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Parentage sibship-inference analyses were conducted using mtDNA sequencing and six microsatellite genotypes of 182 Japanese eel preleptocephali that were collected from one net-tow near the West Mariana Ridge in May 2014. At least 328 parents were involved in producing the 182 preleptocephali, and several parents may have spawned a few times during 3 days of a spawning period. Half-sibs suggested that a few parents mated with 1-3 partners, indicating that the Japanese eel can form spawning aggregations in which several parents mate with each other in the ocean.

Selection acting across environmental gradients, such as latitudes, can cause spatial structuring of genomic variants even within panmictic populations. In this study, we focused on the within-generation latitudinal selection between northernmost and southernmost individuals of the North Pacific population of a tropical eel Anguilla marmorata, which shares its northernmost distribution with a temperate eel Anguilla japonica. Whole-genome sequencing data indicated that the northernmost and southernmost individuals of A. marmorata belong to a single panmictic population, as suggested by previous studies. On the contrary, parts of genomic regions across multiple chromosomes exhibited significant genetic differentiation between the northernmost and southernmost individuals, and in these genomic regions, the genotypes of the northernmost individuals were similar to those of A. japonica. These findings suggested within-generation latitudinal selection of A. marmorata, which might have led to genetic closeness between northernmost A. marmorata and A. japonica.

Rainbow trout, Oncorhynchusmykiss, is one of the most widely introduced fish species in the world, and its impacts on native fishes and ecosystems are of considerable concern. One of the rivers inhabited by rainbow trout in the Shiretoko Peninsula is the Chinishibetsu River, and the origin of rainbow trout in this river is thought to be Lake Chinishibetsu in the upper reaches of the system, where the private stocking of rainbow trout was conducted in the 1960s. However, the basic biology of rainbow trout in the Lake Chinishibetsu area is currently unknown. This study addresses this knowledge gap by examining the biology of rainbow trout in the inlet tributaries of Lake Chinishibetsu based on sampling conducted during the rainbow trout spawning season. A total of 104 rainbow trout, ranging in age from 1+ to 8+ years, were collected from the two inlet tributaries of Lake Chinishibetsu. White-spotted charrSalvelinus leucomaenisand Siberian stone loachBarbatula oreas, neither of which is native to the Shiretoko Peninsula, were also collected and had presumably invaded the area at the same time as the rainbow trout. The sampled rainbow trout included immature and mature males and females. The distribution of fork lengths of mature females was bimodal, and the sex ratio of mature rainbow trout was male-biased. Our results indicate that the rainbow trout population in the inlet tributaries of Lake Chinishibetsu is reproducing continuously and exhibits a dimorphic life history with river residents and lake migrants of both sexes. Additionally, rainbow trout continue to be collected downstream of the Chinishibetsu River, which is the primary habitat for this species in the Shiretoko Peninsula. Therefore, unless rainbow trout are eliminated from Lake Chinishibetsu, which serves as a source of non-native species upstream of the Chinishibetsu River, it will be difficult to control rainbow trout distributions and minimize population sizes on the Shiretoko Peninsula.

International audience; Life-history traits of eels display a high level of phenotypic plasticity in response to large-scale biogeographical drivers, as well as local conditions encountered during the continental phase. Here, we provided a biogeographical snapshot of the variability of life-history traits of eels (Anguilla anguilla), across a large proportion of their natural distribution range. Silver eels (n = 99) were collected across eleven European catchments to investigate how life-history traits vary along geographical and saline habitats, as it was inferred from the Sr:Ca ratio in otoliths. Among 13 life-history traits tested, 3 of them such as total length, body or liver weight were related to geographical coordinates. Overall, eels grow faster in southern Europe and migrate earlier suggesting that the silvering process is related to the local growth conditions more than fish age. The salinity profiles revealed by the otoliths' Sr:Ca ratios indicate that eels with a brackish life-history generally grow faster, reach larger size-at-age, and have a better condition than eels living in freshwater. This observation associated with the lower abundance of the sanguivorous swimbladder nematode, Anguillicola crassus, confirms the importance of brackish areas for sustaining the eel production. A large proportion of the observed variation of life-history traits remained unexplained by the biogeographical trends and salinity condition, which suggests that other drivers act at the catchment scale.

It is known that environmental parameters, such as water depth, size of substrate material, and current velocity, influence the organization of stream fish communities. However, few studies have investigated the effects of these parameters on the sympatric freshwater eels. Here, the habitat use of two anguillid eels, the Indo-Pacific eel (Anguilla marmorata Quoy and Gaimard, 1824) and the Japanese eel (Anguilla japonica Temminck and Schlegel, 1846), coexisting in the same river systems in Japan, was investigated. It was found that the tropical species A. marmorata exclusively used habitats with larger substrate materials, fewer fallen leaves and less leaf detritus, higher current velocity, and lower turbidity than the temperate species A. japonica, and the distributions of the two anguillid eels were clearly segregated. Thus, when both species co-occur in the same river systems, clear flowing mainstems and tributaries are preferred habitats for A. marmorata, whereas stagnant muddy estuaries, backwater areas, irrigation channels, and reservoirs are preferred habitats for A. japonica. These habitat segregations were consistent as body size increased and life-stage developments from elver to yellow eel and did not show ontogenetic shifts. The findings indicated that both species had strong habitat preferences, and these environmental parameters must be considered in the conservation of anguillid eels in sympatric zones.

