two DNA viruses that infect eels, in less than 15 min. • The assay can be a rapid and sensitive tool to diagnose viral infections in eels. • It can be used to develop disease control strategies in the aquaculture industry. [ABSTRACT FROM AUTHOR]

• Pure ohmic heating reduces processing time and energy consumption. • Ohmic heating did not alter parvalbumin's molecular weight. • Ohmic heating effectively reduces parvalbumin allergenicity. We investigated the effects of ohmic heating (OH) on the structural properties and allergenicity of parvalbumin (PV). Compared to other heating methods (water bath heating (WH), OH combined with WH, and OH combined with air thermostatic heating (AH)), pure OH heating expended the least time and total energy. PV sensitization was reduced by approximately 65% by pure OH heating. SDS-PAGE, tricine–SDS-PAGE, and western blotting analyses revealed a molecular weight of sensitized β-PV of about 12 kDa. Band intensity decreased with increasing OH time, and significant changes were observed in amino acid content, secondary structure, microstructure, and dielectric properties. Reducing PV, allergenicity through protein unfolding and secondary structural changes, thereby possibly reducing the allergenicity of eel, provides a theoretical basis for developing hypoallergenic products. [ABSTRACT FROM AUTHOR]

Japanese eel endothelial cell-infecting virus (JEECV) and Anguillid herpesvirus 1 (AngHV-1) are virulent pathogens in eel aquaculture farms in the Republic of Korea and coexist in the same host. To date, no diagnostic method has been developed that detects them simultaneously. We developed a duplex real-time polymerase chain reaction (duplex qPCR) assay to detect both JEECV and AngHV-1 simultaneously in a single reaction. Two sets of specific primers and TaqMan probes were designed to target the large T-antigen gene of JEECV and the DNA polymerase catalytic subunit gene of AngHV-1. The assay was optimized by changing the primer and probe concentrations and the annealing temperature of the reaction. The duplex qPCR could specifically amplify the specific genes of the two viruses with high efficiency and coefficient of determination. The assay could detect both viruses at a limit of detection of 8 fg/μL. The diagnostic performance of the developed assay was determined by analyzing 82 clinical samples (72 infected and 10 healthy) and comparing the results with those of conventional PCR and SYBR Green-based qPCR; the results correlated well with each other. The duplex qPCR showed 100% and 98.6% diagnostic sensitivity for JEECV and AngHV-1, respectively. The diagnostic specificity of this assay was validated using two potential viral strains, red sea bream iridovirus and Ostreid herpesvirus-1 microvariant, reported in the aquaculture industry. In contrast, the methods already developed including conventional PCR and qPCR cannot detect both JEECV and AngHV-1 at the same time as they should perform in separate reactions. Thus, the duplex qPCR developed herein is a sensitive and specific diagnostic tool for the simultaneous detection of JEECV and AngHV-1 and has subsequent applications in the health management of eels. • A novel duplex qPCR assay was developed to detect both JEECV and AngHV-1 simultaneously. • Species-specific primers and TaqMan-based probes were designed to detect JEECV and AngHV-1. • Duplex qPCR assay could detect both viruses at a limit of detection of 8 fg/μL. • The results of this method were in concordance with that of conventional PCR and SYBR Green-based qPCR. [ABSTRACT FROM AUTHOR]

Most cultured Japanese eels (Anguilla japonica) show male sex differentiation; however, natural gonadal sex differentiation has not been evaluated. In this study, this process was characterized in wild eels. Differentiated ovaries and testes were observed after the eels grew to 320 and 300 mm in total length, respectively. The youngest ovary and testis appeared at 3 and 4 years old, respectively; however, undifferentiated gonads were found up to 7 years, suggesting that sex differentiation was triggered by growth rather than aging. gsdf, amh, foxl2b and foxl3b were highly expressed in the testes, whereas figla, sox3, foxn5, zar1, and zp3 were highly expressed in the ovaries. The expression of cyp19a1a and foxl2a did not differ significantly between the testis and ovary. In the ovaries, the cyp19a1a and foxl2a levels were highest in the early stages, suggesting that their function is limited to early ovarian differentiation. The foxn5, zar1 and zp3 levels tended to increase in the later stages, suggesting that they function after the initiation of ovarian differentiation. In undifferentiated gonads, dimorphic gene expression was not observed, suggesting that the molecular sex differentiation phase is short and difficult to detect. These findings provide the first demonstration of the whole course of natural gonadal sex differentiation in eels at molecular and morphological levels.

The abundance of Japanese eel Anguilla japonica has rapidly decreased in recent decades. Following a re-evaluation of the possibility of extinction, the Japanese Ministry of the Environment and the International Union for Conservation of Nature listed the Japanese eel as an endangered species in 2013 and 2014, respectively. However, their abundance and precise distribution have never been clarified owing to their nocturnality and difficulty in their capture. In this study, the distribution of Japanese eels was investigated by monitoring for environmental DNA (eDNA), a non-invasive and efficient detection method. A total of 365 water samples were collected from 265 rivers located throughout Japan. High concentrations of eDNA of Japanese eels were detected in rivers on the Pacific side, but were low in the Sea of Japan side. In particular, very little eDNA amplification was confirmed from Hokkaido and the north of the Sea of Japan. The eDNA distribution in Japanese rivers coincides with the transport of the larvae in the ocean, as estimated by numerical simulations. Generalized linear mixed models were developed to explain the distribution of eDNA concentrations. The total nitrogen concentration emerged as an important factor in the best model. These results indicate that the distribution of Japanese eel is mostly determined by the maritime larval transport, and their survival and growth depend on the abundance of food in the river. The findings of the present study are useful for the management of populations and in the conservation of Japanese eels.

