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The Japanese eel (Anguilla japonica) is a commercially important species in East Asia, the abundance of which has rapidly decreased in recent decades. The fishery resource in the Pearl River basin has mainly deteriorated due to overexploitation and habitat degradation. Knowledge on its genetic status is indispensable for resource management. In this study, we explored the temporal genetic structure of A. japonica on the basis of the concatenated sequences of two mitochondrial fragments (mtDNA) and eight microsatellite markers. A total of nine temporal samples (N = 127) were collected during 2019 and 2021 from Jiangmen City, China, which is located in the Pearl River estuary. mtDNA sequence analysis showed a high level of haplotype diversity, and yielded 124 haplotypes with ranging from 9 to 19 in temporal samples. All microsatellite loci were polymorphic among each of the nine temporal samples, with 150 alleles identified across all samples. Pairwise FST values were low and nonsignificant according to both mtDNA and microsatellite markers. STRUCTURE analysis showed that all temporal samples were not clearly differentiated from each other. The yielded outcomes supported a panmictic pattern in different temporal A. japonica samples. Therefore, our results call for the management of A. japonica as a single unit and joint conservation strategy of the species, since overexploitation in any region will decrease its global resource.

There is growing international concern about declines in populations of anguillid eels, resulting in their inclusion in the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. Monitoring the population dynamics of these species is often challenging, however, owing to their broad distributions and complex, catadromous life histories. Whether environmental DNA (eDNA) analysis could be used to monitor the spatial distribution of anguillid eels in rivers was investigated by conducting basin‐scale surveys of Japanese eels, Anguilla japonica Temminck & Schlegel, 1846, in 10 rivers in Japan, and comparing the results obtained using eDNA analysis with the results obtained using the electrofishing method. Moreover, the relationship between the eDNA concentration and the abundance and biomass of Japanese eels was examined. The eDNA of Japanese eels was detected at 56 (91.8%) of the 61 study sites from which individuals were collected by electrofishing, and at an additional 35 sites where individuals were not directly collected. This indicates that eDNA analysis has a greater sensitivity for detecting the presence of eels, making it a powerful tool for monitoring the spatial distribution of anguillid eels in rivers. A significant, but weak, positive relationship between the eDNA concentration and the abundance and biomass of Japanese eels was also found, suggesting that eDNA analysis may be useful for estimating the abundance and biomass of anguillid eels in rivers. This is the first study to demonstrate the potential usefulness of eDNA analysis for estimating the spatial distribution, abundance, and biomass of Japanese eels in rivers. eDNA analysis will allow anguillid eel populations to be monitored over large spatial and temporal scales using a consistent protocol, with reduced time and effort compared with conventional techniques, providing invaluable information for managing populations of these endangered species.

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., 絶滅危惧種ニホンウナギの分布域を環境DNA解析で推定. 京都大学プレスリリース. 2021-03-03.

The Japanese eel (Anguilla japonica), which was listed as Endangered (EN) under the International Union for the Conservation of Nature due to the declines in its abundance, overfishing, and its narrow range distribution. To better protect this species, we described the complete mitogenome of A. japonica in this study. The mitogenome is 16717bp in length and contains 13PCGs, two rRNA genes (12S rRNA and 16S rRNA), 22 tRNA genes, and a putative control region (CR) and one origin of replication on the light-strand (OL). Moreover, the 13 PCGs encode 3813 amino acids in total, all the protein-coding genes use the initiation codon ATG except COI uses GTG. Most of them have TAA or TAG as the stop codon, except COI uses AGG and two PCGs (COII and ND4) use an incomplete stop codon T. A phylogenetic tree based on the Neighbour Joining method was constructed to provide relationship within Anguilla, which could be a useful basis for conservation of this species.

Japanese eel larvae are passively transported to the East Asian Continental Shelf by the North Equatorial Current, Kuroshio and Kuroshio intrusion currents, and coastal currents. Previous studies have investigated the dispersal characteristics and pathways of Japanese glass eels. However, there are still limitations in these studies. According to long-term (2010–2020) catch data from the Fisheries Agency in Taiwan, the distribution and time series of glass eels recruitment to Taiwan are closely related to the surrounding ocean currents. Recruitment begins in eastern Taiwan via the mainstream Kuroshio and in southern Taiwan via the Taiwan Strait Warm Current. In central Taiwan, recruitment occurs from southern Taiwan, as well as from mainland China via the southern branch of the China Coast Current (CCC). The latest recruitment occurred in northern Taiwan and mainly comprised glass eels from mainland China via the northern branch of the CCC. A stronger monsoon during the La Niña phase could affect the recruitment time series in northern and eastern Taiwan. This study suggests that the recruitment directionality of glass eels is an indicator of the flow field of ocean/coastal currents and elucidates the dispersal characteristics of glass eels in the waters around Taiwan.

