Tropical Anguillid Eels Research Articles

High genetic diversity in anguillid glass eels across three Southeast Asian countries revealed through mitochondrial DNA D-loop sequences, with notes on population structure

Understanding the genetic diversity and population structure of anguillid eels is crucial for effective management. In this study, we used the mitochondrial DNA (mtDNA) D-loop marker to identify anguillid glass eels and assessed their genetic diversity across six sampling sites spanning three Southeast Asian countries. We also examined the population structures of Anguilla marmorata and A. celebesensis. We validated nine distinct species and subspecies of anguillid eels from 465 specimens collected in Indonesia (n = 246), the Philippines (n = 119), and Vietnam (n = 100). High gene diversities were observed across the species, ranging from 0.979 (A. celebesensis) to 1.000 (A. bicolor bicolor, A. marmorata, and A. interioris). Correspondingly, nucleotide diversity values were relatively low, varying from 0.036 (A. marmorata) to 0.057 (A. interioris). A. marmorata (n = 162) displayed a broad distribution, being observed across all six sites, with a significant yet low average fixation index of 0.05755. Pairwise comparisons of FST values between Cimandiri and other locations (except for Cagayan) were relatively high and statistically significant (p < 0.01), indicating significant genetic differentiation of Cimandiri samples compared to most locations in this study. On the other hand, A. celebesensis (n = 110), exhibited a non-significant and low fixation index (0.007), indicating no genetic differentiation in samples from Poso and Poigar Rivers, Indonesia. Our findings highlight the essential need for coordinated management strategies among regions that share common stocks. Additionally, this study underscores the efficacy of the mtDNA D-loop in precisely identifying glass eel species/subspecies and elucidating the genetic diversity and population structure of tropical anguillid eels.

Comparison of Reproductive Aspects of the Tropical Eel Anguilla bicolor (McClelland 1884) in Freshwater and Estuarine Habitats

The tropical anguillid eel, Anguilla bicolor McClelland, experiences significant growth up to the adult size in both freshwater and estuarine ecosystems, encountering salinity gradients that may impact their reproduction. Therefore, this study aimed to investigate the reproductive aspects of A. bicolor collected from Serayu River (freshwater) and Segara Anakan (estuary), Cilacap, Central Java, during July 2020 field surveys. The examined parameters included eye index (EI), fin index (FI), gonadosomatic index (GSI), hepatosomatic index (HSI), and sex ratio of eel specimens in freshwater and estuary. Furthermore, the total length (TL) and body weight (BW) ranged from 260 - 630 mm and 18 - 419 g in freshwater, and 260 - 630 mm and 18 - 447 g in estuary. Significant differences (P&lt;0.01) were observed in the reproductive characteristics between the two habitats (P&lt;0.01), with higher eye index, fin index, gonadosomatic index, and hepatosomatic index in freshwater-caught eels than the values obtained in estuary inhabitants. The mean of these four parameters were 6.99 ± 2.98%, 4.08 ± 0.71%, 1.70 ± 1.13%, and 1.92% ± 1.07 in freshwater, while 3.48 ± 1.60%, 3.45 ± 0.479%, 1.12 ± 0.23%, and 0.28 ± 0.19% were found in estuary. The sex ratio was significantly affected by habitat, with female eels dominating in freshwater (62.5%) compared to estuary (32%). These results suggested that eels in freshwater habitats might exhibit early gonadal maturation compared to those in estuary.

Alternate Day Feeding as a Cost-effective Strategy for Tank Culture of the Pacific Shortfin Eel Anguilla bicolor pacifica

There is a growing interest in the aquaculture of tropical anguillid eels as an export commodity. However, studies on feeding strategies, and the present demand to reduce feed costs need to be addressed to ensure the economic viability of eel farming. In this study, the effects of daily (DF) and alternate day (ADF) feeding on growth, feed utilization, body composition, blood chemistry, liver and intestinal morphology, and economic viability in the Pacific shortfin eel Anguilla bicolor pacifica were examined. Each feeding group of 30 elvers (166.25 ± 16.23 g mean initial wt) were randomly stocked in triplicate 4 m3 outdoor concrete tanks. These were fed for 155 d with formulated eel powder diet (49.77% crude protein; 10.21% crude lipid) made into a paste. Growth and survival were not significantly different between the two feeding groups. However, feed efficiency was improved in the ADF group, with significantly higher protein efficiency ratio (0.63) than in the DF group (0.39). Feed conversion ratio (FCR) was lower in ADF (3.85) than in the DF group (6.44), resulting in a 40% reduction in total feed consumption. Biometric indices, body proximate composition, and blood chemistry were not significantly affected. Likewise, liver and intestinal morphology showed no apparent alterations between groups. Partial costs-and-returns analysis showed that ADF yielded higher net profit and profit index. These results suggest that feeding on alternate days promote compensatory growth, better feed utilization, and normal physiological condition of A. bicolor pacifica, and reduce feed cost in the rearing process. Therefore, alternate day feeding should be promoted as a sound feed management strategy in the tank culture of tropical anguillid eels.