The Japanese eel Anguilla japonica is a prominent and highly valued species in the aquaculture industry in Japan. Sex determination and sex differentiation in eels are significantly affected by environmental factors; however, the molecular mechanisms involved in sex differentiation in eels are largely unknown. In this study, to investigate the gonadal expression profiles of sex-specific genes during and after sexual differentiation in Japanese eel, we induced elvers into predominantly phenotypic males or females by rearing on a control diet or estradiol-17β-treated diet, respectively, during the sex differentiation period. Quantitative real-time PCR showed that forkhead box L2A (foxl2a) and cytochrome P450 aromatase (cyp19a1) were more highly expressed in ovaries than in testes, whereas the expression levels of anti-Müllerian hormone (amh) and gonadal soma-derived factor (gsdf) were significantly higher in testes than in ovaries. Furthermore, foxl2a and cyp19a1 displayed female-specific expression early in the sex differentiation process, while after slightly more growth amh and gsdf displayed male-specific expression during sex differentiation. Together, these results suggest that these genes have important roles in sexual differentiation and development in this species. [ABSTRACT FROM AUTHOR]

Environmental DNA (eDNA) surveys have been conducted to evaluate the distribution and abundance of Japanese eels. However, various environmental and biological factors may influence eDNA concentrations. An experiment was conducted using three water sample replicates (50, 100 and 200 ml) of the same group of eels in a tank that were exposed to sequential nonfeeding/feeding and low/high temperature conditions. Slightly higher concentrations occurred at higher temperature (22–23°C) with nonfeeding, and the highest concentrations occurred when feeding started even though it was in the lower temperature (16–17°C) condition, but sample volume had no effect. [ABSTRACT FROM AUTHOR]

This study was carried out to identify and estimate physiological function of a new type of opsin subfamily present in the retina and whole brain tissues of Japanese eel using RNA–Seq transcriptome method. A total of 18 opsin subfamilies were identified through RNA–seq. The visual opsin family included Rh2, SWS2, FWO, DSO, and Exo-Rhod. The non-visual opsin family included four types of melanopsin subfamily (Opn4x1, Opn4x2, Opn4m1, and Opn4m2), peropsin, two types of neuropsin subfamily (Opn5-like, Opn5), Opn3, three types of TMT opsin subfamily (TMT1, 2, 3), VA-opsin, and parapinopsin. In terms of changes in photoreceptor gene expression in the retina of sexually mature and immature male eels, DSO mRNA increased in the maturation group. Analysis of expression of opsin family gene in male eel brain before and after maturation revealed that DSO and SWS2 expression in terms of visual opsin mRNA increased in the sexually mature group. In terms of non-visual opsin mRNA, parapinopsin mRNA increased whereas that of TMT2 decreased in the fore-brain of the sexually mature group. The mRNA for parapinopsin increased in the mid-brain of the sexually mature group, whereas those of TMT1 and TMT3 increased in the hind-brain of the sexually mature group. DSO mRNA also increased in the retina after sexual maturation, and DSO and SWS2 mRNA increased in whole brain part, suggesting that DSO and SWS2 are closely related to sexual maturation.

The Japanese eel (Anguilla japonica) is a commercially important fish species in East Asia and its recruitment has been rapidly declining since 1990s. Clarifying the genetic population structure of A. japonica is the basis of multinational cooperation on its management and protection, due to its large distribution range. Gene-associated markers have been proved powerful in delineating fine-scale population genetic structure and spatially varying selection. In the present study, we developed 24 polymorphic gene-associated microsatellite markers including 18 loci associated with the genes under selection in the two North Atlantic eel species (Anguilla anguilla and Anguilla rostrata) and 6 loci based on transcript sequences. A total of 13 geographic populations were sampled across its distribution range, including 11 samples from China (9 from China's mainland and 2 from Taiwan region), and 2 samples from Japan. A total of 416 individuals (mostly glass eels) were collected and genotyped at the 24 microsatellites. All measures of differentiation were accordant with a panmictic scenario (FST=-0.001) in A. japonica. No footprints of spatially varying selection were found, indicating that the selection pattern in A. japonica might be different from that in the two North Atlantic eel species. We suggest that A. japonica should be managed as a single unit and management and conservation efforts must be coordinated at the international level, as overexploitation in any region will decrease its recruitment across the whole distributional range. [ABSTRACT FROM AUTHOR]