In mixed fisheries where multiple species are caught, to manage resources sustainably, knowledge about the species-specific vulnerability to fishing is equally or even more important than knowledge of size selectivity of the gear. We compared the vulnerability to bait recreational angling in four salmonid species in natural streams in Japan. The ranking of species-specific angling vulnerability was as follows (from highest to lowest): rainbow trout (Oncorhynchus mykiss), masu salmon (Oncorhynchus masou), white-spotted char (Salvelinus leucomaenis), and southern Asian Dolly Varden (Salvelinus curilus). In all species, larger individuals were more vulnerable to angling, but there were differences in the size dependence between species. In rainbow trout and Dolly Varden (which have a nonanadromous life history in the study area), the probability of being caught monotonically increased with body size, while the vulnerability to angling in masu salmon and white-spotted char (which have an anadromous life history in the study area) showed a domed-shaped pattern. We found that across the species the catch per unit effort showed a hyperstable relationship with population density. Therefore, diminishing local populations are prone to collapse, and this collapse would be hard to foresee based on catch rate data alone.

Colonisation of freshwater habitats by marine animals is a remarkable evolutionary event that has enriched biodiversity in freshwater ecosystems. For successful freshwater colonisation, high physiological plasticity is presumed to be necessary, but its evolutionary basis has not been detailed. Marine-originated amphidromous species, which regularly migrate between freshwater and marine environments, have repeatedly lost migratory behaviour in many lineages, which sometimes triggered species radiation in freshwater habitats. Since amphidromous species typically visit the sea during the larval period, the difficulty in the evolution of larval freshwater tolerance is a bottleneck for freshwater colonisation. To elucidate the key evolutionary changes that enhance the physiological plasticity for freshwater colonisation, we compared larval gene expression changes depending on salinity conditions among three congeneric amphidromous goby species (Gymnogobius) with varying dependences on freshwater habitats. First, an otolith microchemical analysis and rearing experiment under laboratory conditions confirmed the presence of freshwater residents only in G. urotaenia and higher larval survivorship of this species both in seawater and freshwater conditions than the obligate amphidromous G. petschiliensis and G. opperiens. Larval whole-body transcriptome analysis revealed that G. urotaenia exhibited the greatest differences in the expression levels of several osmoregulatory genes, including aqp3, which is critical for water discharge from their body during early fish development. Thus, we obtained the results that consistently support the importance of enhanced osmoregulatory plasticity for establishing freshwater forms, and further identified some important evolutionary changes for larval freshwater adaptation and colonisation in the goby group.

Scientific naming rules of animals are strictly defined by the International Code of Zoological Nomenclature, while those for common names are vague and not well defined. Specifically, the common names of freshwater eels of the genus Anguilla have become confused among scientific papers, pictorial books, and online resources in recent years. This disordered use of common names among freshwater eels demonstrates the urgent need for the standardization of common names for these species. In addition, freshwater eel populations have drastically decreased worldwide over the past few decades, resulting in their listing as endangered species. In the present study, we defined the following five rules for the common names of freshwater eels: to (1) use a representative locality name of the geographic distribution of the species or subspecies, (2) distinguish two sympatric species or subspecies as longfin or shortfin, (3) select a name that would enable the scientific name to be easily recalled, (4) value longstanding former common names if they adhere to the above three rules, and (5) use the shortest name possible. Based on current scientific knowledge and on these rules, we proposed 22 common names for all known 19 species and subspecies of the genus. As a result, 21, 6, 13, and 3 species and subspecies were named based on Rules (1), (2), (3), and (4), respectively.

Alternative life‐history tactics of masu salmon Oncorhynchus masou are well documented. Subsequent to the freshwater parr stage (age ≥ 1 + years), an anadromous form migrates to the sea after smolting, while a resident form matures without seaward migration. In addition to this typical migratory dimorphism, anecdotal reports based on field observations have indicated that some underyearling masu salmon use estuarine waters. However, no empirical evidence indicates saltwater utilisation and subsequent survival in the early parr stage. Here, we used otolith microchemistry to examine whether a portion of masu salmon parr in northern Japan enters coastal habitats. The otolith Sr:Ca ratios of most juveniles collected from six rivers had consistently low values, indicating that masu salmon parr inhabiting these rivers stay only in freshwater. In contrast, in individuals from a steep‐gradient river the Sr:Ca ratios increased at about a 200‐µm distance from the otolith core. These results suggest that some masu salmon parr might use brackish water or sea water temporarily. In addition, three masu salmon parr were found in another steep river where a culvert located only ten metres from the river mouth completely blocked upstream migration for spawning. The Sr:Ca ratios in these fish increased at about >200 µm from the otolith core, indicating the parr had immigrated to the non‐natal river from the sea. Such flexible behaviour at an early life stage may contribute to the spatial expansion of masu salmon, and the movement could moreover help to stabilise its population dynamics.

