Aureispira anguillae sp. nov., isolated from Japanese eel Anguilla japonica leptocephali

A novel filamentous eel-leptocephalus pathogenic marine bacterium, designated strain EL160426T, was isolated from Japanese eel, Anguilla japonica, leptocephali reared at a laboratory in Mie, Japan. In experimental infection studies on eel larvae, the strain EL160426T caused massive larval mortality and was reisolated from moribund leptocephali. Characteristically, observations of infected larvae found that EL160426T forms columnar colonies on the cranial surface of larvae. The novel isolate exhibited growth at 15-30 °C, pH 7-9, and seawater concentrations of 60-150% (W/V). Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain EL160426T was most closely related to Aureispira maritima 59SAT with 97.7% sequence similarity. The whole genome sequence analysis of the strain EL160426T showed that the strain maintained a circular chromosome with a size of approximately 7.58 Mbp and the DNA G + C content was 36.2%. The major respiratory quinone was MK-7 and the predominant cellular fatty acids were 16:0, 20:4 w6c (arachidonic acid), 17:0 iso and 16:0 N alcohol. DNA relatedness between the closest phylogenetic neighbor strain EL160426T and A. maritima (JCM23207T) was less than 13%. On the basis of the polyphasic taxonomic data, the strain represents a novel species of the genus Aureispira, for which the name Aureispira anguillae sp. nov. is proposed. The type strain is EL160426T (= JCM 35024 T = TSD-286 T).

Anguillid Eels as a Model Species for Understanding Endocrinological Influences on the Onset of Spawning Migration of Fishes

Simple Summary

Endocrine regulation has been thought to play a major role in the onset of migration. Anguillid eels provide a good model for studying the onset mechanisms of migrations to breeding areas, because the process of the onset of migration occurs in inland waters. In this review, we summarize information about the silvering process in anguillid eels and the dynamics of mRNA expression of neurohormones and pituitary hormones, thyroid hormones, and sex steroids associated with the onset of the spawning migration. We also provide new results. Because 11-KT drastically increases during silvering, the role of 11-KT in the onset of spawning migration was discussed in detail.

Abstract

Anguillid eels are the iconic example of catadromous fishes, because of their long-distance offshore spawning migrations. They are also a good model for research on the onset mechanisms of migrations to breeding areas, because the migrations begin in inland waters. When eels transform from yellow eels to silver eels, it is called silvering. Silver eels show various synchronous external and internal changes during silvering, that include coloration changes, eye-size increases, and gonadal development, which appear to be pre-adaptations to the oceanic environment and for reproductive maturation. A strong gonadotropic axis activation occurs during silvering, whereas somatotropic and thyrotropic axes are not activated. Among various hormones, 11-ketotestosterone (11-KT) drastically increases during spawning migration onset. Gradual water temperature decreases simulating the autumn migratory season, inducing 11-KT increases. Administration of 11-KT appeared to cause changes related to silvering, such as early-stage oocyte growth and eye enlargement. Moreover, 11-KT may be an endogenous factor that elevates the migratory drive needed for the spawning migration onset. These findings suggested that water temperature decreases cause 11-KT to increase in autumn and this induces silvering and increases migratory drive. In addition, we newly report that 11-KT is associated with a corticotropin-releasing hormone that influences migratory behavior of salmonids. This evidence that 11-KT might be among the most important factors in the spawning migration onset of anguillid eels can help provide useful knowledge for understanding endocrinological mechanisms of the initiation of spawning migrations.

Production of recombinant Japanese eel (Anguilla japonica) growth hormones and their effects on early-stage larvae

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.

Developmental features of Japanese eels, Anguilla japonica, from the late leptocephalus to the yellow eel stages: an early metamorphosis to the eel-like form and a prolonged transition to the juvenile

