A new species of freshwater eel <Emphasis Type="Italic">Anguilla luzonensis</Emphasis> (Teleostei: Anguillidae) from Luzon Island of the Philippines

Anguilla luzonensis, a new species of freshwater eel, family Anguillidae, is described on the basis of 29 specimens collected from the Pinacanauan River system, which is a tributary of the Cagayan River on northern Luzon Island of the Philippines. The new species and A. celebesensis, which is distributed in the same region, both have variegated color marking and broad maxillary bands of teeth, but show statistically significant differences in the following characters of the new species: predorsal-fin length 28.6–33.1% of total length (TL); preanal length 39.6–44.8% of TL; trunk length 26.4–31.3% of TL; distance between the verticals through the anus and origin of the dorsal fin 9.3–13.9% of TL; length of gape 35.5–46.9% of head length; the total number of vertebrae 103–107; and number of abdominal vertebrae 40–42. These morphological differences, together with the present knowledge about anguillid eel taxonomy, genetics, and ecology, strongly support the presence of the new species A. luzonensis in the area of northern Luzon Island of the Philippines.     

Video observation of an eel in the Anguilla japonica spawning area along the West Mariana Ridge

Some anguillid spawning areas are known based on collections of small larvae, but recently for the Japanese eel Anguilla japonica, adult spawners have been caught in trawls and their eggs and preleptocephali collected. The spawning area of A. japonica is located along the western side of the West Mariana Ridge, but the natural spawning behavior of this species or that of any other anguillid species has never been observed. This study reports on the first effort to observe spawning aggregations of anguillid eels that was conducted by the R/V Yokosuka using the Shinkai 6500 submersible and a Deep-Tow camera system in the A. japonica spawning area in July 2012. The submersible was deployed mostly at 200–800 m during daytime and the Deep-Tow was deployed mostly at 130–250 m during nighttime, both in multiple oblique depth tracks along linear transects. Various fishes and invertebrates were seen in the pelagic environment during day and night, but no spawning aggregations were observed. One eel was briefly recorded by a Deep-Tow camera at 20:13 on 17 July (2 days before new moon) at a depth of 179 m. The eel was recorded for <1 s as it passed in front of the camera. Its anterior body and head shape were consistent with a male A. japonica, or possibly a Derichthys serpentinus eel, but not with other mesopelagic eels. Because the tail region of the eel was not visible, species identification was not possible.     

Environmental DNA analysis reveals the spatial distribution,abundance, and biomass of Japanese eels at the river‐basin scale

1. There is growing international concern about declines in populations of anguillid eels, resulting in their inclusion in the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. Monitoring the population dynamics of these species is often challenging, however, owing to their broad distributions and complex, catadromous life histories.
2. Whether environmental DNA (eDNA) analysis could be used to monitor the spatial distribution of anguillid eels in rivers was investigated by conducting basin‐scale surveys of Japanese eels, Anguilla japonica Temminck & Schlegel, 1846, in 10 rivers in Japan, and comparing the results obtained using eDNA analysis with the results obtained using the electrofishing method. Moreover, the relationship between the eDNA concentration and the abundance and biomass of Japanese eels was examined.
3. The eDNA of Japanese eels was detected at 56 (91.8%) of the 61 study sites from which individuals were collected by electrofishing, and at an additional 35 sites where individuals were not directly collected. This indicates that eDNA analysis has a greater sensitivity for detecting the presence of eels, making it a powerful tool for monitoring the spatial distribution of anguillid eels in rivers.
4. A significant, but weak, positive relationship between the eDNA concentration and the abundance and biomass of Japanese eels was also found, suggesting that eDNA analysis may be useful for estimating the abundance and biomass of anguillid eels in rivers.
5. This is the first study to demonstrate the potential usefulness of eDNA analysis for estimating the spatial distribution, abundance, and biomass of Japanese eels in rivers. eDNA analysis will allow anguillid eel populations to be monitored over large spatial and temporal scales using a consistent protocol, with reduced time and effort compared with conventional techniques, providing invaluable information for managing populations of these endangered species.

     

Analysis of the genetic structure of the European eel Anguilla anguilla using the mtDNA D-loop region molecular marker

Sequence variation of the mtDNA D-loop region was analyzed in order to investigate the intraspecific evolution and the population genetic structure of the critically endangered European eel Anguilla anguilla. An additional attempt was made to collect information on the genetic characteristics of groups of eels representing naturally recruited eels to Lithuania and Latvia and introduced into Lithuanian lakes. A total of 148 eels were investigated, and 107 different haplotypes attributed to 39 haplogroups were determined during the study. A dataset comprising 229 sequences was created using the data from both this study and from earlier studies (n = 81). Analysis of this dataset revealed that haplotype diversity was 0.99567, the average number of nucleotide differences was 12.50544, there were 145 polymorphic sites and nucleotide diversity was 0.02426. No significant genetic differentiation was detected between different combinations of samples. However, the population genetic structure of this species could be characterized as a genetic mosaic formed due to the existence of reproductively isolated groups. The existence of a genetic mosaic in this species could be explained by the different evolutionary lineages found in the eel population.     

