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The European and Japanese eel populations have declined significantly in recent decades. To effectively manage and conserve them, gaining a better understanding of their migratory life cycles is important. Previous research on the spawning ecology and larval dispersal of European and Japanese eels has led to many significant discoveries and advancements for their migratory life cycles. However, different scholars hold varying views on their migratory life cycles, especially concerning the European eel, therefore this article aims to provide a comprehensive review of research from multiple disciplines concerning the spawning ecology and larval dispersal of European and Japanese eels and to propose migratory life cycles of these two species. The migratory life cycle of the European eel is as follows: European silver eels undertake a year‐long spawning migration from September to January to reach the Sargasso Sea for spawning before the next spawning season, typically between December and May. After hatching, European eel leptocephali are transported by the Gulf Stream, Frontal Countercurrents, North Atlantic Current, and Azores Current and return to Europe and North Africa for growth. Recruitment of European glass eels mainly occurs between October and June of the following year, and the recruiting season is more concentrated in countries closer to the spawning area and more dispersed in countries farther away. The consistent recruitment pattern and the growth rate of leptocephali suggest a larval transport period, also called larval duration, of around 1 year. Understanding the migratory life cycle of European eels can facilitate the evaluation or development of their conservation measures. [ABSTRACT FROM AUTHOR]

Biofloc technology (BFT) systems heavily rely on microbiota to mitigate ammonia toxicity and manage essential nutrient cycling. Understanding the diversity and functional role of microbiota within BFT-applied aquaculture systems is crucial for ensuring sustainable operations. Though some studies exist on BFT microbiota, research on microbial differences in Japanese eel aquaculture is still limited, hindering the wider application of BFT systems. In this study, we analyzed the characteristics of water quality factors and microbiota in Japanese eel (Anguilla japonica) breeding water, applying the BFT system. Using a metabarcoding approach, the diversity and community structure of aquatic microbiota were compared between BFT and continuous flow (CF) systems. The pH was significantly higher in CF water, while total ammonia nitrogen (TAN) and nitrite (NO2−-N) was higher in BFT water. Alpha diversity was significantly higher in BFT compared to CF systems, and it was correlated significantly with pH and TAN. In both BFT and CF water, the phyla Proteobacteria and Bacteroidota were found to be the most abundant. In the BFT water, a diverse array of bacterial taxa, including BFT-specific clades, were consistently present, while the microbiota in CF water was more variable and contained fewer specific taxa. In addition, bacterial functions related to nitrate reduction, sulfur compound oxidation, and chitinolysis were significantly more abundant in BFT than in CF systems. These findings highlight differences in water quality and microbiota between aquaculture systems, which can inform future research on the use of BFT for sustainable fish farming. [ABSTRACT FROM AUTHOR]

Although the transfer of methane‐derived carbon (MDC) to benthic macroinvertebrates such as chironomid larvae and oligochaetes has been well documented, knowledge of the transfer of MDC to fish is limited.We investigated the MDC contribution to 28 fish species through their growth stages and examined the relationship between fish traits including body size, and MDC contribution to fish within species in Lake Kasumigaura, Japan. We used carbon stable isotope ratios and a two‐source mixing model (methane‐oxidation bacteria [MOB] and particulate organic matter) to estimate the MDC contribution to fish.The MDC contribution to individual fish ranged from 0% to 21.5%. Anguilla japonica showed the highest MDC contribution (mean 3.8%). Fish species were classified into four groups according to two criteria: (a) whether the correlation between the MDC contribution and fish size (within species) was significantly negative (negative vs. not significant; ns); and (b) whether the 90th percentile of MDC contribution was 0% or not. The mean MDC contribution was highest (2.18% ± 2.93%, 12 species) for the group with (a)ns/(b)>0%, including the high contribution species such as Abbottina rivularis (mean 5.5%) and Biwia zezera (mean 9.8%). This group (12 species) included relatively high numbers of benthic habitat use (eight species) and omnivores (nine species). The mean MDC contributions were almost zero for the group with (a)ns/(b)=0% (seven species), which included only two species of benthic habitat use and one species of omnivores. The mean MDC contributions were intermediate for the other groups with (a)negative/(b)>0% (1.30% ± 1.17%, seven species) and (a)negative/(b)=0% (0.10%, two species). The negative correlation between fish size within species and MDC contribution in groups with (a)negative/(b)>0% and (a)negative/(b)=0% indicated that ontogenetic habitat and dietary shifts induced changes in MDC transfer.Our results suggest that 21 out of 28 fish species were involved in the MDC transfer in a shallow lake, but variation in MDC transfer could be explained by fish traits (trophic guild and habitat use). Although the mean MDC contribution to all fish species was low (0.61% ± 1.97%), the roughly estimated annual consumption rate of fish on MOB corresponded to approximately 10% of the MOB production in the sediment, suggesting that fish contribute to methane dynamics from the perspective of biogeochemical cycles. [ABSTRACT FROM AUTHOR]

Estuaries exhibit high physicochemical variability and the properties of estuaries and the constituent segments are not yet systematically understood. This study aims to reveal the spatial heterogeneity of predominant organic sources using carbon and nitrogen stable isotope ratios (δ13C and δ15N, respectively) of Japanese eels (Anguilla japonica), one of the ideal natural samplers. In the Miyakoda River, Japan, our study site, the effectiveness of eel δ13C and δ15N values as standard indices of predominant organic sources was tested by employing the River Continuum Concept. This study then extended the application of these indices to the estuary, i.e., the Hamana Lake system, into which the Miyakoda River flows. Once in the upper estuary, eel δ13C and δ15N values became high, with the latter peaking in this river-estuary continuum, indicating that artificial labile subsidies (nutrients and organic matter) with high δ15N values were rapidly assimilated into the food web. Eel δ15N values decreased again in the middle estuary. Nevertheless, the influence of terrestrial organic subsidies extended into this segment, as evidenced by the low δ13C values of eels. These results suggest that refractory organic matter with low δ15N values, such as plant-derived ones, is slowly assimilated into the food web in the downstream estuarine segments. The higher δ13C values in the lower estuary suggested that the contribution of eelgrass or macroalgae occurred in addition to benthic microalgae. Thus, our results emphasize the need to consider the multiple energy flows to understand the estuary as a continuum. [ABSTRACT FROM AUTHOR]