Tropical eels, Anguilla marmorata, and temperate eels, Anguilla japonica, coexist in rivers on Yakushima Island, southern Japan. Differences in growth rates and habitat use of these species were examined by conducting surveys at 14 stations in seven river systems of the island. Anguilla marmorata (n = 93) were collected from all seven river systems surveyed, four along the eastern coast and three along the western coast, while A. japonica (n = 52) were collected only from the three river systems on the western coast. Only one of the two species was captured at any given sampling station, but both species were collected from a concrete-paved channel of a small river. Anguilla marmorata occurred in fast-flowing river mainstems and tributaries, where the substratum comprised large rocks, while A. japonica frequented irrigation channels and stagnant reservoirs, where the substratum comprised smaller rocks, sand, or mud. Comparison of von Bertalanffy growth curves suggested that A. marmorata in Yakushima Island grows slower than A. japonica and its conspecifics in tropical regions. The mean gonadosomatic indices of female A. marmorata from Yakushima Island were also lower than those of its tropical conspecifics. The slow growth of A. marmorata in Yakushima Island might be a result of the cool and oligotrophic environment near the northern limit of their geographic distribution and environmental tolerance.

A Japanese eel Anguilla japonica was collected off Kozushima Island, one of the volcanic Izu Islands in Japan. The external morphology corresponded to a silvering stage (Silvering Index: S1) with a melanized body coloration of the head, and the tip of the lower jaw to the anus. However, the value of the gonado-somatic index of the female eel was low, and the oocytes were not matured (peri-nucleolus stage). Otolith analyses indicated that the individual had exclusively inhabited the sea for five years without entering freshwater. This record is the first collection of silver-phased Japanese eel in the Kuroshio region, which could be on the spawning migration route of the species, although it was not determined if the eel was settling around the Izu Islands or migrating to the spawning ground.

Early larval growth of the Japanese eel Anguilla japonica reared at five different water temperatures (19, 21, 23, 25, and 27 °C) is described for the first month post-hatching, a critical period of time in the culture of these sensitive larvae. After 3 weeks of culture, larvae reared at 27 °C attained the greatest total length and body height of all temperature treatments. Larvae reared at higher temperatures developed rapidly due to active feeding, despite higher metabolic costs. At lower temperatures of 19–21 °C, sudden mortality 12–16 days post-hatching coincides with the transition of larvae from endogenous to exogenous feeding. A linear relationship between water temperature and otolith diameter for cultured eel larvae otoliths suggests that the mean ambient water temperature experienced by wild larvae during their first month after hatching is about 23 °C. This temperature corresponds to mean water temperatures likely experienced by larvae during diel vertical migrations in the North Equatorial Current. We propose rearing early stage Japanese eel larvae at variable temperatures throughout the light/dark cycle to enhance their survival and growth in captivity.

The unique semi-enclosed Indian Ocean basin includes large Mascarene Plateau banks, offshore coral-reef islands, seasonal equatorial current jets, and cross-basin westward South Equatorial Current (SEC) flow, making it interesting for studying long larval-duration eel larvae (leptocephali) and regional eel biodiversity. Three surveys for leptocephali (in 2003, 2006, 2010) included sampling west of the Mascarene Plateau (west), a major survey and other stations off Sumatra and Java (east), and 2 cross-basin transects across the SEC. The highest numbers of leptocephali species were observed along Sumatra (2003: ~143 species; 2006: 72 species) and south of Java (2010: 69), with intermediate numbers being collected in the western Indian Ocean (2006: 71; 2010: 53) compared to low numbers in the hydrographically variable offshore zones (2006, 2010: 3–27). The larger continental shelf areas along Sumatra including the Mentawai Islands provide more coral reef and other habitats for species such as congrid, muraenid, ophichthid, and chlopsid eels compared to the Mascarene Plateau banks. Some larvae in these areas get transported offshore, but the majority of offshore larvae were of Nemichthyidae and Serrivomeridae mesopelagic eels that were spawning across the basin. Habitat differences between the southern Mascarene Plateau and Sumatra and southern Indonesia along the edge of the high biodiversity Coral Triangle likely explain the higher biodiversity of eel larvae observed along the western side of the basin, which for the Congridae and Ophichthidae included more species than observed previously within the central Indonesian Seas. In addition to local spawning, seasonal currents likely transport larger larvae towards Sumatra from the north or west and larvae may enter the basin from the Indonesian Throughflow in the east, but it is unknown if equatorial jets or the SEC can transport larvae across the whole basin.

The lateral line system and its innervation were examined in the Japanese eel Anguilla japonica. Although the species has seven lateral line canals and 13 superficial neuromast groups, the components are generally similar to those in many other teleosts. The lateral line system of A. japonica is distinctive in having a rostral commissure connecting the left and right supraorbital canals, pouches in the cephalic lateral line canals and superficial neuromasts along the lower lip, and lacking a postotic canal. Four tube-like elements, two along the supratemporal canal and the other two along the temporal portion of the trunk canal, respectively, are also reported. The functional significance of cephalic lateral line pouches, homologies of the four tube-like elements, and other distinctive characters are discussed.

