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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]

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]

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]

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]

The distribution of orexin‐A and orexin‐B immunoreactivity was examined in the pituitaries of the white‐spotted conger (Conger myriaster) and the Japanese eel (Anguilla japonica) using immunohistochemical techniques. Orexin‐A‐immunoreactive puncta were dispersed in the pars nervosa of the neurointermediate lobe, and a part of these puncta were immuno‐positive for galanin. In contrast, orexin‐B immunoreactive cells were observed in the proximal part of the pars distalis, and these cells were immuno‐positive for luteinizing hormone. Considering the present results and previous findings, orexin‐A and orexin‐B may modulate the secretion of α‐melanocyte‐stimulating hormone and growth hormone, respectively, and participate in the regulation of energy homeostasis. [ABSTRACT FROM AUTHOR]

In this study, the levels of plasma estradiol-17β (E 2) in farmed Anguilla japonica were measured to determine their sex. The analyses were performed for two different size groups (large group, Total length (TL): 61–69 cm; small group, TL: 53–60 cm). The anatomical and histological observations showed that the large group consisted of 29% males and 71% females; the small group, 54% males and 45% females. The gonad histology showed that in the large group, 88% of the eels had immature gonads with ongoing sexual differentiation, 12% were mature with completed sexual differentiation. In the small group, 87% of the eels had immature gonads. The plasma E 2 hormone levels were higher in the females of both sizes. In the large group, the average plasma E 2 in females was 415 pg/ml, which was significantly higher than the average of 109 pg/ml in males (P < 0.05). In the small group, the average plasma E 2 hormone level was 618 pg/ml, which was much higher than the average of 108 pg/ml in males. Quantitative real-time PCR showed that zygote arrest 1 (zar 1) and zona pellucida glycoprotein 3 (zp3) were more highly expressed in females than male. In the H-E staining, an eel in the oil droplet containing ovary stage had a high level of plasma E 2 (1500 pg/ml), while an eel with testis in the spermatocyte stage had a low (60 pg/ml). E 2 is a potentially useful tool and could play an important role in sex determination in broodstocks. • Plasma estradiol-17β (E 2) levels in farmed Anguilla japonica could indicate sex. • The plasma E 2 hormone levels were higher in the females. • zar 1 and zp3 expression higher in females. • More reliable supply of broodstocks for artificial seedling production. [ABSTRACT FROM AUTHOR]

We propose a simple index of brain volume (D*L index), calculated using two-dimensional photographs of the brain, captured from the dorsal and lateral views. Here, D and L represent the dorsal and lateral brain areas in the images, respectively. Two distantly related species were assessed. The D*L indices of 11 Japanese eels, Anguilla japonica and 11 yellowfin gobies, Acanthogobius flavimanus were calculated. High correlation coefficients of D*L index with brain weights were obtained from both species, suggesting the validity and reliability of the index. The relative profiles of the olfactory bulb, telencephalon, optic tectum, and cerebellum were calculated using this index. Eels had a larger olfactory bulb and telencephalon, whereas gobies had a larger optic tectum. Principal component analysis showed two separate clusters for eels and gobies. Published images of two Anguilliformes species and four Gobiidae species were used to test the D*L index. Principal component analysis showed separate clusters for Anguilliformes and Gobiidae species. The D*L index is a suitable tool for data collection because it is simple, low-cost, and has reliability and validity. [ABSTRACT FROM AUTHOR]

This study was performed to investigate the effect of microbial supplementation diet on the survival rate and microbiota composition of artificially produced eel larvae. Microorganisms supplemented in the diet were isolated from wild glass eel intestines and identified as Bacillus sp. through 16S rRNA sequencing analysis. In vitro tests confirmed that the strain had no hemolytic activity and virulence genes. Microbial supplemental feeding significantly increased the survival rate of artificially produced eel larvae for 30 days post-hatchling compared with that of the control group. It also caused changes in the α-diversity, β-diversity, and relative abundance of the bacterial communities. Analysis via phylogenetic investigation of communities by reconstruction of unobserved states predicted that these microbial community changes would significantly increase the carbohydrate metabolism, membrane transport, and cellular community pathway of the microbial supplementation group. Therefore, microbial supplementation feeding for eel aquaculture could increase the viability of artificially produced eel larvae and alter the microbial composition to induce metabolic changes. [ABSTRACT FROM AUTHOR]

This study was performed to investigate the effect of microbial supplementation diet on the survival rate and microbiota composition of artificially produced eel larvae. Microorganisms supplemented in the diet were isolated from wild glass eel intestines and identified as Bacillus sp. through 16S rRNA sequencing analysis. In vitro tests confirmed that the strain had no hemolytic activity and virulence genes. Microbial supplemental feeding significantly increased the survival rate of artificially produced eel larvae for 30 days post-hatchling compared with that of the control group. It also caused changes in the α-diversity, β-diversity, and relative abundance of the bacterial communities. Analysis via phylogenetic investigation of communities by reconstruction of unobserved states predicted that these microbial community changes would significantly increase the carbohydrate metabolism, membrane transport, and cellular community pathway of the microbial supplementation group. Therefore, microbial supplementation feeding for eel aquaculture could increase the viability of artificially produced eel larvae and alter the microbial composition to induce metabolic changes.

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.

Selection acting across environmental gradients, such as latitudes, can cause spatial structuring of genomic variants even within panmictic populations. In this study, we focused on the within‐generation latitudinal selection between northernmost and southernmost individuals of the North Pacific population of a tropical eel Anguilla marmorata, which shares its northernmost distribution with a temperate eel Anguilla japonica. Whole‐genome sequencing data indicated that the northernmost and southernmost individuals of A. marmorata belong to a single panmictic population, as suggested by previous studies. On the contrary, parts of genomic regions across multiple chromosomes exhibited significant genetic differentiation between the northernmost and southernmost individuals, and in these genomic regions, the genotypes of the northernmost individuals were similar to those of A. japonica. These findings suggested within‐generation latitudinal selection of A. marmorata, which might have led to genetic closeness between northernmost A. marmorata and A. japonica. [ABSTRACT FROM AUTHOR]

This study provided the first analysis of the distribution of a CXCL14-like chemokine in the Japanese eel (Anguilla japonica) pituitary using immunohistochemical techniques. CXCL14-immnoreactive cells were observed in the rostral part of the pars distalis (RPD) and the pars intermedia (PI). The CXCL14-immunoreactive cells were found to correspond to adrenocorticotropic hormone (ACTH)-producing cells in the RPD and α-melanocyte-stimulating hormone (αMSH)-producing cells in the PI. Therefore, the CXCL14 secreted from the αMSH-producing cells and ACTH-producing cells in the pituitary may participate in the modulation of insulin secretion, as well as energy metabolism, together with αMSH and/or ACTH via endocrine mechanisms. [ABSTRACT FROM AUTHOR]

