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Eel ramps and environmental DNA reveal a preliminary recruitment window of the American Eel in Texas
ABSTRACT
This study is the first to document glass American Eels Anguilla rostrata in Texas, and it provides critical information about recruitment timing along the northern Gulf Coast. The American Eel is a facultative catadromous fish. Data are lacking related to juvenile (glass eel and elver) recruitment into the bays and estuaries of the Gulf of Mexico.
Objective
The goal of the study was to document glass and elver recruitment timing and distribution for American Eels in Texas.
Methods
The study implemented year-round monitoring in rivers and streams using eel ramps and environmental DNA (eDNA) to increase the likelihood of detecting even highly sporadic recruitment events. Thirteen sites were identified based on their connectivity to coastal estuarine waters and the ability to support a gravity-fed eel ramp. Ramps were deployed during June–July 2022 and were checked weekly for 1 year. A subset of nine ramp sites was also monitored for eDNA during each ramp check.
Results
American Eels were caught in one of the 13 eel ramps. In total, 26 juvenile eels (19 glass and 7 elvers) were captured. Captures of glass eels began in January, and captures of elvers ended in May. American Eel DNA was detected at eight of the nine sites that were monitored for eDNA. Environmental DNA detection rates were highest for the only ramp site where eels were captured.
Conclusions
Positive eDNA samples were temporally associated with the recruitment window observed through captures in the ramps. Continued deployment of eel ramps during the estimated recruitment window are planned for 2024 and 2025, with more frequent checks. Continued monitoring will be conducted with the goal of assisting natural resource agencies in determining the conservation and management needs of American Eels in Texas.
Lay Summary
American Eels have a complex life history, and little is known about when and where juvenile eels arrive to coastal water bodies in the Gulf of Mexico. This study is the first to document glass American Eels in Texas, and it provides critical information about recruitment timing along the northern Gulf Coast.
Journal Article
Oceanic spawning ecology of freshwater eels in the western North Pacific
The natural reproductive ecology of freshwater eels remained a mystery even after some of their offshore spawning areas were discovered approximately 100 years ago. In this study, we investigate the spawning ecology of freshwater eels for the first time using collections of eggs, larvae and spawning-condition adults of two species in their shared spawning area in the Pacific. Ovaries of female Japanese eel and giant mottled eel adults were polycyclic, suggesting that freshwater eels can spawn more than once during a spawning season. The first collection of Japanese eel eggs near the West Mariana Ridge where adults and newly hatched larvae were also caught shows that spawning occurs during new moon periods throughout the spawning season. The depths where adults and newly hatched larvae were captured indicate that spawning occurs in shallower layers of 150–200 m and not at great depths. This type of spawning may reduce predation and facilitate reproductive success.
Little is known about the reproductive ecology of freshwater eels. In this article, the authors describe the capture of two species of eels together with eggs and newly hatched larvae, and suggest that spawning takes place during the new moon at shallower depths than previously thought.
Journal Article
Molecular diet analysis of Anguilliformes leptocephalus larvae collected in the western North Pacific
Natural diets of leptocephalus larvae have been enigmatic. In this study, we collected DNA samples from the gut contents and body surface of leptocephali belonging to the five Anguilliform families (Anguillidae, Chlopsidae, Congridae, Muraenidae, and Serrivomeridae) from the northwest Pacific and performed next-generation 18S rDNA sequencing. Wide variety of eukaryotes was detected in both samples, from which eight eukaryotic groups (jellyfish, conoid parasite, tunicate, copepod, krill, segmented worm, fungi, and dinoflagellate) were selected on the basis of abundance. All groups except conoid parasites were common in both the samples. Cnidarian 18S rDNA reads were the most abundant in both the samples; however, the number of samples having cnidarian reads and the read counts were significantly higher in the body surface scraping samples than in the gut content samples, regardless of careful rinsing of the body surface. These results indicate that the cnidarian DNAs are most likely found because of cross contamination from the body surface and/or environment. 18S rDNA read counts of copepod and tunicate in the gut contents were greater than or comparable with those in the body surface scraping samples, which may correspond to the previous observations of fecal pellets and larvacean houses in the leptocephali gut. Thus, the present study supports previous implications that leptocephali utilize detritus materials, so called marine snow.