Organogenesis of Japanese eels (Anguilla japonica) was investigated histologically from the late leptocephalus to the yellow eel stages. Early organogenesis, such as the formation of inner ears and the appearance of round blood cells that might be larval erythrocytes, had already begun at the late leptocephalus stage. During the first developmental phase (M1-M3 stages) of metamorphosing into early glass eels (G1 stage), the formation of gills and lateral muscles progressed conspicuously with a drastic body shape change from leaf-like to eel-like. In contrast, obvious regression in oesophageal muscle and pancreas occurred during metamorphosis. Formation of lateral line canals advanced continuously until the yellow eel stage. When the second developmental phase was initiated at the G1 stage, cone photoreceptor cells appeared, and the formation of oesophageal, stomach and intestinal muscles was initiated. Differentiation of gastric glands began at 1 week after metamorphosis. Erythrocytes increased continuously in density in glass eels and elvers (G1-E2 stages), and the morphological features of cone cells and olfactory epidermal cells became clearer with stage progression. In early elvers (E1 stage), the swimbladder initiated inflation, the stomach fully expanded and the rectal longitudinal fold changed to a circle. Swimbladder gas glands appeared in late elvers (E2 stage). In the yellow eels (juvenile stage), almost all organ structures were formed. These observations indicate that the organogenesis of A. japonica is ongoing after metamorphosis into glass eels, and the M1-E2 stages are considered to be a homologous phase to first metamorphosis, which is a transformation from the larval to the juvenile stages in other teleosts. In comparison to conger eels, the completion of the body shape change to eel-like occurs at the G1 stage, when organogenesis is still in progress, being followed by a prolonged duration of the G1-E2 stages before reaching the yellow eel juvenile stage, which may be a unique characteristic that is related to the early migratory life history of A. japonica.

A Y-linked anti-Müllerian hormone type-II receptor is the sex-determining gene in ayu, Plecoglossus altivelis

Whole-genome duplication and genome compaction are thought to have played important roles in teleost fish evolution. Ayu (or sweetfish), Plecoglossus altivelis, belongs to the superorder Stomiati, order Osmeriformes. Stomiati is phylogenetically classified as sister taxa of Neoteleostei. Thus, ayu holds an important position in the fish tree of life. Although ayu is economically important for the food industry and recreational fishing in Japan, few genomic resources are available for this species. To address this problem, we produced a draft genome sequence of ayu by whole-genome shotgun sequencing and constructed linkage maps using a genotyping-by-sequencing approach. Syntenic analyses of ayu and other teleost fish provided information about chromosomal rearrangements during the divergence of Stomiati, Protacanthopterygii and Neoteleostei. The size of the ayu genome indicates that genome compaction occurred after the divergence of the family Osmeridae. Ayu has an XX/XY sex-determination system for which we identified sex-associated loci by a genome-wide association study by genotyping-by-sequencing and whole-genome resequencing using wild populations. Genome-wide association mapping using wild ayu populations revealed three sex-linked scaffolds (total, 2.03 Mb). Comparison of whole-genome resequencing mapping coverage between males and females identified male-specific regions in sex-linked scaffolds. A duplicate copy of the anti-Müllerian hormone type-II receptor gene (amhr2bY) was found within these male-specific regions, distinct from the autosomal copy of amhr2. Expression of the Y-linked amhr2 gene was male-specific in sox9b-positive somatic cells surrounding germ cells in undifferentiated gonads, whereas autosomal amhr2 transcripts were detected in somatic cells in sexually undifferentiated gonads of both genetic males and females. Loss-of-function mutation for amhr2bY induced male to female sex reversal. Taken together with the known role of Amh and Amhr2 in sex differentiation, these results indicate that the paralog of amhr2 on the ayu Y chromosome determines genetic sex, and the male-specific amh-amhr2 pathway is critical for testicular differentiation in ayu.

Reproductive physiological characteristics of tropical Celebes eels Anguilla celebesensis in relation to downstream migration and ovarian development

Downstream-migrating (n = 64) and non-migrating (n = 21) female Celebes eels Anguilla celebesensis were captured from the Poso Lake-River system on Sulawesi Island, Indonesia, and their reproductive physiological characteristics were examined. A histological observation of the ovaries revealed that most non-migrating eels were at the perinucleolus (43%) or oil-droplet (48%) stage, whereas most migrating eels were at the early vitellogenic (36%) or midvitellogenic (61%) stage. Transcript levels of gonadotropin genes (fshb, lhb) in the pituitary gland and concentrations of sex steroids [11-ketotestosterone (11-KT), testosterone, 17β-oestradiol (E₂ )] in blood plasma of migrating eels were significantly higher than those of non-migrating eels. The fshb messenger (m)RNA levels were lower in perinucleolus and oil-droplet stages and then significantly increased in the early vitellogenic stage. The lhb mRNA levels in vitellogenic-stage eels were significantly higher than those in perinucleolus- and oil-droplet-stage eels. The 11-KT levels of eels at the oil-droplet and vitellogenic stages were significantly higher than those of eels at the perinucleolus stage. The E₂ levels at the vitellogenic stage were significantly higher than those at the perinucleolus and oil-droplet stages. These dynamics of the reproductive hormones represented the physiological background of oogenesis in A. celebesensis that has remarkably well-developed oocytes just before downstream migration.

