• Journal of Practical Agriculture & Fisheries Research
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• v.22 no.2
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• pp.51-58
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• 2020

Continuous mortality in commercially cultured Japanese eel (Anguilla japonica), showing symptoms of dermal ulcerations and focal hemorrhages on the body, occurred on a private farm in November, 2019 in Korea. A series of mortality had been described in one local eel culture farm from November to December in 2019. From the three cases, three isolates of Vibrio spp. were recovered from the blood, ascitic fluid, and kidney of the dead fish, respectively. Based on the 16S rRNA sequence comparisons, the Vibrio isolates from the 1^st and 3^rd cases (strain named 1E1-2 and 3K1-2) were identified as V. fluvialis and the isolate from the 2^nd case was identified as V. plantisponsor (strain named 2A3-1). Moreover, the 16S rRNA-based phylogenetic analysis revealed that strain 1E1-2 and 3K1-2 were most similar to V. fluvialis NBRC 103150^T, and strain 2A3-1 was most similar to V. plantisponsor NBRC103148^T. According to the results of the antibiotic resistance determination, V. fluvialis 1E1-2 showed intermediate resistance to tetracycline and chloramphenicol, and was resistant to trimethoprim-sulfamethoxazole. V. plantisponsor 2A3-1 showed intermediate resistance to ciprofloxacin and levofloxacin, and was resistant to trimethoprim-sulfamethoxazole. V. fluvialis 3K1-2 showed intermediate resistance to tetracycline, and was resistant to ampicillin and trimethoprim-sulfamethoxazole. These results have provided the evidences on the occurrence of antibiotic-resistant Vibrio infection in commercially cultured Japanese eels are present in Korea.

• Fisheries and Aquatic Sciences
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• v.24 no.7
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• pp.260-275
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• 2021

Aquaponics is a cultivation system that combines aquaculture and agricultural hydroponics. This study investigated the productivity of leafy vegetables cultivated in the hybrid biofloc technology-aquaponics (HBFT-AP) using Japanese eel fed two kinds of diets (Mash, commercial powdered feed and extruded pellet [EP]) and hydroponics (HP).The mash was fed to the fish in a type of a dough mixed with water for 6 weeks (Exp1) and switched to the EP containing 2.7% monobasic potassium phosphate (MKP) for subsequent 6 weeks (Exp2). Leafy vegetables of 8 cultivar were employed in the experiment and water quality [dissolved oxygen (DO, mg/L), pH, water temperature (℃), electrical conductivity (EC; ㎲/cm), turbidity (NTU), TAN (NH₃ + NH₄⁺) (mg /L), NO₂-N (mg/L), NO₃-N (mg/L) and PO₄-P (mg/L)] was measured 6 times a week. Leafy vegetable productivity (HBFT-AP vs HP) was compared in respective experiment, which was similar or somewhat higher in HBFT-AP. During the 12-week feeding trial, concentrations of nitrite (NO₂-N) and phosphorus (PO₄-P) were kept low in variability while total ammonia nitrogen (TAN) and NO₃-N levels increased with time in HBFT-AP. At the end of two feeding trials, values of weight gain (WG, %), feed efficiency (FE, %), specific growth rate (SGR, %) and protein efficiency ratio (PER) were higher in Exp2 than in Exp1. As well, higher values in hematocrit (PCV, %), plasma K (mEq/L) and inorganic phosphorus (mg/dL) were found (p < 0.05) in Exp2 where fish were fed the EP (EP: 38.60%, 2.80 mEq/L and 7.04 g/dL; Mash: 33.20%, 1.95 mEq/L and 5.50 g/dL). Leafy vegetables in HBFT-AP using Japanese eel fed the EP with MKP 2.7% had a productivity similar (4 kinds of cultivar) or somewhat higher (4 kinds of cultivar) compared to those in HP. Also, Japanese eel fed the EP showed higher values of Pi (m/dL) and K (mEq/L) in plasma compared to those fed commercial powder diet.

• Journal of Animal Reproduction and Biotechnology
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• v.36 no.4
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• pp.169-174
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• 2021

Mutations in the luteinizing hormone/chorionic gonadotropin receptors (LH/CGRs), representatives of the G protein-coupled receptor family, have been rapidly identified over the last 20 years. This review aims to compare and analyze the data reported the activating and inactivating mutations of the LH/CGRs between human, rat, equine and fish, specifically (Japanese eel Anguilla japonica). Insights obtained through detailed study of these naturally-occurring mutations provide a further update of structure-function relationship of these receptors. Specifically, we present a variety of data on eel LH/CGR. These results provide important information about LH/CGR function in fish and the regulation of mutations of the highly conserved amino acids in glycoprotein hormone receptors.

