Predation on glass eels of Japanese eel <Emphasis Type="Italic">Anguilla japonica</Emphasis> in the Tone River Estuary,Japan

Aquaculture of Japanese eel Anguilla japonica relies upon the natural recruitment of their glass eels (juveniles); however, predation that could influence glass eel recruitment remains unknown. In the present study, we aimed to elucidate the proportion of predation on A. japonica glass eels through stomach content analysis of predatory fishes collected in the estuary region of the Tone River system and its vicinity in Japan. Species of the predated glass eels were identified by DNA barcoding. A total of 270 predatory fishes of 15 taxa was collected over 2 years. The overall proportion of predation on glass eels, genetically identified as Japanese eel, was 0.7%, but this rose to 2.0% when data were limited to fishes caught during the peak months of glass eel recruitment. A glass eel was found in the stomach contents of a channel catfish Ictalurus punctatus, an invasive species in this river system, and a blackfin sea bass Lateolabrax latus. These fishes are therefore considered potential predators of A. japonica glass eels. However, as the proportion of predation was low, and the glass eels represented only small proportions of predator stomach contents, further investigation is needed for a better understanding of predation on A. japonica glass eels, and its effects on the early life stages of this endangered species.     

Rapid identification of eels <Emphasis Type="Italic">Anguilla japonica</Emphasis> and <Emphasis Type="Italic">Anguilla anguilla</Emphasis> by polymerase chain reaction with single nucleotide polymorphism-based specific probes

ABSTRACT:   Standard molecular techniques, such as sequencing and restriction fragment length polymorphism analysis after polymerase chain reaction (PCR) amplification are relatively complicated, and species identification can take a long time when using such techniques. We established a quick method, using PCR with species-specific TaqMan Minor Groove Binder (MGB) probes based on single nucleotide polymorphism (SNP) to distinguish the two eel species Anguilla japonica and Anguilla anguilla . This method can be used in processed products. Partial sequences of the mitochondrial 16S rRNA gene were compared between A. japonica and A. anguilla to design a primer pair common to both A. japonica and A. anguilla and probes specific to A. japonica and A. anguilla . Different fluorescence intensities were produced in two PCR microtubes each containing A. japonica - and A. anguilla -specific probes for one target sample. We observed the fluorescence intensity of PCR products in microtubes under ultraviolet transillumination, with similar results to those obtained by real-time PCR. Therefore, SNP-based PCR is a powerful tool for identifying materials of processed foods from either A. japonica or A. anguilla .     

Occurrence of estuarine and sea eels <Emphasis Type="Italic">Anguilla japonica</Emphasis> and a migrating silver eel <Emphasis Type="Italic">Anguilla anguilla</Emphasis> in the Tokyo Bay area,Japan

Otolith microchemical analyses of the strontium (Sr) and calcium (Ca) concentrations in the eels Anguilla japonica and A. anguilla caught in Tokyo Bay were undertaken to reconstruct the eels’ migratory histories. A. japonica in the yellow stage (immature stage) were caught in a bay without any adjacent rivers or streams. A. anguilla was in the silver stage (early maturing stage), and the eel was confirmed to have just begun spawning migration to the open ocean from Tokyo Bay based on the otolith Sr:Ca ratios, which showed a typical catadromous life history with low Sr:Ca ratio values throughout the eel’s life after recruitment. The mean Sr:Ca ratios in A. japonica from the elver mark to the otolith edge indicated the eels belonged to several general categories of migratory histories, including sea eels (average Sr:Ca ratio ≥6.0 × 10⁻³) and estuarine eels (average Sr:Ca ratio 2.5 to 6.0 × 10⁻³) based on the criteria reported previously in A. japonica. All eels had a certain freshwater life period, although the period was highly variable among fish. These results indicate that A. japonica has a flexible pattern of migration, with the ability to adapt to various habitats and salinities.     

Enhanced local recruitment of glass eel <Emphasis Type="Italic">Anguilla japonica</Emphasis> in Oyodo River,Miyazaki and offshore environmental conditions in 2002

To examine the environmental factors controlling the inshore recruitment dynamics of Anguilla japonica in the Oyodo River, Miyazaki Prefecture, Kyushu, glass eel samplings were carried out using fyke nets during winter (November–March) of 1994–2014. The peak CPUEs (catch per unit effort) were observed between November and February, but differed from year to year. The yearly CPUE was extremely high in 2002, when the sea surface temperature (SST) in the offshore area of the Oyodo River was the lowest in winter of all the sampling years. The negative SST anomaly of less than ? 0.5 °C was sustained in the offshore area during the recruitment season in 2002, which was caused by two combined factors; low air temperature and the Kuroshio path. The oceanographic data showed that the dominant path of the Kuroshio was displaced eastward at 31°N in 2002, which was different from the average Kuroshio path. The eastward displacement of the Kuroshio induced a cyclonic mesoscale eddy in the offshore area of the Oyodo River, resulting in the entrainment of the cold seawater into coastal waters from deep water. The oceanographic condition in relation to the continuous low SST could be favorable for local recruitment of glass eels.     