Growth potential can affect timing of maturity in a long-lived semelparous fish

Many diadromous fishes such as salmon and eels that move between freshwater and the ocean have evolved semelparous reproductive strategies, but both groups display considerable plasticity in characteristics. Factors such as population density and growth, predation risk or reproduction cost have been found to influence timing of maturation. We investigated the relationship between female size at maturity and individual growth trajectories of the long-lived semelparous European eel, Anguilla anguilla A Bayesian model was applied to 338 individual growth trajectories of maturing migration-stage female silver eels from France, Ireland, the Netherlands and Hungary. The results clearly showed that when growth rates declined, the onset of maturation was triggered, and the eels left their growth habitats and migrated to the spawning area. Therefore, female eels tended to attain larger body size when the growth conditions were good enough to risk spending extra time in their growth habitats. This flexible maturation strategy is likely related to the ability to use diverse habitats with widely ranging growth and survival potentials in the catadromous life-history across its wide species range.

Genetic parameters and quantitative trait loci analysis associated with body size and timing at metamorphosis into glass eels in captive-bred Japanese eels (Anguilla japonica)

The Japanese eel (Anguilla japonica) is among the most important aquaculture fish species in Eastern Asia. The present study aimed to identify the genetic parameters underlying body size and the timing at metamorphosis from leptocephali to glass eels in captive-bred Japanese eels, with the intent to foster sustainable development. Larvae from a partly factorial cross (14 sires × 11 dams) were reared until the point of metamorphosis into glass eels. In these organisms, we observed moderate heritability and mild genetic correlations among traits related to body size (h² = 0.16–0.33) and timing at metamorphosis (h² = 0.36–0.41). In an F₁ full-sib family, quantitative trait loci (QTL) mapping for these traits identified one significant (genome-wide P < 0.05) and five suggestive QTLs (chromosome-wide P < 0.05). These results suggest that in the Japanese eel, metamorphic traits exhibit a polygenic genetic structure comprising many QTLs with small effects. In addition, we updated the genetic linkage map for the Japanese eel and integrated it with our newly constructed de novo genome assembly. The information and tools generated from this study will contribute to the development of freshwater eel genetics and genomics.

Migratory Restlessness and the Role of Androgen for Increasing Behavioral Drive in the Spawning Migration of the Japanese eel

Migratory restlessness refers to a type of locomotor activity observed just before the onset of a migration. This behavior is primarily known in birds, where it is considered to be an indicator of the urge for migration. In contrast, little is known about migratory restlessness in fishes. To confirm migratory restlessness in a fish, we measured the locomotor activity of the Japanese eel, Anguilla japonica during its migration season. Migratory-phase silver eels showed higher locomotor activity in aquaria than yellow eels at the non-migratiory growth-phase. Silver eels stayed outside of their shelters for longer durations in dark periods than yellow eels and were active even in light periods when yellow eels were inactive in the shelters. Silver eels had higher levels of the androgen hormone 11-ketotestosterone at the end of experiment than yellow eels. Administration of 11-ketotesosterone to yellow eels induced higher levels of locomotor activity than that observed in non-treated controls. These findings suggest that anguillid eels exhibit migratory restlessness just before their spawning migration and that 11-ketotestosterone may be involved in the onset of this behavior.

Changes in the role of the thyroid axis during metamorphosis of the Japanese eel, Anguilla japonica

To clarify the role of thyroid function during metamorphosis from leptocephalus to glass eel in the Japanese eel, we examined the histology of the thyroid gland and measured whole-body concentrations of thyroid hormones, thyroxine (T4) and triiodothyronine (T3), and thyroid stimulating hormone β-subunit TSH (TSHβ) mRNA expression levels in five stages of artificially hatched eels (leptocephalus, early-metamorphosis, late-metamorphosis, glass eel, and elver). During metamorphosis, the inner colloid of thyroid follicles showed positive immunoreactivity for T4, and both T4 and T3 levels were significantly increased, whereas a small peak of TSHβ mRNA level was observed at the early-metamorphosis stage. Similarly, TSHβ mRNA levels were highest in the glass eel stage, and then decreased markedly in the elver stage. In contrast to TSHβ mRNA expression, thyroid hormones (both T4 and T3) increased further from the glass eel to elver stages. These results indicated that thyroid function in the Japanese eel was active both during and after metamorphosis. Therefore, the thyrotropic axis may play important roles not only in metamorphosis but also in subsequent inshore or upstream migrations.

Profiles of mRNA expression for prolactin, growth hormone, and somatolactin in Japanese eels, Anguilla japonica: The effect of salinity, silvering and seasonal change

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.

