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The objective of this study is to determine the lipid content and fatty acid composition of Anguilla japonica caught in South Korea. Seventy-nine eels caught at four different locations in different seasons were analyzed. Lipid content (1.3–30.0% of muscle weight) was more affected by season than by the location where the fish were caught, being the highest among the eels caught in May. Lipid content was higher in cultured eels than in wild eels. The predominant fatty acids in the eels were C18:1n−9 (24.4–33.0% of the total detected peak areas), C16:0 (17.7–22.0%), C16:1n−7 (7.0–10.2%), C22:6n−3 (2.4–6.8%), C18:1n−7 (2.9–6.2%), C18:2n−6 (0.6–6.1%), C20:5n−3 (1.5–5.1%), C14:0 (2.4–5.1%), C18:0 (3.4–4.5%), and C20:4n−6 (1.0–2.9%). Most fatty acids showed significantly more difference by location than by season. Cultured eels had significantly more saturated fatty acids (31.6–32.9%) and less monounsaturated fatty acids (42.5–43.3%) than wild eels (26.4–28.1% and 44.5–48.0%, respectively). Among the wild eels, estuarine eels had more n−3 fatty acids (12.2 and 14.8%) than lake eels (8.7–12.1%). Principal component analysis showed that the fatty acid composition could be an effective variable to classify fish according to the location where they were caught. [ABSTRACT FROM AUTHOR]

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

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

Outbreaks of ribbon worms observed in 2013, 2015, and 2017-2019 in the Han River Estuary, South Korea, have caused damage to local glass-eel fisheries. The Han River ribbon worms have been identified as Yininemertes pratensis (Sun & Lu, 1998) based on not only morphological characteristics compared with the holotype and paratype specimens, but also DNA sequence comparison with topotypes freshly collected near the Yangtze River mouth, China. Using sequences of six gene markers (18S rRNA, 28S rRNA, histone H3, histone H4, 16S rRNA, and COI), the phylogenetic position of Y. pratensis was inferred among other heteronemerteans based on their sequences obtained from public databases. This analysis firmly placed Y. pratensis as a close relative to Apatronemertes albimaculosa Wilfert & Gibson, 1974, which has been reported from aquarium tanks containing tropical freshwater plants in various parts of the world as well as a wild environment in Panama. [ABSTRACT FROM AUTHOR]

Edwardsiella infections have the potential to induce immense economic losses by affecting multiple fish species, and have been increasingly reported in aquaculture. Edwardsiella tarda , Edwardsiella hoshinae, and Edwardsiella ictaluri have all been previously associated with mass mortality. Recent technological advances in bacterial identification have identified E. piscicida and E. anguillarum as important pathogens affecting global fisheries. Strains previously identified as E. tarda have been re-grouped based on new sequence data and phylogenetic studies. E. piscicida was classified as a novel species in 2012 and has since been reported to have caused mortality in diverse fish species in numerous countries that are potentially more threatening than the mortality caused by E. tarda. Eels are one of the most consumed fish species in Asia, and Anguilla japonica is a commonly reared species in fisheries. However, due to increasing global eel consumption, attempts to culture marbled eel (A. marmorata) have been recently carried out in many Asian countries including Korea. The external clinical signs of Edwardsiella piscicida - infected marbled eel (A. marmorata) were enlarged liver, hemorrhagic congestion at the base of fins and on the skin, as well as hemorrhagic ascites and multifocal abscesses were found in the liver. Histopathology on E. piscicida- infected tissues revealed bacteremia, multifocal necrotizing hepatitis with vasculitis and splenitis with vasculitis. This study reports the first E. piscicida infection case responsible for mass mortality in A. marmorata cultured in Korea. Our study focused on the isolation and histopathological characterization of this bacterial strain to better understand its damage on A. marmorata. • Edwardsiella piscicida was isolated from diseased marbled eels. • With increased eel consumption, Anguilla japonica is substituted with A. marmorata. • This is the first report on E. piscicida infection in marbled eel cultured in Korea. • Epidemiological studies on marbled eel diseases are vital for breeding Korean eels. [ABSTRACT FROM AUTHOR]

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.

Simple Summary: Anguilla japonica is an IUCN red list species facing a natural reduction in population due to environmental changes and artificial decline due to various human activities (disturbance in foraging, river, etc.). While many of these disturbances mainly occur in freshwater areas, studies on the ecology of A. japonica in the freshwater environment are limited compared to those of Anguilla anguilla and Anguilla rostrata. Freshwater is an important habitat for eels to grow to the spawning period. With insufficient numbers of adults supplied to the sea for spawning, eels may eventually become extinct. This study aimed to provide ecological information of the continental phase in the freshwater ecosystem by examining diel activity along with the movement patterns of eels in the pure freshwater environment based on the results of monitoring. In the future, it is necessary to establish a protection and management strategy for the conservation of eels living in rivers based on the results. To investigate movement patterns and diel activities of Anguilla japonica in the freshwater ecosystem, we applied acoustic telemetry on A. japonica in the Geum River, a large river in South Korea. The acoustic tags were attached on 19 individuals of A. japonica (12 with a depth sensor) in May and October 2015 and tracked at approximately 100-km sections from an estuary barrage by 20 automatic listening stations. Only four individuals showed longitudinal movement (mean, 5.2 km), and others were detected by the receivers at release sites; therefore, A. japonica showed high site fidelity. We did not identify seaward migration during the study period (May to November). Conversely, A. japonica showed active diel movement. The number of detections (p = 0.002) and movement distance (p = 0.004) were higher at night-time (18:00–06:00). As most individuals were actively moving at nighttime, we confirmed that A. japonica is nocturnal, although few individuals also showed daytime movement. Although the population and habitats of A. japonica have been decreasing simultaneously, the East-Asian countries are still severely exploiting rivers and streams to use water resources, and result in habitat simplification generated. Therefore, these results contribute to effective A. japonica management regarding habitat and population conservation and restoration. [ABSTRACT FROM AUTHOR]