The present study aims to reveal the influence of path fluctuations of the western boundary current (Kuroshio) and other regional environmental factors on coastal recruitment of transported organisms. As a model species, we chose a catadromous eel species, Japanese eel Anguilla japonica, which is transported poleward by the Kuroshio in early life stages. We analyzed a 50-year record of A. japonica juvenile (glass eel) catch from an estuary (Lake Hamana, Japan) with regional environmental data, using multiple linear regression with stepwise variable selection. Kuroshio path indicator (fishing-season mean of sea level difference between two tide stations), year, or their interaction showed moderate to strong evidence of an effect on the natural log-transformed catch of glass eels in the regression models. Strong evidence of an interaction effect between the Kuroshio path indicator and year was also found on the natural log-transformed catch-per-unit-effort. The estuarine recruitment of A. japonica glass eels was suggested to be positively influenced by nearshore Kuroshio position, but the influence has been lost in recent decades. The present study provides statistically robust evidence of the environment–recruitment relationship (western boundary current–recruitment link) for coastal species. This finding enhances our understanding of coastal recruitment of organisms transported by western boundary currents and is potentially essential in forecasting recruitment for fisheries management and conservation. Furthermore, our findings suggest that stock decline may lead to poorer understanding of environment–recruitment relationship. [ABSTRACT FROM AUTHOR]

The environmental DNA (eDNA) technique is a convenient and powerful tool to detect rare species. Knowledge of the degradation rate of eDNA in water is important for understanding how degradation influences the presence and/or estimate biomass of aquatic animals. We developed a new set of species-specific primers and probe to detect eDNA of Japanese eel Anguilla japonica, which is a commercially important and endangered species, and then conducted a laboratory experiment to quantify the temperature-dependent degradation of emitted eDNA. Eels were held in tanks at five different temperature levels from 10 to 30 °C and water from each tank was sampled and kept in bottles at each temperature over 6 days. The concentration of eDNA was measured every day and the results showed that temperature (T) had a significant and positive effect on the degradation rate (k) as k = 0.02T + 0.18. Improved understanding of the effect of temperature on degradation rates would help data interpretations and adjustments would increase the reliability of eDNA analysis in future studies. [ABSTRACT FROM AUTHOR]

A 12‐week feeding trial was conducted to investigate the effects of two dietary probiotics; Bacillus subtilis KCTC 2217 or Bacillus licheniformis KCCM 11775 with two prebiotics; mannan oligosaccharide (MOS) or fructooligosaccharide (FOS) in Japanese eel. Fish averaging 12.8 ± 0.47 g (mean ± SD) were randomly distributed into five treatments with triplicate tanks. A basal control diet (CON) and four synbiotic diets supplementing B. subtilis + MOS (BSM), B. subtilis + FOS (BSF), B. licheniformis + MOS (BLM), and B. licheniformis + FOS (BLF). Weight gain and specific growth rate of fish fed all synbiotic diets were higher than those of fish fed CON. Immune‐related gene expression of heat shock protein 70 and immunoglobulin M of fish fed BSF and BLM were significantly higher than those of fish fed CON. Fish fed BSF and BLM had significantly higher intestinal villi length than those of fish fed BLF and CON (p <.05). Disease resistance against Aeromonas hydrophila of fish fed all synbiotic diets were significantly higher than those of fish fed CON (p <.05). Therefore, these results indicated that dietary B. subtilis with FOS (BSF) and B. licheniformis with MOS (BLM) could have beneficial effects on intestinal morphology, and immune‐related gene expression in Japanese eel. [ABSTRACT FROM AUTHOR]

Background: Gills of euryhaline fishes possess great physiological and structural plasticity to adapt to large changes in external osmolality and to participate in ion uptake/excretion, which is essential for the re-establishment of fluid and electrolyte homeostasis. The osmoregulatory plasticity of gills provides an excellent model to study the role of microRNAs (miRs) in adaptive osmotic responses. The present study is to characterize an ex-vivo gill filament culture and using omics approach, to decipher the interaction between tonicity-responsive miRs and gene targets, in orchestrating the osmotic stress-induced responses. Results: Ex-vivo gill filament culture was exposed to Leibovitz's L-15 medium (300 mOsmol l− 1) or the medium with an adjusted osmolality of 600 mOsmol l− 1 for 4, 8 and 24 h. Hypertonic responsive genes, including osmotic stress transcriptional factor, Na+/Cl−-taurine transporter, Na+/H+ exchange regulatory cofactor, cystic fibrosis transmembrane regulator, inward rectifying K+ channel, Na+/K+-ATPase, and calcium-transporting ATPase were significantly upregulated, while the hypo-osmotic gene, V-type proton ATPase was downregulated. The data illustrated that the ex-vivo gill filament culture exhibited distinctive responses to hyperosmotic challenge. In the hyperosmotic treatment, four key factors (i.e. drosha RNase III endonuclease, exportin-5, dicer ribonuclease III and argonaute-2) involved in miR biogenesis were dysregulated (P < 0.05). Transcriptome and miR-sequencing of gill filament samples at 4 and 8 h were conducted and two downregulated miRs, miR-29b-3p and miR-200b-3p were identified. An inhibition of miR-29b-3p and miR-200b-3p in primary gill cell culture led to an upregulation of 100 and 93 gene transcripts, respectively. Commonly upregulated gene transcripts from the hyperosmotic experiments and miR-inhibition studies, were overlaid, in which two miR-29b-3p target-genes [Krueppel-like factor 4 (klf4), Homeobox protein Meis2] and one miR-200b-3p target-gene (slc17a5) were identified. Integrated miR-mRNA-omics analysis revealed the specific binding of miR-29b-3p on Klf4 and miR-200b-3p on slc17a5. The target-genes are known to regulate differentiation of gill ionocytes and cellular osmolality. Conclusions: In this study, we have characterized the hypo-osmoregulatory responses and unraveled the modulation of miR-biogenesis factors/the dysregulation of miRs, using ex-vivo gill filament culture. MicroRNA-messenger RNA interactome analysis of miR-29b-3p and miR-200b-3p revealed the gene targets are essential for osmotic stress responses. [ABSTRACT FROM AUTHOR]