Sustainable aquaculture of the Japanese eel Anguilla japonica requires an understanding of the physiological conditions of the fish under culture conditions. Therefore, we examined the effects of tentative stressors such as background color and rearing density on stress-related hormones in juvenile Japanese eel. In Experiment 1, fish were divided into white- or black-coated tanks and reared for 35 days. Plasma cortisol levels were significantly higher in the white-acclimated fish on day 35. No significant differences were observed between the groups in hypothalamic CRH mRNA levels. In Experiment 2, fish were divided into three rearing density groups (0.5 kg/m2, 1.2 kg/m2, and 2.4 kg/m2) and reared for 28 days. Plasma cortisol levels were significantly lower in the low-density-acclimated fish than in the medium- and high-density-acclimated fish. However, no significant differences were observed in hypothalamic CRH mRNA levels. Evaluation of plasma cortisol levels indicates that a white background and high rearing density induce more stress for juvenile Japanese eel. [ABSTRACT FROM AUTHOR]

Following the decline of Japanese eel catches and the EU European eel trade ban, other regions, such as Southeast Asia, have become increasingly important sources of juvenile anguillid species for aquaculture. Although appropriate fishery management of tropical eel species is urgently required, data collection by the government or scientific authorities is extremely difficult, because tropical glass eel fisheries are small-scale fisheries conducted in the Global South, where expertise and resources are sometimes limited. To collect data on glass eel fisheries, participatory stock assessments were conducted in West Java, Indonesia, where a Fishery Improvement Project (FIP) and an Aquaculture Improvement Project (AIP) were launched in collaboration with industries, middlemen, fishermen, NGOs, national and local agencies, and scientists. This participatory study successfully collected data on more than 3000 daily fishing practices from approximately 400 fishermen in a single water system in 2019. Even though the assessment was not species-specific, the data collected allowed estimation of the natural mortality rate as being 0.011 day−1. The annual exploitation rate of the glass eel fisheries was estimated as 24.9%, based on a generalised depletion model developed for the data-limited situation where only fishery catch and effort data are available. The data collection scheme for glass eel fisheries was disseminated by local and national agencies to glass eel fishermen in Indonesia and contributed to developing the Eel Conservation National Action Plan. [ABSTRACT FROM AUTHOR]

Anguilla japonica, also known as Japanese eel, is an integral part of Japanese culture from centuries. This fish spawns in the Mariana islands, and subsequently the larval stages migrate towards east Asia. Whereas, the sexually maturing eels return back to the spawning grounds in the ocean. Being commercially vital, the glass eels are intensively captured to be used in aquaculture systems along east Asia. This practice along with the climate change contributes to a significant decline in the Japanese eel population. Further, to counteract the increasing demand of Japanese eel in domestic market, their mass production through aquaculture systems require a higher amount of innovation in terms of their feeding and rearing technology. Here we have summed up all the important information about the Japanese eel based on the thorough survey of already published literature. Overall, this review will be useful for a better understanding of the currently available information pertaining to the species. [ABSTRACT FROM AUTHOR]

Japanese eels (Anguilla japonica) were trained on a Morris-type spatial learning task. There were four tubes in a pool, but the eels could hide in only one of these. The eels learned the position of the open tube, and maintained their performance when the pool was rotated to remove possible intra-maze cues. The eels could not maintain their performance in a dark room, suggesting that spatial learning involved extra-maze visual cues. When the position of the open tube was randomly changed every day, the performance of the eels in finding the open tube did not improve. [ABSTRACT FROM AUTHOR]

We report wavelength‐specific thresholds of leptocephali of Japanese eels Anguilla japonica determined from their negative‐phototactic behaviour. Leptocephali are most sensitive to wavelengths 400–500 nm and at very short wavelengths. Their visual sensitivity decreases more sharply at wavelengths >500 nm than it does at wavelengths <400 nm. The spectral sensitivity of leptocephali adapts to the optical conditions of their habitat. The mean visual sensitivity threshold of leptocephali is 7.22 × 10−4 μmol m−2 s−1 between 400 and 500 nm. Based on visual sensitivity thresholds of 475 nm, the most transparent wavelength in waters where these leptocephali occur, the daytime depth of occurrence of these larvae may exceed 250 m. LEDs emitting light of wavelength 625 nm in culture environments would minimise disturbance to leptocephali during facility maintenance. [ABSTRACT FROM AUTHOR]

Mass mortality occurred at an Anguilla japonica eel farm equipped with a recirculating aquaculture system in Gimcheon, Korea, from late spring to early summer 2015. The cumulative 3‐month mortality was 16% (approximately 24,300–150,000 fish). The majority of affected fish displayed ulcerative lesions that progressed to petechial haemorrhages and small white granulomas in the major organs. A Gram‐positive, acid‐fast, nonmotile bacterium was isolated from internal organ lesions. Phylogenetic analysis of 16S rRNA identified the species as Nocardia seriolae and the strain was designated EM150506. Afterwards, naïve eels were injected with 1.8 × 107 colony‐forming units per fish to confirm the strain's pathogenicity, which resulted in a 20% mortality rate within 4 weeks. However, surviving fish still exhibited white N. seriolae colonies in internal organs. To our knowledge, this is the first report of a N. seriolae infection in cultured eel. [ABSTRACT FROM AUTHOR]

Atrial and B-type natriuretic peptides (ANP and BNP) are cardiac hormones important for cardiovascular and body fluid regulation. In some teleost species, an additional member of the natriuretic peptide family, ventricular NP (VNP), has been identified. In this study, we examine tissue distribution of these three NPs in the eel heart. Quantitative real-time PCR showed that anp is almost exclusively expressed in atria, bnp equally in atria and ventricles and vnp three-fold more in ventricles than in atria. The amount of bnp transcript overall in the heart was 1/10 those of anp and vnp. There was no difference in transcript levels between freshwater and seawater-acclimated fishes. Immunohistochemistry using specific antisera and in situ hybridization using gene-specific probes showed that NP signals were detected in most atrial and ventricular myocytes with some regional differences in density. Because of high sequence similarity of the three NPs, each of the three NP antisera individually was pre-incubated with 10−8 M of the other two non-targeted cardiac NPs to increase the specificity. A few atrial myocytes contained all three NPs in the same cell. Immuno-electron microscopy identified many dense-core vesicles containing ANP in atria and VNP in ventricles and some vesicles contained both ANP and VNP as demonstrated using pre-absorbed antisera. Based on these data and those of previous studies, we suggest that in eels ANP is secreted from atria in a regulatory pathway and VNP from ventricles in a constitutive pathway. In addition, VNP, not BNP, is the principal ventricular hormone in eels. [ABSTRACT FROM AUTHOR]