Anosmic Japanese eel <Emphasis Type="Italic">Anguilla japonica</Emphasis> can no longer detect magnetic fields

ABSTRACT:   Magnetosensitivity in anosmic Japanese eel Anguilla japonica was examined by conditioning and electrocardiography. Five eels from a freshwater farm in Kagoshima, Japan, were made anosmic by injection of boiling petroleum jelly (130–140°C) into the nasal cavity, a technique widely used in studies on olfaction and olfaction-related behavior of eels. Another five eels were not injected with boiling petroleum jelly and served as controls. Conditioning consisted of exposure of eels to an imposed magnetic field of 192 473 nT. After complete conditioning of the eels, the imposed magnetic field was reduced from 192 473 nT to 12 663 nT during the tests. The control eels showed a significant conditioned response (i.e. slowing of the heart beat) to the magnetic field after one set of conditioning runs. The anosmic eels did not respond to the magnetic field, presumably due to damage of the olfactory organ from the jelly injection. The results suggest that Japanese eels have a magnetosensitive organ in or around the nares. At 1 month after the anosmia treatment, there was no visible trauma to the olfactory lamellae of anosmic eels.     

Effects of silvering state on induced maturation and spawning in wild female Japanese eel <Emphasis Type="Italic">Anguilla japonica</Emphasis>

ABSTRACT:   The effects of silvering state of wild female Japanese eels Anguilla japonica on the success of induced maturation and the following spawning were examined. Thirty-eight females, collected in Mikawa Bay, were divided into four stages based on their silvering state: yellow (Y1), late-yellow (Y2), silver (S1) and late silver eels (S2). Despite injections of salmon pituitary extract (SPE) through the standard technique, Y1 and Y2 eels did not respond to the treatment with undeveloped gonad (gonad-somatic index [GSI]: 0.3–0.9), and all these females died by 5 weeks, probably due to an abnormal physiological condition. Most S1 (81%) and S2 eels (100%) matured completely (GSI: 17.8–51.4), and finally spawned successfully (69% for S1, 89% for S2). S2 eels fully matured with oocytes of over 750 μm in diameter by significantly smaller number of injections of SPE (5–6 times) than the case of S1 eels (6–8 times). The amount of eggs released by S2 eels (0.65 ± 0.11 g/fish per body weight [BW]) was significantly larger than those by S1 eels (0.54 ± 0.09 g/fish per BW). There was no difference in fertilization and hatching rates between eggs released by S1 eels and those of S2 eels. These results indicate that the success of induced maturation and spawning in wild female Japanese eels depends on their silvering state, and matured eggs can be obtained efficiently through the use of S2 eels rather than other stages.     

Biomass fluctuation of two dominant lanternfish <Emphasis Type="Italic">Diaphus</Emphasis><Emphasis Type="Italic">garmani</Emphasis> and <Emphasis Type="Italic">D. chrysorhynchus</Emphasis> with environmental changes in the East China Sea

Acoustic surveys have been conducted for estimating the biomass of commercially important fish (e.g., anchovy, jack mackerel), lanternfish (Diaphus garmani and D. chrysorhynchus), and pearlside (Maurolicus japonicus) in summer in the East China Sea (ECS) since 1997. The biomass of lanternfish and pearlside was 2.26–19.16 times that of commercially important fish, and these species represented substantial biomass in the ECS. Though there were no correlations between biomass of pearlside and environmental indices, significant correlations between biomass of lanternfish and southern oscillation index (SOI) in March (positive correlation), arctic oscillation (AO) in March (negative) and October (positive), monsoon index (MOI) in February (positive), and Kuroshio flow mass in winter (positive) were observed. Weak AO and strong MOI would cool down the sea temperature and would lead to increased primary and secondary production in the ECS, thereby enhancing larval survival of lanternfish. The SOI would affect the Kuroshio meander in the ECS, and strong SOI and Kuroshio flow mass would transport larvae of lanternfish to the present survey area. This is the first report on the lanternfish standing stock and its fluctuation in the ECS.     

