Ecology and evolution of migration in the freshwater eels of the genus Anguilla Schrank, 1798

Migratory life cycle of Anguilla anguilla: a mirror symmetry with A. japonica

The European and Japanese eel populations have declined significantly in recent decades. To effectively manage and conserve them, gaining a better understanding of their migratory life cycles is important. Previous research on the spawning ecology and larval dispersal of European and Japanese eels has led to many significant discoveries and advancements for their migratory life cycles. However, different scholars hold varying views on their migratory life cycles, especially concerning the European eel, therefore this article aims to provide a comprehensive review of research from multiple disciplines concerning the spawning ecology and larval dispersal of European and Japanese eels and to propose migratory life cycles of these two species. The migratory life cycle of the European eel is as follows: European silver eels undertake a year-long spawning migration from September to January to reach the Sargasso Sea for spawning before the next spawning season, typically between December and May. After hatching, European eel leptocephali are transported by the Gulf Stream, Frontal Countercurrents, North Atlantic Current, and Azores Current and return to Europe and North Africa for growth. Recruitment of European glass eels mainly occurs between October and June of the following year, and the recruiting season is more concentrated in countries closer to the spawning area and more dispersed in countries farther away. The consistent recruitment pattern and the growth rate of leptocephali suggest a larval transport period, also called larval duration, of around 1 year. Understanding the migratory life cycle of European eels can facilitate the evaluation or development of their conservation measures.

Highly contiguous genome assembly and gene annotation of the short-finned eel (Anguilla bicolor pacifica)

In East Asia, anguillid eels are commercially important. However, unlike other species, they have not been successfully cultivated throughout their lifecycle. Facing population decline due to overharvesting and environmental pressures, the industry is turning to alternatives, such as Anguilla bicolor pacifica (short-finned eel). However, genomic data for short-finned eels are unavailable. Here, we present in-depth whole-genome sequencing results for short-finned eel obtained using two sequencing platforms (PacBio Revio, and Illumina). In this study, we achieved a highly contiguous genome assembly of the short-finned eel, comprising 19 pseudochromosomes encompassing 99.76% of the 1.087 Gb genome sequence with an N50 of 16.88 and 61.07 Mb from contig and scaffold, respectively. Transcripts from four different tissues led to the annotation of 23,095 protein-coding genes in the eel genome, 98.66% of which were functionally annotated. This high-quality genome assembly, along with the annotation data, provides a foundation for future functional genomic studies of short-finned eels.

Genetic architecture of long-distance migration and population genomics of the endangered Japanese eel

The Japanese eel (Anguilla japonica), a flagship anguillid species for conservation, is known for its long-distance-oriented migration. However, our understanding of the genetic architecture underlying long-distance migration and population genomic characteristics of A. japonica is still limited. Here, we generated a high-quality chromosome-level genome assembly and conducted whole-genome resequencing of 218 individuals to explore these aspects. Strong signals of selection were found on genes involved in long-distance aerobic exercise and navigation, which might be associated with evolutionary adaptation to long-distance migrations. Low genetic diversity was detected, which might result from genetic drift associated with demographic declines. Both mitochondrial and nuclear genomic datasets supported the existence of a single panmictic population for Japanese eel, despite signals of single-generation selection. Candidate genes for local selection involved in functions like development and circadian rhythm. The findings can provide insights to adaptative evolution to long-distance migration and inform conservation efforts for A. japonica.

Characterizing Sex Ratios of American Eels ( Anguilla rostrata ) in Louisiana

The American eel ( Anguilla rostrata) inhabits Louisiana waterways; however, little is known about their life history, population abundance, or behavior. Eels under 400 mm require histologic evaluation to determine sex. We have processed eel gonad samples from 40 sampling locations across Louisiana, as well as across size categories to aid in establishing a sex determination protocol. One hundred and eighteen (118) eel samples have been histologically analyzed to date. The histologic data compliments morphometric, location, and ageing data collected by the Louisiana Department of Wildlife and Fisheries to build an initial understanding of the biological characteristics of American eels in Louisiana.