Artificial maturation and ovulation of Japanese eel do not always result in the production of good-quality eggs. The molecular basis for compromised egg quality in artificially matured Japanese eel remains unknown. In this study, we investigated the relationship between the localization of six maternal transcripts, grip2, dazl, sybu, trim36, pou5f3 and npm2, and egg quality in artificially matured Japanese eel oocytes and eggs. We observed that localization of these messenger RNAs (mRNAs) in oocytes from the perinucleolus stage to the migratory nucleus stage is not associated with egg quality. In contrast, in ovulated eggs, we observed two different mRNA localization patterns, which we designated type A (mRNA localized in the cortex) and type B (mRNA distributed more centrally). Furthermore, individuals containing many type B eggs tended to have low fertilization and hatching rates, but this was not reflected in the target gene mRNA levels. Our results suggest that the loss of quality in eggs from artificially matured Japanese eel is caused by abnormal mRNA localization even if there is no difference in mRNA levels. [ABSTRACT FROM AUTHOR]

Retinoic acid-inducible gene-I (RIG-I) is a cytoplasmic viral RNA sensor that triggers the production of type I interferons (IFNs) and proinflammatory cytokines during viral infection. RIG-I gene has been identified previously in Japanese eel, Anguilla japonica. In the present study, we have characterized a novel isoform of RIG-I (designated as AjRIG-Ib) and its truncated variant (AjRIG-Ibv). The AjRIG-Ib encodes 940 amino acids (aa) consisting of two N-terminal caspase activation and recruitment domains (CARDs), a DEX(D/H) box RNA helicase domain, and a C-terminal regulatory domain (CTD). The AjRIG-Ibv encodes a protein of 843 aa, that shares similar structural organization with AjRIG-Ib, but lacking CTD. The gene expression analyses showed that AjRIG-Ib and AjRIG-Ibv were detectable in all tissues/organs examined, and AjRIG-Ib was the predominant form. The mRNA level of AjRIG-Ibv was upregulated rapidly at 8 h after the Poly I:C injection, and the significant increase of AjRIG-Ib was observed at 16 and 24 h post-injection (hpi). Laser confocal microscopy showed that AjRIG-Ib and AjRIG-Ibv were both located in cytoplasm. In addition, the overexpression of AjRIG-Ib or AjRIG-Ibv led to the increased activity of IFN promoter in transient transfection assay. Taken together, our results indicated that AjRIG-Ib and AjRIG-Ibv may play cooperative or somewhat complementary roles in coordinating the antiviral response in fish. • A novel isoform of RIG-I and its truncated variant lacking the C-terminal domain were identified in Japanese eel. • The two variants share similar sub-cellular localization and IFN promoter inducibility. • The expression of truncated RIG-I variant increased rapidly upon Poly I:C stimulation. • The two RIG-I variants may play cooperative or complementary roles in antiviral response. [ABSTRACT FROM AUTHOR]

Morphological deformities of the jaw and vertebrae arise as hindrances to seed production of Japanese eel Anguilla japonica. To obtain basic information related to development in these areas, we analyzed eel larva skeletal system formation by staining with alizarin red S. Results demonstrated that dentary and maxillary ossification at the jaw begins by 10 days after hatching, about 8 mm total length. Subsequent progression of ossification is slow until metamorphosis. For instance, ossification of the angular and quadrate, which are connected to the dentary, begins remarkably later than ossification of the dentary. Ossification of the head develops quickly during metamorphosis and produces a primitive skull in glass eels. Ossification of the vertebral centrum progresses rapidly from the posterior end to the anterior-most during metamorphosis. Our results demonstrate that the process of ossification at the head is shared among some species of Elopomorpha, but that ossification of vertebral centra differed. Furthermore, our results imply consistency of the period between ossification and the occurrence of some morphological deformities around the jaw and vertebrae. [ABSTRACT FROM AUTHOR]

The Yangtze River and its branches are important habitats for the Japanese Eel (Anguilla japonica), however, there are few data on gonadal development and silvering indices of eels during the downstream spawning migration. This study investigates gonadal development in 121 Japanese eels caught downstream from the Jingjiang Reach of the Yangtze River in October 2012. We found that in females the ovaries of most of the out-migrating population developed earlier than the testes in males, that is, the ovaries reached stage 3 (F3) or early stage 4 (eF4), while the testes were at stage 2 (M2) or stage 3 (M3). Gonadal development in eels collected in 2012 was earlier than that of samples caught from the same region in 2007–2008 and from the Pearl River Estuary in 1972–1973. The average age, body size and gonadal-somatic index of females were significantly greater than those in males. For both females and males, there were no significant differences between the two stages of gonadal development in any of the morphological indices, which indicates that gonadal development of the Japanese Eel is independent of body size. These results help us to understand the spawning population of Japanese Eels in the Yangtze River.