Immune-endocrine interactions are an important pathogen resistance mechanism in fish. We review the immune-endocrine interactions in the Japanese eel, Anguilla japonica , with special reference to high throughput gene sequencing. These data may be relevant to the significant decrease in the eel harvest in recent years and will aid in the selection of appropriate disease-resistant strains for aquaculture. More than 1000 sequences that whose expression in elvers responded to air exposure were identified through comprehensive gene expression analysis using next-generation sequencing. These included transcription factors within the MAPK pathway. Significant changes in expression after air exposure were detected by quantitative polymerase chain reaction analysis in many genes related to disease resistance. These factors include innate immune system factors and cytokines that interact with the endocrine system during the stress response. Other applications of immune-endocrine interactions in eel culture are discussed. [ABSTRACT FROM AUTHOR]

Green fluorescent protein (GFP) from eel ( Anguilla japonica ) muscle (eelGFP) is unique in the vertebrates and requires bilirubin as a ligand to emit fluorescence. This study was performed to clarify the physiological function of the unique GFP. Investigation of susceptibility to oxidative stress was carried out using three types of cell lines including jellyfish ( Aequorea coerulescens ) GFP (jfGFP)-, or eel GFP (eelGFP)-expressing HEK293 cells, and control vector-transfected HEK293 cells. Binding of eelGFP to bilirubin was confirmed by the observation of green fluorescence in HEK293-eelGFP cells. The growth rate was compared with the three types of cells in the presence or absence of phenol red which possessed antioxidant activity. The growth rates of HEK293-CV and HEK293-jfGFP under phenol red-free conditions were reduced to 52 and 31% of those under phenol red. Under the phenol red-free condition, HEK293-eelGFP had a growth rate of approximately 70% of the phenol red-containing condition. The eelGFP-expressing cells were approximately 2-fold resistant to oxidative stress such as H 2 O 2 exposure. The fluorescence intensity partially decreased or disappeared after exposure to H 2 O 2 , and heterogeneous intensity of fluorescence was also observed in isolated eel skeletal muscle cells. These results suggested eelGFP, but not jfGFP, coupled with bilirubin provided the antioxidant activity to the cells as compared to non-bound free bilirubin. [ABSTRACT FROM AUTHOR]

Introduction: Eels are animals commonly used in zoological research, as these species have a unique catadromous life history and belong to a phylogenetically ancient group of Teleostei. However, eel reproduction is difficult to investigate, since mature samples are not easily obtainable in the wild. In this study, we tested daggertooth pike conger (Muraenesox cinereus), an Anguilliformes species, as a potential model for the investigation of the reproductive biology of eels. Seventy individuals were caught between June and October, which is supposed to be their spawning season, from inshore of the Seto Inland Sea. Results: The lengths and ages of samples ranged from 510 to 1239 mm and three to nine years, respectively, and the sex ratio was skewed towards females (96 % of the total sample). The gonado-somatic index of the females peaked in July. Histological observation revealed that these ovaries were similar to those of other eel species and contained matured oocytes (migratory-nucleus stage), suggesting that pike conger spawn inshore in July. The plasma concentrations of sex steroid hormones (estradiol-17β and 11-keto-testosterone) in females gradually increased during maturation and decreased after spawning, indicating the involvement of these hormones in oogenesis of pike conger. Conclusions: The present study is the first to report on characteristics of natural oogenesis in pike conger. Because naturally maturing samples can easily be captured, daggertooth pike conger may represent an excellent model for the study of reproduction in Anguilliformes. [ABSTRACT FROM AUTHOR]

Abstract: Osmoregulation in fish has been a classical research topic for several decades. Salmon and eels are the widely used model animals because of their wide distribution in different geographical locations and spawning migration between fresh- and salt-water habitats. Numerous fish osmoregulatory hormones and ion transporters were identified for their essential roles in acclimation and adaptation to waters of different salinities. Because of the lack of a genomic database, the scope of most studies, however, is very limited. Recently, our group reported the first high-throughput transcriptomic and proteomic studies to identify hyperosmotic-responsive genes/proteins in gills of Japanese eels. In this study, we aimed to decipher changes in hypo-osmotic-responsive proteins in fish acclimating from seawater (SW) to freshwater (FW) conditions. We collected gill samples from SW-adapted and SW-to-FW-acclimating fish. The respective gill proteins were extracted and labeled using isobaric tags for relative and absolute quantitation (iTRAQ) and analyzed using a high-resolution mass spectrometer. In the short-term transfer from SW to FW, 51 hypo-responsive proteins were detected, and 24 unique hypo-osmotic-responsive proteins were identified (15 up-regulated and nine down-regulated proteins). Our data support the use of an omics approach to facilitate the application of functional genomics in non-model organisms. Biological significance: By combining transcriptomic and proteomic approaches, the study has provided the most comprehensive, targeted investigation of eel gill hypo-osmotic responsive proteins that provides molecular insights of osmoregulation mechanisms in a non-model organism, eel. This article is part of a Special Issue entitled: Proteomics of non-model organisms. [Copyright &y& Elsevier]

The stress-related corticotropin-releasing hormone (CRH) was first identified by isolation of its cDNA from the brain of the Japanese eel Anguilla japonica. CRH cDNA encodes a signal peptide, a cryptic peptide and CRH (41 amino acids). The sequence homology to mammalian CRH is high. Next, the distribution of CRH-immunoreactive (ir) cell bodies and fibers in the brain and pituitary were examined by immunohistochemistry. CRH-ir cell bodies were detected in several brain regions, e.g., nucleus preopticus pars magnocellularis, nucleus preopticus pars gigantocellularis and formatio reticularis superius. In the brain, CRH-ir fibers were distributed not only in the hypothalamus but also in various regions. Some CRH-ir fibers projected to adrenocorticotropic hormone (ACTH) cells in the rostral pars distalis of the pituitary and also the α-melanocyte-stimulating hormone (α-MSH) cells in the pars intermedia of the pituitary. Finally, the neuroanatomical relationship between the CRH neurons and gonadotropin-releasing hormone (GnRH) neurons was examined by dual-label immunohistochemistry. CRH-ir fibers were found to be in close contact with GnRH-ir cell bodies in the hypothalamus and in the midbrain tegmentum and GnRH-ir fibers were in close contact with CRH-ir cell bodies in the nucleus preopticus pars magnocellularis. These results suggest that CRH has some physiological functions other than the stimulation of ACTH and α-MSH secretion and that reciprocal connections may exist between the CRH neurons and GnRH neurons in the brain of the Japanese eel. [ABSTRACT FROM AUTHOR]