Host and geography impact virus diversity in New Zealand's longfin and shortfin eels

The fishing and aquaculture industry is vital for global food security, yet viral diseases can result in mass fish die-off events. Determining the viromes of traditionally understudied species, such as fish, enhances our understanding of the global virosphere and the factors that influence virome composition and disease emergence. Very little is known about the viruses present in New Zealand's native fish species, including the shortfin eel (Anguilla australis) and the longfin eel (Anguilla dieffenbachii), both of which are fished culturally by Māori (the indigenous population of New Zealand) and commercially. Through a total RNA metatranscriptomic analysis of longfin and shortfin eels across three different geographic locations in the South Island of New Zealand, we aimed to determine whether viruses had jumped between the two eel species and whether eel virome composition was impacted by life stage, species, and geographic location. We identified nine viral species spanning eight different families, thereby enhancing our understanding of eel virus diversity in New Zealand and the host range of these viral families. Viruses of the family Flaviviridae (genus Hepacivirus) were widespread and found in both longfin and shortfin eels, indicative of cross-species transmission or virus-host co-divergence. Notably, both host specificity and geographic location appeared to influence eel virome composition, highlighting the complex interaction between viruses, hosts, and their ecosystems. This study broadens our understanding of viromes in aquatic hosts and highlights the importance of gaining baseline knowledge of fish viral abundance and diversity, particularly in aquatic species that are facing population declines.

Anguillid eels

Anguillid eels have fascinated humans for centuries, but our knowledge of these mysterious fish is still scant. There are 19 species or subspecies in the genus Anguilla, which are found globally, except in the eastern Pacific and southern Atlantic. Their common label 'freshwater eels' is a misnomer - all anguillids are facultatively catadromous, born in marine environments, developing in continental waters, with a proportion never entering freshwater at all. Anguillid eels have several life history traits that have allowed them to exploit a broad range of habitats. As such, anguillid eels play an important ecological role in both marine and freshwater environments as well as being commercially valuable. Because of this, anguillid eels are under threat from multiple stressors, such as barriers to migration, pollution, parasites, disease, climate change and unsustainable exploitation. Six species are listed as Threatened in the Red List of Threatened Species, and four are listed as Data Deficient. Strengthening conservation and management of these species is essential, and further research provides an exciting opportunity to develop a greater understanding of this mysterious clade of fish.

A chromosome-level reference genome assembly of the Reeve's moray eel (Gymnothorax reevesii)

Due to potentially hostile behaviors and elusive habitats, moray eels (Muraenidae) as one group of apex predators in coral reefs all across the globe have not been well investigated. Here, we constructed a chromosome-level genome assembly for the representative Reeve's moray eel (Gymnothorax reevesii). This haplotype genome assembly is 2.17 Gb in length, and 97.87% of the sequences are anchored into 21 chromosomes. It contains 56.34% repetitive sequences and 23,812 protein-coding genes, of which 96.77% are functionally annotated. This sequenced marine species in Anguilliformes makes a good complement to the genetic resource of eel genomes. It not only provides a genetic resource for in-depth studies of the Reeve's moray eel, but also enables deep-going genomic comparisons among various eels.

Configuration of gut bacterial community profile and their potential functionality in the digestive tract of the wild and cultivated Indonesian shortfin elver-phase eels (Anguilla bicolor bicolor McClelland, 1844)

This study aimed to explore the bacteria present in the digestive tracts of wild and cultivated Indonesian shortfin eel during the elver phase. The eel has high export potential due to its vitamin and micronutrient content, but slow growth and vulnerability to collapse in farm conditions hinder its cultivation. The microbiota in the eel's digestive tract is crucial for its health, particularly during the elver phase. This study used Next Generation Sequencing to analyze the community structure and diversity of bacteria in the eels' digestive tracts, focusing on the V3-V4 regions of the 16S rRNA gene. Mothur software was used for data analysis and PAST v.3.26 was used to calculate alpha diversity. The results showed that Proteobacteria (64.18%) and Firmicutes (33.55%) were the predominant phyla in the digestive tract of cultivated eels, while Bacteroidetes (54.16%), Firmicutes (14.71%), and Fusobacteria (10.56%) were predominant in wild eels. The most prevalent genera in cultivated and wild elver were Plesiomonas and Cetobacterium, respectively. The microbiota in the digestive tract of cultivated eels was diverse despite uneven distribution. The KEGG database analysis revealed that the primary function of the microbiome was to facilitate the eel's absorption of nutrients by contributing significantly to the metabolism of carbohydrates and amino acids. This study's findings can aid in assessing eel health and improving eel farming conditions.