• Journal of Aquaculture
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• v.18 no.3
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• pp.135-141
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• 2005

A 16-week feeding trial was conducted to estimate the optimum dietary protein to energy ratio (P/E ratio, mg/kcal) in juvenile Japanese eel, Anguilla japonica. Six experimental diets were formulated with three energy levels and two protein levels at each energy level. Three energy levels of 3800, 4150 and 4500 kcal per kg diets were included at 45 and 50% crude protein (CP) levels, respectively $(_{120}P_{45},\;_{110}P_{45},\;_{100}P_{45},\;_{130}P_{50},\;_{120}P_{50},\;and\;_{110}P_{50})$. After four weeks of the conditioning period, fish initially averaging $15.0{\pm}3g\;(means{\pm}SD)$ were randomly distributed into each tank as groups of 20 fish. Each diet was fed to fish in three randomly selected tanks at a rate of $2{\sim}3%$ wet body weight per day in the recirculated system. Weight gain (WG) and specific growth rate of fish fed diet $_{100}P_{45}$ were significantly higher (P<0.05) than those of fish fed the other diets. WG of fish fed diet $_{120}P_{50}$ was also significantly higher than those of fish fed diets $_{130}P_{50}$ and $_{110}P_{50}$. Feed efficiency ratio of fish fed diets $_{100}P_{45}$ and $_{110}P_{45}$ were significantly higher (P<0.05) than those of fish fed other diets. These results suggest that the optimum P/E ratio may be 100 mg/kcal with 45% protein diets, and 120 mg/Kcal 50% protein diets for the maximum growth of juvenile Japanese eel under the experimental condition.

• Fisheries and Aquatic Sciences
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• v.25 no.1
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• pp.12-19
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• 2022

In the present study, we investigated the effects of recombinant eel gonadotropins (rec-GTHs) on maturation induction in immature ovarian culture in vitro and sex steroid hormones in vivo in the Japanese eel Anguilla japonica. To study the in vitro effects of rec-GTHs on estradiol-17β (E2) production in immature ovarian tissues, ovarian tissues were incubated with different doses of rec-follicle-stimulating hormone (rec-FSH) or rec-luteinizing hormone (rec-LH). The results revealed that the E2 levels in the rec-FSH (0.1, 0.5, or 1 ㎍/mL)- and rec-LH (0.1 or 0.5 ㎍/mL)-treated groups were significantly higher than those in the female eels from the control group. Furthermore, to investigate the in vivo effects of rec-GTHs on the gonadosomatic index (GSI) and plasma sex steroid hormone levels, the eels were injected intraperitoneally with eel's ringer (control), salmon pituitary extract (SPE; for female eels), human chorionic gonadotropin (hCG; for male eels), rec-FSH, rec-LH, and rec-FSH + rec-LH once a week. The results revealed that except for the SPE and the hCG groups, none of the groups exhibited a significant difference in GSI values. However, in vivo plasma E2 levels increased at the end of 4 weeks after rec-FSH treatment in female eels. Based on these results, it is suggested that rec-GTHs may have a positive effect on sexual maturation in female eels; however, further studies on complementary rec-protein production systems and additional glycosylation of rec-hormones are needed to elucidate hormone bioactivity in vivo and in vitro.

• Development and Reproduction
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• v.25 no.4
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• pp.225-234
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• 2021

The present study aimed to investigate the mechanism of cell surface receptor loss by two constitutively activating mutants (designated L469R, and D590Y) and two inactivating mutants (D417N and Y558F) of the luteinizing hormone receptor (LHR) in the Japanese eel Anguilla japonica, known to naturally occur in human LHR transmembrane domains. We investigated cell surface receptor loss using an enzyme-linked immunosorbent assay in HEK 293 cells. The expression level of wild-type eel LHR was considered to be 100%, and the expression levels of L469R and D417N were 97% and 101%, respectively, whereas the expression levels of D590Y and Y558F slightly increased to approximately 110% and 106%, respectively. The constitutively activating mutants L469R and D590Y exhibited a decrease in cell surface loss in a manner similar to that of wild-type eel LHR. The rates of loss of cell surface agonist-receptor complexes were observed to be very rapid (2.6-6.2 min) in both the wild-type eel LHR and activating mutants. However, cell surface receptor loss in the cells expressing inactivating mutants D417N and Y558F was slightly observed in the cells expressing inactivating mutants D417N and Y558F, despite treatment with a high concentration of agonist. These results provide important information on LHR function in fish and the regulation of mutations of highly conserved amino acids in glycoprotein hormone receptors.