Edwardsiella tarda is the predominant bacterium in farm-cultured eel in Korea. Here, we evaluated the heterogeneity of 37 E. tarda isolates derived from Japanese eel with various origins (olive flounder, common carp and ornamental fish) between 2003 and 2010. Regardless of origins, the biochemical characteristics of E. tarda isolates were homogenous except hydrogen sulfide production, citrate utilization and mannitol fermentation. Based on the phylogenetic analysis of 16S r RNA, E. tarda isolates could be classified into two subgroups and displayed a close relation with Edwardsiella ictaluri and Edwardsiella hosinae lineages, suggesting that the subgroup I has been a predominant type in the Jeonnam and Jeonbuk provinces. I- CeuI-based pulsed-field gel electrophoresis ( PFGE) typing showed that the isolates from Japanese eels belonged to 11 pulsotypes, indicating that the presence of highly genomic diversity. Additionally, two isolates, ET-060 and ET-191, showed a high frequency of virulence genes (100%) and caused 90% and 60% mortality in Japanese eel, respectively. This finding suggests a substantial congruence of virulence gene profiles and pathogenicity. Our results demonstrate that the intraspecific diversity within E. tarda strains from Japanese eel has been in prior existence. Significance and Impact of the Study Based on the biochemical characteristics, the phylogenetic property of the 16S r RNA gene and PFGE types of Edwardsiella tarda, we could identify the intraspecific diversity of isolates from Japanese eel, Anguilla japonica in Korea. In addition, this study describes the strong congruence of virulence-related genes and pathogenicity, suggesting that the virulence profile may be useful tool for prediction of pathogenicity. [ABSTRACT FROM AUTHOR]

Abstract: Osmoregulation is critical for the survival of fishes that migrate between freshwater (FW) and seawater (SW). The eel, as a catadromous fish, has been studied for decades to reveal the mechanisms of osmoregulation. These studies, however, have been limited by the lack of a genomic database to decipher the mechanism of osmoregulation at a molecular level. In this study, using high-throughput transcriptomic and proteomic technologies, we have provided the first genome-wide study to identify hyperosmotic responsive proteins in the gills of the Japanese eel. Deep sequencing using the 454 platform produced over 660,000 reads with a mean length of 385bp. For the proteomic study, we collected gill samples from three different treatment groups of fish that had fully adapted to FW/SW or were transferred from FW to SW for 6h. The respective group of gill proteins were extracted and labeled using an isobaric tag for relative and absolute quantitation (iTRAQ) using LTQ-Orbitrap, a high resolution mass spectrometer. Among the 1519 proteins identified from the gill samples, 96 proteins were differentially expressed between FW and SW adapted fish. Nineteen hyperosmotic responsive proteins were detected (10 up-regulated and 9 down-regulated proteins) after 6h post FW to SW transfer. Biological significance: The study has provided the most comprehensive, targeted investigation of eel gill proteins to date, and shown the powerfulness of combining transcriptomic and proteomic approaches to provide molecular insights of osmoregulation mechanisms in a non-model organism, eel. [Copyright &y& Elsevier]

Abstract: Seventy Aeromonas strains were identified by phylogenetic analysis using housekeeping genes (gyrB and rpoD) in order to investigate etiological agents for aeromoniasis in farmed eels (Anguilla japonica). The phylogenetic analysis showed that Aeromonas aquariorum (n =22, 31.4%) was the predominant species among the investigated eel strains, followed by Aeromonas caviae (n =16, 22.9%), A. veronii (n =13, 18.6%), A. hydrophila (n =12, 17.1%), A. jandaei (n =4, 5.7%), A. media (n =2, 2.9%), and A. trota (n =1, 1.4%). The potential virulence of the present strains was estimated by performing PCR assays using the following seven virulence genes: cytotoxic enterotoxin (act), two cytotonic enterotoxins (alt and ast), glycerophospholipid:cholesterol acyltransferase (gcaT), DNase (exu), lipase (lip), and flagellin (fla). The detection rates of act, alt, ast, gcaT, exu, lip, and fla among all 70 strains were 91.4%, 55.7%, 27.1%, 97.1%, 95.7%, 100%, and 98.6%, respectively. In genotyping of enterotoxin genes, act +/alt +/ast +, act +/alt +/ast −, and act +/alt −/ast − genotypes were prevalent in A. hydrophila (8/12 strains), A. aquariorum (13/22 strains), and A. caviae (14/16 strains), respectively, suggesting a high heterogeneity among Aeromonas species. In this study, A. aquariorum, which has been an unrecorded species in Korea, can be an etiological agent for aeromoniasis of eel. [Copyright &y& Elsevier]

Reduction in eel resources and catches of glass eels as seedlings for aquaculture have been a serious concern in recent years in both Europe and East Asia. Thus, technical advancement to produce eel seeds for artificial cultivation is most desired. Fundamental information on oocyte maturation and ovulation and its application to artificial induction of sexual maturation are needed to produce good quality seeds of the Japanese eel. This review introduces hormonal mechanisms of cytoplasmic maturation (such as hydration, lipid coalescence, and clearing of the ooplasm) and the maturational competence (the ability to respond to maturation-inducing steroid) and nuclear maturation (germinal vesicle breakdown). In addition, previous and newly developed methods for induction of spawning have been described. [ABSTRACT FROM AUTHOR]

Abstract: For understanding the functions of the growth hormone (GH)/prolactin (PRL)/somatolactin (SL) family of hormones, we examined pituitary mRNA expression of these hormones in anguillid eels in relation to salinity difference, silvering, and seasonal change. Female Japanese eels (Anguilla japonica) were collected in the brackish Hamana Lake and its freshwater rivers from July to December. To clarify the effect of salinity, the habitat use history of the eels were determined using otolith microchemistry. Expression levels of mRNA of each hormone were determined using real time PCR. Although GH and PRL have been known to be osmoregulatory hormones, there were no consistent differences in expression levels of these hormones between different salinity habitats. In contrast, SL mRNA expression was higher in eels from freshwater rivers than from the brackish lake. GH mRNA expression clearly decreased during silvering, whereas PRL and SL mRNA expression did not change. We also showed that PRL mRNA and SL mRNA decreased in the brackish lake and PRL mRNA increased in freshwater rivers from autumn to early winter. These findings provide basic knowledge for a further understanding of the role of these hormones. [Copyright &y& Elsevier]