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.

During the long migration from river habitats to the spawning ground, the Japanese eel undergoes sexual maturation. This spawning migration occurs concurrently with morphological changes, such as increases in eye size; however, the mechanisms by which sex steroids and their receptors influence these changes in peripheral tissues remain unclear. The aim of this study was to investigate changes in the eyes of female Japanese eels during sexual maturation, and our research focused on estrogen receptor (ER)α and ERβ transcripts. During ovarian development, the gonadosomatic index increased and yolk-laden oocytes developed rapidly. These changes occurred in conjunction with a steady increase in plasma levels of estradiol-17β (E2). Concomitant increases in transcript levels of ERα and ERβ in eye, brain, pituitary, and ovary were also observed. Fluorescence in-situ hybridization analyses revealed that ERα and ERβ transcripts were present in the choriocapillary layer and photoreceptor layer of the eyes, and the analysis also revealed that their signals in these layers became stronger in mature females compared to those observed in immature females, suggesting that under the influence of gonadotropins, morphological changes in the eyes are regulated by E2 through the activation of its receptors. In conclusion, E2 plays a crucial role in physiological adaptations that occur in peripheral tissues during the spawning migration.

Genomic DNAs were extracted from the muscle of twenty-one specimens of three eel species collected in Anguilla japonica (AJ), Muraenesox cinereus (MC) and Conger myriaster (CM) from the Yellow Sea, respectively. In the present study, 7 oligonucleotides primers generated 191 specific loci in the AJ species, 226 in the (MC) species and 181 in the CM species, respectively. The primer BION-02 generated the most loci (a total of 83), with an average of 11.86 in the AJ species. The specific loci generated by oligonucleotides primers exhibited inter-individual-specific characteristics, thus revealing DNA polymorphisms. With regard to average bandsharing value (BS) results, individuals from Conger myriaster species (0.808) exhibited higher bandsharing values than did individuals from Muraenesox cinereus species (0.729) (p

To describe the histological characteristics of the oocyte chorion in wild adult and artificially matured Japanese eels, we investigated changes in chorion thickness during artificially induced oogenesis and compared the chorion thickness and ultrastructure between wild and artificially matured eels. In artificially maturing eels, the chorion thickness and volume increased significantly with increasing follicle diameter, peaking at approximately 450 A mu m; beyond this point, the chorion thinned significantly, whereas there were no significant changes in volume. A significant positive correlation was observed between the number of salmon pituitary extract (SPE) injections and chorion thickness. In wild post-spawning adult eels, chorion thickness varied among individuals, and two had chorions that were significantly thinner than those of artificially matured eels. Ultrastructural examination revealed electron-dense layers in the chorions of wild post-spawning adult eels, as was seen in artificially matured eels. This result is inconsistent with our hypothesis that the formation of an electron-dense layer is unique to artificially maturing eels due to repeated SPE injections. These results suggest that the formation cycle of the chorion might be affected by SPE injections in artificially maturing eels, whereas that of wild eels might be synchronized with behavioral and/or environmental fluctuations that occur during the oceanic spawning migration.

The Japanese eel Anguilla japonica is an important aquaculture fish species in the East Asian countries of Japan, China, Korea, and Taiwan. All glass eel fry are captured from the wild and understanding the recruitment patterns of the glass eel is important. The larvae of A. japonica are passively transported to the East Asian Continental Shelf by the North Equatorial Current, the Kuroshio, the Kuroshio intrusion currents, and coastal currents. In each location, recruitment time is diverse: It is November in Taiwan and April in the Yalu River. How the glass eels reach recruitment areas remains poorly understood. Here, we combine information from larval ages based on otolith increments, simulated drifting paths on the East Asian Continental Shelf, and main fishing seasons in each location of East Asia. We identify five main recruitment blocks: (1) The main Kuroshio, (2) The Taiwan Strait Warm Current, (3) The Taiwan Warm Current, (4) The Yellow Sea Warm Current and (5) The branch of Yellow Sea Warm Current. The counted age of the glass eels is significantly underestimated for the later recruits, possibly due to the cessation of the otolith edge growth under low water temperatures. This study clarifies the eel&rsquo, s larval characteristics and transport mechanisms in the East Asia Continental Shelf, providing important information for its recruitment dynamics in the marine stage.

