Novel tissue thickening around the notochord sheath found in deformed Japanese eel Anguilla japonica leptocephali.

Even though reared leptocephalus larvae of the Japanese eel Anguilla japonica have a high incidence of notochord deformities (>60%), the cause is unknown. We performed histological examinations of the notochord and associated organs in reared larvae to better understand the process causing notochord deformation in eel larvae. In deformed larvae, unknown tissue thickening was discovered near the notochord sheath.

Migration Behavior of Silver Eels within their Tomini Bay Spawning Area.

The tropical Celebes eel, , has a short migration between its spawning and growth habitats. Its spawning areas were hypothesized to be in Tomini Bay and the Celebes Sea after collecting their small leptocephali. However, there is no information about the silver eel oceanic spawning migration behavior of .

Preliminary insight into parental contributions to Japanese eel (Anguilla japonica) preleptocephali spawned on different nights.

Parentage sibship-inference analyses were conducted using mtDNA sequencing and six microsatellite genotypes of 182 Japanese eel preleptocephali that were collected from one net-tow near the West Mariana Ridge in May 2014. At least 328 parents were involved in producing the 182 preleptocephali, and several parents may have spawned a few times during 3 days of a spawning period. Half-sibs suggested that a few parents mated with 1-3 partners, indicating that the Japanese eel can form spawning aggregations in which several parents mate with each other in the ocean.

Distribution of anguillid leptocephali and possible spawning areas in the South Pacific Ocean.

Seven South Pacific anguillid eel species live from New Guinea to French Polynesia, but their spawning areas and life histories are mostly unknown despite previous sampling surveys. A July-October 2016 research cruise was conducted to study the spawning areas and times, and larval distributions of South Pacific anguillid eels, which included a short 155°E station-line northeast of New Guinea and five long transects (5-25°S, 160°E-140°W) crossing the South Equatorial (SEC) and other currents. This survey collected nearly 4000 anguilliform leptocephali at 179 stations using an Isaacs-Kidd Midwater Trawl accompanied by 104 CTD casts.

Capacity for freshwater acclimation and differences in the transcription of ion transporter genes underlying different migratory life histories of Takifugu fish.

The genus Takifugu is a group of approximately 20 species of puffer fishes living in a wide range of salinity environments around East Asian countries. This group presents a broad spectrum of evolutionary stages adapted to anadromy as a result of speciation that occurred a short time (2-5 million years) ago on an evolutionary timescale. This group thus can be considered as a model for studying the evolutionary mechanisms of anadromy.

New clues on the Atlantic eels spawning behavior and area: the Mid-Atlantic Ridge hypothesis.

The Sargasso Sea has long been considered as the only spawning area for Atlantic eels, despite the absence of direct observations. The present study raises a novel scenario, deviating from Schmidt's dogma, begins with a review of historical and recent observations that were combined to build up a global theory on spawning ecology and migration behavior of Atlantic eels. From this, it is argued that a favorable spawning area could be located eastward of Sargasso Sea at the intersection between the Mid-Atlantic Ridge and the oceanic fronts.

Morphological, ecological and physiological characteristics of downstream-migrating and non-migrating Pacific bicolor eels Anguilla bicolor pacifica.

This is the first study describing the morphological, ecological and physiological characteristics of two downstream-migrating and two non-migrating female Pacific bicolor eels, Anguilla bicolor pacifica. The total length and age of the downstream-migrating eels were 1005 mm and 10 years and 1110 mm and 11 years old, respectively, and those of the non-migrating eels were 892 mm and 8 years and 805 mm and 9 years, respectively. Silvering-related characteristics (silvering index, eye index, pectoral-fin index, gut-somatic index and swimbladder-somatic index) and reproductive physiological characteristics (gonado-somatic index, follicle diameter, oocyte stage, transcription of gonadotropins and concentration of sex steroids) of the migrating eels were more advanced than those of the non-migrating eels.

Ghrelin and food acquisition in wild and cultured Japanese eel (Anguilla japonica).

To clarify the relationships between growth, endocrine status and habitat characteristics in Japanese eel (Anguilla japonica), plasma and stomach mRNA levels of ghrelin were examined in wild eels captured in the river and the bay, and in cultured eels during and after experimental fasting. Wild juvenile eels captured in freshwater habitats within the river showed significantly higher plasma and stomach mRNA levels of ghrelin than did fish obtained from brackish-water habitats within the bay. In cultured eels experimentally fasted for 4 weeks, plasma and stomach mRNA levels of ghrelin increased.