Water‐choice experiments were conducted using glass eels of Anguilla japonica and A. marmorata, which coexist in subtropical East Asian rivers. The results of the present study, together with those of previous water‐choice experiments, suggest that salinity enhances the odor preferences of glass eels. Compared to A. marmorata glass eels, A. japonica glass eels were more strongly attracted to water collected from a leaf detritus‐accumulating backwater area than to normal river water under the same salinity conditions. These results suggest that interspecific differences in odor preferences may facilitate their habitat segregation. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Hydrobiology is the property of Editorial Department of Journal of Hydrobiology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Simple Summary: Artificial sex hormones such as 17α-methyltestosterone (MT) are used as useful tools for sex determination of major cultured fish species. However, the metamorphosis and embryology by MT treatment in Japanese eel Anguilla japonica remain largely unknown. In this study, MT induced male characteristics and morphological changes in juvenile eels, particularly a shortened snout length and pigmentation of the fin. Additionally, we successfully identified and demonstrated the involvement of specific genes associated with the determination of pigmentation, such as melanocortin 1 receptor (MC1R), tyrosinase (Tyr), and dopachrome tautomerase (DCT), following MT treatment. Our study presents the sex differentiation, deformities, and pigmentation development in the juvenile eels caused by MT treatment, providing basic knowledge for both growth and development studies analyzing sex steroids. 17α-methyltestosterone (MT) is a synthetic steroid used to induce masculinization when administered during the larval stage of fish. However, the side effects of MT on eel are still poorly understood and, in this study, we examined the various effects of MT on juvenile eel A. japonica (100.63 ± 8.56 mm total length (TL)). To further investigate growth and sex differentiation, juvenile eels (n = 1000) were exposed to 25 µg/g MT for 6 months. We analyzed growth-related factors, sex steroid hormones, skin pigmentation, and color-related gene expression. Through this study, we found a 90% sex conversion of juvenile eels to males using MT treatment. In the MT-treated eel group (285.97 ± 26.21 mm TL) where sexual maturity was induced, spermatogonia stages were observed in the gonads. In contrast, the control group (395.97 ± 27.72 mm TL) exhibited an 80% immaturity rate, with only 20% of the subjects that were rapidly developing displaying early oogonia. ELISA analysis results showed that the level of growth hormone, which is known to be secreted from spermatogonia, did not change as a result of MT treatment. We confirmed that MT delayed growth and caused morphological changes, particularly a shortened snout length and pigmentation of the fin. The total length, body weight, and snout length were considerably lower in the experimental group than in the control group. In addition, in histological analysis we also observed that some of the MT-treated group (5 out of 10 fish) showed liver atrophy and inflammation, and physiological analysis showed that the cortisol concentration increased in the MT-treated eels. Interestingly, we found that some pigment color-related genes, such as melanocortin 1 receptor (MC1R), tyrosinase (Tyr), and dopachrome tautomerase (DCT), were significantly overexpressed in the fins of MT-treated eels. These results suggest that the treatment of A. japonica larvae with MT induced masculinization but also causes growth side effects from the use of synthetic hormones. [ABSTRACT FROM AUTHOR]

IKKε is an IκB kinase participating in the control of NF-κB and type I IFN signal pathways in mammals. However, the function of IKKε in regulating immune response is largely unknown in teleost. Herein, an IKKε homologue named AjIKKε was characterized in Japanese eel (Anguilla japonica). AjIKKε has an N-terminal kinase domain, a ubiquitin-like domain, and a coiled coil-containing domain (CC), which is conserved and similar to its counterpart in mammals. Expression analysis showed that AjIKKε could be up-regulated in kidney, spleen, and particularly in liver under the stimulation of poly I:C, LPS, and Aeromonas hydrophila infection. In vitro, the mRNA levels of AjIKKε were significantly provoked in eel liver cells stimulated by LPS and poly I:C, or the different concentrations of A. hydrophila. The overexpression of AjIKKε could not only induce a significantly higher level of promoter activity of human NF-κB, AP-1, and IFN-β in a dose-dependent manner but also up-regulate the activation of promoters of Japanese eel cRel, AP1, IL6, IFN4, IRF3, and IRF7 in HEK293 cells. RNAi studies showed that after AjIKKε was knocked down, the expression levels of IL1, IL6, TNFα, c-Jun, IFN2, IFN3, MX1, MX2, and IRF3 genes were significantly down-regulated in liver, spleen, and kidney of Japanese eels. In addition, the mutants of AjIKKε-K39A, AjIKKε-S174A, and AjIKKε-ΔCC failed to activate Japanese eel IFN4, IRF3 and human IFN-β promoters in HEK293 cells. Collectively, these results suggest that AjIKKε may function as a positive regulator of NF-κB, MAPK, and IRF3-mediated type I IFN signaling pathways related to immune response evoked by bacterial and viral infection.

Abstract Background Anguillid eels spend their larval period as leptocephalus larvae that have a unique and specialized body form with leaf-like and transparent features, and they undergo drastic metamorphosis to juvenile glass eels. Less is known about the transition of leptocephali to the glass eel stage, because it is difficult to catch the metamorphosing larvae in the open ocean. However, recent advances in rearing techniques for the Japanese eel have made it possible to study the larval metamorphosis of anguillid eels. In the present study, we investigated the dynamics of gene expression during the metamorphosis of Japanese eel leptocephali using RNA sequencing. Results During metamorphosis, Japanese eels were classified into 7 developmental stages according to their morphological characteristics, and RNA sequencing was used to collect gene expression data from each stage. A total of 354.8 million clean reads were generated from the body and 365.5 million from the head, after the processing of raw reads. For filtering of genes that characterize developmental stages, a classification model created by a Random Forest algorithm was built. Using the importance of explanatory variables feature obtained from the created model, we identified 46 genes selected in the body and 169 genes selected in the head that were defined as the “most characteristic genes” during eel metamorphosis. Next, network analysis and subsequently gene clustering were conducted using the most characteristic genes and their correlated genes, and then 6 clusters in the body and 5 clusters in the head were constructed. Then, the characteristics of the clusters were revealed by Gene Ontology (GO) enrichment analysis. The expression patterns and GO terms of each stage were consistent with previous observations and experiments during the larval metamorphosis of the Japanese eel. Conclusion Genome and transcriptome resources have been generated for metamorphosing Japanese eels. Genes that characterized metamorphosis of the Japanese eel were identified through statistical modeling by a Random Forest algorithm. The functions of these genes were consistent with previous observations and experiments during the metamorphosis of anguillid eels.

The starter diet for Japanese eel (Anguilla japonica) has always been a difficult problem for the realization of total artificial reproduction. Therefore, this research analyzed the nutritional composition of artificially fertilized eggs, and transcriptome of samples from early hatchlings of fry to better understand nutrients requirements. The composition of crude lipid and crude protein in fertilized eggs was 7.24% ± 0.32% and 10.56% ± 0.41%, respectively. Seven kinds of essential amino acids (EAA) were detected but took a comparable lower content (3.19%) than other marine fish eggs. We randomly assembled 265.74 million clean reads and identified 1751 differentially expressed genes (DEGs) (P < 0.01) from pre-leptocephalus larvae. A total of 23 KEGG pathways related to the digestive and metabolic system were detected. Genes related to the secretion pathway of saliva, pancreatic juice and other digestive juices were significantly changed. Transcriptome analysis showed that as larvae aged, glycolytic metabolism and the transcription level of hexokinase (HK) increased significantly (day 0 to 12). This study will facilitate future studies on the nutrition of A. japonica larvae and other biological traits to reproductive research. [ABSTRACT FROM AUTHOR]

