Yu-San Han

Synchronous changes of morphology and gonadal development of silvering Japanese eel Anguilla japonica

Abstract The gonadal development of Japanese eel Anguilla japonica during silvering was examined via gonadal histology and morphometric characteristics. Specimens were collected from the Kaoping River of Taiwan between November 1998 and November 2001. Female eels predominated, constituting 87.6% of the sex-determined eels. The degree of gonadal development was assessed by skin coloration, with female eels divided into yellow, pre-silver, and silver phases. Males were divided into yellow and silver phases because of small sample size. Silver phase eels predominated in the winter. Mean (±S.E.) total length of silver-phase eels was significantly larger in females (642.2±10.4) than males (564.8±14.6) (p

Evaluation of Multi-Scale Climate Effects on Annual Recruitment Levels of the Japanese Eel, Anguilla japonica, to Taiwan

Long-term (1967–2008) glass eel catches were used to investigate climatic effects on the annual recruitment of Japanese eel to Taiwan. Specifically, three prevailing hypotheses that potentially explain the annual recruitment were evaluated. Hypothesis 1: high precipitation shifts the salinity front northward, resulting in favorable spawning locations. Hypothesis 2: a southward shift of the position of the North Equatorial Current (NEC) bifurcation provides a favorable larval transport route. Hypothesis 3: ocean conditions (eddy activities and productivity) along the larval migration route influence larval survival. Results of time series regression and wavelet analyses suggest that Hypothesis 1 is not supported, as the glass eel catches exhibited a negative relationship with precipitation. Hypothesis 2 is plausible. However, the catches are correlated with the NEC bifurcation with a one-year lag. Considering the time needed for larval transport (only four to six months), the one-year lag correlation does not support the direct transport hypothesis. Hypothesis 3 is supported indirectly by the results. Significant correlations were found between catches and climate indices that affect ocean productivity and eddy activities, such as the Quasi Biennial Oscillation (QBO), North Pacific Gyre Oscillation (NPGO), Pacific Decadal Oscillation (PDO), and Western Pacific Oscillation (WPO). Wavelet analysis reveals three periodicities of eel catches: 2.7, 5.4, and 10.3 years. The interannual coherence with QBO and the Niño 3.4 region suggests that the shorter-term climate variability is modulated zonally by equatorial dynamics. The low-frequency coherence with WPO, PDO, and NPGO demonstrates the decadal modulation of meridional teleconnection via ocean–atmosphere interactions. Furthermore, WPO and QBO are linked to solar activities. These results imply that the Japanese eel recruitment may be influenced by multi-timescale climate variability. Our findings call for investigation of extra-tropical ocean dynamics that affect survival of eels during transport, in addition to the existing efforts to study the equatorial system.

Thiopurine analogues inhibit papain-like protease of severe acute respiratory syndrome coronavirus

Abstract The papain-like protease of severe acute respiratory syndrome coronavirus (PLpro) (EC 3.4.22.46) is essential for the viral life cycle and therefore represents an important antiviral target. We have identified 6MP and 6TG as reversible and slow-binding inhibitors of SARS-CoV PLpro, which is the first report about small molecule reversible inhibitors of PLpro. The inhibition mechanism was investigated by kinetic measurements and computer docking. Both compounds are competitive, selective, and reversible inhibitors of the PLpro with K is values ∼10 to 20μM. A structure–function relationship study has identified the thiocarbonyl moiety of 6MP or 6TG as the active pharmacophore essential for these inhibitions, which has not been reported before. The inhibition is selective because these compounds do not exert significant inhibitory effects against other cysteine proteases, including SARS-CoV 3CLpro and several cathepsins. Thus, our results present the first potential chemical leads against SARS-CoV PLpro, which might be used as lead compounds for further optimization to enhance their potency against SARS-CoV. Both 6MP and 6TG are still used extensively in clinics, especially for children with acute lymphoblastic or myeloblastic leukemia. In light of the possible inhibition against subset of cysteine proteases, our study has emphasized the importance to study in depth these drug actions in vivo.