• Journal of Life Science
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• v.23 no.10
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• pp.1288-1294
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• 2013

Knowledge of morphological changes in eel larvae is very important for artificial rearing of eel larvae. In this study, we investigated the morphological structure of the head region and histological changes of the eye retina in artificially reared larvae at various stages and in glass eel just after metamorphosis. Structural changes were observed in the upper jaw (maxilla) and the lower jaw (mandible) after 100 dah (day after hatchery) and after metamorphosis. Teeth had degenerated by the time of completion of metamorphosis. Major histological changes observed in the eye retina were the formation of the outer plexiform layer and the outer nuclear layer from 100 dah larva and a change in the rod cell layer after metamorphosis. The cornea was not observed at 10 dah in the eel larva. More information is needed on the early developmental stages of eel larvae to enable mass production of glass eels. The results obtained in the present research will be useful when developing novel rearing programs for eel larvae.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.47 no.5
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• pp.597-602
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• 2014

We determined the effects of starvation, water temperature, and water flow on the onset of metamorphosis in leptocephali of the Japanese eel Anguilla japonica. Leptocephali larger than 50 mm (ca. 200 days old) were reared in 10-L tanks under different feeding, water temperature, and water flow regimes. Fasted leptocephali metamorphosed earlier and faster than did the fed ones. Metamorphosis was faster in the high water flow tank (1.2 L/min) than in the tank with flow at 0.6 L/min. The duration of metamorphosis was slightly shorter at $26^{\circ}C$ than at $23^{\circ}C$. To conclude, starvation induced the metamorphosis of artificially produced leptocephali larger than 50 mm TL. Increased water flow and water temperature both appear to induce and shorten metamorphosis.

• Journal of Fisheries and Marine Sciences Education
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• v.20 no.1
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• pp.90-94
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• 2008

Growth and water quality in Japanese eel, Anguilla japonica ($6.0{\pm}0.9g$) fed extruded and paste type diets were compared for 8 weeks. Fish in the duplicate tanks were fed with one of three experimental diets: two different extruded pellet diets and one paste type diet. Six FRP tanks were used with stocking density of 3.6 kg per tank (3 m3). Restricted feeds (ca. 2% of body weight) were served twice daily. Weight gains were checked at the end of every two-week interval, and water qualities in terms of NH4-N and NO2-N were determined daily one hour after feeding in the morning. Water temperature, dissolved oxygen, and pH were constantly kept within the optimum range, while fifty percent of total water volume was changed daily. Eels fed the extruded pellet diets showed significantly better (P<0.05) growth performance than did eels fed the paste type diet. The amount of feed waste from the paste type diet was significantly greater than those from the extruded pellet diets (P<0.05). Ammonia (NH4-N) waste was significantly higher from the paste diet than from the extruded pellet diets (P<0.05). These results show that extruded pellet diets are better than the paste type diet for growth and water quality management in eel culture.

• Journal of Environmental Science International
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• v.20 no.8
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• pp.943-951
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• 2011

We investigated the effects of temperature changes on the oxygen consumption rhythm in Japanese eels, Anguilla japonica, using an automatic intermittent flow respirometer (AIFR). The endogenous rhythm of the oxygen consumption rate (OCR) in the eels (n = 18; 44-74 cm, 145-690 g), freshly collected by bag net from estuaries, was nearly synchronous with the tidal pattern of the estuarine collection site. The magnitude of mean OCR (mOCR) of eels showed variable range of 82.2 - 116.5 ml $O_2\;kg^{-1}ww \;h^{-1}$ under constant conditions. In case of increasing temperature from 25 to $38^{\circ}C$, the OCR of eels exhibited a gradually increasing trend with a rhythmic pattern until $36^{\circ}C$. Above $36^{\circ}C$ the rhythms of the OCR dampened and the OCR decreased rapidly at around $36-37^{\circ}C$. The OCR of the eels exhibited the maximum value at $38^{\circ}C$, and then it sharply decreased. The results suggested that the critical thermal maximum (CTM) regarding the endogenous rhythms of the eels was at around $36-37^{\circ}C$ when water temperature increased at $0.5^{\circ}C$/14 h following the acclimation at $25^{\circ}C$. In case of decreasing temperature ($0.5^{\circ}C$/14 h) from 25 to $0^{\circ}C$, the OCR of the eels displayed a abrupt decrease up to $23^{\circ}C$, and between at 23 and $20^{\circ}C$, there was an agitation which showed a slight increase in the OCR with a duration of 1-2 days. Below $9^{\circ}C$, the OCR rhythm of the eels showed a constant state regardless of temperature decreasing. These results suggest that the Japanese eel has an upper incipient lethal temperature at $36^{\circ}C$, with a lower thermal limit at $9^{\circ}C$. The biochemical aspects of the eels influenced by water temperature need to be further studied.