Tertiary treatment of domestic wastewater using zeolite ceramics and aquatic plants

In this study, we examined tertiary treatment of domestic wastewater using zeolite ceramics and aquatic plants, especially reeds, Phragmites australis. The experiment was made at real domestic wastewater treatment facilities, and comparison of treatment performance was made between the method with zeolite ceramics and that with pebble stones as conventional way. SEM observation of the ceramics' surface was also made to examine its possibility as the habitat of bacteria. The results obtained are as follows. Through the tertiary treatment experiment, it was suggested that the water purification system with zeolite ceramics and reeds could keep higher nitrogen removal efficiency for a long time. Zeolite ceramics would be useful when nitrogen compound, NH(4)-N in particular, in the influent was higher. Under SEM observation, bacteria-like objects were observed on the ceramics' surface. Appropriate operation and maintenance would be needed to keep long-term performance of both the NH(4) (+) absorption and nitrogen removal with use of zeolite ceramics.

Real-time control strategy for simultaneous nitrogen and phosphorus removal using aerobic granular sludge

To achieve stable and simultaneous removal of nitrogen and phosphorus using aerobic granular sludge in a sequencing batch reactor, a real-time control strategy was established, where time derivatives of electric conductivity (EC) and pH were monitored to facilitate the determinations of ends of phosphate release, nitrification and denitrification as well as corresponding optimum time-lengths of anaerobic, oxic, and anoxic phases in treatment cycles. Although biomass concentration in a reactor drastically fluctuated at the startup period because of very short sludge settling time for the formation of aerobic granular sludge, cycle length for proper treatment was automatically adjusted in this control system. Even when characteristics of influent wastewater markedly fluctuated, stable nitrogen and phosphorus removal was successfully attained both before and at pseudo-steady-state. Effluent concentrations of NH4-N, NOx-N and PO4-P were always lower than 0.3 mg/L. On the other hand, when time lengths of the anaerobic/oxic/anoxic phases were fixed, stable nitrogen and phosphorus removal was not accomplished. Therefore, it is clear that the designed control system is very effective to obtain stable treatment performance in simultaneous nitrogen and phosphorus removal by aerobic granular sludge.

Automatic control strategy for biological nitrogen removal of low C/N wastewater in a sequencing batch reactor

To establish an automatic control system of external carbon addition in biological nitrogen removal, a bench-scale sequencing batch reactor with real-time control strategy was designed in this study. An oxidation-reduction potential (ORP) profile was used for automatic control of external carbon addition. The mean removal efficiency of total organic carbon was over 98%. Complete denitrification in an anoxic phase and complete denitrification and nitrification in anoxic and oxic phases were accomplished, respectively, because the oxic and anoxic periods were also appropriately controlled with ORP and pH profiles, respectively. Mean removal efficiency of total nitrogen was over 95%. When concentration of influent wastewater was changed, volume of additional carbon was automatically changed with the influent fluctuation, and flexible hydraulic retention time was achieved in this system.

Effect of C/N ratio on nitrous oxide emission from swine wastewater treatment process

To evaluate control parameters for nitrous oxide (N2O) emissions in the swine wastewater treatment process, the N2O emission was compared in the activated sludge from SBRs acclimated in different carbon/nitrogen (C/N) ratios. N2O emission from a denitrification phase was very strongly dependent on C/N ratio of swine wastewater, and the total N2O emission in the operating condition of BOD5/TN ratio of 2.6 was approximately 270 times greater than that in BOD5/TN ratio of 4.5. However, the effect of C/N ratio on N2O emission amount from nitrification was not significant in an oxic phase study. It is considered that stabilization of the C/N ratio through optimal solid-liquid separation of slurry or use of an external carbon source is indispensable for effective N2O emission control from nitrogen removal process of swine wastewater.

Enhanced nitrogen removal using C/N load adjustment and real-time control strategy in sequencing batch reactors for swine wastewater treatment

The laboratory-scale sequencing batch reactor (SBR) was used to study the effectiveness of an integrated strategy of real time control with C/N ratio adjustment for practical swine wastewater treatment. Swine waste was used as the external carbon source for continuous treatment in the SBR reactors. Oxidation-reduction potential and pH were used as parameters to control the continuous denitrification and nitrification process, respectively. A constant effluent quality could be obtained, despite drastic variations in the characteristics of influent wastewater. Also, a relatively complete removal of nutrients was always ensured, since the optimum quantity of the external carbon source could be provided for complete denitrification, and a flexible hydraulic retention time was achieved by the successful real-time control strategy. The average removal efficiencies of total organic carbon and nitrogen were over 94% and 95%, respectively.