Massive encapsulation of larval Anguillicoloides crassus in the intestinal wall of Japanese eels

BackgroundWithin the last 25 years, after the introduction of the swimbladder nematode Anguillicoloides crassus from East-Asia to Europe, a body of work has aggregated on the host parasite interactions in the acquired host Anguilla anguilla. Despite the emerging evolutionary interest there is still a lack of knowledge about host parasite relations of A. crassus in its natural host Anguilla japonica. We examined the Anguillicoloides infections of wild-caught Japanese eels as well as from aquacultured specimens in Taiwan with respect to the fate of migratory L3 larvae and performed infection experiments with Japanese eels.ResultsInside the intestinal wall of cultured eels, where the infective pressure was higher than among wild eels, we found large numbers of granuloma-like cysts. In a few eels these cysts contained nematodes still recognizable as L3 larvae of A. crassus, while in most cases the content of these capsules was degraded to amorphous matter. Occurrence of these objects was correlated with the number of encapsulated larvae in the swimbladder wall. We were able to show, that the cysts contained disintegrated L3 larvae by amplification and subsequent sequencing of large subunit ribosomal rRNA. Furthermore we identified repeated infections with high doses of larvae as prerequisites for the processes of encapsulation in infection experiments.ConclusionUnder high infective pressure a large percentage of L3 larvae of A. crassus coming from the gut lumen are eliminated by the natural host within its intestinal tissue. It is possible to reproduce this condition in infection experiments. We provide a fast, easy and reliable PCR-based method for identification of encapsulated swimbladder parasites.

Sympatric Spawning but Allopatric Distribution of Anguilla japonica and Anguilla marmorata: Temperature- and Oceanic Current-Dependent Sieving

Anguilla japonica and Anguilla marmorata share overlapping spawning sites, similar drifting routes, and comparable larval durations. However, they exhibit allopatric geographical distributions in East Asia. To clarify this ecological discrepancy, glass eels from estuaries in Taiwan, the Philippines, Indonesia, and China were collected monthly, and the survival rate of A. marmorata under varying water salinities and temperatures was examined. The composition ratio of these 2 eel species showed a significant latitude cline, matching the 24°C sea surface temperature isotherm in winter. Both species had opposing temperature preferences for recruitment. A. marmorata prefer high water temperatures and die at low water temperatures. In contrast, A. japonica can endure low water temperatures, but their recruitment is inhibited by high water temperatures. Thus, A. japonica glass eels, which mainly spawn in summer, are preferably recruited to Taiwan, China, Korea, and Japan by the Kuroshio and its branch waters in winter. Meanwhile, A. marmorata glass eels, which spawn throughout the year, are mostly screened out in East Asia in areas with low-temperature coastal waters in winter. During summer, the strong northward currents from the South China Sea and Changjiang River discharge markedly block the Kuroshio invasion and thus restrict the approach of A. marmorata glass eels to the coasts of China and Korea. The differences in the preferences of the recruitment temperature for glass eels combined with the availability of oceanic currents shape the real geographic distribution of Anguilla japonica and Anguilla marmorata, making them “temperate” and “tropical” eels, respectively.

Seasonal Changes in Species Composition of Glass Eels of the Genus Anguilla (Teleostei: Anguillidae) Recruiting to the Cagayan River, Luzon Island, the Philippines

Abstract: Tropical species of anguillid glass eels have recently been exploited to be a substitute for the commercially important Anguilla japonica or A. anguilla, so information about their life histories is needed to facilitate conservation. To understand ecological aspects of glass eel recruitment patterns of tropical eels, Anguilla spp., monthly monitoring of species compositions at the mouth of the Cagayan River in northern Luzon in the Philippines was carried out at different time periods from May 2008 to September 2009 and from November 2011 to December 2012. Species identifications were made for 32,178 glass eels using morphology and/or genetics, and a total of five anguillid species, Anguilla bicolor pacifica, A. celebesensis, A. japonica, A. luzonensis, and A. marmorata, were found. In both time periods A. marmorata was dominant followed by A. luzonensis and A. bicolor pacifica, but the proportion of A. marmorata decreased from 55.1% in 2008–2009 to 41.2% in 2011–2012, with A. bicolor pacifica increasing from 3.9% to 26.3% and A. luzonensis staying relatively stable (41.0%, 32.5%). Two other species, A. japonica and A. celebesensis, that were reported in previous studies from the Cagayan River were extremely rare. This study indicated that selective glass eel catch of a commercially preferred tropical species in this area is not feasible, because the harvest will include a high proportion of bycatch of economically less-important species, potentially having considerable impacts on their populations.

De Novo Assembly of the Whole Transcriptome of the Wild Embryo, Preleptocephalus, Leptocephalus, and Glass Eel of Anguilla japonica and Deciphering the Digestive and Absorptive Capacities during Early Development.