Water-choice trial experiments revealed that Anguilla japonica glass eels collected in southern Japan possess strong preferences for fresh water and agricultural water. Their locomotor activity and preference for fresh water were higher and stronger, respectively, in this study when compared to previous studies conducted at lower temperatures. These results suggest that their locomotor activity and preference for fresh water is influenced by water temperature. The attraction to agricultural water indicates their upstream migration and habitat selection could be influenced by agricultural water.

Oxygen isotope ratios of the core region of otoliths were examined in Anguilla japonica glass eels collected from two rivers in Japan to verify the possible temperature and depth layer experienced by these eels when they were at the egg stage in their spawning area. To determine the relationship between otolith δ18O values and water temperature, the otoliths of glass eels reared under four different temperatures (15, 20, 25 and 30 °C) were analyzed. The otolith δ18O values showed an inverse relationship to ambient water temperature. Linear regression of the fractionation between otolith oxygen isotopic ratio from the δ18O of seawater and water temperature produced a precisely determined relationship from 15 to 30 °C: δ18Ootolith,PDB − δ18Oseawater,SMOW = −0.153 × T (°C) + 1.418. The δ18O core,PDB values of the otolith core region of the glass eels from the two locations were −2.53 ± 0.12 and −2.59 ± 0.07 respectively, and could be converted to water temperatures of 26.3 ± 0.8 °C and 26.7 ± 0.4 °C, respectively, using the equation and assuming a seawater δ18Oseawater,SMOW = 0.06‰. The water depth corresponding to these temperatures is ∼150 m in the water column in the spawning area of Japanese eels, which corresponds to the upper-most part of the thermocline and chlorophyll maximum in the vertical hydrographic profile. These results were consistent with the field studies that egg development after the beginning of otolith formation and hatching occurs around the upper-most part of thermocline, suggesting that stable isotope micro-analysis is a powerful method to extrapolate unknown spawning ecology of fishes.

The Indonesian Seas are at the center of the Coral Triangle, which has the highest marine biodiversity in the world, and the region is under threat from climate change. Freshwater habitats in the region have a high number of anguillid eels compared to other regions of the world, but it is more difficult to capture marine eels to assess their biodiversity. Catches of leptocephali from 5 internationally collaborative surveys for eel larvae (leptocephali) in the Coral Triangle have collected about 126-169 species of larvae, which indicates that the Coral Triangle region likely has the highest marine eel biodiversity in the world based on comparisons to similar larval surveys in the Indian, Pacific, and Atlantic oceans (29-107 species). These marine eel species inhabit a wide range of benthic and pelagic habitats, but how they might be affected by climate changes such as ocean warming has not been considered. Anguillid eels in the Coral Triangle region could be affected mainly by changes in rainfall patterns that could affect their freshwater growth stage or their reproductive maturation patterns and migration. Effects on marine eels would depend on the types of habitats where they live, with the least impacts occurring for deep benthic or pelagic species. Marine eels that live in shallow habitats would be most affected if warming seas and coral bleaching reduce the types of prey species they depend on. Based on their possible association with coral reef habitats, eels of the families Muraenidae and Chlopsidae appear to the most likely types of eels to be impacted by changes in community structure resulting from coral bleaching. All leptocephali species live in the ocean surface layer where they feed on marine snow, so warmer ocean temperatures might reduce the amount or quality of marine snow that is available, resulting in lower larval survival rates. Further studies on eel biodiversity and habitat use will provide more insight into the possible loss of endemic species in the Coral Triangle due to climate change, but presently it is unclear how many species of eels may be directly affected by climate change.

The Indonesian Seas in the center of the Coral Triangle have high marine biodiversity, and a unique area is Sulawesi Island that has the large semi-enclosed Tomini Bay, including the Togian Islands, formed within the island. A sampling survey for leptocephali was conducted in March 2010 at 48 stations in the southern Celebes Sea and in Tomini Bay that collected 2056 leptocephali (6.0–319 mm) of at least 11 anguilliform and 3 elopomorph families and ∼126 species. About 44 Muraenidae, 26 Ophichthidae, 25 Congridae, and 11 Chlopsidae species and 5 species of each of the other families were collected. The congrid, Ariosoma scheelei, the chlopsid, Kaupichthys, serrivomerids, Nemichthys and an ophichthid species of Neenchelys were the most abundant species/taxa. Comparisons of the two sampling areas found that ∼105 species were among the 1090 leptocephali from 21 tows in Tomini Bay and ∼78 species were among the 966 leptocephali from 30 Celebes Sea tows. Species such as chlopsids (25% of leptocephali in Tomini Bay), some muraenids, congrids, moringuids, and serrivomerids were more abundant in Tomini Bay than in the Celebes Sea, and some rare species were only collected there. The wide size range of most taxa indicated that many species spawn in the bay and their larvae are likely retained there. Large eddies in the Celebes Sea appeared to have transported leptocephali offshore. The high biodiversity of marine eel larvae suggest the unique semi-enclosed Tomini Bay may offer a good environment for spawning and self-recruitment, which may partly explain the high number of species of some families. More research is needed to evaluate the species composition of eels in Tomini bay and the wider region and to facilitate understanding and conservation of the diverse marine habitats of the Indonesian Seas.