Evaluation of the pigmentation stages and body proportions from the glass eel to yellow eel in Anguilla japonica

The pigmentation stages of Anguilla japonica were classified from the glass eel to yellow eel stage using 412 wild-caught eels from Hamana Lake and detailed developmental observations of 10 laboratory-reared eels. The sequential appearance of pigment was similar to that in A. anguilla, except for delayed pigment formation on the nerve cord of A. japonica. The general classification of pigmentation stages (V_A–VI_B) in A. anguilla was applicable to A. japonica, but the V_B stage could be separated into two stages (V_B1, V_B2) in A. japonica. The completion of guanine deposition on the intra-abdominal membrane may be a trait to discriminate the yellow eel stage (VII) from the VI_B stage. In natural conditions, wild-caught 0-age eels (n = 3,298) did not increase in size and retained a slender body form until VI_A4, and then became thicker during VI_B. The transition between body forms and growth may correspond to their ecological change from using passive transport in glass eels with tidal flow into rivers, to their settlement and further movements of elvers upstream. Considering this behavioral transition and pigmentation progression, it is proposed that the terms "glass eel" and "elver" could be defined as stages V_A–VI_A4 and VI_B, respectively.     

Composition and distribution of freshwater eels <Emphasis Type="Italic">Anguilla</Emphasis> spp. in Vietnam

A sampling survey to determine the composition and distribution of freshwater eel species was conducted in 12 provinces along the coast of Vietnam from October 2014 to February 2015. The sampled provinces spanned the country from central Vietnam to the south, but freshwater eels were only found in the central part of Vietnam. Species identification of 77 specimens, collected from different habitats (rivers and estuaries), was performed by analysis of both morphological characters and nucleotide sequences of mitochondrial cytochrome oxidase subunit I (COI). The 77 specimens were morphologically and genetically identified as 58 Anguilla marmorata, 17 A. bicolor pacifica, and two A. japonica. This is the first definitive survey about the composition and distribution of freshwater eels in Vietnam where distribution has been hitherto unknown.     

Genetic identification and morphology of naturally spawned eggs of the Japanese eel Anguilla japonica collected in the western North Pacific

Eggs of the Japanese eel Anguilla japonica collected in the western North Pacific were identified by onboard species-specific polymerase chain reaction (PCR) and DNA nucleotide sequencing after the cruise. Fish eggs of various species were collected by large plankton net tows at 12 stations along the southern part of the West Mariana Ridge on 19–25 May 2009. A total of 43 fish eggs were distinguished morphologically as possibly being of A. japonica. Thirty-one of those were analyzed by PCR, which included 15 eggs collected at 12°50–55′N, 141°15–20′E (in 5 tows) that showed positive results. The 16S ribosomal RNA (rRNA) gene sequences of eggs determined after the cruise indicated that 31 A. japonica eggs had been collected. The remaining eggs were of mesopelagic eel species (Serrivomeridae and Derichthyidae), or unidentified species. The morphological characteristics of the A. japonica eggs were consistent with those of artificially spawned eggs, except they had a slightly larger diameter. The egg diameter range did not overlap with those of mesopelagic eels of the Serrivomeridae, which often spawn in the same area as A. japonica. These results suggest that egg diameter and embryo shape can be used to morphologically identify naturally spawned A. japonica eggs.     

Late arrival of Anguilla japonica glass eels at the Sagami River estuary in two recent consecutive year classes: ecology and socio-economic impacts

To determine the actual status of the recruiting glass eel stock of Anguilla japonica, we conducted a continuous monthly monitoring program for 2?years between November 2009 and October 2011 at the Sagami River estuary, Japan. A total of 114 and 372 A. japonica glass eels were observed in the 2009?C2010 and 2010?C2011 seasons, respectively. Recruitment patterns were the same in both years, starting in early winter (December in 2009 and November in 2010), increasing slightly until March, decreasing in April, but attaining the largest peak in June, after which no glass eels were observed during August?COctober. The Japanese eel has been known to spawn mainly in the summer, and glass eels recruit to their freshwater growth habitats during the winter to early spring. Our results clearly demonstrate an unexpected late arrival of glass eels in the early summer for two recent consecutive year classes. The summer recruitment found in our study indicates the unusual phenology of the Japanese eel, which may be a possible response to recent climate change.     