IFNAR1 (type I IFN receptor) plays a crucial role in mammalian host immunity against virus invasion. However, the immune function of the fish teleost IFNAR1, which is also named CRFB5, remains largely unknown. Here, we report the successful cloning of the whole cDNA of the Japanese eel (Anguilla japonica) CRFB5 homolog AjCRFB5b. AjCRFB5b is similar in its three-dimensional structure to grass carp, zebrafish, and humans. It comprises typically two fibronectin-3 domains and a transmembrane region. Expression analysis revealed that AjCRFB5b was widely expressed in all tissues detected, especially in the liver. In vivo, AjCRFB5b was significantly induced in the liver by Aeromonas hydrophila and poly I:C. In vitro, enhanced expression of AjCRFB5b was found in Japanese eel liver cells upon Aeromonas hydrophila infection or poly I:C stimulation. Fluorescence microscopy results showed that EGFP-AjCRFB5b proteins were distributed in the cytoplasm, some in a condensed form in Japanese eel liver cells. Luciferase reporter assays demonstrated that AjCRFB5b overexpression not only significantly induced human IFN-β promoter activity in HEK293 cells and FHM cells but also strongly upregulated immune-related genes in Japanese eel liver cells, including IFN1 (Interferon1), IFN4, IRF5 (Interferon regulatory factor5), and IRF7 of the type I IFN signaling pathway, as well as one of the essential antimicrobial peptides LEAP-2. In addition, RNA interference (RNAi) results showed that knocking down AjCRFB5b caused all five genes to be significantly downregulated in Japanese eel liver cells. Our findings indicate that AjCRFB5b positively regulates host antiviral immune responses by the IRF5- and IRF7-mediated type I IFN signaling pathways and that it also participates in the host immune response to bacterial infection through the production of antimicrobial peptides mediated by LEAP-2. [ABSTRACT FROM AUTHOR]

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]

In rearing systems for the Japanese eel Anguilla japonica, although it is assumed that microorganisms influence larval survival and mortality, particularly during the early stages of growth, the effects of bacterial communities on larval survival have yet to be sufficiently determined. In this study, we compared the bacterial communities associated with larval survival at three stages of eel growth. To artificially alter bacterial communities and assess larval survival, eel larvae were treated with 11 types of antibiotic, and larval survival and bacterial characteristics were compared between the antibiotic-treated and antibiotic-free control groups. Throughout the three growth stages, eels treated with four antibiotics (polymyxin B, tetracycline, novobiocin, and erythromycin) had survival rates higher than those in the control groups. The bacterial communities of surviving larvae in the control and antibiotic groups and dead larvae in the control groups were subsequently analyzed using 16S rRNA gene amplicon sequencing. PERMANOVA analysis indicated that these three larval groups were characterized by significantly different bacterial communities. We identified 14 biomarker amplicon sequence variants (ASVs) of bacterial genera such as Oceanobacter, Alcanivorax, Marinobacter, Roseibium, and Sneathiella that were enriched in surviving larvae in the antibiotic treatment groups. In contrast, all four biomarker ASVs enriched in dead larvae of the control groups were from bacteria in the genus Vibrio. Moreover, 52 bacterial strains corresponding to nine biomarkers were isolated using a culture method. To the best of our knowledge, this is the first study to evaluate the bacterial communities associated with the survival and mortality of larvae in during the early stages of Japanese eel growth and to isolate biomarker bacterial strains. These findings will provide valuable insights for enhancing larval survival in the eel larval rearing systems from a microbiological perspective. [ABSTRACT FROM AUTHOR]

The feedback regulatory effects of estrogen (E2) and androgen (T) on the gonadotropin-releasing hormone (GnRH) and gonadotropin (GtH) within the brain–pituitary–gonad (BPG) axis in eels with undeveloped ovaries were investigated through in vivo studies. However, the regulatory role of the BPG axis only became apparent during ovary development in the migratory stage. To further elucidate the direct feedback regulation of the BPG axis, female Anguilla japonica underwent artificial induction of vitellogenesis, and the regulation of BPG axis tissues by GtH (human chorionic gonadotropin, hCG), E2, and T was explored through in vitro exposure. The mRNA expression levels of GnRH (mGnRH), GtH (fshb and lhb), and steroid biosynthesis enzymes (cyp11a1, hsd3b, cyp17a1, and cyp17a2) in the diencephalon, pituitary, and ovary, respectively, were determined. The results showed that the expression level of mGnRH in the diencephalon was significantly downregulated by 0.1 IU/mL hCG but upregulated by both 1 nM E2 and higher concentrations of T, suggesting a direct positive feedback regulation of E2 on mGnRH. In the pituitary, the expression levels of fshb and lhb were upregulated by E2, while fshb was suppressed by T. In the ovaries, the expression of cyp11a1 and hsd3b was upregulated by 1 nM E2, whereas T exposure resulted in an opposite effect. Cyp17a1 mRNA levels did not differ significantly with E2 treatment but were upregulated by 1 nM T. These findings suggest that low concentrations of E2 exhibited positive feedback regulation on all three levels (diencephalon, pituitary, and ovary) of the BPG axis, while T showed weaker and differential feedback regulation in BPG axis tissues. Overall, this study's results revealed the direct feedback regulation of hCG, E2, and T on the BPG axis in eels, a phylogenetic base of teleosts. [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]

Investigating the composition and functional analysis of the intestinal microbiome of wild leptocephalus larvae has significant scientific and practical value for predicting their nutritional needs and developing appropriate initial diet. The study identified the species of leptocephalus larvae captured during the 2022 and 2023 scientific expeditions of the "Songhang" research vessel in Western and Central Pacific Ocean using DNA barcoding technology. Additionally, 16S rDNA high-throughput sequencing was employed to sequence the intestinal samples of the leptocephalus larvae, aiming to analyze the intestinal microbial community structure and identify dominant microflora and their related functions. Results showed that 14 of 16 captured leptocephalus larvae were identified to the species level, including Ariosoma majus, Thalassenchelys coheni, Ariosoma megalops, Gnathophis longicauda, Ariosoma meeki, Muraenidae sp. KY17-483, and Gymnothorax zonipectis. The other two leptocephalus larvae were identified to the genus level, Gymnothorax and Gnathophis. Sequencing revealed that the dominant bacterial groups in the intestinal microbiota of pacific leptocephalus larvae were Proteobacteria, Bacteroidetes and Firmicutes. At the genus classification level, Psychrobacter and Acinetobacter exhibited relatively high abundance. However, there were some differences in the intestinal microbiota between different species and sampling sites. Furthermore, functional prediction analysis showed that intestinal microbiota involved in metabolism had the highest relative abundance, and functional pathways related to 'amino acid transport and metabolism' were the most enriched. These data suggested that leptocephalus larvae have a higher demand for protein. Our results preliminarily elucidated the diverse composition of the intestinal microbiota in leptocephalus larvae, providing a reference for studying their feeding habits and the development of starter feeds during the artificial breeding of Japanese eels. [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]