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.

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.

Molecular confirmation of the occurrence of Anguilla interioris (Actinopterygii: Anguilliformes) in North Maluku of Indonesia and mitochondrial DNA haplotype diversity among existing specimens

Nineteen species of Anguilla Schrank, 1798 are globally distributed in the world, however knowledge on the biogeography, species diversity and ecology of the 13 species of tropical anguillids in the Indo-Pacific region is highly limited. This study examined the diversity of tropical anguillids found in North Maluku of East Indonesia, which is known to have unique and highly heterogeneous habitats, complex oceanography, high biodiversity, and representativeness of Asian and Australian fauna. By means of molecular identification, two tropical anguillid eels, A. marmorata Quoy & Gaimard, 1824 and A. interioris Whitely, 1938, were confirmed to be distributed in North Maluku. This study also examined the mitochondrial DNA haplotype diversity of A. interioris, as it could contribute to our understanding of the biogeography and life history of this eel species. Our molecular analyses showed the presence of the same haplotypes along the different locations in the Indo-Pacific region. Although more samples and DNA markers are required to provide more support, the results suggest that the larvae of A. interioris from potentially different spawning sites in the Indian and Pacific oceans could be mixed together due to the complexity of oceanic currents, and when these migrating larvae reach maturity, they would likely spawn with the local eels.

Important questions to progress science and sustainable management of anguillid eels

AbstractAnguillid eels are found globally in fresh, transitional and saline waters and have played an important role in human life for centuries. The population status of several species is now of significant concern. The threats to populations include direct exploitation at different life stages, blockages to migratory routes by dams and other structures, changes in river basin management that impact habitat carrying capacity and suitability, pollution, climate change, diseases and parasites. While much has been done to understand eel biology and ecology, a major challenge is to identify the key research and management questions so that effective and targeted studies can be designed to inform conservation, management and policy. We gathered 30 experts in the field of eel biology and management to review the current state of knowledge for anguillid eel species and to identify the main topics for research. The identified research topics fell into three themes: (a) Lifecycle and Biology; (b) Impacts and (c) Management. Although tropical anguillid eels are by far the least well understood, significant knowledge gaps exist for all species. Considerable progress has been made in the last 20 years, but the status of many species remains of great concern, particularly for northern temperate species. Without improved engagement and coordination at the regional, national and international level, the situation is unlikely to improve. Further, adaptive management mechanisms to respond to developments in science, policy and our knowledge of potential threats are required to ensure the future of these important and enigmatic species.

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.

Locomotor activity and growth response of glass eel Anguilla marmorata exposed to different salinity levels

Temperate eels are well known for partial migration where contingents may remain settled in marine habitats or move between seawater and brackish and freshwater habitats during their sub-adult life. This behaviour is less described in tropical Anguillid eels, particularly as it relates to salinity preference, habitat selection, and growth. In this study, glass eels Anguilla marmorata were collected from an estuary near the Cagayan River, northern Philippines, and observed for their behaviour and growth response in freshwater, brackish water, and seawater. A. marmorata glass eels spent most of their time in freshwater and brackish water and the least amount of time in seawater throughout the 30-min trial. Some individuals showed switching behaviour between salinity choices, but freshwater was the final selection. Growth-in-length was moderately and significantly lower for seawater-reared glass eels than those reared in freshwater and brackish water. Glass eels reared in freshwater had the highest survival, followed by those reared in brackish water and seawater. Habitat choice, growth, and survival trials all indicated that A. marmorata glass eels found in the Cagayan River have a major preference for freshwater and brackish water over seawater. These findings raise concerns with regard to the increased fishing pressure of eels, water pollution, and quarrying which is concentrated in freshwater habitats.