During the long migration from river habitats to the spawning ground, the Japanese eel undergoes sexual maturation. This spawning migration occurs concurrently with morphological changes, such as increases in eye size, however, the mechanisms by which sex steroids and their receptors influence these changes in peripheral tissues remain unclear. The aim of this study was to investigate changes in the eyes of female Japanese eels during sexual maturation, and our research focused on estrogen receptor (ER)&alpha, and ER&beta, transcripts. During ovarian development, the gonadosomatic index increased and yolk-laden oocytes developed rapidly. These changes occurred in conjunction with a steady increase in plasma levels of estradiol-17&beta, (E2). Concomitant increases in transcript levels of ER&alpha, in eye, brain, pituitary, and ovary were also observed. Fluorescence in-situ hybridization analyses revealed that ER&alpha, transcripts were present in the choriocapillary layer and photoreceptor layer of the eyes, and the analysis also revealed that their signals in these layers became stronger in mature females compared to those observed in immature females, suggesting that under the influence of gonadotropins, morphological changes in the eyes are regulated by E2 through the activation of its receptors. In conclusion, E2 plays a crucial role in physiological adaptations that occur in peripheral tissues during the spawning migration.

Spontaneous polyploids and mosaics have often been observed in artificially propagated larvae of the Japanese eel, Anguilla japonica. However, the mechanisms responsible for such unusual cytotypes are unclear. In this study, we examined the relationship of such polyploidization and mosaicisms in larvae resulting from artificial propagation to egg quality (fertilization rate and hatching rate) and viability of larvae, and then clarified the inducing factors and the mechanism for occurrence of such phenomena. Eggs stripped from females after induced maturation were artificially inseminated with sperm pre-cultured with artificial seminal plasma. Ploidy was determined by measuring the relative DNA content of the nuclei with flow cytometry. Of 968 embryos from 32 full-sib families, 9.1% were determined to be abnormal, most of which were triploids (86.5% of abnormal embryos); others were haploids (1.1%), aneuploids (2.3%), and mosaics (10.1%). The percentage of normal diploids from each family varied between 56.3% and 100% (90.9±11.7%, n=32). A significant positive correlation was found between the fertilization rate (P

Background Anguilla japonica (Japanese eel) is currently one of the most important research subjects in eastern Asia aquaculture. Enigmatic life cycle of the organism makes study of artificial reproduction extremely limited. Henceforth genomic and transcriptomic resources of eels are urgently needed to help solving the problems surrounding this organism across multiple fields. We hereby provide a reconstructed transcriptome from deep sequencing of juvenile (glass eels) whole body samples. The provided expressed sequence tags were used to annotate the currently available draft genome sequence. Homologous information derived from the annotation result was applied to improve the group of scaffolds into available linkage groups. Results With the transcriptome sequence data combined with publicly available expressed sequence tags evidences, 18,121 genes were structurally and functionally annotated on the draft genome. Among them, 3,921 genes were located in the 19 linkage groups. 137 scaffolds covering 13 million bases were grouped into the linkage groups in additional to the original partial linkage groups, increasing the linkage group coverage from 13 to 14 %. Conclusions This annotation provide information of the coding regions of the genes supported by transcriptome based evidence. The derived homologous evidences pave the way for phylogenetic analysis of important genetic traits and the improvement of the genome assembly. Electronic supplementary material The online version of this article (doi:10.1186/s12864-015-2306-6) contains supplementary material, which is available to authorized users.