Spatial learning-induced <Emphasis Type="Italic">egr</Emphasis>-<Emphasis Type="Italic">1</Emphasis> expression in telencephalon of gold fish <Emphasis Type="Italic">Carassius auratus</Emphasis>

The immediate-early gene (egr-1) expression was used to examine the neuron’s response in telencephalon of goldfish during spatial learning in small space. Fishes were pre-exposed in the experimental apparatus and trained to pick food from the tray in a rectangular-shaped arena. The apparatus was divided into identical compartments comprising three gates to provide different spatial tasks. After the fish learned to pass through the gate one, two more gates were introduced one by one. Fish made more number of attempts and took longer time (P < 0.05) to pass through the first gate than the gate two or three. This active learning induces the expression of egr-1 in telencephalon as established by western blot analysis. Subsequently, the fish learn quickly to cross the similar type of second and third gate and make fewer errors with a corresponding decline in the level of egr-1 expression. As the fish learned to pass through all the three gates, third gate was replaced by modified gate three. Interestingly, the level of egr-1 expression increased again, when the fish exhibit a high exploratory behavior to cross the modified gate three. The present study shows that egr-1 expression is induced in the telencephalon of goldfish while intensively acquiring geometric spatial information to pass through the gates.     

Effects of potential probiotic <Emphasis Type="Italic">Lactococcus lactis</Emphasis> subsp. <Emphasis Type="Italic">lactis</Emphasis> on digestive enzymatic activities of live feed <Emphasis Type="Italic">Artemia franciscana</Emphasis>

In the present study, cellulase, protease, lipase and amylase activities were performed to investigate the effects of Lactococcus lactis subsp. lactis CF4MRS bioencapsulation of Artemia franciscana. Our results show that cellulase activities (total cellulase—FPase activity, exoglucanase and endoglucanase—CMCase activity) were significantly higher (P < 0.05) in A. franciscana tissue homogenates compared to those in the control group after 8 h of L. lactic bioencapsulation. Notably, an exception case was found in β-D-glucosidase activity, whereby the cellulase activity was not significantly different (P > 0.05) compared to the control group. Administrations of L. lactis at cell concentration of 10⁸ CFU mL^?1 showed considerable improvement of other important enzymatic activities such as amylase, protease and lipase in A. franciscana. The amylase/protease ratio in probiotic-treated A. franciscana was recorded at 0.343, approximately two times higher than those without probiotic administration (0.184). In contrary, amylase/lipase ratio showed half of a reduction (0.330) in L. lactis-administrated A. franciscana compared to the control (0.614). Our study suggests that important digestive enzymes, e.g., cellulase, amylase, protease and lipase, can be enhanced through bioencapsulation of A. franciscana with L. lactis subsp. lactis, which could in turn lead to further stimulation of endogenous enzymes in the fish and shrimp larvae.     

Characterization of <Emphasis Type="Italic">kiss2</Emphasis> and <Emphasis Type="Italic">kissr2</Emphasis> genes and the regulation of kisspeptin on the HPG axis in <Emphasis Type="Italic">Cynoglossus semilaevis</Emphasis>

Reproduction allows organisms to produce offspring. Animals shift from immature juveniles into mature adults and become capable of sexual reproduction during puberty, which culminates in the first spermiation and sperm hydration or ovulation. Reproduction is closely related to the precise control of the hypothalamic–pituitary–gonadal (HPG) axis. Kisspeptin peptides are considered as the important regulator of HPG axis in mammalian. However, the current understanding of kisspeptin in flatfish is not comprehensive. In this study, we cloned and analyzed the kiss2 and kissr2 genes in Cynoglossus semilaevis. Interesting alternative splicing in the 5′-untranslated regions (UTR) of the Cskissr2 gene was found. The expression profiles of Cskiss2 and Cskissr2 showed relative high messenger RNA (mRNA) levels at the late gastrula stage during embryonic development, at total length = 40 mm during early gonadal differentiation, and in the brains and gonads of all investigated tissues. These results suggested that the kisspeptin system participated in embryogenesis and in the regulation of gonadal differentiation and development. Considering that the control and regulatory mechanisms of kisspeptin in the central reproductive axis are still unclear, we documented that the intramuscular injection of kisspeptin caused different sGnRH and cGnRH mRNA levels in a dose- and tissue-dependent manner. The mRNA expressions of FSH and LH were stimulated in the ovary and were inhibited in the testis under the kisspeptin treatments. These results provided foundations for understanding the roles of kisspeptin in the neuroendocrine system in fish. The manipulation of the kisspeptin system may provide new opportunities to control the gonadal development and even reproduction in fish.     