Habitat segregation and migration in tropical anguillid eels, Anguilla bengalensis bengalensis and A. bicolor bicolor

Anguillid eels of the genus Anguilla, which have a unique catadromous life history, are widely distributed across many parts of the world. However, little research has been conducted on the behavioural mechanisms of habitat segregation between sympatric species in tropical anguillid eels. To understand the ecological and behavioural mechanisms involved in the life history and migration of tropical anguillid eels, strontium (Sr):calcium (Ca) ratios were examined in otoliths of A. bengalensis bengalensis (41 specimens) and A. bicolor bicolor (130 specimens) collected from ten rivers in northwestern Peninsular Malaysia. The otolith Sr:Ca ratios revealed different habitat use between the two species. The broad range of otolith Sr:Ca ratios and habitat shift found in A. bicolor bicolor suggested that its habitat utilization was opportunistic in environments of varying salinity. A. bicolor bicolor prefers to live in the midstream to downstream areas with tidal influences. A. bengalensis bengalensis, however, was found to only reside in freshwater environments throughout their continental growth. A. bengalensis bengalensis tends to live in upstream area with no tidal influence. Their habitat use, migratory history, and habitat distribution indicate that habitat segregation occurs between the two species, leading to the different habitat preferences in tropical river systems.

Molecular and morphological evidence for the identity of the giant mottled eel, Anguilla marmorata in Southeast Asia

Tropical anguillid eels account for two-thirds of all species in the genus Anguilla, however information on the species diversity, biogeography, and life histories is very limited. The anguillid eels found in Southeast Asian waters, i.e. Malaysia, Thailand and Vietnam were identified using molecular genetic analysis after morphological observation. This is the first record of the occurrence of A. marmorata in Thailand and Vietnam that was confirmed by both morphological and molecular genetic analyses. The molecular phylogenetic tree and the haplotype network of A. marmorata in Malaysia, Thailand and Vietnam suggested that the eel might be transported from the western North Pacific spawning area. We also propose possible dispersion and migration of A. marmorata into Southeast Asian waters.

Occurrence and ecological implication of a tropical anguillid eel, Anguilla marmorata, in Brunei Darussalam, Borneo Island

Tropical anguillid eels account for two-thirds of the 19 species in Anguilla Schrank, 1798. However, information on the species diversity, geographical distribution, and life histories of the tropical eels is very limited. Recent studies suggested that morphological species identification of the tropical anguillid eels should be validated by molecular analysis for accurate identification. After surveying for three years, two anguillid eels were found in Brunei Darussalam, Borneo Island. They were firstly identified as Anguilla marmorata Quoy &amp;amp; Gaimard, 1824 using morphological analysis and further gene analysis of cytochrome c oxidase subunit I (COI) confirmed the species identification. This study is the first comprehensive description of A. marmorata in Brunei Darussalam, Borneo Island. Furthermore, it is also the first study to validate two anguillid eels collected from the tropical Bonin Islands of Japan as A. marmorata by means of morphological and COI analyses. The molecular phylogenetic tree and haplotype network analyses suggest that A. marmorata found in Brunei Darussalam would belong to the North Pacific population of the westernmost distribution.

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

Jun Aoyama, Sam Wouthuyzen, Michael J. Miller, Hagi Y. Sugeha, Mari Kuroki, Shun Watanabe, Augy Syahailatua, Fadly Y. Tantu, Seishi Hagihara, Triyanto, Tsuguo Otake, and Katsumi Tsukamoto (2018) 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. Twenty-eight anguillid leptocephali were collected at 13 different stations, with genetic identification indicating that 3 species of eels had spawned in the two areas. Larvae were more abundant in the Celebes Sea (N = 21; 16.0-52.1 mm TL) than in Tomini Bay (N = 7; 9.6-54.8 mm). The abundant 16-21 mm size-class of Anguilla bornensis in the Celebes Sea indicated that species had recently spawned there, and spawning had also occurred in Tomini Bay by A. celebesensis (17.4 mm). These data and previous life history information suggest that A. celebesensis may have two spawning seasons in the Celebes Sea, but only one main spawning season in Tomini Bay. Anguilla borneensis may spawn at several times of the year in the Celebes Sea. Anguilla marmorata and A. biocolor pacifica spawn outside the Indonesian Seas, with A. marmorata recruiting in large numbers in the Sulawesi Island region during much of the year. Other spawning locations of A. celebesensis and A. interioris likely exist in Indonesian waters. Therefore, further research is needed to understand the reproductive ecologies and biodiversity of the tropical anguillid eels in each region of Indonesia in relation to geographic and climatic factors.