Research efforts to achieve the production of artificial seedlings of the Japanese eel Anguilla japonica have progressed in recent decades. However, morphological deformities have been frequently observed in reared leptocephali and glass eels. We examined the effect of water current velocities (5.7–8.3 cm/s) on the body size and morphology of reared leptocephali and metamorphosed glass eels. As the current velocity increased, the size of leptocephali became smaller and the occurrence rate of notochord curvature increased. However, even in low velocities, water current had a long-term negative impact on their morphology. Sixty-five percent of metamorphosed glass eels had one of the eight types of vertebral deformities: compression, luxation, fusion, brachyspina, modification, lordosis, kyphosis, or scoliosis. Although their occurrence rate was unrelated to current velocity, there was a tendency for some deformities to be localised in a certain area of the vertebral column. In particular, compression frequently occurred in caudal vertebrae in faster currents. Most vertebral deformities began before the completion of metamorphosis. Therefore, appropriate management of the water current during the leptocephalus stage is important for establishing mass production of morphologically normal glass eels.

Wild anguillid eel larvae inhabit the ocean during their early life stages and never experience low-salinity water until the glass eel stage. The larvae show less mortality in half-diluted seawater than in full-strength seawater in captivity; however, physiological influences of environmental salinity on eel larvae have not been clarified. In this study, we compared the distributional and functional features of ionocytes between Japanese eel larvae acclimated to half-diluted and full-strength seawater. The mean tissue fluid osmolality in larvae acclimated to half-diluted seawater (300 mOsm/kg H2O) was slightly lower than in those (344 mOsm/kg H2O) acclimated to full-strength seawater. The density and opening size of ionocytes in the skin were not significantly different between the two salinities. Na+/K+-ATPase-immunoreactive ionocytes showed Na+/H+ exchanger-3 (NHE3) and cystic fibrosis transmembrane conductance regulator (CFTR) immunoreactions in their apical region and Na+/K+/2Cl- cotransporter-1 (NKCC1) immunoreaction in their basolateral region, suggesting that the skin ionocytes are involved in salt secretion in both salinities. In transmission electron microscopic observation, the ionocytes of larvae in full-strength seawater were characterized by the electron-dense cytoplasm, expanded tubular system and well-developed mitochondria, compared with those in half-diluted seawater, suggesting that the salt-secreting function was more activated in full-strength seawater than in half-diluted seawater. These results suggest that the energy metabolism cost of ion regulation could be lower in the intermediate salinity environment, which is closer to their osmolality than full-strength seawater. Hence, it is hypothesized that the saving of energy required for osmoregulation in half-diluted seawater could be favorable to better survival and growth of artificial eel larvae.

International audience; ABSTRACT: The feeding ecology of leptocephali has remained poorly understood because they apparently feed on particulate organic matter (POM), which varies in composition, and it is unclear which components of the POM they assimilate. The δ13C and δ15N stable isotope (SI) and fatty acid (FA) compositions of 3 families of leptocephali and POM were compared in 3 latitudinal current zones of the western South Pacific. The δ15N signatures of leptocephali and POM overlapped, with both having their lowest values in the southern current zone. POM in general (across all zones) contained 38 FAs and was rich in saturated FAs (SFA) (16:0, 18:0, 14:0), while leptocephali contained 50 FAs, with high proportions of 16:0, and higher contributions of 22:6ω3, 20:5ω3, 18:1ω9, 16:1ω7 and other FAs than found in the POM. Serrivomeridae leptocephali in the north had higher δ15N signatures and were also distinguished from Nemichthyidae and Muraenidae larvae by their FA compositions (higher SFAs, lower 22:6ω3 and 20:5ω3). Although SI signatures of the Serrivomeridae larvae did not clearly vary with size, 16:0 and 18:0 FA proportions decreased with increasing larval size, and 22:6ω3 and 16:1ω7 increased in larger larvae. Correspondences between the latitudinal variations in nitrogen SI signatures and FA compositions of POM with those of leptocephali and the presence of FA markers of both autotrophic and heterotrophic organisms were consistent with leptocephali feeding on POM. POM can contain various materials from primary producers and heterotrophic microorganisms, but differences in the SI signatures and FA compositions in leptocephali remain to be explained through further research

The anguillid eels are catadromous fishes that migrate between marine and freshwater habitats. The long migration of eel larvae, called leptocephali, as long as thousands of kilometers in the ocean is important to determine their recruitment successes. The leptocephali in the ocean have a pelagic lifestyle totally different from the benthic one of glass eels and yellow eels in rivers. It is known that eel leptocephali have ionocytes on the body surface that may maintain ionic and osmotic status in the internal environment; however, detailed morphology and function of ionocytes in leptocephali are still unknown. In the present study, we aimed 1) to clarify the morphological features of the epidermis in Japanese eel Anguilla japonica leptocephali cultured in hyper-osmotic condition, and 2) to examine the ion-transporting functions of ionocytes of both leptocephali and yellow eels. Na+/K+-ATPase-immunoreactive ionocytes were distributed all over the body surface of leptocephali. Ionocytes were in contact with external environments through their apical membrane, which was located at the boundary of pavement cells. Na+/K+-ATPase-immunopositive cells were not observed in the skin of seawater-acclimated yellow eels. In ionocytes of the larval skin, the apical membrane appeared as a slightly projecting disk with a microvilli-like structure. Meanwhile, the apical membrane of gill ionocytes of yellow eels formed a concave surface. In ionocytes of leptocephali, mitochondria were enlarged and the tubular system was well developed, as compared with those of the gill of yellow eels. Ionocytes of leptocephali showed CFTR immunoreaction in their apical region and NKCC1 immunoreaction in their basolateral region, suggesting that the skin ionocytes are involved in salt secretion. These results support the notion that Japanese eel maintain their ion balance through skin ionocytes during early life stages, and that the skin ionocytes of leptocephali disappear in yellow eel stages after the formation of functional gills and gill ionocytes.