Anguilla japonica Temminck & Schlegel, 1846 R����� Anguilla japonica Temminck & Schlegel, 1846: 258 (type locality: Japan). Oshima, 1919: 263; Chen, 1956: 73; Liaw, 1960: 4; Chen & Weng, 1967: 5; Chen, 1969: 124; Shen, 1984 a: 103; Chen & Yu, 1986: 242; Shen et al., 1993: 97, fig. 14 - 5; Shao et al., 1994 a: 273; Chen & Fang, 1999: 51; Chen, 2004: 26; Lin, 2007: 82; Chen, 2009: 6; Ho et al., 2010:table 1; Chou & Kao, 2011: 34; Ho & Shao, 2011: 21, table 1; Chang et al., 2011: 1; Chang et al., 2013: 44. Anguilla bostoniensis (not of Lesueur, 1817):? G��nther, 1870: 31. * Anguilla remifera Jordan & Evermann, 1902: 325, fig. 7 (type locality: Taiwan). Jordan & Richardson, 1909: 171, fig. 8. Anguilla sinensis McClelland, 1843: Chen, 1956: 73. Muraena japonica (Temminck & Schlegel, 1846): Fowler, 1932 a: 49, fig. 1. Remarks. G��nther (1870) recorded Anguilla bostoniensis based on several half-grown specimens collected from Taiwan, but their identities are not verified since whereabouts of these specimens are unknown., Published as part of Ho, Hsuan-Ching, Smith, David G., Mccosker, John E., Hibino, Yusuke, Loh, Kar-Hoe, Tighe, Kenneth A. & Shao, Kwang-Tsao, 2015, Annotated checklist of eels (orders Anguilliformes and Saccopharyngiformes) from Taiwan, pp. 140-189 in Zootaxa 4060 (1) on page 142, DOI: 10.11646/zootaxa.4060.1.16, http://zenodo.org/record/243651, {"references":["Temminck, C. J. & Schlegel, H. (1846) Pisces. In: Fauna Japonica, sive descriptio animalium quae in itinere per Japoniam suscepto annis 1823 - 30 collegit, notis observationibus et adumbrationibus illustravit P. F. de Siebold. Part 1 (10 - 14), 173 - 269.","Oshima, M. (1919) Contributions to the study of the fresh water fishes of the island of Formosa. Annals of the Carnegie Museum, 12 (2 - 4), 169 - 328.","Chen, J. T. - F. (1956) A synopsis of the vertebrates of Taiwan. Kaiming Press, Taipei, 292 pp. [In Chinese]","Liaw, W. - K. (1960) Study on the fishes from Ma-kung, Pescadores Island (Penghu). Report of Laboratory of Fishery Biology, Taiwan Fisheries Research Institute, 13, 1 - 26.","Chen, J. T. - F. & Weng, H. T. - C. (1967) A review of the apodal fishes of Taiwan. Biological Bulletin of Tunghai University, Ichthyology Series, 6, 1 - 86.","Chen, J. T. - F. (1969) A synopsis of the vertebrates of Taiwan. Volume 1. Commerical Books Co., Taipei, 548 pp.","Shen, S. - C. (1984 a) Synopsis of fishes of Taiwan. Southern Materials Center, Taipei, Taiwan, 533 pp.","Chen, J. T. - F. & Yu, M. - J. (1986) A synopsis of the vertebrates of Taiwan, revised and enlarged edition. Commercial Press, Taipei, 1092 pp. [in Chinese]","Shen, S. - C., Lee, S. - C., Shao, K. - T., Mok, H. - K., Chen, C. - T. & Chen, C. - H. (1993) Fishes of Taiwan. Department of Zoology, National Taiwan University, Taipei, 960 pp.","Shao K. - T., Chen, J. - P., Ho, L. - T., Lin, C. - P., Kao, P. - H., Lin, P. - L. & Chen, L. - S. (1994 a) Checklist and distributional pattern of the fishes of the Pescadores Islands. In: Faculty of Fisheries, Kasetsart University (Ed.) Proceeding of the 4 th Indo-Pacific fish Conference: systematics and evolution of Indo-Pacific fishes. Kasetsart University, Bangkok, pp. 267 - 280.","Chen, I. - S. & Fang, L. - S. (1999) The freshwater and estuarine fishes of Taiwan. National Museum of Marine Biology & Aquarium, Pingtung, 287 pp.","Chen, C. - H. (2004) Checklist of fishes of Penghu. FRI special publication, 4, 1 - 175.","Lin, C. - J. (2007) Freshwater fishes and shrimps of Taiwan. Commonwealth Publishing Group, Taipei, 239 pp. [in Chinese]","McCosker, J. E., Chen, W. - L. & Chen, H. - M. (2009) Comments on the snake-eel genus Xyrias (Anguilliformes: Ophichthidae) with the description of a new species. Zootaxa, 2289, 61 - 67.","Chou. T. - M. & Kao, J. - C. (2011) The freshwater and estuarine fish of Taiwan. Morningstar press, Taichong, 384 pp.","Ho, H. - C. & Shao, K. - T. (2011) Annotated checklist and type catalog of fish genera and species described from Taiwan. Zootaxa, 2957, 1 - 74.","Chang, G. - C., Hung, Y. - S. & Liu, F. - K. (2011) How many fresh eel species in Taiwan? Fishery Research Institute E-Paper, 66, 1. [in Chinese]","Chang, G. - C., Hung, Y. - S. & Liu, F. - K. (2013) Identification of the elver, Anguilla celebesensis from the surrounding waters of Taiwan. Journal of Taiwan Fisheries Research, 21 (1), 43 - 48. [in Chinese]","Gunther, A. (1870) Catalogue of the Physostomi, containing the families Gymnotidae, Symbranchidae, Muraenidae, Pegasidae, and of the Lophobranchii, Plectognathi, Dipnoi, Ganoidei, Chondropterygii, Cyclostomata, Leptocardii, in the British Museum. Catalogue of the fishes in the British Museum, 8, 1 - 549.","Jordan, D. S. & Evermann, B. W. (1902) Notes on a collection of fishes from the island of Formosa. Proceedings of the United States National Museum, 25, 315 - 368. http: // dx. doi. org / 10.5479 / si. 00963801.25 - 1289.315","Jordan, D. S. & Richardson, R. E. (1909) A catalogue of the fishes of the island of Formosa, or Taiwan, based on the collections of Dr. Hans Sauter. Memoirs of the Carnegie Museum, 4, 159 - 204.","McClelland, J. (1843) Description of a collection of fishes made at Chusan and Ningpo in China, by Dr. G. R. Playfair, Surgeon of the Phlegethon, war steamer, during the late military operations in that country. Calcutta Journal of Natural History, 4 (4), 390 - 41. [plates. 21 - 25]","Fowler, H. W. (1932 a) A synopsis of the fishes of China. Part III. The eels. Hong Kong Naturalist, 3 (1), 46 - 63."]}