Follitropin, an important gonadotropin hormone, participates in vitellogenesis and spermatogenesis. Equine chorionic gonadotropin (eCG) can induce gonadotropin hormone activity in non-equid species and exhibits a long biological half-life. Here, we report the production, using silkworm larval and pupal systems, of biologically active recombinant hybrid-type follitropins based on the coding sequence of the eCG C-terminal peptide (CTP) between the mature β- and α-chains of eel. The three constructs, rJeFSH, rJeFSH·eCG, and rJeFSH·2xeCG were produced and verified to be N- or O-glycosylated and secreted mature peptides. Although rJeFSH·eCG contains more elaborate O-linked carbohydrate chains than rJeFSH, it elicited no significant in vitro oocyte maturation, which may be a result of insufficient terminal sialylation of its N-and O-linked carbohydrate chains. Then, a hybrid of rJeFSH·2xeCG extended with two eCG CTP. Furthermore, the receptor binding assay revealed potency of rJeFSH and rJeFSH·2xeCG to be a few folds greater than that of rJeFSH·eCG. The findings of this study will be useful for the development of more efficient GTHs in teleosts, including eels, when various modifications with two or more extended eCG CTP produced by silkworm are included. [ABSTRACT FROM AUTHOR]

Dmrt1, doublesex- and mab-3-related transcription factor-1, has been suggested to play critical roles in male gonadogenesis, testicular differentiation and development, including spermatogenesis, among different vertebrates. Vasa is a putative molecular marker of germ cells in vertebrates. In this study, we cloned the full-length dmrt1 cDNA from Japanese eel, and the protein comprised 290 amino acids and presented an extremely conserved Doublesex and Mab-3 (DM) domain. Vasa proteins were expressed in gonadal germ cells in a stage-specific manner, and were expressed at high levels in PGC and spermatogonia, low levels in spermatocytes, and were absent in spermatids and spermatozoa of Japanese eels. Dmrt1 proteins were abundantly expressed in spermatogonia B cells, spermatocytes, spermatids, but not in spermatozoa, spermatogonia A and Sertoli cells. To our knowledge, this study is the first to show a restricted expression pattern for the Dmrt1 protein in spermatogonia B cells, but not spermatogonia A cells, of teleosts. Therefore, Dmrt1 might play vital roles at the specific stages during spermatogenesis from spermatogonia B cells to spermatids in the Japanese eel. Moreover, the Dmrt1 protein exhibited a restricted localization in differentiating oogonia in the early differentiating gonad (ovary-like structure) of male Japanese eels and in E2-induced feminized Japanese eels. We proposed that dmrt1 may be not only required for spermatogenesis but might also play a role in oogenesis in the Japanese eel. [ABSTRACT FROM AUTHOR]

Japanese eel (Anguilla japonica) is an important food source in East Asia whose population has dramatically declined since the 1970s. Despite past analysis with DNA sequencing, microsatellite and isozyme methods, management decisions remain hampered by contradictory findings. For example, it remains unresolved whether Japanese eels are a single panmictic population or whether they harbor significant substructure. Accurate assessment of population genetic substructure, both spatial and temporal, is essential for determining the relevant number of distinct management units appropriate for this species. In the present study, we assayed genetic variation genome-wide using Restriction Site Associated DNA Sequencing (RAD-seq) technology to analyze the population genetic structure of Japanese eels. For analysis of temporal isolation, five "cohort" samples were collected yearly from 2005 to 2009 in the Yangtze River Estuary. For analysis of spatial structure, five "arrival wave" samples were collected in China in 2009, and two arrival wave samples were collected in Japan in 2001. In each cohort of each arrival wave, five individuals were collected for a total of 55 eels sampled. In total, 214,210 loci were identified from these individuals, 106,652 of which satisfied quality checks and were retained for further analysis. There was relatively little population differentiation between arrival waves and cohorts collected either at different locations during the same year (Fst = 0.077) or at the same location collected over subsequent years (Fst = 0.082), and locations displayed no consistent isolation-by-distance. [ABSTRACT FROM AUTHOR]

The widely distributed East Asian Japanese eel Anguilla japonica constitutes a single genetically homogeneous population with a single spawning area near the West Mariana Ridge. Otolith 87Sr/86Sr ratios of adults (categorized as "river", "estuarine" or "sea" eels, according to their habitat use history determined from otolith Sr:Ca ratio analysis) collected from the spawning area in 2008 and 2009 were examined in an attempt to determine their juvenile growth areas. In addition, 87Sr/86Sr ratios of water samples from rivers in China, Taiwan, Korea, and Japan were determined. Otolith 87Sr/86Sr ratios of the "river", "estuarine" and "sea" eels were 0.707793, 0.708580-0.709944 and 0.709068, respectively, and water sample ratios from China, Korea, Taiwan, and Japan were 0.7104320-0.7141010, 0.7190826-0.7227976, 0.7115523-0.7146914 and 0.706191-0.712484, respectively. "River" and "estuarine" eels, which had otolith 87Sr/86Sr ratios less than 0.7092 (seawater ratio), appeared to have inhabited Japanese rivers and/or estuaries because similarly low isotope ratios were recorded only from Japan. However, the juvenile growth areas of other eels were unknown, as their origins could not be determined from otolith 87Sr/86Sr ratios alone, and required further information regarding otolith elemental and isotope compositions. [ABSTRACT FROM AUTHOR]