Abstract: The Japanese eel is a much appreciated research object and very important for Asian aquaculture; however, its genomic resources are still limited. We have used a streamlined bioinformatics pipeline for the de novo assembly of the genome sequence of the Japanese eel from raw Illumina sequence reads. The total assembled genome has a size of 1.15Gbp, which is divided over 323,776 scaffolds with an N50 of 52,849bp, a minimum scaffold size of 200bp and a maximum scaffold size of 1.14Mbp. Direct comparison of a representative set of scaffolds revealed that all the Hox genes and their intergenic distances are almost perfectly conserved between the European and the Japanese eel. The first draft genome sequence of an organism strongly catalyzes research progress in multiple fields. Therefore, the Japanese eel genome sequence will provide a rich resource of data for all scientists working on this important fish species. [Copyright &y& Elsevier]

Sulfate (S042-) is maintained at ∼1 mmoll-1 in teleost fishes that are exposed to media of varying S042- concentrations. We first measured plasma S042- concentration in euryhaline fishes that adapt to both S042-poor freshwater (<0.5mmoll-1) and S042-enriched seawater (30mmoll-1). Unlike Mozambique tilapia and chum salmon, Japanese eels maintained higher plasma S042-concentration in freshwater (6.2+2.3 mmol I-1) than in seawater (0.7±0.1 mmoll-1). We then analyzed the whole-body S042- budget using 35S042-. 35S042- influx in seawater-adapted eels occurred by 84.5% via body surfaces and 15.5% via digestive tracts. The S042- influx was higher in seawater eels (1.55nmolkg-1 h-1) than in freshwater eels (0.09nmolkg-1 min-1), but it was facilitated in freshwater eels when the difference in S042- concentrations between plasma and environment was taken into account (freshwater eels, 6.2 vs 0.3mmoll-1; seawater eels, 0.7 vs 30mmoll-1). One hour after injection of 35S042- into the blood of seawater eels, the kidney excreted ∼97% of the ionized form, whereas the radioactivity increased gradually in the medium and the rectal fluid more than 3h after injection. As the radioactivity was poorly adsorbed by anion-exchange resin, 35S042- in the blood may be incorporated into cells and excreted by the intestine, gills and skin, probably as mucus. These results show that freshwater eels take up S042- actively from the environment, but seawater eels cope with the obligatory influx of S042- through the gills by excreting excess S042- via the kidney and in mucus. [ABSTRACT FROM AUTHOR]

Abstract: In order to improve the hatching rate of eggs from artificially matured female eels, the environmental temperature during late stages of oocyte development was lowered from 20°C (control) to 18°C. Thus, female eels in which artificial maturation was induced at 20°C using chum salmon pituitary extract (SPE) were transferred to water at either 20 or 18°C at the beginning of the late stage of maturation. Biopsy by cannulation was carried out on the basis of an increase in body weight index (BWI; BWI=body weight×100/initial body weight). If the eel was judged to be nearing final maturation, a priming dose of SPE was administered, followed by a dose of maturation-inducing hormone (MIH). The average hatching rates of the eggs from the eels maintained at 20 and 18°C were 16.6% (n =10) and 45.9% (n =11), respectively. The hatching rates at 18°C were significantly higher than those at 20°C. At 20°C, the optimal timing of cannulation may often be missed due to both the rapid rate of oocyte development in the late stage and the unreliability of the BWI-based timing criterion. At 18°C, rapid development of maturing oocytes in the late stage was prevented, and the optimal ovulation induction period was prolonged. Accordingly, the likelihood for optimal timing of cannulation was increased, as reflected in high hatching rates of fertilised eggs. On the basis of these results, a temperature of 18°C in late stages of oocyte development is recommended to help in improving and stabilising the hatching rate. [Copyright &y& Elsevier]

Abstract: This study aimed at investigating the role of sexual steroids in the regulation of the expression of the single aromatase gene and steroid receptor subtypes in the brain–pituitary–ovarian axis of the Japanese eel. Unlike other teleosts, which possess duplicated genes for aromatase, cyp19a1a and cyp19a1b, expressed in the gonads and in the brain, respectively, eel species possess a single cyp19a1. Phylogenetic analysis indicated that eel brain/gonadal cyp19a1 branches at the basis of both teleost gonadal cyp19a1a and brain cyp19a1b clades. Female eels treated with catfish pituitary homogenate (CPH) to induce sexual maturation showed an increase in the expression of cyp19a1 and aromatase enzymatic activity in the brain and in the ovaries. Treatments with sex steroids (estradiol-17β, E2 or testosterone, T) revealed that the increase in cyp19a1 expression in the brain may result from E2-specific induction. In contrast, the increase in cyp19a1 expression in the ovaries of CPH-treated eels is a result of steroid-independent control, probably from a direct effect of gonadotropins contained in the pituitary extract. Analysis of the expression of estrogen and androgen receptor subtypes, esr-α, esr-β, ar-α and ar-β, in eels treated with CPH or sex steroids revealed differential regulations. In CPH-treated eels, the expression of esr-α and ar-α was significantly increased in the brain, while the expression of ar-α and ar-β was increased in the ovaries. No change was observed in esr-β in any organ. Steroid treatments induced an upregulation by E2 of esr-α, but not esr-β expression, in the brain, pituitary and ovaries, while no autoregulation by T of its own receptors could be observed. These results reveal both steroid-dependent and -independent mechanisms in the regulation of cyp19a1 and steroid receptor subtype expression in the eel. [Copyright &y& Elsevier]

The article discusses the results of a study that investigated the characteristics of Edwardsiella tarda found on Anguilla japonica, farm-cultured eels in Korea. A discussion on materials and methods is given, including parasitical examination, bacterial isolation, molecular sequencing and identification and characterization of biochemical and enzyme activities. Data are provided, based on investigated samples, showing the occurrence rate and percentage of edwardsiellosis cases that have mixed infections. A comparison of the biochemical characteristics of Edwardsiella tarda found in farm-cultured eels from those isolated from other kinds of fish is given.