Copyright of Journal of Hydrobiology is the property of Editorial Department of Journal of Hydrobiology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Synaptic zinc ions (Zn2+) play an important role in the development of vascular dementia (VD) and Parkinson's disease (PD). In this article, we reviewed the current comprehension of the Zn2+-induced neurotoxicity that leads to the pathogenesis of these neuronal diseases. Zn2+-induced neurotoxicity was investigated by using immortalised hypothalamic neurons (GT1-7 cells). This cell line is useful for the development of a rapid and convenient screening system for investigating Zn2+-induced neurotoxicity. GT1-7 cells were also used to search for substances that prevent Zn2+-induced neurotoxicity. Among the tested substances was a protective substance in the extract of Japanese eel (Anguilla japonica), and we determined its structure to be like carnosine (β-alanylhistidine). Carnosine may be a therapeutic drug for VD and PD. Furthermore, we reviewed the molecular mechanisms that involve the role of carnosine as an endogenous protector and its protective effect against Zn2+-induced cytotoxicity and discussed the prospects for the future therapeutic applications of this dipeptide for neurodegenerative diseases and dementia. [ABSTRACT FROM AUTHOR]

Objective: Feed additives have attracted increased attention in aquaculture due to their ability to modulate fish gut microbiota, resulting in improved fish growth and immunity. This study assessed the effects of two synbiotics in Japanese Eel Anguilla japonica: Bacillus subtilis with mannooligosaccharides (MOS) and Enterococcus faecium with fructooligosaccharides (FOS). Methods: Six diets, including a control (CON) diet, oxytetracycline (OTC) diet, and four synbiotic diets (B. subtilis at 1 × 106 or 1 × 107 colony‐forming units [CFU]/g with MOS at 5 g/kg [BS6MO and BS7MO; collectively, BSMOS diets] and E. faecium at 1 × 106 or 1 × 107 CFU/g with FOS at 5 g/kg [EF6FO and EF7FO; collectively, EFFOS diets]), were fed to triplicate groups of 20 fish (average weight ± SD = 6.00 ± 0.07 g) for 8 weeks. Result: Fish fed the BSMOS diets showed significantly higher weight gain, specific growth rate (SGR), and feed efficiency compared to fish fed the CON and OTC diets, but the values were not significantly different from those of fish fed the EFFOS diets. Weight gain and SGR of fish that were given EFFOS diets were not significantly different from those of fish fed all other diets. Fish fed the OTC diet showed a higher mean aspartate aminotransferase level, although the difference was not statistically significant. The myeloperoxidase activity of fish fed the BS7MO diet was significantly higher than those of fish receiving all other diets, and the superoxide dismutase activity of fish fed the BS7MO diet was also significantly higher than that of fish fed the EF7FO diet. Overall, the BSMOS synbiotic diets were significantly more effective than the CON diet in enhancing fish survival against a Vibrio anguillarum challenge. Conclusion: Our findings suggest that synbiotics can be a preferable alternative to antibiotics in aquaculture. Impact statementThe Japanese Eel is an economically important fish in Korea, but its aquaculture is fraught with disease challenges and slow growth. By adding synbiotics (blends of prebiotics and probiotics) to the Japanese Eel's diet, growth and immunity were improved, suggesting a potential alternative to antibiotics. [ABSTRACT FROM AUTHOR]

In recent years, the use of fish-derived probiotics in aquaculture has become more widespread. However, research on Anguilla japonica-derived probiotics is still limited. To evaluate the potential of probiotics for disease control in eel aquaculture, isolates were obtained from the intestinal tract of healthy Anguilla japonica. These isolates were assessed for their adhesion properties, inhibition of pathogen adhesion, and hydrolytic enzyme production. Morphological characteristics and 16S rRNA sequence analysis were used for identification. Results showed that the AJQ03 strain adhered to the intestinal mucus and inhibited common pathogenic bacteria through adhesion inhibition, and further produced amylase, lipase, protease, and cellulase. Based on morphological characteristics and 16S rRNA sequencing, AJQ03 was identified as Bacillus subtilis. The strain demonstrated tolerance to various extreme conditions, as well as survival in simulated gastrointestinal fluids and superior growth in intestinal fluid compared to Luria-Bertani (LB) broth. In vitro safety tests showed that AJQ03 was not resistant to 32 antibiotics and exhibited γ hemolysis on blood plate. In vivo safety tests demonstrated a 100% survival rate for the fish, with stable organ indices, reduced bacterial loads in the liver and spleen, and complete bacterial clearance by day 7 without residue. Intestinal bacterial load results confirmed effective colonization by strain AJQ03. Analysis of the impact of AJQ03 on the gut microbiota of A. japonica revealed a significant increase in the relative abundance of Bacillus at the genus level, corroborating the colonization efficiency of AJQ03. Additionally, the relative abundances of Klebsiella, Pseudomonas, and Aeromonas were significantly lower compared to the controls, indicating that strain AJQ03 effectively reduced harmful bacteria and improved gut microbiota composition. This study confirms that B. subtilis AJQ03, isolated from the intestine of A. japonica, can serve as a probiotic candidate in A. japonica aquaculture.

The natriuretic peptide (NP) family consists of cardiac NPs (ANP, BNP, and VNP) and brain NPs (CNPs) in teleosts. In addition to CNP1-4, a paralogue of CNP4 (named CNP4b) was recently discovered in basal teleosts including Japanese eel. Mammals have lost most Cnps during the evolution, but teleost cnps were conserved and diversified, suggesting that CNPs are important hormones for maintaining brain functions in teleost. The present study evaluated the potency of each Japanese eel CNP to their NP receptors (NPR-A, NPR-B, NPR-C, and NPR-D) overexpressed in CHO cells. A comprehensive brain map of cnps- and nprs-expressing neurons in Japanese eel was constructed by integrating the localization results obtained by in situ hybridization. The result showed that CHO cells expressing NPR-A and NPR-B induced strong cGMP productions after stimulation by cardiac and brain NPs, respectively. Regarding brain distribution of cnps, cnp1 is engaged in the ventral telencephalic area and periventricular area including the parvocellular preoptic nucleus (Pp), anterior/posterior tuberal nuclei, and periventricular gray zone of the optic tectum. cnp3 is found in the habenular nucleus and prolactin cells in the pituitary. cnp4 is expressed in the ventral telencephalic area, while cnp4b is expressed in the motoneurons in the medullary area. Such CNP isoform-specific localizations suggest that function of each CNP has diverged in the eel brain. Furthermore, the Pp lacking the blood-brain barrier expressed both npra and nprb, suggesting that endocrine and paracrine NPs interplay for regulating the Pp functions in Japanese eels. [ABSTRACT FROM AUTHOR]