Aquaculture has experienced tremendous growth over the last few decades as global production has increased from 16 million tons in 1990 to 110 million tons in 2016. Meanwhile, the modern aquaculture industry is interested in the production of endangered species with great economic potential. Japanese eel (Anguilla japonica) is one of the most valuable species in East Asia with high market demand. But, unfortunately, this fish is critically endangered largely due to excessive harvesting at early life stages and deteriorating environmental conditions. Therefore, a sustainable aquaculture strategy is necessary to conserve and rehabilitate Japanese eel stocks while meeting market demand. In order to achieve this goal, efficient artificial breeding of Japanese eel and knowledge of nutritional requirements during every life stage is critical. The present review will cover recent research and technology in the areas of aquaculture and artificial breeding, and provide comprehensive information regarding the nutrition of Japanese eel. The information provided in this study is intended for researchers working on the aquaculture of Japanese eel by identifying the current knowledge and research gaps. This review is also helpful to feed manufacturers in producing practical diets for different life stages of Japanese eel. [ABSTRACT FROM AUTHOR]

Abstract Although Japanese eel, Anguilla japonica, is one of the most important fish species in East Asia, only one study has assessed Japanese eel stocks to date. That study estimated the stock status and dynamics of this species mainly based on Japanese fishery data on recruiting glass eel, growing-phase yellow eel, and maturing silver eel. That study also used data on biological characteristics, such as the growth, maturity and natural mortality of both Japanese eel and European eel, A. anguilla , to develop an age- and sex-structured production model. However, Japanese fishery statistics for glass eel are not an appropriate data source for stock assessment because there are non-negligible illegal and unreported glass eel catches occur in Japan. Moreover, yellow and silver eel fishery data in Japan are biased by those pertaining to stocked individuals, although most of the data are for eels originating from Japan as opposed to other countries and regions within the species distribution range. In addition to these problems in data collection, the biological parameters might be biased by contamination from stocked individuals. Therefore, it is difficult to assess the stock status of Japanese eels using an age- and sex-structured production model based on official Japanese eel fisheries statistics. The legality and traceability of glass eel fisheries and eel trading should be improved to enable appropriate stock assessment in future, and data on catch per unit effort (CPUE) in areas where eel stocking is not conducted should be collected throughout the species distribution range. [ABSTRACT FROM AUTHOR]

Abstract Type I IFNs are a family of cytokines with antiviral, anti-proliferative and immune-modulatory functions. In this study, four type I IFNs (termed AjIFN1-4) have been cloned from the Japanese eel, Anguilla japonica. The open reading frames of AjIFN1-4 are 552, 534, 546 and 561 bp in length, encoding 183, 177, 181, and 186 amino acids (aa), respectively. Sequence comparison and phylogenetic analysis results revealed that AjIFN1 and AjIFN2 belong to group one (2C-containing) IFNs, while AjIFN3 and AjIFN4 belong to group two (4C-containing) IFNs. Syntenic comparison showed that chromosome block duplication and rearrangement events might have occurred at IFN loci in different teleost lineages. Expression analysis revealed the rapid induction of AjIFNl and AjIFN2 in response to poly I:C stimulation, while AjIFN3 and AjIFN4 were predominantly expressed at later time points. Two Mx promoter reporter assays were conducted to assess the Mx-inducing capability of AjIFN1-4. It is shown that the overexpression of AjIFN1-4 all promoted the luciferase activity of MxB reporter, but the activity of MxC reporter increased only in cells transfected with AjIFN1. Collectively, it is suggested that teleost IFNs were evolved independently in different lineages of fish and may function differently in teleost antiviral immunity. Highlights • Four type I IFN genes, AjIFN1-4, were cloned from the Japanese eel. • Chromosome duplication and rearrangement occurred at IFN loci in teleost lineages. • Expression of AjIFN1 and AjIFN2 increased rapidly in response to poly I:C stimulation. • AjIFN3and AjIFN4 were expressed predominantly at later time points. • The overexpression of AjIFN1-4 all promoted the luciferase activity of Mx reporter. [ABSTRACT FROM AUTHOR]