The use of clove oil as an anaesthetic for the longfinned eel, Anguilla reinhardtii (Steindachner)

To handle large river eels during procedures such as measuring and tagging for field and aquaculture studies, they must be anaesthetized. During our initial biological studies of Anguilla reinhardtii (Steindachner) it was found that the anaesthetic benzocaine was relatively expensive and elicited a variable response, even when used at relatively high concentrations. Human health risks are also a concern when using benzocaine, as some of the eels may later be sold for human consumption. Therefore, experiments were done to evaluate the use of clove oil (a safe, naturally occurring product) for anaesthesia of this species at a range of temperatures (17 and 25 °C) and salinities (0–32 g L^?1). It was found that clove oil provided a suitable anaesthetic response through this wide range of temperatures and salinities. Response times were found to be relatively variable for both benzocaine and clove oil. This variability may be related to stress, environmental factors, or the condition of the fish. Clove oil is recommended for anaesthesia of anguillid eels because it is effective, relatively inexpensive, and poses little risk to human health.     

A novel semi-multiplex PCR assay for identification of tropical eels of genus Anguilla in Indonesian waters

A one-step semi-multiplex polymerase chain reaction (PCR) is proposed to distinguish seven species and subspecies of tropical eels, i.e., Anguilla celebesensis, A. interioris, A. nebulosa nebulosa, A. marmorata, A. borneensis, A. bicolor bicolor, and A. bicolor pacifica, known in Indonesian waters. Seven pairs of species-specific primers, including two forward and seven reverse sequences, were designed after alignment of published and new complete mitochondrial cytochrome b (1,140 bp) and 16S ribosomal RNA (rRNA) (1,120 bp) sequences. All species-specific primer pairs are included in one PCR mix, but only one pair of them can amplify a specific fragment from the template DNA that is analyzed. The semi-multiplex PCR amplified a fragment of 230 bp for A. b. bicolor, 372 bp for A. n. nebulosa, 450 bp for A. borneensis, 620 bp for A. marmorata, 670 bp for A. b. pacifica, 720 bp for A. celebesensis, and 795 bp for A. interioris, which are then separated by DNA agarose gel electrophoresis.     

Anthropogenic spreading of anguillid herpesvirus 1 by stocking of infected farmed European eels,Anguilla anguilla (L.), in the Schlei fjord in northern Germany

The Schlei fjord in northern Germany is the recipient water of a comprehensive eel, Anguilla anguilla (L.), stocking programme. Since 2015, stocked eels become alizarin red S marked, but to date no control mechanism is implemented in this stock enhancement measure to prevent anthropogenic spreading of diseases. Consequentially, it was possible that farmed stocking cohorts of 2015 and 2016 (in total ca. 1040 kg) were subsequently tested positive for anguillid herpesvirus 1 (AngHV 1). For this study, 100 eels [total length (TL) 24.3–72.9 cm, age ca. 1–6 years] were caught in 2016 and investigated with regard to AngHV 1 infection, parasite load (Anguillicoloides crassus) and body conditions. 68% of the eels were found to be virus positive while larger specimens were more often infected. In addition, a fitted generalized linear model (area under the curve = 0.741) demonstrated that an increase in individual TL is accompanied with an increased risk of clinically relevant virus loads. Anguillicoloides crassus turned out to be an important stressor for eels, because parasite and virus load revealed a significant positive correlation. The results of this study evidently show the urgent need of a disease containment strategy for eel stocking programmes.     

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.     

Discovering the dominance of the non-native European eel in the upper reaches of the Tone River system,Japan

To investigate the presence of non-native anguillid eels in Japanese waters, 141 eels were collected from seven sampling sites throughout the Tone River system. Genetic species identification showed an extraordinary dominance of the non-native European eel Anguilla anguilla in the uppermost site of the study area. Estimation of age from otoliths suggested that the European eels were introduced into the river in the 1990s, corresponding to previous reports from other Japanese water systems. Comparison of the von Bertalanffy growth curve parameters indicated that the European eels caught in the Tone River system appeared to have a similar or even higher growth rate than the same species in the original habitats in Europe. The long-term inhabitation and the normal development of European eels in Japanese waters suggest that regulations prohibiting the release of non-native eels and safeguards against accidental escape from culture ponds must be strictly maintained in order to ensure the conservation of the native Japanese eel.     