• Recombinant Japanese eel growth hormones with CTP were produced. • Recombinant Gh was found to promote the growth of eel larvae. • Addition of CTP to recombinant Gh enhanced growth promoting effect. • Recombinant Gh may have negative effects on the Gh receptors and Igf-I. Growth hormone (Gh) regulates somatic growth in fishes, particularly through the Gh - insulin-like growth factor-I (Igf-I) axis. In this study, recombinant Japanese eel Ghs with or without C-terminal peptides of human chorionic gonadotropin (CTP), which are known to prolong the half-life, were produced using the HEK 293 and CHO expression system. The effect of recombinant Gh administration to eel larvae on their somatic growth was investigated in short-term feeding experiments, and it was found that three types of recombinant Ghs with CTP (CTP-reGh, reGh-CTP and reGh-CTP × 2) were more effective in promoting somatic growth in eel larvae than recombinant Ghs without CTP. Among the three recombinant Ghs with CTP, reGh-CTP × 2 had the highest growth-promoting effects, however only when provided in the short term. After long-term administration of reGh-CTP × 2, there was no difference in growth between the Gh administrated group and the control group. The survival rate of eel larvae were not affected by recombinant Ghs. In addition, the mRNA expression of gh , Gh receptors, Igf-I and IGF-II were measured by quantitative real-time PCR, and significant reductions in the expression of gh, Gh receptors and Igf-I were observed. These findings provide useful tools to study the mechanisms of somatic growth and increase understanding of Gh regulation in anguillid eel larvae. [ABSTRACT FROM AUTHOR]

Furazolidone, an antibacterial drug that was once widely used in the livestock industry and aquaculture, is now prohibited in numerous countries. It is difficult to detect residual furazolidone because it is readily metabolized in animal tissues but, by using and liquid chromatography coupled with tandem mass spectrometry, its metabolite, 3-amino-2-oxazolidinone (AOZ) can be detected. Here we describe the validity of an enzyme-linked immunosorbent assay (ELISA) kit to detect AOZ in Japanese eel Anguilla japonica tissue. ELISA is capable of detecting AOZ at 1.0 μg/kg in an eel sample with excellent accuracy and precision. Our results show that ELISA is suitable for regulatory purposes and for studying the fate of AOZ residues in eel treated with furazolidone. To measure the persistence of AOZ in eel tissues, eels (1.4–6.5 g) were immersed in tanks containing 2 and 10 mg furazolidone/L for 3 h, and then maintained in a tank supplying well water for the next 160 days. The half-lives of AOZ, calculated from the linear terminal part of the excretion curve, were 25.0 days in muscle and 21.6 days in liver from fish exposed to 2 mg/L furazolidone. In the eels treated with 10 mg/L furazolidone, by contrast, high levels of AOZ were detected in liver and muscle, but the half-lives of AOZ were similar to those in fish treated with 2 mg/L furazolidone. The half-lives of AOZ in eel tissues were prolonged by the condition of low water temperature. [ABSTRACT FROM AUTHOR]

Abstract: Guanylyl cyclase C (GC-C) is a single transmembrane receptor for a family of intestinal hormones, guanylins. In the eel, we previously identified three guanylins, whose gene expression was enhanced in the intestine after transfer from fresh water to seawater. However, only limited information is available about the structure and function of their receptor(s). In the present study, we cloned full-length cDNAs encoding two isoforms of GC-C, named GC-C1 and GC-C2, from eel intestine. The predicted GC-C proteins consisted of extracellular ligand-binding domain, membrane-spanning domain, kinase-like domain and cyclase catalytic domain, in which GC-C-specific sequences were largely conserved. Phylogenetic analyses showed that the cloned membrane GCs are grouped with the GC-C of other vertebrates but not with GC-A and GC-B. However, eel GC-Cs appear to have undergone unique structural evolution compared with other GC-Cs. The three eel guanylins (guanylin, uroguanylin and renoguanylin), but not eel atrial natriuretic peptide, stimulated cGMP production dose-dependently in COS cells expressing either of the cloned cDNAs, providing functional support for assignment as eel guanylin receptors. The potency order for cGMP production was uroguanylin>guanylin⩾renoguanylin for GC-C1; guanylin⩾renoguanylin>uroguanylin for GC-C2. The distinctive ligand selectivity was consistent with the low homology (53%) of the extracellular domain of the two GC-Cs compared with that observed for other domains (74–90%). Both GC-C genes were expressed in the alimentary tract (esophagus, stomach and intestine) and kidney, and their expression was higher in the intestine of seawater-adapted eels than that of freshwater eels just as observed with the guanylin genes. However, the expression of the receptor genes was unchanged for 24h after transfer of eels from fresh water to seawater or vice versa, showing slower response of the receptors to salinity changes than their ligands. Collectively, the multiple guanylin—GC-C system may be involved as a paracrine factor in seawater adaptation at the intestine and kidney of the eel. [Copyright &y& Elsevier]

Abstract: To clarify the physiological functions of follicle-stimulating hormone (FSH) during oogenesis in Japanese eel, Anguilla japonica, the steroidogenic activities of recombinant Japanese eel FSH (rjeFSH) were assessed in the eel ovary. Female eel were injected with salmon pituitary homogenate to enhance the ovarian development, and the ovaries at different developmental stages were subjected to steroidogenic bioassay. These ovaries could be classified into three types according to oocyte growth and development of ovarian follicular cells. The type-A ovary possessed poorly developed follicular cells around pre- or early vitellogenic oocytes, and rjeFSH did not induce sex steroid secretion. Testosterone (T) secretion was stimulated by rjeFSH in the type-B ovary with developed theca cells and undeveloped granulosa cells around early to mid-vitellogenic oocytes, whereas estradiol-17β (E2) secretion was not enhanced. The rjeFSH stimulated both T and E2 secretion in a dose-dependent manner from the type-C ovary with fully developed theca and granulosa cells around mid-vitellogenic oocytes. Salmon GTH fraction (sGTH) and a membrane permeable cAMP analogue, 8-bromo-cAMP (8-Br-cAMP) also enhanced T and E2 secretion from the type-C ovary. Human chorionic gonadotropin (hCG) similarly enhanced T secretion, but failed to stimulate E2 secretion from the type-C ovary, suggesting different effects on steroidogenic activities between eel FSH and hCG in eel ovary. There was a positive correlation between the oocyte diameter and E2 secretion from eel ovaries stimulated by rjeFSH. These results suggest that aromatase activity is accelerated by eel FSH in the granulosa cells, which develop following theca cell development in this species. [Copyright &y& Elsevier]