<Emphasis Type="Italic">Streptococcus agalactiae</Emphasis> from tilapia (<Emphasis Type="Italic">Oreochromis</Emphasis> sp.) transmitted to a new host,bighead carp (<Emphasis Type="Italic">Aristichthys nobilis</Emphasis>), in China

Several outbreaks of Streptococcus agalactiae infection of bighead carp (Aristichthys nobilis) were observed in China. The molecular epidemiology and pathogenicity of S. agalactiae in bighead carp and tilapia (Oreochromis sp.) is poorly understood. In the present study, we identified S. agalactiae strains isolated from diseased bighead carp using the API 20 Strep kit and 16S rDNA sequencing and determined whether these strains came from tilapia. Of the 46 identified S. agalactiae strains, 24 strains were successfully isolated from diseased bighead carps, 20 S. agalactiae strains were isolated from tilapia, and two S. agalactiae strains were isolated from tiger frog (Hoplobatrachus chinensis). The results of molecular typing, including multilocus sequence typing, molecular serotyping, surface protein gene detection, and virulence-related gene detection showed that the 44 strains from bighead carp and tilapia were highly similar, whereas different from tiger frog GBS strains. Remarkably, the bighead carp strain Hn1404 showed high virulence in bighead carp and zebrafish. Moreover, this strain was pathogenic to Nile tilapia (Oreochromis niloticus). In addition, comparative genomic analysis showed that isolate Hn1404 had a close relationship with the bighead carp and tilapia S. agalactiae strains. All the analyses of the genetic characteristics of bighead carp and tilapia strains showed that tilapia S. agalactiae strains could be transmitted to other fish species such as bighead carp.     

Immunological effects of glucan and <Emphasis Type="Italic">Lactobacillus rhamnosus</Emphasis> GG,a probiotic bacterium,on Nile tilapia <Emphasis Type="Italic">Oreochromis niloticus</Emphasis> intestine with oral <Emphasis Type="Italic">Aeromonas</Emphasis> challenges

The present study histologically examined the effects of glucan-containing and Lactobacillus rhamnosus GG (LGG)-containing diets on intestinal damages inflicted on Nile tilapia by Aeromonas challenges. Tilapia were fed control, glucan, and LGG diets for 2 weeks and were subsequently challenged with Aeromonas. The intestines were then histologically examined at 1, 7, 14, and 21 days post-infection. Mortality following the challenge was lower for the fish fed the glucan and LGG diets. The intestines of these groups also showed increased inflammatory cell infiltration and reduced intestinal damage from Aeromonas. Moreover, inflammatory cell infiltration occurred more rapidly in the glucan-fed than in the LGG-fed fish following the challenge. Before the challenge, the dominant mucous cell was the acid type in all the tests. After the challenge, the main mucus cell type in the proximal intestine of the glucan-fed fish shifted to AB-PAS double-staining cells, while in the LGG-fed fish, it remained the acid type throughout the test period, and the number of double-staining cells was smaller than in the control fish after the challenge. Thus, the different mucous cell and inflammatory cell responses show that glucan and LGG might have different immunostimulative effects, although they both reduced the intestinal damage following Aeromonas challenges.     

Influence of the dietary inclusion of <Emphasis Type="Italic">Gracilaria cornea</Emphasis> and <Emphasis Type="Italic">Ulva rigida</Emphasis> on the biodiversity of the intestinal microbiota of <Emphasis Type="Italic">Sparus aurata</Emphasis> juveniles

Partial substitutions of fish meal by 5, 15, or 25 % of Gracilaria cornea or Ulva rigida in experimental diets were evaluated to study their effects on biodiversity of intestinal microbiota composition in gilthead seabream (Sparus aurata) juveniles. The diets were offered to duplicate groups of 15 juvenile fish (14.0 ± 0.5 g) for 70 days, and at the end of the experiment the intestinal microbiota from four specimens of each treatment was analysed by denaturing gel gradient electrophoresis. Results showed that the substitution of fish meal by algae meal induced important modifications in the intestinal microbiota community, as a big reduction of the biodiversity when the highest percentage (25 %) of U. rigida was included. On the contrary, an increase on the number of species was detected when a 15 % of algae was included. Various Lactobacillus delbrueckii subspecies were selectively stimulated when G. cornea was included in the feed, and other bacterial species, such as those included in the Vibrio genus, were reduced.     

Influence of temperature and feeding regimes on growth and notochord deformity in reared <Emphasis Type="Italic">Anguilla japonica</Emphasis> leptocephali

The high mortality rate of reared Japanese eel Anguilla japonica larvae is largely due to lower growth rate and the higher rate of deformed larvae. To establish an effective rearing protocol for this species, we examined the effects of water temperature and feeding regimes on their growth and notochord kyphosis. Larvae at 165 days post hatching were reared for 28 days at mean temperatures of 24, 25 and 27 °C, and were fed 4 or 6 times per day. Larval growth rate was significantly higher in larvae reared at 24–25 °C and fed 6 times per day. However, growth rate was significantly reduced at 27 °C, suggesting a shortage of metabolic energy due to an elevated cost of the higher basal metabolic rate at higher temperatures and low nutritional performance of currently used artificial diet. Notochord kyphosis was promoted by elevated water temperature, and two-way ANOVA showed that water temperature and feeding frequency had combined effects on the deformity. These findings suggest the importance of concurrently manipulating both environmental and nutritional factors to produce healthy eel larvae in captivity.