Evaluation of absorbability of poorly water-soluble drugs: validity of the use of additives

Apparent membrane permeation coefficients (Papp) of poorly water-soluble drugs such as indomethacin (IDM) and triamterene (TAT) were obtained by the chamber method using an isolated rat intestinal tissue after solubilization of the drugs by additives. For the additives, sodium deoxycholate (DOC), polyethylene glycol 600 (PEG 600), dimethylsulfoxide (DMSO), ethanol (EtOH), propylene glycol (PG), and rat bile were examined. Their concentrations were determined in ranges considered to be appropriate from the results of in vivo experiments and physiological findings. From the correspondence between this membrane permeability and in vivo bioavailability, we evaluated the validity of our in vitro experiment. On the basis of these evaluations, it was shown that 5% DMSO and 10% PEG 600, which did not affect the membrane integrity, were most appropriate additives for chamber experiments. Papp of IDM was greater than that of TAT, indicating that the order corresponded with that of in vivo bioavailability after oral administration of their PEG 600 solutions. Accordingly, it was concluded that Papp obtained by our in vitro system can be used to assess the in vivo bioavailability.

Mucoadhesion of polystyrene nanoparticles having surface hydrophilic polymeric chains in the gastrointestinal tract

The mucoadhesion of polystyrene nanoparticles having surface hydrophilic polymeric chains in the gastrointestinal (GI) tract was investigated in rats. Radiolabeled nanoparticles were synthesized by adding hydrophobic 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine in the final process of nanoparticle preparation. The radioiodonated diazirine seemed to be incorporated in the hydrophobic polystyrene core of nanoparticles. The incorporation rate was less than 10%, irrespective of nanoparticle type. The diazirine incorporated in nanoparticles exhibited little leakage from them even though they were mixed with a solution corresponding to GI juice. The change in blood ionized calcium concentration after oral administration of salmon calcitonin (sCT) with nanoparticles showed that the in vivo enhancement of sCT absorption by radiolabeled nanoparticles was the same as that by non-labeled nanoparticles. The GI transit rates of nanoparticles having surface poly(N-isopropylacrylamide), poly(vinylamine) and poly(methacrylic acid) chains, which can improve sCT absorption, were slower than that of nanoparticles covered by poly(N-vinylacetamide), which does not enhance sCT absorption at all. These slow transit rates were probably the result of mucoadhesion of nanoparticles. The strength of mucoadhesion depended on the structure of the hydrophilic polymeric chains on the nanoparticle surface. The mucoadhesion of poly(N-isopropylacrylamide) nanoparticles, which most strongly enhanced sCT absorption, was stronger than that of ionic nanoparticles, and poly(N-vinylacetamide) nanoparticles probably did not adhere to the GI mucosa. These findings demonstrated that there is a good correlation between mucoadhesion and enhancement of sCT absorption.

Biopharmaceutical evaluation of the liposomes prepared by rehydration of freeze-dried empty liposomes (FDELs) with an aqueous solution of a drug

We have evaluated a method for preparation of a dispersion of liposomes encapsulating a drug, namely rehydration of freeze-dried empty (not containing drug) liposomes with an aqueous drug solution (FDEL method). In the present study, we characterized and compared this method with the conventional method using a lipid composition of DPPC-DPPG-cholesterol in a molar ratio of 27:3:20. Two hydrophilic compounds, [3H]-inulin and [3H]-mannitol, were used as model drugs. Liposomal preparations by the FDEL method had an encapsulation efficiency of 2.9 and 6.7% for [3H]-inulin and [3H]-mannitol, respectively, when rehydrated and incubated at 70 degrees C. Since non-specific adsorption of these markers to liposomal membrane is negligible, this method produces liposomes which encapsulate a drug in the intravesicular space. One-tenth of the marker encapsulated in the liposomes prepared by the FDEL method (F-liposomes) was released very rapidly on incubation with rat plasma, followed by the slow release of the remaining fraction thereafter. No such rapid-release phase was observed for the liposomes prepared by the conventional method (C-liposomes). This suggests the existence of two types of encapsulation, loose encapsulation and tight encapsulation, in F-liposomes at least. Pharmacokinetic parameters of marker encapsulated tightly in F-liposomes were comparable to those in C-liposomes. It is likely that amphipathic drugs such as doxorubicin are incorporated into liposomes more easily than inulin and mannitol when formulated by the FDEL method. These results therefore suggest that the FDEL method is useful in the preparation of a liposomal formulation of a drug.