Abstract: Eggs from maturation-induced Japanese eels, Anguilla japonica, often sink in seawater immediately after artificial insemination and do not hatch. In the present study, the specific gravity of unfertilized eggs from individual females was measured in isotonic (about 310 mOsm/kg) and hypertonic (about 875 mOsm/kg) saline solutions and the relation of specific gravity to egg quality was examined. Egg specific gravity under isotonic conditions showed a significant negative correlation with egg fertility, hatchability and water content, suggesting that inadequate hydration of oocytes during final maturation, which leads to insufficient egg buoyancy, is one of the causes of poor egg quality. Some of the eggs that showed lower specific gravity than seawater under isotonic conditions exhibited higher specific gravity than seawater under hypertonic conditions, indicating that the buoyancy acquired by the eggs is sometimes lost after their transfer to seawater. Only eggs that retained low specific gravity under hypertonic as well as isotonic conditions exhibited high fertility and hatchability. Taken together with data from other studies, these results suggest that the poor quality of eggs that sink immediately after artificial insemination is attributable to at least two causes: the failure of oocytes to acquire sufficient buoyancy during maturation and the loss of buoyancy in seawater because of the inability of egg osmoregulation. [Copyright &y& Elsevier]

In order to distinguish the two eel speciesAnguilla japonicaandA. anguillairrespective of unprocessed and processed samples, a quick, convenient method using polymerase chain reaction (PCR) with a species-specific primer was established. The comparison of partial sequences of the mitochondrial 16S ribosomal RNA gene betweenA. japonicaandA. anguillafacilitated designing of a primer pair common to bothA. japonicaandA. anguillaand a primer specific toA. japonica. PCR was carried out with the three primers for total 110 specimens ofA. japonicaandA. anguilla. PCR products with the species-specific primer showed two bands forA. japonicaand one band forA. anguillain an agarose gel electrophoresis. This analytical procedure required only a short period and is more convenient than PCR-restriction fragment length polymorphism, one of the standard methods employed for fish species identification. [ABSTRACT FROM AUTHOR]

A method was developed for assessing fertility, hatchability and larval survival in the Japanese eel, Anguilla japonica. Fertilized eggs were stocked in 24-, 48-, and 96-well plates at one egg to one well and maintained at 23 °C without removing dead larvae, changing the water or feeding. Upon hatching, the larvae were often trapped on the surface of the water where they died (surface death). The remaining larvae then showed rather stable survival in the three types of plates beyond the yolk-sac stage, which was completed by day 8 after hatching (DAH). Polyethylene glycol 6000 (PEG 6000) added to the rearing water at 1 or 10 μg/ml markedly suppressed surface death without apparent effect on hatchability, the survival profile or abnormality. These results indicated that the survival rate can be determined easily by stocking the fertilized eggs individually in the wells of microplates with water containing PEG 6000 and by rearing the hatched larvae until the completion of yolk absorption, with little management. By this method, a series of parameters for evaluating the egg quality, rates of fertilization, hatching, survival and abnormality were determined in the artificially maturation-induced Japanese eel: unstable fertility and frequent abnormal development were revealed to be impediments to constant production of larvae at first feeding. [Copyright &y& Elsevier]

The effects of angiotensin II (ANG II) and natriuretic peptides (NPs) of the eel (ANP, atrial natriuretic peptide; CNP, C-type natriuretic peptide; and VNP, ventricular natriuretic peptide) on prolactin (PRL188 and PRL177) and growth hormone (GH) release from the organ-cultured tilapia pituitary were examined. Eel ANG II at concentrations greater than 1 nM stimulated the release of PRL188 and PRL177 in a dose-related manner during the first hour of incubation. Significant stimulation by 100 nM ANG II on PRL177 release was observed until 4 h of incubation, and on PRL188 release until 12 h. No effect of ANG II was seen on GH release. None of the NPs altered the release of PRLs at any time point. On the other hand, eel VNP at concentrations greater than 1 nM stimulated GH release in a dose-related manner after 4 h, and significant stimulation was observed until 48 h. Eel CNP was less effective than eel VNP; significant stimulation of GH release was observed at 1 and 10 nM during 24–48 h of incubation. No significant effect of eel ANP on GH release was seen at any concentration. ANG II had no effect on GH release at any time point. There was no change in mRNA levels of PRLs or GH in the pituitaries incubated with ANG II for 8 h or those incubated with the NPs for 48 h. These results indicate rapid and short-lasting stimulation by ANG II on PRL release and slow and long-lasting stimulation by VNP and CNP on GH release from the tilapia pituitary. [Copyright &y& Elsevier]

To reveal the ontogeny of pancreatic exocrine function in the early larval stage of eel, cDNAs encoding major pancreatic enzymes, trypsinogen, amylase and lipase were identified from the Japanese eel Anguilla japonica and their expression pattern in larvae was analyzed. The cloned eel trypsinogen precursor consisted of 224 amino acids and showed 82.2% identity to trypsinogen-2 of winter flounder Pleuronectes americanus . The eel amylase precursor consisted of 512 amino acids and showed 77% identity to winter flounder amylase. Eel pancreatic lipase was composed of 470 amino acids and had 58.3% of identity to human pancreatic lipase. In the eel larvae, mRNA expression of trypsinogen and amylase was first detected at 6 days post-hatching (d.p.h.), and the expression level increased between 7 and 8 d.p.h. In contrast, mRNA expression of lipase was first detected at 8 d.p.h. Eel larvae start to feed actively at 8 d.p.h. Thus, it was indicated that eel pancreas starts to synthesize digestive enzymes at 6 d.p.h. and acquires full function by the onset of exogenous feeding at 8 d.p.h. [ABSTRACT FROM AUTHOR]

The objectives were to investigate the regulation of brain mammalian gonadotropin-releasing hormone (mGnRH), pituitary gonadotropin II (GTH II), growth hormone (GH), and gonadosomatic index (GSI) using sex steroids and salmon pituitary extract in aquacultured Japanese eels, Anguilla japonica. Yellow Japanese eels were collected from culture ponds and treated at 2-week intervals with various doses of estradiol-17β (E2), testosterone (T), and 5α-dihydrotestosterone (DHT), or weekly for varying periods with salmon pituitary extract. GSI was not significantly stimulated in females after three steroid injections over a 6-week period, nor in males after six injections over a 12-week period (E2, T, and DHT; for each steroid with two doses, 0.75 and 3.75 mg/kg body weight). Salmon pituitary extract (12 mg/kg body weight) induced a progressive increase in GSI after 3-, 6-, 9- and 14-injections over a 14-week period. E2 and pituitary extract significantly increased pituitary GTH II content in a dose-dependent manner but did not affect pituitary GH content. Androgens (T and especially DHT) inhibited pituitary GH content. Androgens also stimulated pituitary GTH II content but the effects were much weaker than E2. Sex steroids did not stimulate brain mGnRH content. Brain and pituitary mGnRH content, and pituitary GTH II content were significantly increased in maturing eels (GSI>20) injected with salmon pituitary extract. Pituitary and plasma GH levels were not different between pituitary extract-treated eels and the control group. Factors other than mGnRH are suggested to mediate the in vivo estrogen effects on pituitary GTH II in cultured yellow eels. [Copyright &y& Elsevier]