Estimation of the spawning time of Japanese eels in the open ocean.

To understand the spawning ecology of the Japanese eel, the spawning time of this species was estimated based on measurements of the ascending speed of eggs and previously obtained data. Two types of water temperature parameters were calculated assuming an arbitrary spawning time. The 'incubation temperature' of 53 eggs collected in the spawning area was estimated based on the developmental stage of each egg and experimentally determined relationships between water temperature and incubation duration.

Evaluation of downward movements of Japanese eel Anguilla japonica inhabiting brackish water areas.

This study evaluated the size and age distributions and otolith microchemistry of the Japanese eel Anguilla japonica in freshwater and brackish water areas in the Aki and Tsuchikawa rivers for 1 year, and in brackish water areas in the Asahi River for 3 years to understand the movements of Japanese eels between continental habitats of different salinity after recruitment (n = 759). For all three rivers, the total length (L ) and age distributions were consistent; yellow eels captured in the upper brackish water (Aki River: 353.5 ± 77.

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 (E )] in blood plasma of migrating eels were significantly higher than those of non-migrating eels.

Changes in PRL Gene Expression During Upstream Movement of the Japanese Eel, .

Changes in mRNA levels of prolactin (PRL) during upstream movement were examined in juvenile Japanese eels, . Glass eels and elvers were collected from 2007 to 2009 near the entrance of Hamana Lake, and in a small inflowing stream, the Egawa River. Quantification of mRNA was performed by real-time PCR and expressed as whole-body content.

Freshwater entry behaviour of a non-migratory stenohaline marine fish Takifugu snyderi.

We conducted salinity choice trials with the stenohaline marine species Takifugu snyderi to test their freshwater (FW) entry frequency in relation to starvation. The fish preferred to enter non-natal FW rather than remain in seawater. No relationship was detected between starvation and FW entry behaviour.

Influence of ocean circulation and the Kuroshio large meander on the 2018 Japanese eel recruitment season.

Japanese eel (Anguilla japonica) recruitment to Japan was very low during the early 2017-2018 recruitment season when most glass eels are usually caught, but catches increased in the late recruitment season when recruitment usually decreases. Concurrently, the Kuroshio meander south of Japan had formed again after the previous event ended in 2005. The role of the large meander and ocean circulation features such as the North Equatorial Current (NEC) in the unusual 2017-2018 Japanese eel recruitment timing-pattern was investigated using a three-dimensional particle tracking model that simulated swimming behaviors of virtual larvae (v-larvae) in addition to their drift in ocean currents.

New PCR primers for metabarcoding environmental DNA from freshwater eels, genus Anguilla.

Freshwater eels of the genus Anguilla comprise 16 species that include three subspecies and are characterized by their unique catadromous life cycles. Their life histories and nocturnal life styles make it difficult to observe them in freshwater and marine habitats. To investigate their distribution and ecology in aquatic environments, we developed new PCR primers for metabarcoding environmental DNA (eDNA) from Anguilla.

Release of eDNA by different life history stages and during spawning activities of laboratory-reared Japanese eels for interpretation of oceanic survey data.

To assist in detection of offshore spawning activities of the Japanese eel Anguilla japonica and facilitate interpretation of results of environmental DNA (eDNA) analysis in their spawning area, we examined the eDNA concentration released by each life history stage of artificially reared Japanese eels in the laboratory using quantitative real-time PCR (qPCR). We also compared eDNA concentrations between before and after artificially induced spawning activities. eDNA was not detected from three 30 L seawater tanks containing each single fertilized egg, but eDNA was found from other tanks each containing single individuals of larval stages (preleptocephalus and leptocephalus), juvenile stages (glass eel, elver and yellow eel) or adult stage (silver eel).

Spawning by the European eel across 2000 km of the Sargasso Sea.

It has been known for about a century that European eels have a unique life history that includes offshore spawning in the Sargasso Sea about 5000-7000 km away from their juvenile and adult habitats in Europe and northern Africa. Recently hatched eel larvae were historically collected during Danish, German and American surveys in specific areas in the southern Sargasso Sea. During a 31 day period of March and April 2014, Danish and German research ships sampled for European eel larvae along 15 alternating transects of stations across the Sargasso Sea.