Japanese eel endothelial cells-infecting virus (JEECV) has spread in eel farms and caused serious economic loss. In this study, we examined the prevalence of JEECV infection in 100 wild Japanese eel (Anguilla japonica) elvers caught from Yamaguchi prefecture, Japan, using quantitative PCR and conventional PCR. Total genomic DNA was obtained from the cranial quarter of the body in 70 of 100 eels and from the gill in the remaining. Of 30 gill samples, 20 were analyzed after pooling with other samples, and the remaining 10 were analyzed separately. A single positive result for JEECV was detected following analysis of the 10 separately analyzed samples. This result constitutes the first report of JEECV infection in wild A. japonica elvers.

This study was conducted to re-evaluate the dietary vitamin C requirement in juvenile eel, Anguilla japonica by using L-ascorbyl-2-monophosphate (AMP) as the vitamin C source. Five semi-purified experimental diets were formulated to contain 0 (AMP(0)), 30 (AMP(24)), 60 (AMP(52)), 120 (AMP(108)) and 1,200 (AMP(1137)) mg AMP kg(-1) diet on a dry matter basis. Casein and defatted fish meal were used as the main protein sources in the semi-purified experimental diets. After a 4-week conditioning period, fish initially averaging 15 +/- 0.3 g (mean +/- SD) were randomly distributed to each aquarium as triplicate groups of 20 fish each. One of five experimental diets was fed on a DM basis to fish in three randomly selected aquaria, at a rate of 3% of total body weight, twice a day. At the end of the feeding trial, weight gain (WG) and specific growth rate (SGR) for fish fed AMP(52) and AMP(108) were significantly higher than those recorded for fish fed the control diet (p