Japanese eel (Anguilla japonica) is a commercially important fish species in Asia. Understanding factors like photoperiod, temperature, and lunar cycles is crucial for successful aquaculture and managing its reproduction. Melatonin and dopamine (DA) are essential for regulating reproduction in vertebrates, including fish. This study investigated the effects of melatonin and DA on the reproductive system of mature male Japanese eels to better understand reproductive regulation in fish. To clarify the effects of these hormones on sexual maturation in eels, a critical stage in the reproductive process, sexual maturation was induced by injecting human chorionic gonadotropin, which stimulates the production of sex hormones. To check the effect of melatonin and DA on sexual maturation, DA, melatonin, and DA + domperidone were intraperitoneally injected into fish from each group (six per treatment) at a dose of 1 mg/kg body weight. The fish were then examined using quantitative RT‐PCR by comparing the messenger RNA level of reproduction‐related genes (gonadotropin releasing hormone 1; gnrh1, gonadotropin releasing hormone 2; gnrh2, follicle stimulating hormone; fshβ, luteinizing hormone; lhβ and DA receptor 2b; d2b), involved in the gonadotropic axis in eels, to those that received a control injection. The results indicate significant differences in the expression levels of gnrh1, gnrh2 and d2b in the brain and d2b, fshβ, lhβ in the pituitary at different stages of sexual maturation. Melatonin appears to enhance the production of sex gonadotropins, whereas DA inhibits them. These findings suggest an interaction between melatonin and DA in regulating reproduction in Japanese eels. Research Highlights: Melatonin enhances gonadotropin genes, while dopamine inhibits it in Japanese eels, revealing a dynamic interplay in their reproductive regulation [ABSTRACT FROM AUTHOR]

The only diet that has been capable of rearing eel larvae to the glass eel stage in captivity is the shark egg (SE)-based diet (SE). This study investigated the potential of alternative dietary components, namely hen egg yolk (HEY), milk proteins, and fish protein hydrolysate (FPH), on the growth and survival of eel larvae. In the first experiment, a diet containing HEY and skimmed milk powder (HS) was compared to SE. There were no significant differences in growth and survival rate between the two diets except in the early part of the experiment period. In the second experiment, HS was modified by the addition of FPH and casein (FC), and larval performance was evaluated for three dietary regimens (SE, HS, and FC). The performance of larvae fed HS and FC was found to be comparable or superior to those fed SE. It was observed that larvae fed the alternative diets to SE were able to progress to glass eels. However, more skeletal abnormalities were observed in HS in experiment 1. The results of this study indicate that a combination of HEY and milk proteins is suitable as a larval eel diet, and that it has the potential to replace SE. [ABSTRACT FROM AUTHOR]

Biofloc technology (BFT) systems heavily rely on microbiota to mitigate ammonia toxicity and manage essential nutrient cycling. Understanding the diversity and functional role of microbiota within BFT-applied aquaculture systems is crucial for ensuring sustainable operations. Though some studies exist on BFT microbiota, research on microbial differences in Japanese eel aquaculture is still limited, hindering the wider application of BFT systems. In this study, we analyzed the characteristics of water quality factors and microbiota in Japanese eel (Anguilla japonica) breeding water, applying the BFT system. Using a metabarcoding approach, the diversity and community structure of aquatic microbiota were compared between BFT and continuous flow (CF) systems. The pH was significantly higher in CF water, while total ammonia nitrogen (TAN) and nitrite (NO2−-N) was higher in BFT water. Alpha diversity was significantly higher in BFT compared to CF systems, and it was correlated significantly with pH and TAN. In both BFT and CF water, the phyla Proteobacteria and Bacteroidota were found to be the most abundant. In the BFT water, a diverse array of bacterial taxa, including BFT-specific clades, were consistently present, while the microbiota in CF water was more variable and contained fewer specific taxa. In addition, bacterial functions related to nitrate reduction, sulfur compound oxidation, and chitinolysis were significantly more abundant in BFT than in CF systems. These findings highlight differences in water quality and microbiota between aquaculture systems, which can inform future research on the use of BFT for sustainable fish farming.

Eels have fascinated biologists for centuries due to their amazing long‐distance migrations between freshwater habitats and very distant ocean spawning areas. The migratory life histories of the Japanese eel, Anguilla japonica, in the waters of south China are not very clear despite its ecological importance, and the need for fishery regulation and management. In this study, strontium (Sr) and calcium (Ca) microchemical profiles of the otoliths of silver eels were measured by X‐ray electron probe microanalysis based on data collected from different habitats (including freshwater and brackish habitats), in the large subtropical Pearl River. The corresponding habitat preference characteristics were further analysed using redundancy analysis (RDA). A total of 195 Japanese eels were collected over 6 years. The collected individuals ranged from 180 to 771 mm in total length and from 8 to 612 g in body weight. Two‐dimensional pictures of the Sr:Ca concentrations in otoliths revealed that the A. japonica in the Pearl River are almost entirely river eels, spending the majority of their lives in fresh water without exposure to salt water, while the catadromous migration time has delayed about 1 month in the Pearl River estuary in the past 20 years. RDA analysis further indicated that juveniles and adults preferred water with high salinity and high tide levels. Youth preferred habitats with high river fractals. Our findings contribute to a growing body of evidence showing that the eels are extremely scarce currently and conservation measures against them are imminent, including the protection of brackish and freshwater areas where they live in south China. [ABSTRACT FROM AUTHOR]

Simple Summary: The Japanese eel, a high-valued commercial fish species in the Yangtze River ecosystem, faces a significant decline in the resource. In the present study, we reported our 11-year investigation to track the migratory patterns of glass eels of Anguilla japonica into the Yangtze estuary. We identified two distinct annual peak arrival times which coincide with cooler water temperatures, i.e., the first peak at 6–8 °C and the second at 10–12 °C, both in sync with the strong spring tides of the lunar cycle. Despite rebounded population in a few years, an auto-regressive integrated moving average model analysis reveals a concerning trend: since 2016, rising temperatures have been associated with delayed recruitment. This change poses a threat to their migratory routes and life cycle stability. Underlining the critical nature of our research, our findings call for immediate conservation efforts to counteract the impact of climate change on the eel and to safeguard the biodiversity of the river ecosystem, a valuable asset to society. The decline of Japanese eel (Anguilla japonica) populations in the Yangtze River estuary represents a critical conservation concern. Eleven-years of daily catch data during recruitment periods (i.e., January–April, 2012–2022) indicate that annual catch averaged from 153 to 1108 eels, and show a bimodal pattern in glass eel arrivals. Utilizing seasonal-trend decomposition and generalized additive models, we demonstrated a strong correlation between catch abundance, optimal water temperatures, and lunar cycles. An auto-regressive integrated moving average (ARIMA) model predicts an increase in glass eel numbers for 2023–2024 but also points to a concerning trend of delayed recruitment timing since 2016, attributable to the 0.48 °C per decade rise in sea surface temperatures. This delay correlates with a significant decrease in the average body weight of glass eels, suggesting potential energy deficits that may hinder successful upstream migration. This study not only furthers our understanding of glass eel recruitment dynamics but also underscores the urgent need for targeted conservation measures. Additionally, it highlights the importance of sustained, detailed monitoring to mitigate the detrimental effects of climate change on these eels, vital for preserving the Yangtze River's ecological integrity. [ABSTRACT FROM AUTHOR]