SUMMARY: Development of skin lectin and the lymphoid tissues were studied in the leptocephalus larvae of the Japanese eel Anguilla japonica. The specimens ranging in total length from 11 to 58 mm were captured at 12–19°N, 131–137°E and at 21–23°N, 123–128°E in the Pacific Ocean. The skin of leptocephali contained extremely active lectin which agglutinated rabbit red blood cells the same as adults. Club cells, known as lectin secreting cells, were also recognized in the epidermis of leptocephali, although the shape was not an elongated club form but oval. The cells were confirmed to contain lectin in the secretory vacuole by an immunofluorescence technique. The lectin in the cells was also recognized in the 8 days post-hatch preleptocephalous larvae, which was obtained from an artificially spawned eel, suggesting the importance of the lectin in the early larval development. In contrast, lymphoid tissues concerning immune functions showed delayed development with the exception of the thymus. No blood cells were seen in the kidney and spleen, even in a large specimen. Only a few undifferentiated leukocyte-like cells could be observed; some of them showed phagocytic figures. The thymus along with many thymocytes could be seen in the smallest leptocephalus of total length (TL) 11 mm. T cells may have a function in defense mechanisms during the leptocephalus stage, although other immune cells were still underdeveloped. [ABSTRACT FROM AUTHOR]

SUMMARY:We generated 196 expressed sequence tags (EST) from a Japanese eel Anguilla japonica spleen cDNA library. Of these, 90 EST (46%) showed no homology to known genes, while 106 EST (54%) showed homology to known genes and transcripts. Of the 106 homologous EST, 13 were homologous to 12 different immune-related genes, 46 were homologous to 33 different ribosomal proteins, three were homologous to the 28S rRNA gene, 10 were homologous to eight different mitochondrial genes and the remaining 34 encoded 28 different identified genes. Six identified EST are similar to previously reported EST of Japanese flounder Paralichthys olivaceus spleen cDNA. These analyses demonstrate the utility of EST and sequence-tagged clones in comparative studies of gene expression in the fish spleen. [ABSTRACT FROM AUTHOR]

The gonochoristic feature with environmental sex determination that occurs during the yellow stage in the eel provides an interesting model to investigate the mechanisms of gonadal development. We previously studied various sex-related genes during gonadal sex differentiation in Japanese eels. In the present study, the members of transforming growth factor beta (TGF-β) superfamily were investigated. Transcript levels of anti-Müllerian hormone, its receptor, gonadal soma-derived factor (amh, amhr2, and gsdf, respectively) measured by real-time polymerase chain reaction (qPCR) showed a strong sexual dimorphism. Transcripts were dominantly expressed in the testis, and their levels significantly increased with testicular differentiation. In contrast, the expressions of amh, amhr2, and gsdf transcripts were low in the ovary of E2-feminized female eels. In situ hybridization detected gsdf (but not amh) transcript signals in undifferentiated gonads. amh and gsdf signals were localized to Sertoli cells and had increased significantly with testicular differentiation. Weak gsdf and no amh signals were detected in early ovaries of E2-feminized female eels. Transcript levels of amh and gsdf (not amhr2) decreased during human chorionic gonadotropin (HCG)-induced spermatogenesis in males. This study suggests that amh, amhr2, and especially gsdf might be involved in the gene pathway regulating testicular differentiation of Japanese eels. [ABSTRACT FROM AUTHOR]

During the course of cloning and characterization of natriuretic peptide receptor‐A (NPR‐A) from the euryhaline fish eel, Anguilla japonica, we identified a splice variant with unique structural properties that affect ligand‐inducible intrinsic guanylate cyclase activity. The variant, generated from a splice between a cryptic donor site and the normal acceptor site, lacked nine amino acid residues (VFTKTGYYK) in the kinase‐like regulatory domain. This deletion of a very short segment resulted in the complete loss of the ligand inducibility of the cyclase activity. The nine‐amino acid segment may therefore be useful as a target for studies aimed at clarifying the mechanism of activation of the guanylate cyclase domain. Characterization of the normal form of eel NPR‐A also led to the following interesting findings. Although eel NPR‐A had a domain structure very similar to that of mammalian counterparts, it lacked the third cysteine residue in the extracellular domain which is conserved among mammalian NPR‐A molecules. The eel receptor bound both amidated and nonamidated eel atrial natriuretic peptide (eANP) with high affinity but, when assayed for ligand‐inducible cGMP generation, it responded efficiently only to physiological concentrations of the amidated ligand, suggesting that the biologically active form is the amidated eANP, and the nonamidated form acts as a partial antagonist; similarly, nonhomologous rat ligands behaved like antagonists toward the eel receptor in the concentration range 0.1–10 nm. The receptor message was found to be relatively abundant in the osmoregulatory organs such as the gill, kidney, intestine and urinary bladder. [ABSTRACT FROM AUTHOR]

The effects of temperature and salinity on the concentration ratios of strontium (Sr) to calcium (Ca) within the sagittal otoliths of elvers of the Japanese eel, Anguilla japonica, were studied by spot analysis using a wavelength dispersive X-ray electron microprobe. A total of 340 elvers were used: 100 elvers were reared for 15 days under various salinity conditions (freshwater, one-third seawater, two-thirds seawater and pure seawater at 22 °C: 240 elvers were reared for 58 days under various water temperature conditions (12, 17, 22. 27 °C) in either freshwater or pure seawater. Otolith Sr/Ca ratios were found to be positively correlated with water salinity. On the other hand, the Sr/Ca ratios were not found to be significantly different among the various temperature groups. The above results strongly suggest that the physiological mechanism of incorporation of Sr and Ca within the otolith of an eurythermal fish, Japanese eel, does not change within this range of temperatures (12-27 °C). [ABSTRACT FROM AUTHOR]