Mucosal galectin genes in all freshwater eels of the genus Anguilla.

In this study, we determined the genomic DNA sequences of the mucosal galectin-encoding genes from all 19 species and subspecies of the genus Anguilla. The nucleotide sequences of the galectin genes were c. 2.

Effect of larval swimming in the western North Pacific subtropical gyre on the recruitment success of the Japanese eel.

The possible effect of directional larval swimming on the recruitment success of the Japanese eel, Anguilla japonica, was examined with a three-dimensional particle-tracking ocean circulation model using horizontal northwestward swimming and diel vertical migration (DVM). Four separate experiments included virtual larvae (v-larvae) movement from the spawning area over 290 days (total migration) and 160 days (stage A), from the STCC eddy region in 70 days (stage B), and from the origin of the Kuroshio in 60 days (stage C) to evaluate the effect of directional swimming and DVM compared to simple drifting. Passive or random swimming were not the most effective strategies for larvae dispersing from the spawning area because most v-larvae remained south of 20°N without entering the Kuroshio.

Resolving deep-sea pelagic saccopharyngiform eel mysteries: Identification of Neocyema and Monognathidae leptocephali and establishment of a new fish family "Neocyematidae" based on larvae, adults and mitogenomic gene orders.

Deep-sea midwater "saccopharyngiform" eels of the families Cyematidae, Monognathidae, Eurypharyngidae and Saccopharyngidae (order Anguilliformes) are extraordinary fishes having major skeletal reductions and modifications compared to the general anguilliform body structure. Little is known about most aspects of the systematics, phylogeny, and ecology of these families, and few of the approximately 30 species described from adult specimens have been matched with their leptotocephalus larvae. Based on mitogenomic sequence data from rare new specimens, we show that the long-speculated-about larval form referred to as "Leptocephalus holti", which was thought to possibly be the larva of the rare orange-colored eels of Neocyema (5 known specimens; speculated to belong to the Cyematidae) are actually the larvae of the one-jaw eels of the family Monognathidae.

Reproductive Ecology and Biodiversity of Freshwater Eels around Sulawesi Island Indonesia.

Sulawesi Island of north-central Indonesia is located in a region where at least 6 species of tropical anguillid eels are present, but the reproductive ecology and biodiversity of these eels in each area of the Indonesian archipelago remains poorly understood. Some information about these species was obtained from collections of their leptocephalus larvae made during several times of the year and from year-round collections of their recruitment-stage glass eels at a few locations. A sampling survey of anguillid leptocephali was conducted in March 2010 in both the Celebes Sea and Tomini Bay of Sulawesi Island to learn about the biodiversity and reproductive ecology of the eels in the region.

Interspecific difference in downstream migratory season between two tropical eels, Anguilla celebesensis and Anguilla marmorata.

Downstream-migrating Anguilla celebesensis eels were predominant relative to Anguilla marmorata in October, November, December, January and February (75.9-92%), while no A. celebesensis occurred and A.

Interspecific and sexual differences in riverine distribution of tropical eels Anguilla spp.

A total of 261 individuals of the four tropical eel species, Anguilla celebesensis, Anguilla marmorata, Anguilla bicolor pacifica and Anguilla interioris, were collected from 12 locations around Sulawesi Island, Indonesia, to gain knowledge about the riverine distribution of tropical eels. Anguilla marmorata was predominant in the lower reaches of Poso River (94·4% of total eel catch in the sampling area), Poso Lake (93·3%), three small inlet rivers of Tomini Bay (100%) and Laa River (92·3%). Anguilla celebesensis occurred frequently in the inlet rivers of Poso Lake (63·5%).

Tracking Silver Eels Along the West Marina Ridge Using Pop-up Archival Transmitting Tags.

Japanese eels were tagged in order to understand their behavior in their spawning area. Three silver eels (EEL-A, B, C: TL792, 898, 992 mm) were tagged with pop-up satellite archival transmitting tags (PSATs) and released at different locations near/in their spawning area along the southern part of the West Mariana Ridge. EEL-A showed premature tag pop-up with mostly disordered records and the EEL-C tag did not pop up, while EEL-B showed stable diel vertical migrations during 31 of the 43 days it was tracked.