Bacterial infections are a significant concern in eel aquaculture. Edwardsiella causes severe systemic infections and high mortality rates. This study aimed to demonstrate that synbiotic oral administration could improve eels' gut environment, health, and aquaculture productivity. After 1 month of administration in Japanese eel Anguilla japonica using synbiotics consisting of prebiotic 1-kestose, a fructooligosaccharide with a degree of polymerization of 3, and the probiotic Lactiplantibacillus plantarum FM8, isolated from fermented food, we discovered that feed efficiency was improved by more than 20%. This may partly be because the synbiotics significantly increased intestinal acetate concentrations (P = 0.0144). 16S rRNA gene amplicon sequencing and linear discriminant analysis effect size revealed that the relative abundance of the Romboutsia was significantly higher, and that of the Edwardsiella was significantly lower in the eel intestine of the synbiotic group (P = 0.0166 and 0.0497, respectively). The BLAST search revealed that all amplicon sequence variants classified as genus Edwardsiella had > 98% identity with the 16S rRNA gene of Edwardsiella tarda, Edwardsiella piscicida, or Edwardsiella anguillarum. Our synbiotic administration in eels exhibited a breakthrough effect in significantly reducing Edwardsiella and increasing acetate concentrations in the intestine, suggesting that our synbiotics could reduce the incidence of eel diseases and improve aquaculture productivity. [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 chemical composition of muscles from Japanese eels cultured in hot-spring water was determined and compared with that of eels cultured in well water. The proximate composition did not differ significantly between the culture conditions, but the hepatosomatic indices of the hot-spring water eels were significantly higher than those of the well water eels. Amino acid levels were higher in the hot-spring water eels, and the carnosine content of the hot-spring eels was lower than that of the controls. Our results suggest that hot-spring water is a promising medium for eel culture, and its use may bring energy cost savings. [ABSTRACT FROM AUTHOR]

Daytime habitat use by Japanese eel (Anguilla japonica) at the yellow eel stage (total length ≥ 15 cm) and the newly recruited glass/elver stage in spring (total length 5–7 cm) was examined in three sites of different types of streams in southwestern Japan. Although habitat use by yellow eels was examined in summer and winter, no distinct difference was found between the two seasons. Yellow eels preferred lower current velocity in all the three sites in both seasons. Cover use by yellow eels differed among the three sites having different cover-type availability. In a coarse-substrate mountain stream, the majority of yellow eels used cobble/boulders as cover, whereas in the other two fine-substrate streams, where coarse substrates were rare, large parts of yellow eels burrowed into fine sediments. In addition, undercut bank and bank vegetation were substantially used and preferred by yellow eels depending on the study site, suggesting the importance of natural bank habitat. The majority of glass/elver eels used sand substrates with current velocity < 10 cm·s−1 in all the three sites, suggesting their clear preference for such habitat. However, the preference for sand shown in this study contrasts with a general trend established by previous experimental studies that glass/elver eels prefer stony substrates over sand. Our results contrary to the general trend could be attributed to glass/elver eels displaying a stronger preference for slow currents over stony substrates, implying that current velocity has a dominant effect in habitat use by glass/elver eels under natural conditions. [ABSTRACT FROM AUTHOR]

The feedback regulatory effects of estrogen (E2) and androgen (T) on the gonadotropin-releasing hormone (GnRH) and gonadotropin (GtH) within the brain–pituitary–gonad (BPG) axis in eels with undeveloped ovaries were investigated through in vivo studies. However, the regulatory role of the BPG axis only became apparent during ovary development in the migratory stage. To further elucidate the direct feedback regulation of the BPG axis, female Anguilla japonica underwent artificial induction of vitellogenesis, and the regulation of BPG axis tissues by GtH (human chorionic gonadotropin, hCG), E2, and T was explored through in vitro exposure. The mRNA expression levels of GnRH (mGnRH), GtH (fshb and lhb), and steroid biosynthesis enzymes (cyp11a1, hsd3b, cyp17a1, and cyp17a2) in the diencephalon, pituitary, and ovary, respectively, were determined. The results showed that the expression level of mGnRH in the diencephalon was significantly downregulated by 0.1 IU/mL hCG but upregulated by both 1 nM E2 and higher concentrations of T, suggesting a direct positive feedback regulation of E2 on mGnRH. In the pituitary, the expression levels of fshb and lhb were upregulated by E2, while fshb was suppressed by T. In the ovaries, the expression of cyp11a1 and hsd3b was upregulated by 1 nM E2, whereas T exposure resulted in an opposite effect. Cyp17a1 mRNA levels did not differ significantly with E2 treatment but were upregulated by 1 nM T. These findings suggest that low concentrations of E2 exhibited positive feedback regulation on all three levels (diencephalon, pituitary, and ovary) of the BPG axis, while T showed weaker and differential feedback regulation in BPG axis tissues. Overall, this study’s results revealed the direct feedback regulation of hCG, E2, and T on the BPG axis in eels, a phylogenetic base of teleosts.

Japanese eel (Anguilla japonica) and giant mottled eel (Anguilla marmorata) are the main eel species reared in recirculating aquaculture system (RAS) in Taiwan. RAS increases the rearing density of eel. On the other hand, it spreads pathogens and diseases more efficiently and uncontrollably. Currently, eel farmers use antibiotics to control disease in RAS, but their misuse can lead to resistance and food-safety issues. Spermidine carbon quantum dots (CQDSpds), a newly developed carbon nanomaterial with positive charges on the surface, can bind to the negatively charged bacterial cell membrane to destroy bacteria and not cause drug resistance. This research investigated the effects of different concentrations of CQDSpds as paste feed additives on the growth performance, intestinal morphology, immune-related gene expression, and disease resistance of Japanese and giant marble eels. The results showed the highest final body weight, weight gain, and specific growth rate of Japanese eel and giant mottled eel fed with 1 ppm and 0.5 ppm CQDSpds added. Compared with the control group, 1 ppm CQDSpds group showed higher lysozyme expression in Japanese eels. In the part of intestinal morphology, there was not a significant difference in intestinal villus height and intestinal morphology between the CQDSpds groups and the control group in both eel species. The survival rate of each group supplemented with CQDSpds was higher than the control group after the E. tarda challenge test, among which the highest were found at 1 ppm group in Japanese eel and 0.5 ppm group in giant mottled eel. By this research, the additive of CQDSpds in eel culturing could improve the growth and immune performance with not caused an impact on the intestine of both eels, which can be a potential feed additive in intensive eel farming.