Ages at maturation of tropical freshwater eels, Anguilla bicolor bicolor and A. bengalensis bengalensis

ABSTRACTFreshwater eels are one of the world’s most valuable cultured fish species. However, there is little information available on the biological characteristics of tropical anguillid eels genus Anguilla in the silver (maturation) stage even though these tropical eels account for two-thirds of all species in the genus. In order to understand the life history of tropical anguillid eels, Anguilla bicolor bicolor and A. bengalensis bengalensis which are found in Malaysian natural waters, age at maturation was studied. Their total length, body weight and maturation were significantly increased and advanced as their age became older. The ages at maturation (stages IV and V) of A. bicolor bicolor and A. bengalensis bengalensis ranged from 5.5 to 8.5 years and from 6.5 to 10.5, respectively. Ages of maturation in the tropical eels were earlier than that of other temperate eels A. anguilla, A. rostrata, A. australis and A. dieffenbachii, reported previously, although it was similar to that of the temperate eel A. japonica. The higher growth rate found in the tropical eels associated with the tropical habitat environment might induce the earlier maturation than the temperate species.

Fecundity of the tropical catadromous eels Anguilla bicolor bicolor, A. bengalensis bengalensis and A. marmorata

Maturation is one of the most important ontogenetic transitions in an individual’s life. However, the reproductive ecology of the tropical anguillid eel genus Anguilla at the onset of oceanic spawning migration is poorly understood. To understand the reproductive ecology, the fecundity of the tropical eels Anguilla bicolor bicolor, A. bengalensis bengalensis and A. marmorata was examined using advanced migrating silver eels (Stage IV and V). A close linear relationship was found between total length and fecundity in A. bengalensis bengalensis. The fecundities of A. bicolor bicolor (0.55 to 4.96 × 106), A. bengalensis bengalensis (0.33–1.72 × 106) and A. marmorata (0.99 × 106) were within the range of those observed in temperate eels.

Occurrence and the ecological implication of a tropical anguillid eel Anguilla marmorata from peninsular Malaysia.

Recent studies suggested that accurate species identification in the tropical anguillid eels based on morphological examination requires confirmation by molecular genetic analysis. Previous studies found that two tropical anguillid eels, Anguilla bicolor bicolor and A. bengalensis bengalensis, were found in peninsular Malaysia (West Malaysia) based on morphological and molecular genetic analyses. This study is the first record of A. marmorata in peninsular Malaysia confirmed by both morphological and molecular genetic analyses. The present study also suggests that accurate tropical eel species identification is difficult by morphological identification alone; therefore, molecular genetic analysis is needed for precise species confirmation.

Diversity, distribution and different habitat use among the tropical freshwater eels of genus Anguilla

Along with the mysteries of their ecology, freshwater eels have fascinated biologists for centuries. However, information concerning species diversity, geographic distribution, and life histories of the tropical anguillid eels in the Indo-Pacific region are highly limited. Comprehensive research on the species composition, distribution and habitat use among tropical anguillid eels in the Peninsular Malaysia were conducted for four years. A total of 463 specimens were collected in the northwestern peninsular area. The dominant species was A. bicolor bicolor constituting of 88.1% of the total eels, the second one was A. bengalensis bengalensis at 11.7%, while A. marmorata was the least abundant at 0.2%. A. bicolor bicolor was widely distributed from upstream to downstream areas of the rivers. In comparison, A. bengalensis bengalensis preferred to reside from the upstream to midstream areas with no tidal zones, cooler water temperatures and higher elevation areas. The habitat preference might be different between sites due to inter-species interactions and intra-specific plasticity to local environmental conditions. These results suggest that habitat use in the tropical anguillid eels might be more influenced by ambient environmental factors, such as salinity, temperature, elevation, river size and carrying capacity, than ecological competition, such as interspecific competition.

Evidence supporting the occurrence and the ecological implication of giant mottled eel, Anguilla marmorata (Actinopterygii: Anguilliformes: Anguillidae), from Sabah, Borneo Island

Although tropical anguillid eels account for two-thirds of all species in the genus Anguilla, the information on the species diversity, geographic distribution, and life histories of the tropical eels is very limited. Recent studies suggested that accurate species identification in the tropical anguillid eels needs a validation by molecular genetic analysis after morphological observation. Two anguillid eels found in Sabah, Borneo Island, were firstly identified as Anguilla marmorata Quoy et Gaimard, 1824 using morphological analysis and further analysis of mitochondrial 16S ribosomal RNA (16S rRNA) sequences confirmed the morphological species identification. The presently reported study represents the first description of A. marmorata in Sabah, Borneo Island. One-year survey suggests that representatives of A. marmorata found in the region might belong to the North Pacific population in the westernmost distribution.