Exotic silver eels Anguilla anguilla in Japanese waters: seaward migration and environmental factors

To understand the ecology of the exotic silver European eel Anguilla anguilla introduced into Japanese waters, the migratory behavior of 106 specimens captured in the coastal waters of Japan between April 1997 and March 2002 was analyzed. Their migratory behavior was apparently correlated with various environmental factors, particularly photoperiod, water temperature, lunar phase, and passage of atmospheric depressions, and was similar to the behavior of the species in European waters. These findings suggest that transplanted European eels retain their ability to respond to environmental cues for seaward migration in similar temperate habitats. The timing of the migration of silver European eels coincided with that of the native Japanese eels A. japonica, suggesting that the silver European eel was synchronized physiologically with the native eel by the same environmental factors.     

Presence of viruses in wild eels Anguilla anguilla L,from the Albufera Lake (Spain)

A virological analysis was conducted on wild eels from the Albufera Lake (Spain). A total of 179 individuals at different growth stages were collected in two different surveys (2004 and 2008). Presence of anguillid herpesvirus (AngHV‐1), aquabirnavirus and betanodavirus was confirmed by PCR procedures in both surveys, although the number of detections was clearly higher in 2008 (83% of the eels analysed resulted positive for virus presence). AngHV‐1 was the viral agent most frequently detected, followed by aquabirnaviruses. Betanodaviruses were detected by the first time in wild eels, and although the detections were only made by nested PCR, high percentage of positives were achieved. In addition, in 2008, seven aquabirnaviruses were isolated. Phylogenetic analysis performed using partial sequences of both genomic segments of aquabirnaviruses indicated that the seven isolates could be typed as WB (genogroup I) on the basis of segment A sequences, but when segment B was used six of them clustered with C1 strain (genogroup V) and one was typed as Ab (genogroup II). These results indicate natural reassortment between different strains of aquabirnaviruses in the eels. Although betanodaviruses were not isolated in cell culture, the analysis of the sequence of the nested PCR product indicated that they clustered with SJNNV genotype. The diversity of viral agents and the high level of viral detections suggest that viral infections may play a more prominent role in the decline of the European eel than initially thought.     

Evidence of niche segregation between freshwater eels and conger eels in Kojima Bay, Japan

The aim of this study was to examine the possible interspecific interactions between the freshwater eel Anguilla japonica and the marine conger eel Conger myriaster. Spatial distribution, foraging time, and diet were compared between 380 A. japonica and 221 C. myriaster juveniles in a large brackish water bay and a tributary river estuary. Almost no C. myriaster were caught in the estuary, but both species coexisted in the bay at similar densities. For both species, percentages of eels with stomach contents were greater in the morning than in the afternoon, indicating they forage at night. Both species mainly fed on benthic crustaceans in the bay, but the size of the main prey species, mud shrimp Upogebia major, was significantly larger in A. japonica. The difference in prey size appears to be related to the eel body size (A. japonica: 559.5 ± 88.0 mm, C. myriaster: 356.4 ± 56.0 mm), which probably reflects the differences in the life history-related movements of these eels, with A. japonica using the bay at large sizes and C. myriaster using the bay as a nursery area before moving to deeper water. The differences in their migratory and movement patterns may result in niche segregation in the brackish bay where both species coexist.     

Synthesis and possible function of 11-ketotestosterone during oogenesis in eel (Anguilla spp.)

The control of 11-ketotestosterone (11-KT) biosynthesis and its physiological roles were examined in female Japanese eel (Anguilla japonica) and New Zealand longfinned eel (Anguilla dieffenbachii). 11-KT was detected in serum of female eels of both species. Among various tissues from Japanese eel, the ovary had the greatest capacity to synthesize 11-KT in vitro. In addition, the oocyte diameters of eels treated with 11-KT had increased significantly. Furthermore, these oocytes were found to have an increased number of oil droplets. These findings suggest that 11-KT in female eels may be mostly of ovarian origin and that this androgen appears to play an important role in controlling pre-vitellogenic oocyte growth.     

First capture of post-spawning female of the Japanese eel Anguilla japonica at the southern West Mariana Ridge

Two adult male freshwater eels, Anguilla japonica, were captured in June 2008 in the West Mariana Ridge (13°N, 142°E) in the North Pacific, but collections of females have yet to be reported. In September 2008, we successfully caught two adult female A. japonica, 55.5 and 66.2 cm in total length, in the adjacent but northern area (14°N, 143°E). Six newly hatched eel larvae (pre-leptocephali) were also collected by subsequent plankton sampling conducted near the female catch area. Female adults appeared to be in the post-spawning state, probably a considerable time after spawning, since a small number of remarkably regressed oocytes (50–250 μm in diameter) were observed in the ovaries. Capture of post-spawning female eels and newly hatched larvae near the Suruga Seamount (14°N) together with the previous collection of mature males in the southern area (13°N) corroborates that the area along the West Mariana Ridge is the spawning area of this species, but suggests that the eel spawning may occur over a wider area than previously expected.