Abstract This study evaluated the synergistic effects of dietary Bacillus subtilis WB60 and mannanoligosaccharide (MOS) in juvenile Japanese eel, Anguilla japonica. Seven treatment diets were formulated to contain three different levels of B. subtilis (0.0, 0.5, and 1.0 × 107 CFU/g diet denoted as BS 0 , BS 0.5 , and BS 1 , respectively) with two MOS levels (0 and 5 g/kg diet denoted as M 0 and M 5 , respectively), and one diet with oxytetracycline (OTC) at 5 g/kg diet. Each diet (BS 0 M 0 (CON), BS 0 M 5 , BS 0.5 M 0, BS 0.5 M 5, BS 1 M 0, BS 1 M 5 , and OTC) was fed to triplicate groups of 20 fish averaging 9.00 ± 0.11 g (mean ± SD) for eight weeks. Average weight gain, feed efficiency, specific growth rate and protein efficiency ratio of fish fed the BS 0.5 M 5 and BS 1 M 5 diets were significantly higher than those of fish fed CON, BS 0.5 M 0 and OTC diets (P < 0.05). Significant increases in the nonspecific enzymatic activities (e.g., lysozyme and myeloperoxidase) were detected from fish fed the BS 0.5 M 5 , BS 1 M 5 , and OTC diets compared to the CON, BS 0.5 M 0 , and BS 0 M 5 diets (P < 0.05). Whereas, immunoglobulin M expressions were recorded significantly higher for fish fed the BS 0.5 M 5 and BS 1 M 5 diets compared to those of fish fed the other diets (P < 0.05). Also, heat shock protein 70 mRNA levels of fish fed BS 0.5 M 5 and BS 1 M 5 diets were significantly higher than those of fish fed the CON diet (P < 0.05). Histological observations of the intestinal morphology showed healthier gut for fish fed BS 0.5 M 5 and BS 1 M 5 diets than those fed CON, BS 0 M 5 , and OTC diets. Additionally, resistance to bacterial challenge with Vibrio anguillarum was recorded significantly lower for fish fed the CON diet than those fed other diets (P > 0.05). Therefore, the results for growth performance, non-specific immune responses, intestinal morphology, and disease resistance demonstrated that supplementation of B. subtilis at 0.5 × 107 CFU/g diet and mannanoligosaccharide at 5 g/kg diet could have beneficial synergistic effects in Japanese eel. The isolated probiotic from eel and the selected prebiotic could lead to the development of a specific and potential synbiotic in Japanese eel aquaculture. Highlights •Higher growth and immunity could be achieved by the administration of Busilus subitlis and mannanoligosaccharide. •Dietary synbiotic could have more benefits than probiotic and prebiotic alone. •Specific synbiotic has the potential to replace chemical antibiotics in Japanese eel aquaculture. [ABSTRACT FROM AUTHOR]

The article presents a study which reveals that Nocardia seriolae is causative agent for nocardia infection in cultured Japanese eel, Anguilla japonica. It mentions that N. seriolae is positive Gram‐positive bacteria containing a large amount of mycolic acid and reacts positively with Ziehl–Neelsen stain. It states that pyknotic nuclei of epithelial cells associated with coagulative necrosis and Langhans giant cells were observed in the primary lamella of gill.

The natural occurrence of Japanese eels Anguilla japonica in Japan's highly altered catchments is a critical measure of habitat restoration and ecosystem health, yet this metric may be obscured by the incidence of co‐occurring cultured eels. Distribution of wild and stocked A. japonica in the lower reaches of the Tone River cachment was investigated using otolith oxygen and carbon stable‐isotope ratios (δ18O and δ13C). We developed a discrimination model to classify wild and cultured eels for 560 individuals and applied it to eels captured in the lower reaches of the Tone River catchment. Wild eels were found at all study sites of this part of the catchment, indicating natural recruitment of the species occurred from the estuary to third‐order streams at least as far as Lake Imbanuma. The proportion of wild eels ranged from 86 to 100% at river and stream sites and was only 23% at upstream Lake Imbanuma sites. This difference probably depends on whether or not eels were released near the corresponding site. More than 50% of stocked eels occurred at sites proximate to release locations, indicating limited dispersal by cultured eels. [ABSTRACT FROM AUTHOR]

The aim of this study was to develop sperm cryopreservation protocols for large-scale fertilization in Japanese eel ( Anguilla japonica ), by testing the feasibility of combining large-volume straws and a low sperm dilution ratio. Experiments were conducted using both 0.25-ml straws and 2.5- and 5-ml straws (macrotubes) for sperm cryopreservation. Cooling rates in macrotubes were measured at three internal positions and at different heights above the surface of liquid nitrogen (LN). There was no significant difference in the cooling rate among internal positions in the straws. The cooling rate could be adjusted to the appropriate range for Japanese eel sperm (6.3 °C–8.6 °C min −1 ) by positioning the 2.5- and 5.0-ml straw 0.5–5.0 cm above the surface of LN. We succeeded in scaling up the storage volume from 0.25 to 4.5 ml (18-fold) without decreasing the motility using a 5.0-ml straw. The number of storable motile spermatozoa was the highest at a range of dilution ratios (sperm to extender, v /v) from 1:1 to 1:5. Although frozen-thawed sperm showed a significant decrease in the proportion of motile sperm to egg compared with than fresh sperm, there was no significant difference in the fertilization rate, and fertilized eggs with cryopreserved sperm did not affect the hatching rate or survival rate at 7 days after hatching. Using a segmented linear analysis, we found that 10,180 ± 3077 motile spermatozoa per egg are required to maximize fertilization success. We estimated that spermatozoa stored in a 2.5- and 5.0-ml straw can fertilize approximately 184,185 and 331,532 eggs, respectively. Furthermore, our investigation of the effect of long-term storage in LN on the sperm quality showed that the storage period does not affect the post-thaw motility rate even after 2 years of storage. Our results demonstrate that cryopreservation techniques for Japanese eel sperm using 2.5- and 5.0-ml straws could be used for large-scale fertilization programs. [ABSTRACT FROM AUTHOR]