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.

Several surveys and studies have examined the Atlantic anguillid eels’ larval distributions, but little is known about their larval growth rates. Otoliths of 17 European eel Anguilla anguilla (8.8–46.0 mm) and 19 American eel Anguilla rostrata (9.8–59.9 mm) leptocephali collected in the Sargasso Sea (25–31°N, 58–70°W) in March and April 2011 were analyzed and their spawning times and larval growth rates were estimated. Ages calculated from the number of otolith increments of European and American eel larvae showed ranges of 10–127 days and 14–233 days, respectively. Linear relationships between age and total length indicated early larval growth rates of 0.31 mm/day for the European eel and 0.35 mm/day for the American eel. This suggested slower growth rates in low temperatures in the Sargasso Sea compared to other anguillid species in the Indo-Pacific, where water temperatures are higher. The back-calculated hatching dates of small leptocephali (8.8–26.7 mm) were in February and March 2011. More American eels hatched in February and more European eels hatched in March. The hatching times of two larger European eel leptocephali (38.7 and 46.0 mm) and a larger American eel leptocephalus (59.9 mm) were back-calculated to November and December 2010 and July 2010, respectively, suggesting hatching times outside of the primary spawning seasons. These novel observations provide important information on the timing of spawning and larval growth characteristics of Atlantic eels, which would benefit from validation by additional otolith studies of leptocephali.

Early life-history traits of all 19 anguillid eel species and subspecies were examined to help understand the evolutionary processes of their oceanic migrations in comparison with their migration distances and the geography of their species ranges. Tropical species were found to have fewer myomeres, greater body depths, higher growth rates, shorter larval durations, and smaller maximum larval sizes than temperate species. The relationships among larval characteristics such as growth rate, age at metamorphosis, and maximum larval size differed among tropical and temperate species and corresponded with the maximum latitudes of their species ranges. Temperate eel leptocephali with slow growth and large maximum size with slender bodies appear to be specialized for long migrations and dispersal over a wide range of distances to higher latitudes, while having flexible sizes of metamorphosis and recruitment. Tropical species with faster growth metamorphose earlier at a relatively fixed size, which would facilitate larval retention near their species ranges at low latitudes. Changes in the early life-history traits of tropical eels appear to have occurred during the evolution of longer migrations as they entered temperate regions.

Temperature-driven life-history modifications by adaptation occur in ectotherms, and therefore, life-history modifications by adaptation need to be taken into consideration when predicting population responses to the climate change. Partial migration is a common form of life-history diversity in which a population contains both migratory and resident behaviours. Salmonid fish exhibit a wide range of life-history diversity and, in particular, partial migration. We evaluated the effect of temperature-driven life-history modifications on population dynamics in partially migratory masu salmon (Oncorhynchus masou) by field observations and theoretical models. Field observations revealed that spatial patterns of alternative migratory tactics were associated with temperature gradients. The occurrence of resident males increased, whereas the proportion of migrant males and the proportion of delayed migrants including both sexes decreased with increasing temperature and, thereby, with improved early growth conditions. The expected fitness for each migratory tactic was computed in a life-history model with early growth conditions as a function. Individual fitness was maximized by adopting resident tactics under favourable early growth conditions, early migrant tactics under intermediate early growth conditions and delayed migrant tactics under unfavourable early growth conditions. The results suggest that individuals exhibited a status-dependent conditional strategy, that is, the adoption of alternative migratory tactics is influenced by the status of individuals to make the best of a situation. A simulation model suggests that increased residency by males to increased temperature leads to a substantial decrease in the number of migrants. Moreover, the decrease in the number of delayed (older) migrants with increasing temperature magnified fluctuations in abundance. Our findings indicate the importance of temperature-driven life-history modifications for predicting dynamics of natural populations under climate warming.

To clarify the role of thyroid function during metamorphosis from leptocephalus to glass eel in the Japanese eel, we examined the histology of the thyroid gland and measured whole-body concentrations of thyroid hormones, thyroxine (T4) and triiodothyronine (T3), and thyroid stimulating hormone β-subunit TSH (TSHβ) mRNA expression levels in five stages of artificially hatched eels (leptocephalus, early-metamorphosis, late-metamorphosis, glass eel, and elver). During metamorphosis, the inner colloid of thyroid follicles showed positive immunoreactivity for T4, and both T4 and T3 levels were significantly increased, whereas a small peak of TSHβ mRNA level was observed at the early-metamorphosis stage. Similarly, TSHβ mRNA levels were highest in the glass eel stage, and then decreased markedly in the elver stage. In contrast to TSHβ mRNA expression, thyroid hormones (both T4 and T3) increased further from the glass eel to elver stages. These results indicated that thyroid function in the Japanese eel was active both during and after metamorphosis. Therefore, the thyrotropic axis may play important roles not only in metamorphosis but also in subsequent inshore or upstream migrations. J. Exp. Zool. 321A: 357–364, 2014. © 2014 Wiley Periodicals, Inc.