Spatiotemporal Variability of Trace Elements Fingerprints in Otoliths of Japanese Eel (Anguilla japonica) and Its Use in Tracing Geographic Origin

To secure traceability along supply chains of foodstuffs, the spatiotemporal variability of trace elements' fingerprints (TEF) in fish otoliths provides a powerful tool to determine and discriminate the origin. Spatiotemporal variability of TEF was examined in a commercially important seafood, Japanese eel (Anguilla japonica), by means of laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). Six elemental ratios (Na:Ca, Mg:Ca, P:Ca, K:Ca, Sr:Ca, and Ba:Ca) were determined in the otoliths of specimens originating from four aquaculture farms to examine the spatial variability and from one wild habitat over three years to examine the temporal variation. Significant temporal variation was found in Mg:Ca and Sr:Ca ratios; however, discriminant function analysis showed a lower temporal variation (50%) for the three years. Spatial variations were significant in Sr:Ca and Ba:Ca ratios, and discriminant function analysis showed high (80%) spatial variation among the four farms. Otolith TEF in the Japanese eel showed specific spatial variation among aquaculture farms but intangible temporal variation, suggesting the otolith TEF reflect each aquaculture environment. The present study shows that otolith TEF can be a reliable tool to discriminate the geographic origin of the Japanese eel.

Role of estuarine habitats for the feeding ecology of the European eel (Anguilla anguilla L.)

This study aims to characterize and compare the feeding ecology of the European eels (Anguilla anguilla L.) during the continental phase (i.e. yellow and silver) along a salinity gradient (i.e. lower, middle and upper) in six northern France estuaries (i.e. brackish water). The diet and stable isotopic (i.e. δ15N and δ13C values) compositions of eels collected with a fyke net in six estuaries (Slack, Wimereux, Liane, Canche, Authie and Somme estuaries) located along the French coast of the eastern English Channel per season over a year were described by combining gut content and stable isotope analyses. Eel guts were dominated by typical BW prey, Malacostraca and Actinopterygii (54% and 40%, respectively), with the gammare Gammarus zaddachi and the green crab Carcinus maenas (38% and 14%, respectively), and smaller yellow eels of A. anguilla and juvenile European flounder, Platichthys flesus (19% and 14%, respectively) being the most frequently found in their guts. The δ13C values of a majority of eels confirmed the sea- and brackish water-specific carbon resources. Dietary and isotopic niche revealed no clear change between total length, silvering stages and seasons, but a significant difference between salinity gradients and estuaries. Eels δ13C values showed significant enrichment from upper to lower along the estuaries while the δ15N values showed an inverse effect, with the lowest values in the lower part and highest in the upper part. Higher variability in δ13C values in larger estuaries suggested that eels feed on a wide range of food sources than in smaller estuaries. While eels in the smaller estuaries fed mainly on Actinopterygii prey, eels in the larger ones had a lower trophic level (i.e. δ15N values) and fed mainly on Malacostraca prey. This spatial difference in dietary and isotopic niche is discussed in relation to biological structure of eel and environmental variables.

Contribution of migratory types to the reproduction of migrating silver eels in a tropical eel, Anguilla bicolor bicolor

The recent noticeable depletion of the catadromous eel populations has created global concerns for the eel stocks. Thus, the demand for tropical catadromous eels, which are a major alternate target species to compensate temperate species, is sharply increasing. However, eminently little is known regarding the biology and ecology of tropical eels across the Indo-Pacific region. To elucidate the contribution to reproduction among migratory types in a tropical eel, Anguilla bicolor bicolor, which is currently considered a major commercial target species, the otolith microchemistry was examined in silver (matured) stage eels. The broad range of otolith Sr:Ca ratios indicated that the habitat use was opportunistic among sea, brackish and fresh waters after recruitment to the continental habitats. Two migratory types, estuarine resident and marine resident eels, were found, but no freshwater resident eel, which is considered as the typical catadromous migration, was found. The lack of freshwater resident eels among all silver eels suggests that the estuarine and marine migrants colonizing adjacent to coastal areas might facilitate a substantial contribution to reproduction for the following generation in this area.