Natural stocks of Japanese eel (Anguilla japonica) have decreased drastically because of overfishing, habitat destruction, and changes in the ocean environment over the past few decades. However, to date, artificial mass production of glass eels is far from reality because of the lack of appropriate feed for the eel larvae. In this study, wild glass eel, leptocephali, preleptocephali, and embryos were collected to conduct RNA-seq. Approximately 279 million reads were generated and assembled into 224,043 transcripts. The transcript levels of genes coding for digestive enzymes and nutrient transporters were investigated to estimate the capacities for nutrient digestion and absorption during early development. The results showed that the transcript levels of protein digestion enzymes were higher than those of carbohydrate and lipid digestion enzymes in the preleptocephali and leptocephali, and the transcript levels of amino acid transporters were also higher than those of glucose and fructose transporters and the cholesterol transporter. In addition, the transcript levels of glucose and fructose transporters were significantly raising in the leptocephali. Moreover, the transcript levels of protein, carbohydrate, and lipid digestion enzymes were balanced in the glass eel, but the transcript levels of amino acid transporters were higher than those of glucose and cholesterol transporters. These findings implied that preleptocephali and leptocephali prefer high-protein food, and the nutritional requirements of monosaccharides and lipids for the eel larvae vary with growth. An online database (http://molas.iis.sinica.edu.tw/jpeel/) that will provide the sequences and the annotated results of assembled transcripts was established for the eel research community.

Rapid evolution of Anguillicola crassus in Europe: species diagnostic traits are plastic and evolutionarily labile

BackgroundSince its introduction from Taiwan to Europe around 1980, Anguillicola crassus, a natural parasite of the Japanese eel (Anguilla japonica), has acquired the European eel (Anguilla anguilla) as a novel definitive host. In this host the nematode differs noticeably in its body mass and reproductive capacity from its Asian conspecifics. We conducted a common garden experiment under a reciprocal transplant design to investigate whether differences in species-diagnostic morphological traits exist between two European and one Asian population of A. crassus and if yes whether these have a genetically fixed component.ResultsWe found that worms from Germany, Poland and Taiwan differ in the size and shape of their body, oesophagus and buccal capsule. These changes are induced by both phenotypic plasticity and genetic divergence: in the European eel, nematodes from Europe as well as from Taiwan responded plastically with larger body and oesophagus dimensions compared to infections in the Japanese eel. Interestingly, the oesophagus simultaneously shows a high degree of genetically based changes being largest in the Polish strain kept in A. anguilla. In addition, the size and shape of the buccal capsule has undergone a rapid evolutionary change. Polish nematodes evolved a genetically fixed larger buccal capsule than the German and Taiwanese populations. The German strain had the smallest buccal capsule.ConclusionsThis study provides evidence for the genetic divergence of morphological traits in A. crassus which evolved over a timescale of about 30 years. Within Europe and in the European eel host these alternations affect characters used as diagnostic markers for species differentiation. Thus we provide an explanation of the discrepancy between morphological and molecular features reported for the parasitic nematode featured here, demanding general caution in morphological diagnosis of parasites discovered in new hosts.

Effects of metamorphosis timing and the larval growth rate on the latitudinal distribution of sympatric freshwater eels, Anguilla japonica and A. marmorata, in the western North Pacific

BackgroundEarly life history traits of the temperate eel Anguilla japonica and tropical eel Anguilla marmorata were examined to determine the possible reason why these two species have similar spawning areas and oceanic larval transport in the North Equatorial Current and yet are recruited to different but partly overlapping continental growth habitats in northern East Asia. To understand the segregative migration of these two sympatric eel species, their glass eels were collected from nine estuaries in the Philippines, Taiwan, Japan, and China. The age at metamorphosis from leptocephalus to glass eel (Tm), the age at estuarine arrival (Tt), the time between metamorphosis and estuarine arrival (Tt−m), and the growth rate (Gt) of glass eels were calculated from daily growth increments in their otoliths.ResultsResults indicated that the Gt was faster and the Tm was younger in A. marmorata than in A. japonica. On the other hand, fish length and the Tt at estuarine arrival were larger in A. japonica than in A. marmorata, indicating that elvers of A. japonica experience a longer oceanic drift than those of A. marmorata. In addition, the Tt−m also indicated that A. japonica experienced a longer coastal migration than A. marmorata.ConclusionThis study validated that the Tm and Gt seem to play important roles in the segregative migration and latitudinal distribution of these two sympatric freshwater eel species in the northwestern Pacific.

Validation of the Occurrence of Short-finned Eel Anguilla bicolor pacifica in Natural Waters of Taiwan

Previous studies indicated that elvers of the short-finned eel Anguilla bicolor pacifica Schmidt occurred in the estuaries of Taiwan. However, no studies have validated that juveniles and adults exist in Taiwans natural waters. We report that six short-finned eels, ranging 32-68 cm in total length, were collected from the Kaoping and Tungkang Rivers of southern Taiwan during May 1998 to July 1999. Polymerase chain reaction (PCR) amplification and sequencing of the mitochondrial cytochrome b gene (mtCyt-b) confirmed this species to be A. bicolor pacifica. Accordingly, A. bicolor pacifica is a native eel species of Taiwan.