Freshwater eels of the Anguillidae are diadromous because they migrate between ocean and freshwater environments, but other anguilliform fishes are generally considered to be strictly marine species. A few marine eels of the Muraenidae and Ophichthidae have occasionally been found in freshwater or estuaries, indicating that anguillids are not the only anguilliform eels that can use freshwater in some parts of the world. The moray eel Gymnothorax polyuranodon is one species that is known to be present in freshwater in the Indo-Pacific, but its life history is unknown. One way to evaluate what types of habitats are used by fishes is to determine the ratio of strontium (Sr) to calcium (Ca) in their otoliths, because this can show if they have used freshwater or saltwater environments. To evaluate the patterns of freshwater use by this unusual species of marine eel, the otolith Sr/Ca ratios of four G. polyuranodon (275–344 mm) caught in a freshwater stream of Fiji were analyzed. The consistently low Sr/Ca values (0–4) indicated upstream movement after settlement and freshwater or estuarine residence of all four individuals. These eels did not appear to have entered freshwater just for a short time period, which is consistent with other reports that this species is present in estuarine and freshwater habitats. This suggests that G. polyuranodon may be a catadromous species of marine eel. The similarities and differences between the life histories of anguillid eels and the few marine eels that have evolved the ability to invade freshwater habitats is discussed in relation to the evolutionary origin of diadromy in anguilliform fishes that originated in the marine environment.

We evaluated the migratory patterns of white-spotted charr in eastern Hokkaido, Japan using: (1) field observation of fish, (2) salinity-recording archival tags, and (3) analysis of otolith Sr and Ca concentrations. Field observations suggest that the majority of anadromous white-spotted charr outmigrate to the sea in spring (April to June) and ascend the river between late summer and autumn (August to November). Salinity records from archival tags revealed a fluctuating pattern consisting of both low and high values within a month, indicating frequent short-term movements between freshwater (including non-natal rivers) and brackish/salt water habitats. Otolith Sr:Ca ratios exhibited a sharp rise at about 40–50% of the otolith radius, which was followed by several spikes consisting of both low and high values, indicating that fish migrated between freshwater and marine environments multiple times during their life history. However, the peaks in Sr:Ca ratios were generally associated with annulus zones on the otolith, indicating that the anadromous migration was generally an annual event.

Anguillid eels have been observed, studied, fished, traded and consumed by humans for centuries. As a consequence, societies have developed various cultural phenomena regarding cuisine, folklore, art, literature, legend. In recent years, however, some populations of eels have been in sharp decline across the globe, some species are now considered to be endangered, and there is an ever increasing need to share knowledge amongst the scientific, management and conservation community to protect these enigmatic fish, to reverse declines and elsewhere to prevent their occurrence. To facilitate this knowledge exchange, a congress session focussed on anguillid eels was held at the sixth World Fisheries Congress in May 2012 at the Edinburgh International Conference Centre. This special issue of Ecology of Freshwater Fish contains a number of the papers presented at the congress session, each focussed on a particular scientific problem. While the scientific study of eels is required to understand and manage eel resources, and to which end the majority of this volume is related, it is also necessary to take the cultural and social aspects of eels into account when considering how to ensure that this fascinating family of fishes has a sustainable future.

A new anguillid eel species, Anguilla luzonensis, was recently discovered on Luzon Island of the northern Philippines, but little is known about its life history. DNA identification was used to determine that five leptocephali of this species (29.2–51.2 mm) were collected offshore in the western North Pacific (13°–17.5° N, 125°–141° E) in April, June, and July between 2002 and 2009. One leptocephalus was caught west of Luzon and four were close to the spawning areas of Anguilla japonica and Anguilla marmorata. Otolith microstructure showed that the leptocephali were up to 103–138 days old. Glass eels from northern Luzon in previous studies that were thought to be Anguilla celebesensis now appear likely to have been A. luzonensis, and they were estimated by otolith analysis to have long larval durations similar to those of A. marmorata. Estimated hatching dates of these glass eels and the A. luzonensis leptocephali were both in the February to May season. Offshore presence of leptocephali of A. l...

