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Two species of cusk eels, Neobythites nanhaiensis Chen, Zhao, Yang, Zhang, He, Zhang, Wang et Zhong, 2026 and Grammonus robustus Smith et Radcliffe, 1913 are reported based on specimens collected from Nha Trang, Vietnam. The specimens of N. nanhaiensis represent the northernmost record of this species and the specimen of G. robustus fills the distribution gap between the Philippines and eastern Australia. Detailed descriptions of both species and sagittal otoliths are provided and compared to data of other specimens. Moreover, we found that the original description of N. nanhaiensis was partly based on damaged specimens (tail regenerated), making the counts of dorsal-, anal-, and caudal-fin rays, and vertebrae, and some measurements unreliable, and the true variation of this species remains unknown. Keywords: biodiversity, distributional gap, ichthyology, morphology, redescription sagittal otolith

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.

In the area of the Neretva delta in the eastern Adriatic, where the European eel, Anguilla anguilla (Linnaeus, 1758) has been traditionally fished for centuries, a decline in its population has been observed, as in most of Europe. Despite several studies, systematic monitoring was not performed, and the causes of population decline are attributed to anthropogenic stressors, mainly overfishing and interventions that disrupt the migration. With the stock at a low level, there is a need for a detailed assessment of biological data and the determination of the “zero state” of the eel population in the areas where monitoring was not previously performed, such as the Neretva delta. This data would serve as a basis for the development of an appropriate monitoring and eel management plan. One of the under-researched aspects is still the eel’s morphology, which is closely related to all basic life functions. The aim of this work was to analyze in detail the morphological parameters of yellow and silver eels from the mouth of the Neretva River in different seasons and the relationships between the measured morphometric parameters and physiological indicators and to compare them with previously published results for different life stages across Europe. The samples were collected during spring, summer and autumn of 2021, and winter of 2022. Yellow eels were present in the catch throughout the sampling period, while silver eels were caught in the autumn and winter. Yellow and silver eels were significantly different regarding 22 morphometric measures that were analyzed. Isometric growth was recorded for yellow eels in the spring and autumn of 2021, and positive allometric growth was recorded for yellow eels in the summer and silver eels in the autumn of 2021 and winter of 2022. PCA showed that the main factor that separates the eels grouped by life stage in different seasons is the intestine length (IL), whereas the rest of the factors (weight—W; intestine weight—IW; liver weight—LW; and total length—TL) affect the groupings almost equally. Seasonal averages of the condition factor (CF) for yellow and silver eels did not differ statistically. Three indicators were used to describe intestine morphology: relative gut weight (RGW), relative gut length (RGL), and Zihler’s index (ZHI); and the only statistically significant difference between yellow and silver eels was recorded for the RGW. The hepatosomatic index (HSI) was significantly different between silver eels in winter and yellow eels in spring. In addition to supplementing the already known facts, this paper provides new information on the functional morphology of the European eel. Monitoring of these characteristics is crucial for management of the European eel fisheries as they are directly related to functional performance and affect the ability to maintain sustainable populations in anthropogenically altered environments.

We investigated the mechanism of signal transduction using inactivating (R476H) and activating (D576G) mutants of luteinizing hormone receptor (LHR) of eel at the conserved regions of intracellular loops II and III, respectively, naturally occurring in mammalian LHR. The expression of D576G and R476H mutants was approximately 58% and 59%, respectively, on the cell surface compared to those of eel LHR-wild type (wt). In eel LHR-wt, cAMP production increased upon agonist stimulation. Cells expressing eel LHR-D576G, a highly conserved aspartic acid residue, exhibited a 5.8-fold increase in basal cAMP response; however, the maximal cAMP response by high-agonist stimulation was approximately 0.62-fold. Mutation of a highly conserved arginine residue in the second intracellular loop of eel LHR (LHR-R476H) completely impaired the cAMP response. The rate of loss in cell-surface expression of eel LHR-wt and D576G mutant was similar to the agonist recombinant (rec)-eel LH after 30 min. However, the mutants presented rates of loss higher than eel LHR-wt did upon rec-eCG treatment. Therefore, the activating mutant constitutively induced cAMP signaling. The inactivating mutation resulted in the loss of LHR expression on the cell surface and no cAMP signaling. These data provide valuable information regarding the structure–function relationship of LHR–LH complexes.

The Florida Everglades is a critically important, but highly threatened ecosystem that is becoming increasingly susceptible to the invasion of non-native species. This study investigated the ecological role of the invasive peacock eel ( Macrognathus siamensis ) within this ecosystem using 15 years of electrofishing data and stable isotope analysis. We investigated the population trends of peacock eels at the marsh-mangrove ecotone of the Shark River Estuary, the environmental factors contributing to their abundance, and the potential interactions they may have with native fish assemblages and coastal food webs. We used stable isotope analysis to provide insights into the basal resource contribution to peacock eels and hypervolume analysis to determine peacock eel trophic niche size and overlap with native species. Results of this study found that peacock eel abundance has rapidly increased, and their populations are strongly related to hydroclimatic regimes. Peacock eel abundance was positively associated with warmer water temperatures and greater marsh inundation periods. The trophic niche of peacock eels was significantly smaller in volume than that of native sunfishes ( Lepomis spp.) indicating lower intraspecific resource use variability and suggesting a limited potential for inter-specific competition with these taxa. However, in recent years, the catch of peacock eels has outnumbered the catch of all native sunfishes combined. The feeding habits and pervasiveness of peacock eels in the coastal Everglades could lead to a decrease in abundance of benthic prey items targeted by peacock eels and alter food web dynamics in the system. Based on these data, peacock eel populations are predicted to continue to increase, highlighting the importance of continued monitoring of their potential impact on native fish assemblages and food webs.

Adult American eels (Anguilla rostrata) are vulnerable to hydropower turbine mortality during outmigration from growth habitat in inland waters to the ocean where they spawn. Imaging sonar is a reliable and proven technology for monitoring of fish passage and migration; however, there is no efficient automated method for eel detection. We designed a deep learning model for automated detection of adult American eels from sonar data. The method employs convolution neural network (CNN) to distinguish between 14 images of eels and non-eel objects. Prior to image classification with CNN, background subtraction and wavelet denoising were applied to enhance sonar images. The CNN model was first trained and tested on data obtained from a laboratory experiment, which yielded overall accuracies of >98% for image-based classification. Then, the model was trained and tested on field data that were obtained near the Iroquois Dam located on the St. Lawrence River; the accuracy achieved was commensurate with that of human experts.

Freshwater eels (Anguillidae) are facing numerous anthropogenic impacts that have led to declines in abundance for nearly all species. They have a complex life history that includes obligate migration and flexible habitat use, characteristics which have generated much research interest in the family. Eels also have high cultural and economic value, further incentivizing the study of key species. We reviewed the scientific literature on anguillids from the last four decades, analyzing research trends among and within species of the genus Anguilla. We identified a shift in research focus from largely biological towards more applied management and conservation topics, an increased emphasis on migration and fish passage considerations, and a tendency towards research on glass eels and silver eels over other life stages. We also identified a significant disparity in research effort between temperate species and tropical species, with a scarcity of knowledge on the latter. Finally, we described several key knowledge gaps about community-based interactions of eels, notably their roles as predator, prey, and ecosystem connector, and highlight opportunities for early career researchers to establish research programs within the field of anguillid research.

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.

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.