Economic loss due to viral endothelial cell necrosis of eel (VECNE) of Anguilla japonica is a serious problem for the cultured Japanese eel market. However, the viral genome responsible for VECNE is unknown. We recently developed a rapid determination system for viral nucleic acid sequences (RDV) to determine viral genome sequences. In this study, viral DNA fragments were obtained using RDV, and approximately 15-kbp circular full genome sequences were determined using a next-generation sequencing system, overlapping PCR, and Southern blot analysis. One open reading frame (ORF) was homologous to the large T-antigen of polyomavirus; other ORFs have no homology with any nucleic or amino acid sequences of polyomavirus. Therefore, as this DNA virus might comprise a novel virus family, we provisionally named it Japanese eel endothelial cells-infecting virus (JEECV). JEECV was detected in both naturally and experimentally infected eels, suggesting that JEECV potentially causes VECNE.

Members of the genus Nocardia are widespread in diverse environments; a wide range of Nocardia species are known to cause nocardiosis in several animals, including cat, dog, fish, and humans. Of the pathogenic Nocardia species, N. seriolae is known to cause disease in cultured fish, resulting in major economic loss. We isolated two N. seriolae strains, CK‐14008 and EM15050, from diseased fish and sequenced their genomes using the PacBio sequencing platform. To identify their genomic features, we compared their genomes with those of other Nocardia species. Phylogenetic analysis showed that N. seriolae shares a common ancestor with a putative human pathogenic Nocardia species. Moreover, N. seriolae strains were phylogenetically divided into four clusters according to host fish families. Through genome comparison, we observed that the putative pathogenic Nocardia strains had additional genes for iron acquisition. Dozens of antibiotic resistance genes were detected in the genomes of N. seriolae strains; most of the antibiotics were involved in the inhibition of the biosynthesis of proteins or cell walls. Our results demonstrated the virulence features and antibiotic resistance of fish pathogenic N. seriolae strains at the genomic level. These results may be useful to develop strategies for the prevention of fish nocardiosis.

International audience; Eel species have a striking life cycle with a blockade of puberty until the oceanic migration. We report the first molecular data on eel gonadotropin receptors. The partial sequences cloned covered two-third of the open reading frame and included most of the extracellular and transmembrane domains. Phylogenetic analysis partitioned the two eel gonadotropin receptors into the two teleost FSHR and LHR clusters, respectively. Real-time quantitative RT-PCR was used to quantify the expression of eel gonadotropins and their receptors. Similar levels of pituitary FSH-beta and LH-beta transcripts were found in the immature previtellogenic female eels. In contrast, ovarian FSHR mRNA level was at 100- to 185-fold higher than that of LHR. This revealed that FSHR rather LHR would mediate gonadotropin stimulation of the early stages of ovarian growth. Chronic treatment with fish pituitary homogenates, applied to induce eel sexual maturation, stimulated pituitary LH-beta but suppressed FSH-beta transcripts. In the ovaries, both FSHR and LHR mRNA were significantly increased in experimentally matured eels. Treatments with sexual steroids showed a stimulatory effect of estradiol-17beta (E(2)) on pituitary LH-beta mRNA levels, while FSH-beta transcripts were suppressed by E(2) or testosterone (T). In contrast, neither E(2) nor T-treatment had any significant effect on ovarian FSHR nor LHR transcripts. This suggests that steroid feedbacks may be responsible for the opposite regulation of pituitary gonadotropins in experimentally matured eels, but are not involved in the regulation of gonadotropin receptors. In conclusion, these are the first data on the sequence, expression and regulation of gonadotropin receptors in the eel. They provide new foundation for basic and applied research on eel reproduction.

Genetic improvement of the Japanese eel ( Anguilla japonica ) can be achieved by artificially controlling its life cycle using recent advances in reproductive biology. In this study, we developed 43 microsatellite loci to confirm Mendelian inheritance at 10 of them as well at 16 previously reported in two full-sib families produced by artificial insemination. In order to establish a base for aquaculture genetics of this species in the near future, these microsatellite loci were mapped in relation to the centromere by half-tetrad analysis using four artificially induced triploid families. The second division segregation frequency ( y ) of the microsatellite loci ranged from 0.008 to 0.968 (mean ± SD = 0.645 ± 0.298). These results suggest the presence of strong chiasma interference in the eel. Significant differences were observed for the map distances of microsatellite loci between the two isolation procedures. Microsatellites isolated using the enrichment procedure were mapped to various sites starting from the centromere to the telomere, whereas those from the conventional size-selected library showed a tendency to be distributed in the telomeric region.