Early Life History and Recruitment Processes of a Tropical Anguillid Eel Anguilla marmorata to the Pacific Coast, as Revealed by Otolith Sr:Ca Ratios and Microstructure

Simple Summary

Examination of strontium:calcium (Sr:Ca) ratios in otolith has elucidated substantial information on the life history of fishes. This study has found that a drastic decline in otolith Sr:Ca ratios, indicated the initiation of metamorphosis from larva to juvenile in a tropical anguillid eel Anguilla marmorata. Based on the criterion, the current study revealed the early life history and recruitment processes of A. marmorata. Furthermore, the larval transportation, dispersion processes and recruitment dynamics to the Pacific coast of A. marmorata are determined by means of the otolith microchemical analysis in combination with abiotic parameters such as oceanic currents.

Abstract

Recent progress in otolith microchemistry especially in strontium:calcium (Sr:Ca) ratios has revealed significant features of life histories in fishes. A catadromous eel, Anguilla marmorata, has the widest distribution among anguillid eels throughout the Indo-Pacific region. However, its dispersal and recruitment mechanisms in the ocean are still unknown. The temporal and spatial variations of early life history characteristics in a tropical anguillid eel A. marmorata were examined by means of otolith Sr:Ca ratios and microstructure to understand the larval transport and recruitment processes to the coasts in the Pacific region. Durations of the larval stage and age at recruitment to the southern part of Japan ranged from 79 to 157 d and 113 to 192, respectively. No significant differences were found between recruitment months in those parameters. The early life characteristics such as larval duration and age at recruitment were constant throughout the recruitment period in the southern part of Japan. The early life history characteristics in combination with the oceanic current regime possibly determine the larval transportation and dispersion processes and further recruitment dynamics to the Pacific coast of A. marmorata. The present study also provides useful information on its biogeographic distribution in the species as determined by otolith Sr:Ca ratios and microstructure.

Contrasting patterns of genetic population structure in tropical freshwater eels of genus Anguilla in the Indo-Pacific

Freshwater eels, genus Anguilla, have a distinctive catadromous life history, which could be associated with certain oceanic current systems and offshore spawning sites. Thus, migration and dispersion patterns are believed to be important factors influencing the population structure of each species. Temperate eel species are well studied, while little research has been conducted on the tropical counterparts that comprise two-thirds of all eel species. The population structure of three tropical species, A. marmorata, A. bicolor bicolor and A. bengalensis bengalensis, which are distributed widely in the Indo-Pacific region, were explored by means of DNA sequence analysis of mitochondrial cytochrome c oxidase subunit I (COI). This study suggests that A. bicolor bicolor might have two genetically distinct populations (fixation index, F_ST = 0.891; p < 0.001) that co-occur geographically in the Indo-Pacific region, while A. marmorata and A. bengalensis bengalensis might have a panmictic-population structure in this region. This study is the first to explore the population genetic structure of A. bengalensis bengalensis. The present results also suggest plausible dispersion and migration of these tropical species into their continental habitats.

DNA barcoding of a tropical anguillid eel, Anguilla bicolor (Actinopterygii: Anguilliformes), in Indo-Pacific region and notes on its population structure

The tropical anguillid eel, Anguilla bicolor McCelland, 1844, includes two subspecies, Anguilla bicolor bicolor McCelland, 1844 and Anguilla bicolor pacifica Schmidt, 1928, and is distributed across the Indo-Pacific region. Although A. bicolor is widely distributed and recognized as an important fish resource in the Indo-Pacific region, few studies have been conducted on its genetic variation and population structure. DNA barcoding of A. bicolor specimens collected in the Indo-Pacific region was carried out in this study using mitochondrial cytochrome c oxidase subunit I. Anguilla bicolor was found to diverge genetically, which supported its classification into two different subspecies. In addition, our study showed that A. bicolor bicolor had two genetically distinct populations/groups, and these different populations co-occur geographically in Indonesia and Malaysia in the eastern Indian Ocean. Our findings suggest that the eel larvae might be transported from at least two geographically different spawning grounds in the Indian Ocean, and then recruited to and settled in the same habitats in Indonesian and Malaysian waters. The molecular evidence calls for further research on the life history, stock assessment and protection of the populations of A. bicolor bicolor in Indonesia and Malaysia.