Current status and concerns on tropical anguillid eel stocks

@font-face { font-family: "ＭＳ 明朝"; }@font-face { font-family: "ＭＳ 明朝"; }@font-face { font-family: "@ＭＳ 明朝"; }@font-face { font-family: "Calibri"; }p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0mm 0mm 0.0001pt; text-align: justify; font-size: 12pt; font-family: Calibri; }.MsoChpDefault { font-size: 11pt; font-family: Calibri; }.MsoPapDefault { margin-bottom: 6pt; }div.WordSection1 { } Freshwater eels of genus Anguilla have fascinated biologists for centuries due to the spectacular long-distance migrations between the eels’ freshwater habitats and their spawning areas far out in the ocean and the mysteries of their ecology. The spawning areas of Atlantic eels and Japanese eel were located far offshore in the Atlantic Ocean and the Pacific Ocean, respectively, and their reproduction took place thousands of kilometres away from their growth habitats. Phylogenetic studies have revealed that freshwater eels originated in the Indonesian region. However, remarkably little is known about the life histories of tropical freshwater eels despite the fact that tropical eels are key to understanding the nature of primitive forms of catadromous migration. Recently, the juvenile abundance has declined dramatically: by 99% for the European eel and by 80% for the Japanese eel. Recruitment of the American eel near the species’ northern limit has virtually ceased. Other eel species, including Australian and New Zealand eels, also show indications of decline. The main problem is that all young eels used in cultivation are wild juveniles (glass eels and elvers), which are captured in estuaries. Almost all (90 %) of the total world eel supply comes from aquaculture. Therefore, the supply of eel resources for human consumption is completely dependent on wild catch. Commercial eel industries are now considering tropical eels as possible replacement for European and Japanese eels to compensate for declining stocks. However, useful scientific research and information on the biology and stock assessments of tropical eels are lacking, a situation quite different from that for other temperate freshwater eels, which have been well studied for several decades with trends and recruitment patterns being on record. Nevertheless, the present tropical eel catch has been reported as being less than half that of 20 years ago. The present trends in eel stocks and utilization for human consumption suggest that all eel populations will decline to numbers that fall outside safe biological limits and will be seriously threatened with extinction without protection and conservation from strict enforcement of local and international laws.

CURRENT STATUS AND PROBLEMS OF THE CATCH STATISTICS ON ANGUILLID EEL FISHERY IN INDONESIA

To compensate the decline of the populations of temperate anguillid eels, tropical anguillid eels become getting attention of East Asian eel market in recent years. Many eel farms have been established in Java Island to culture tropical anguillid eels intending to export the products to East Asia. Since eel farming is reliant on wild-caught anguillid eels such as glass eels, elvers and yellow eels, these eel seeds have been captured in various places in Indonesia. However, it is still unknown that how much of tropical anguillid eels are caught as seeds for eel farming. This study showed two different patterns of the commodity chains of eel seeds from both Sukabumi Regency and Bengkulu Province to the eel farms in Java Island. Official catch statistics on anguillid eels found in both Sukabumi Regency and Bengkulu Province were also analyzed on their features and problems underlied. Considering the sustainable use of anguillid eel resources and critical stances on exploitation of eel seeds from all over the world, the Indonesian government should take an immediate action for developing the national catch statistics on anguillid eel fishery as soon as possible.

Genetic and migratory evidence for sympatric spawning of tropical Pacific eels from Vanuatu

The spawning areas of tropical anguillid eels in the South Pacific are poorly known, and more information about their life histories is needed to facilitate conservation. We genetically characterized 83 out of 84 eels caught on Gaua Island (Vanuatu) and tagged 8 eels with pop-up satellite transmitters. Based on morphological evidence, 32 eels were identified as Anguilla marmorata, 45 as A. megastoma and 7 as A. obscura. Thirteen of these eels possessed a mitochondrial DNA sequence (control region, 527 bp) or nuclear haplotype (GTH2b, 268 bp) conflicting with their species designation. These individuals also had multi-locus genotypes (6 microsatellite loci) intermediate between the species, and 9 of these eels further possessed heterozygote genotypes at species-diagnostic nuclear single nucleotide polymorphisms (SNPs). We classified these individuals as possibly admixed between A. marmorata and A. megastoma. One A. marmorata and one A. megastoma migrated 634 and 874 km, respectively, towards the border between the South Equatorial Current and the South Equatorial Counter Current. Both species descended from around 200 m depth at night to 750 m during the day. Lunar cycle affected the upper limit of migration depths of both species. The tags remained attached for 3 and 5 mo and surfaced <300 km from the pop-up location of a previously tagged A. marmorata. A salinity maximum at the pop-up locations corresponding to the upper nighttime eel migration depths may serve as a seamark of the spawning area. The similar pop-up locations of both species and the evidence for admixture suggest that these tropical eels share a sympatric spawning area.