The extraction process of Japanese eel skin collagen was optimized using response surface methodology, and its physicochemical properties and allergenicity were investigated. Based on the single-factor experiment, the best extraction process was obtained based on the acid-enzymatic extraction method, with the material-liquid ratio, the concentration of extraction solution and the extraction time as the investigating factors and the collagen yield as the index. The results showed that the collagen contained five bands and belonged to type I collagen, and the percentages of essential amino acids and medicinal amino acids were 19. 61% and 11. 04%, respectively. The thermal transition temperature and thermal denaturation temperature of collagen were 38. 5 and 28. 3 ℃, respectively. The enzyme-linked immunosorbent reaction and simulated gastrointestinal fluid experiments showed that collagen has certain allergenic properties, providing a theoretical basis for further studies on eel skin collagen. [ABSTRACT FROM AUTHOR]

Understanding the habitat requirements of fishes throughout their life cycle is essential for effective conservation. Despite the growing need to conserve anguillid eels, the current understanding of the ontogenetic habitat shifts during their riverine stages remains limited, particularly for the Japanese eel Anguilla japonica and the Indo‐Pacific eel Anguilla marmorata.Here, changes in habitat use during the riverine stages of these two eel species were investigated in the southern Japan rivers, where they coexist. Eels were classified into total length (TL) classes at 10 cm intervals, and generalized additive models were constructed using these classes as an ordinal, quasi‐continuous explanatory variable.The models showed that the habitat use of the two species changed with growth; both species shifted to deeper and more lentic habitats as they grew larger. Small‐ and mid‐sized (<25 and 25–55 cm in TL, respectively) A. japonica used more lentic habitats with smaller substrates, ranging from sand/silt to cobbles, than small‐ and mid‐sized A. marmorata, which mainly used lotic habitats with cobble and boulder substrates. However, compared with small‐sized conspecifics, mid‐sized A. japonica mainly used habitats with larger substrates, such as pebbles and cobbles, suggesting that the interspecific overlap in habitat use is relatively high for mid‐sized individuals of both species. Large A. marmorata (≥85 cm) mainly used habitats with smaller substrates as they grew larger.The results suggest that using a fine body size classification and including these classes as a quasi‐continuous explanatory variable in habitat models capture gradual changes in habitat use, improving habitat use evaluation of anguillid eels. Preserving the environmental heterogeneity of river habitats in terms of water depth, current velocity and substrate size would facilitate habitat segregation of both conspecifics and allospecifics in the same rivers, thereby promoting the conservation of anguillid eels. [ABSTRACT FROM AUTHOR]

Environmental DNA (eDNA) analysis is a biological survey method that has drawn much attention in recent years. However, the results of eDNA analysis and capture surveys often do not completely match, and the validity of the eDNA analysis needs to be verified. Verification of eDNA metabarcoding was conducted in a river in Fukushima Prefecture, Japan, in comparison with capture survey data. Most of the captured species were detected, and 13 uncaptured lineages (two genera and 11 species) were detected in the eDNAs. Some rare species detected in the eDNAs were also identified, including exotic eels and an endangered yet out-of-range bitterling fish. To confirm the validity of the exotic Anguilla spp. detected in eDNAs, mitochondrial Cytb sequencing was performed on captured eel specimens. All eel specimens were sequenced as the native Anguilla japonica, indicating a small biomass of the exotic species difficult to capture. Our results clearly indicated the eDNA analysis as a powerful tool for detecting possible habitats of rare fish species without disturbance to the natural environment. [ABSTRACT FROM AUTHOR]

The Japanese eel (Anguilla japonica) is the most prevalent freshwater eel species in Taiwan. However, its population has undergone a significant decline in recent decades due to factors such as overfishing, habitat destruction, and the effects of climate change. Urgent action is needed to conserve this species. Before implementing conservation measures, it is imperative to ascertain the distribution of Japanese eels in Taiwan's rivers. This study's primary objective was to assess the effectiveness of eDNA analysis as a method for detecting Japanese eels. To achieve this goal, we compared eDNA analysis data with results obtained from electrofishing, with the Fengshan and Shimen Rivers serving as our designated test sites. Additionally, we collected water samples from 34 other rivers across Taiwan to comprehensively assess the species' wider distribution using eDNA analysis. Our findings demonstrated eDNA analysis's viability for detecting Japanese eels. Of the 36 rivers tested, Japanese eel DNA was detected in samples from 21 rivers, scattered across northern, eastern, southern, and western Taiwan, with no specific concentration in any region. We also noted reduced detectability of Japanese eel DNA in highly polluted rivers, indicating that river pollution may have a potential impact on their population. In the future, expanding eDNA analysis to more rivers could identify additional rivers that Japanese eels inhabit. Subsequently, resource management and conservation efforts can be focused on these identified habitats. Furthermore, developing advanced eDNA-based methods for estimating the abundance or biomass of Japanese eels could enhance the flexibility of management and conservation measures. [ABSTRACT FROM AUTHOR]

Simple Summary: In this study, we investigated the response mechanism of type I interferon receptor in the Japanese eel (Anguilla japonica) after an immune challenge, and we also explored the key role of type I interferon receptor in its immune response mechanism, which will provide new evidence for the study of type I interferon receptor in vertebrates, especially in teleost. IFNAR1, one of the type I IFN receptors, is crucial to mammalian host defense against viral invasion. However, largely unknown is the immunological role of the fish teleost protein IFNAR1, also known as CRFB5. We have successfully cloned the whole cDNA of the Japanese eel's (Anguilla japonica) CRFB5a homolog, AjCRFB5a. The two fibronectin-3 domains and the transmembrane region (238–260 aa) of AjCRFB5a are normally present, and it shares a three-dimensional structure with zebrafish, Asian arowana, and humans. According to expression analyses, AjCRFB5a is highly expressed in all tissues found, particularly the liver and intestine. In vivo, Aeromonas hydrophila, LPS, and the viral mimic poly I:C all dramatically increased AjCRFB5a expression in the liver. Japanese eel liver cells were reported to express AjCRFB5a more strongly in vitro after being exposed to Aeromonas hydrophila or being stimulated with poly I: C. The membranes of Japanese eel liver cells contained EGFP-AjCRFB5a proteins, some of which were condensed, according to the results of fluorescence microscopy. Luciferase reporter assays showed that AjCRFB5a overexpression strongly increased the expression of immune-related genes in Japanese eel liver cells, such as IFN1, IFN2, IFN3, IFN4, IRF3, IRF5, and IRF7 of the type I IFN signaling pathway, as well as one of the essential antimicrobial peptides LEAP2, in addition to significantly inducing human IFN-promoter activities in HEK293 cells. Additionally, RNA interference (RNAi) data demonstrated that knocking down AjCRFB5a caused all eight of those genes to drastically lower their expression in Japanese eel liver cells, as well as to variable degrees in the kidney, spleen, liver, and intestine. Our findings together showed that AjCRFB5a participates in the host immune response to bacterial infection by inducing antimicrobial peptides mediated by LEAP2 and favorably modulates host antiviral immune responses by activating IRF3 and IRF7-driven type I IFN signaling pathways. [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]