Journal Article
Opportunistic spawning of tropical anguillid eels Anguilla bicolor bicolor and A. bengalensis bengalensis
Information on the spawning migration, spawning ecology and life history of tropical eels in the Indo-Pacific region is very limited. The physiological and morphological characteristics of tropical freshwater eels,
Anguilla bicolor bicolor
and
A. bengalensis bengalensis
collected in Malaysia were examined in relation to their downstream migration patterns. A total of 455 eels were collected over monthly intervals between February 2014 and January 2016 and we examined both gonadosomatic index and gonad histology features. In both species, close positive relationships between advanced maturation stages and eye, fin, gonadosomatic indexes were found in males and females.
A. bengalensis bengalensis
was found to be larger and heavier than
A. bicolor bicolor
at the time of seaward migration. The final stage of maturation for seaward spawning migration occurred throughout the year in
A. bicolor bicolor
, although that of
A. bengalensis bengalensis
was estimated to six months due to the limited number of samples. These results suggest that year-round spawning in the open ocean occurs in the tropical eel. This non-seasonal spawning ecology is notably different from that of temperate eels, which are known to follow a well-defined spawning season, with spawning migrations generally taking place during autumn months.
Journal Article
Global consumption of threatened freshwater eels revealed by integrating DNA barcoding, production data, and trade statistics
Fisheries resources depend on natural ecosystems, yet their sustainable management is often limited by uneven regional capacities and the pressures of international trade. High demand from certain regions can lead to overexploitation in others, highlighting the need to understand global consumption patterns of key aquatic species. This study introduces an integrated approach that combines DNA barcoding of freshwater eel (
Anguilla
spp.) products collected from end markets in 11 countries/regions with global production and trade statistics. We estimate that over 99% of eels consumed worldwide belong to three IUCN-listed threatened species: the American eel, Japanese eel, and European eel. Consumption was heavily concentrated in East Asia—particularly China, Japan, and South Korea—where supply volumes far exceed those of other regions. Our approach yields the most comprehensive quantitative global estimate to date of eel species composition in consumption, offering essential insights for the conservation and sustainable management of this highly exploited group.
Journal Article
Comparative analysis of complete mitochondrial genomes of fourteen moray eels (Anguilliformes: Muraenidae) and primary exploration of their phylogenetic relationship and temperature adaptation
Moray eels (Anguilliformes: Muraenidae) are commonly found in subtropical and tropical waters, where they favor sheltered habitats and occasionally exhibit aggressive behavior. Nowadays, mitochondrial genomes (mitogenomes) information of moray eels remains poorly understood, and analysis of gene re-arrangements and evolutionary history within Anguilloidei remain largely unknown. In this study, mitochondrial genomes from 12
Gymnothorax spp.
and 2
Echidna spp
species were sequenced, assembled and annotated, and most of which were not published before. Our results showed that the mitochondrial genomes of the 14 species were approximately 16.5 kb in length, with GC content ranging from 41.1 to 46.1%. The mitochondrial genomes of these 12
Gymnothorax spp.
and 2
Echidna spp
are circular molecules that encode a total of 37 genes, including 13 protein-coding genes (PCGs), 22 tRNA genes, 2 rRNA (small and large subunit rRNA encoding genes) and a non-coding control region (CR). In addition, three types of gene arrangement have been identified for the first time in the mitogenomes of Anguilloidei. Phylogenetic relationships of 51 moray eel mitogenomes based on the protein-coding genes indicate that mitogenome sequences reveals the higher-order relationships of different Anguilloidei. Gene order and phylogenetic data are valuable for elucidating the evolutionary connections among various Anguilloidei species.
Cox3
exhibited the smallest non-synonymous mutation value among all the 13 protein-coding genes, indicating that the role of Cox3 in maintaining energy metabolism under warm water conditions is a hypothesis based on observed evolutionary conservation. The complete information of mitogenomes in these 12
Gymnothorax spp.
and 2
Echidna spp
species provides genetic markers for species identification and temperature adaptation studies in Anguilloidei.
Journal Article