Myeloid differentiation factor 88 (MyD88) is a key adaptor protein required for the signaling of all Toll-like receptors except TLR3, which results to the interaction of activated TLR complexes via C-terminal TIR domain and the binding of downstream kinase via N-terminal death domain. In this study, the MyD88 gene from the Japanese eel ( Anguilla japonica ) was identified. The open reading frame of AjMyD88 was 918 bp in length, encoding a protein composed of conserved N-terminal death domain and C-terminal TIR domain, respectively. Multiple alignment revealed highly conserved sites across all examined vertebrate lineages in death and TIR domains. Site-directed mutagenesis and luciferase analysis revealed that the W78A, L91A and L95A mutations in death domain had modest impairment of their ability in activating NF-κB promoter. The expression level of AjMyD88 was investigated by real-time PCR in response to poly I:C stimulation and Edwardsiella tarda infection. Significantly increased MyD88 expression was observed at early phase in all tested tissues/organs in response to E. tarda infection and slight increase was detected in intestine and gill at 16 hpi and in head kidney, spleen and liver at 24 hpi after poly I:C stimulation. Immunofluorescence staining revealed that AjMyD88 is present as condensed forms in the cytoplasm. Taken together, sequence characterization, gene expression and cellular distribution data obtained in this study suggest that AjMyD88, similar to its mammalian ortholog, plays an important role in eel immune response against bacteria. [ABSTRACT FROM AUTHOR]

The growing popularity of Japanese cuisine worldwide has caused an increasing demand for freshwater eels (family Anguillidae), usually termed 'unagi' and commonly utilised in sushi and donburi dishes. The trade of the Critically Endangered European eel (Anguilla anguilla) is now restricted and this has resulted in illegal trade. Here, DNA barcoding was used to identify the species of eel sold in processed eel products worldwide, including a focus on Europe and North America. For the first time, traditional jellied and stewed eel products from the UK are also barcoded. A total of 114 novel samples were analysed alongside previously published data to evaluate differences in species composition between locations, over time and in unagi versus more traditional UK eel products. The results revealed more European eel in East Asia than in Europe and more Japanese eel (Anguilla japonica) in the UK than East Asia – demonstrating a stark contrast between the native species and those commonly on sale (although variation in product type and eel recruitment influence this). This suggests thousands of food miles are potentially hidden in unagi products. Considering the limitations on export of the European eel, the numbers identified in products outside of its natural range bring into question the legality of supply. An increased prevalence of endangered American eel (Anguilla rostrata) was also shown globally. Future efforts should focus on implementing robust traceability systems, including using genetic methods, to deter illegal trade within the industry. • 114 eel products DNA barcoded revealing American eel as the most commonly sold. • Significant regional difference in eel species sold across the globe. • Meta-analysis identifies high frequencies European eel in many regions. • Sales of European eel identified bring into question the legality of supply. [ABSTRACT FROM AUTHOR]

Abstract: The present experiment was conducted to evaluate the efficacy of an additive derived from the nature as an alternative of dietary antibiotic in Japanese eel, Anguilla japonica. Six experimental diets were formulated to contain no antibiotics or additive (yellow loess/YL) (control/CON), three graded levels of yellow loess at 5 (YL5), 10 (YL10) and 20 g/kg (YL20), oxytetracycline at 5 (OTC) and amoxicillin at 10 g/kg amoxicillin (AMX) of diet. Weight gain (WG) and specific growth rate (SGR) from fish fed CON or YL5 diets were significantly lower than those of fish fed YL20 or OTC diets. Among non‐specific enzyme, lysozyme activity of fish fed YL20, OTC or AMX was detected to be significantly higher than that from fish fed CON or YL5 diets, whereas superoxide dismutase (SOD) activity of the fish fed CON was significantly lower than that for fish fed other experimental diets. Challenge test with bacteria, Edwardsiella tarda, showed improved disease resistance among the fish fed different levels of natural additive without any statistical difference from those fed antibiotics (OTC and/or AMX) supplemented diets. Therefore, these results demonstrated the potential of natural feed additive, yellow loess to replace oxytetracycline and/or amoxicillin in Japanese eel, A. japonica. [ABSTRACT FROM AUTHOR]

The present study evaluated the effects of internal illuminance and shelter shape on shelter selection by Japanese eels to enable the preservation or construction of suitable shelters for the Japanese eel. Japanese eels were able to distinguish a 1.25-fold difference in illumination inside the shelters, and preferred darker shelters. When the internal illumination of two shelters with the same shape was less than one-tenth of the ambient illumination (about 400 lx), shelter selection by Japanese eels was not affected by internal illuminance, even when there was a 10-fold difference in internal illumination between shelters. The width of the shelter was not important, but Japanese eels preferred a deep shelter with a low ceiling and walls that spread to a ‘dead end’. This has important implications on the creation of suitable shelters for Japanese eels. [ABSTRACT FROM AUTHOR]