Major histocompatibility complex (MHC) molecules play a critical role in the immune response of vertebrate animals by presenting foreign antigens to T lymphocytes. In this study, we first cloned and identified a classical and a novel nonclassical MHC I α molecules from Japanese eel (Anguilla japonica), named as AjMHC I-UBA and AjMHC I-L, respectively. The full-length cDNA of AjMHC I-UBA contains an open reading frame (ORF) of 1047 bp encoding a predicted protein of 348 amino acids, while AjMHC I-L 1089 bp encodes 362 amino acids. The multiple alignment of the amino acid sequence showed that AjMHC I-UBA and AjMHC I-L consist of an N-terminal MHC I superfamily domain within α1 and α2 helices, an IgC-MHC I α3 domain, and a transmembrane region close to the C-terminal, which are similar to other fish and mammal species. Molecular polymorphism analysis showed that eight different major alleles of AjMHC I-UBA, named as AjMHC I-UBA*0101～1001, were identified from six Japanese eel individuals. Furthermore, a distinguishing signature of the nonclassical L-lineage genes specific motif “HINMTL”, including an N-glycosylation site (NXS/T), was present in the α3 domain of AjMHC I-L sequences. Although the predicted three-dimensional structures of AjMHC I-UBA and AjMHC I-L are similar to that of human MHC I α, phylogenetic analysis showed that these two protein molecules belong to classical MHC I UBA gene of U-lineage and nonclassical MHC I gene of L-lineage, respectively. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that the highest expression of AjMHC I-UBA and AjMHC I-L was found in the intestine and the expression level of AjMHC I-UBA in all of the examined tissues was significantly higher than that of AjMHC I-L. The expressions of AjMHC I-UBA and AjMHC I-L in liver, kidney and spleen were significantly induced following injection with the viral mimic poly I:C, LPS, and Aeromonas hydrophila infection. In vitro, the AjMHC I-UBA and AjMHC I-L transcripts of Japanese eel liver cells were significantly enhanced by the treatment of poly I:C or the stimulation of the high concentration of A. hydrophila. Subcellular localization showed that under natural conditions, AjMHC I-UBA and AjMHC I-L were uniformly distributed in the cytoplasm, and aggregated into spots or flakes in the cytoplasm after the stimulation of poly I:C or LPS. These results collectively suggested that AjMHC I-UBA and AjMHC I-L are important components possibly involved in Japanese eel defense against viral and bacterial infection. Taken together, these findings provide valuable insight into the immune function of MHC class I in the immune system of teleost.

Predation shapes diversity in the defensive tactics of prey. One specialized defensive tactic is to escape the digestive system of the predator after capture 1,2,3,4,5,6,7,8. While most of these defensive tactics involve passive ejection alive from predators' mouths and vents 1,2,3,4,5 , active escape from the digestive tracts of predators has recently been observed in certain invertebrate species 6,7 and fish 8. However, no study has yet uncovered the behavioral patterns and escape routes of the prey within a predator's digestive tract. Here, we report the sequential escape processes of the Japanese eel Anguilla japonica from capture to escape via the gills of predatory fish Odontobutis obscura using an X-ray video system. All captured eels had at least one portion of their bodies swallowed into the stomach of the predator. Surprisingly, after being swallowed, most individuals attempted to escape by going back up the digestive tract towards the esophagus and gill, and some of them succeeded in escaping via the predator's gill. Some eels, whose entire bodies were completely inside the stomach, exhibited circling behavior along the stomach, seemingly searching for possible escape routes. An electro-anesthetization experiment revealed that eels utilize various escape routes through gill clefts, rather than just one. Japanese eels can escape via the gill of the predatory fish after being captured. Hasegawa et al. examine the behavioral patterns and escape processes of eels within the predator's digestive tract using an X-ray video system and demonstrate that eels escape from the predator's stomach by going back up the digestive tract towards the gill. [ABSTRACT FROM AUTHOR]

There is growing concern about the potential impact of artificial light at night (ALAN) on the ecology of various taxonomic groups and their associated ecological processes. Freshwater anguillid eels have nocturnal and crepuscular foraging behaviors in estuarine and riverine habitats that are likely affected by ALAN, but little is known about possible effects. We examined changes in foraging activity of Japanese eels, Anguilla japonica, in relation to ALAN using catch data collected during recreational fishing activities. There were significant differences in the relationship between the frequency of eel catches and times from sunset among different illuminance conditions. The frequency of eel catchs peaked at dusk in the lower illuminance conditions, whereas eel catch times were delayed and most eels were caught more than 120 min after sunset at fishing sites highly exposed to artificial lights. Eels were not caught in lighted conditions but were caught after the artificial lights were turned off, although it is not clear if foraging reduction in lighted duration could be compensated for. However, at two sites located near cylindrical refuge traps, eels were caught independent of the lighted durations and sunset times. These preliminary results suggest that ALAN has caused reductions or delay of foraging activity of Japanese eels in urbanized estuarine and riverine habitats, but the presence of preferable refuge habitats may influence the effects of artificial light. Future studies examining the abiotic and biotic environmental factors more rigorously are needed that can contribute to the effective conservation of eel populations and aquatic communities. [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]

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Japanese eels (Anguilla japonica) are a typical migratory fish species with high commercial importance. The Pearl River estuary in southern China is an important natural growing ground for Japanese glass eels, but limited information on Japanese glass eel population characteristics is available, despite their ecological importance. In this paper, we examined the annual patterns of the occurrence of Japanese glass eels in the Pearl River estuary from 2011 to 2022. The most frequently occurring Japanese glass eel's total length is 5.3 cm. The collecting period extended from December to February, and the collection catch-per-unit-effort (CPUE) decreased significantly from 2011 to 2022. The generalized linear model (GLM) indicated that daily changes in Japanese glass eel collection were significantly affected by tidal range, water temperature, and lunar distance. The catch peak appeared when the tidal range rose to 1.7 m, and the water temperature dropped below 8 °C on the full moon days. Overall CPUE analysis showed no significant periodic and inter-annual variability in the period 2011–2022, with the ARIMA model suggesting that the CPUE is expected to remain stable but low in the coming years (2023–2026), although recruitment ultimately depends on the overall spawning stock. [ABSTRACT FROM AUTHOR]

Beneficial microorganisms can increase nutrient digestion and absorption in farmed fish. This study investigates the effects of supplemental feeding of Bacillus species isolated from the intestines of wild glass eels on the growth, survival, and gene expression of farm-raised eel larvae for 30 days after hatching. Three species of Bacillus (B. velezensis, AJBV; B. subtilis, AJBS; B. licheniformis, AJBL) without hemolytic activity were isolated, and an experiment compared the growth of eel larvae fed an artificial diet supplemented with each Bacillus species. There were no significant differences in the total length and body depth of eel larvae at 30 days after hatching in all groups. During the feeding period, 149 eels survived from the initial 1000 in the control group. On the other hand, 240, 178, and 141 eels survived in the AJBV, AJBS, and AJBL groups, respectively. However, there were no significant differences in survival rates despite the difference in the number of surviving eel larvae among the groups. In the comparison of gene expression of genes involved with growth (growth hormone, growth hormone receptor 1, insulin-like growth factor II-2) and those involved with digestive enzymes (amylase, trypsin, lipase), there were also no significant differences among the groups. Our results confirm that dietary supplementation with each of the three host-associated Bacillus does not affect the growth and survival rates of eel larvae reared on an artificial diet up to the first 30 days after hatching, nor does it significantly affect related gene expression. [ABSTRACT FROM AUTHOR]