Artificial reproduction and subsequent improved culture of the Japanese eel Anguilla japonica have not yet been achieved, but are expected in the near future. To allow the development of aquaculture activities, chromosome manipulation techniques are necessary for the genetic improvement of the eel. In the present paper suitable conditions were determined for heat shock treatment for the induction of triploidy. This was done using fertilized eggs obtained from gametes of cultured eels that were induced to mature by repeated injections of salmon pituitary extract, followed by an injection of 17α,20β-dihydroxy-4-pregnen-3-one to induce ovulation in the female eels and by repeated injections of human chorionic gonadotropin in the male eels. Three variables (i.e. timing after fertilization, temperature, and duration of the shock) were varied to maximize the yield of triploid larvae by examining hatching and triploid rates of treated groups. Ploidy was determined by DNA content flow cytometry and/or chromosome counting. The triploid yield was the highest (70%), when a 37°C heat shock was applied for 3 min, 10 min after fertilization. Under such conditions the rate of triploidy reached 70–100% and the rate of hatching was 5.2–24.6% when the control hatching rate was 28.5% on average.

injections of carp pituitary extract (CPE, 5 pit. / fish / 10 days) induced ovarian maturation in female eel, gonadosomatic index (GSI) increasing significantly to 43-55 %. 4 injections of CPE (1-2 pit. / fish / 10 days) induced spermatogenesis and spermiation in male eel, GSI increasing to 2.1-2.8 %. After serial injections of CPE, serum gonadotropin (GtH) levels increased significantly, indicating that the exogeneous GtH stimulated gonadal development and maturation in the Japanese eel. 8 implantations of 17 a-methyltesto sterone (MT) or androstenedio ne (ADSD) (50 ug / g body weight /15 days) stimulated ovarian maturation in female eel, GSI increasing significantly to 38-49 %. 4 implantations of MT or ADSD (50 ug / g body weight /15 days) induced spermatogenesis and spermiation in the male eel, GSI increasing to 2.1-3.5 %. MT or ADSD implantations significantly elevated pituitary and serum GtH contents. ADSD or MT implantations resulted in the significant elevation of serum estradiol levels in female eel and serum testosterone levels in male eels. In female eel, ADSD implantations significantly increased mammalean-libe-g onadotropin-rele asing hormone (mGnRH) content in brain and pituitary. These results demonstrated that the positive feedback effects of sex steroids on the brain (hypothalamus)-p ituitary-gonad axis play an important role on the induction of gonadal development and maturation in the Japanese eel, and the endogeneous GtH is the key hormone during these physiological processes. In the matured female eel, ovulation and spawning can be induced 18-21 hours following an injection of CPE alone or in combination with (D-Ala 6, Pro 9 -NEt)-lutein izing hormone-releasin g hormone (LHRH-A) and dopamine antagonist domperidone (DOM).

A new natriuretic peptide, which exhibits the entire spectrum of actions known to be characteristics of atrial and brain natriuretic peptides (ANP and BNP), was isolated from eel cardiac ventricles and has been named ventricular natriuretic peptide (VNP). The primary structure of eel VNP is characterized by its uniquely long C- terminal ‘tall’ that extends from the second half-cystine. Thus, eel VNP appears to be a novel natriuretic peptide of a type not found in mammals. With respect to natriuretic (rat) and vasodepressor (rat and eel) activities, eel VNP is much more potent than human ANP in eels and almost equipotent in rats. Strong tachyphylaxis is observed for the vasodepressor effect in both rats and eels, whereas it is not observed for the natriuretic effect in rats.

Ten eels (Anguilla japonica) from a fish farm in Korea were examined and diagnosed with a Heterosporis infection. The gross lesions on the trunk were uneven and the concave parts were pasty. Histopathologically, lyses of the trunk muscles, degenerative muscle fibers and the scattered spores were observed. The sporophorocyst (SPC) contained several spores with a variety of shapes. Some SPC were disrupted and the spores in the SPC were scattered in the muscle tissues. Macrophages existed near the scattered spores. Electron microscopy revealed special structures such as sporophorocyst containing various developmental parasitic stages such as meronts, sporonts, sporophorous vesicles and spores.