The migratory history of tapertail anchovy Coilia nasus in the Yangtze River Estuary, China was investigated using otolith Sr:Ca ratios and two-dimensional images of the Sr level from an X-ray electron probe microanalyzer (EPMA). The results showed that 17 of the 22 young-of-the-year (YOY) specimens had low Sr:Ca ratios (1.2–2.4 × 10−3;1.5 ± 0.3 × 10−3) at the central otolith area, indicating their riverine origin and initial freshwater residence. In addition, 11 of the 14 adult specimens had low Sr:Ca ratios (1.3–2.2 × 10−3; 1.7 ± 0.4 × 10−3) at the central otolith area but showed alternating changes between high (>4.0 × 10−3) and low (

The distribution of all larval stages of the Japanese eel, Anguilla japonica, were examined using historical catch records and original data in the western North Pacific (WNP) to evaluate existing information about the larval distribution and migration of this species. A total of 148 preleptocephali, 2547 leptocephali, 6 metamorphosing larvae, and 21 glass eels were collected during 37 cruises over a 52-year period (1956–2007). Sampling effort was spatio-temporally biased in latitude/longitude among seasons with sampling effort being concentrated near the western margin of the subtropical gyre near Taiwan in the winter season and extensive effort occurring near the spawning area to the east near the seamount chain of the West Mariana Ridge in summer during the spawning season. The distribution of preleptocephali (4.2–8.7 mm) was limited to a narrow area around 14°N, 142°E just west of the southern part of the seamount chain, while leptocephali (7.7–62.0 mm) were widely distributed at increasing size westward in the North Equatorial Current (NEC) to the region east of Taiwan. Metamorphosing larvae (52.7–61.2 mm) were collected only in the area 21–26°N, 121–129°E to the east of Taiwan, while glass eels (51.3–61.2 mm) occurred only within or west of the Kuroshio. These distributions suggest that leptocephali begin to metamorphose within or just east of the Kuroshio, then after completion of metamorphosis the glass eels detrain from the current and migrate inshore. The relationship between catch date and body size of leptocephali suggested that the spawning season is from April to August, but further sampling is needed to eliminate possible effects of sampling bias. This analysis is consistent with the existing hypothesis that Japanese eel larvae born near the West Mariana Ridge are transported westward in the NEC and then transfer to the Kuroshio to recruit to East Asia, although more sampling effort is needed for later stage larvae in the NEC bifurcation region to help understand the larval migration in relation to the possible impacts of ocean–atmosphere changes.

Extensive collections were made of the larvae of the temperate Japanese eel Anguilla japonica and the tropical giant mottled eel Anguilla marmorata in an overlapping area of the North Equatorial Current region of the western North Pacific Ocean. Collections of 189 A. marmorata and > 2500 A. japonica larvae during nine surveys from 1991 to 2007 showed that these two anguillid eels have similar spawning areas just west of the southern West Mariana Ridge. In July to August 2006 and August 2007, morphologically and genetically identified A. marmorata preleptocephali were mainly collected between 14.5-15 degrees N and 142-142.5 degrees E, where A. japonica preleptocephali were also caught in some of the same net tows. Fewer A. marmorata preleptocephali, however, were collected (n = 31) compared to those of A. japonica (n = c. 165), and fewer small larvae of A. marmorata were collected per tow than A. japonica (n = 1-10 and 1-294, respectively), suggesting relatively smaller spawning aggregations of A. marmorata. The distribution of preleptocephali and small larvae was wider in longitude in A. marmorata (131- 143 degrees E) than in A. japonica (137-143 degrees E), while the latitudinal range was almost the same (12-17 degrees N). Although spawning by these two species overlaps both spatially and temporally, the tropical eels of the North Pacific population of A. marmorata probably have a much longer spawning season with fewer spawners, at least in summer, and recruit to a much wider latitudinal range of growth habitats.

Remarkably little is known about the life histories of the many tropical anguillid eels distributed across the Indo-Pacific region, and since the Danish expedition to study eels in the region in 1928 and 1929, research on these eels has only begun again in recent years. Sampling for anguillid leptocephali in the Indonesian Seas has been carried out recently to learn about the spawning ecology and larval distributions of tropical eels there. The leptocephali of Anguilla marmorata, Anguilla bicolor pacifica, Anguilla borneensis, Anguilla interioris, and Anguilla celebesensis were collected around Sulawesi Island both in May 2001 and October of 2002. The development of genetic identification techniques has enabled these leptocephali to be identified to species, and their distributions and sizes during different seasons indicated that there are differing life history patterns among sympatric species in the region. A. celebesensis was found to have been spawning in Tomini Bay of northeastern Sulawesi Island in March and April 2001, but apparently, no spawning had occurred in the late summer and fall of 2002. Studies on anguillid glass eels have suggested that tropical anguillids may spawn throughout much of year, but our research on leptocephali in Tomini Bay and data on the downstream migration of tropical anguillids in the major tributary to Tomini Bay indicate that A. celebesensis may have a distinct seasonal pattern of spawning possibly related to the regional monsoon cycles. This is the first evidence of seasonality of spawning in tropical anguillid eels whose life histories are only just beginning to be revealed.

The timing of catches of anguillid glass eels and their otolith microstructure and microchemistry were studied in southwest Iceland, where the European eel, Anguilla anguilla and American eel, A. rostrata have been thought to live sympatrically, to learn about their early life history and the possible mechanism of the separation between these two species ranges. Catches at the site studied suggest that glass eels may have started upstream migration as the river temperature warmed in late June and early July. The glass eels were mitochondrially identified into two species, A. anguilla and A. rostrata, although the latter were likely hybrids between the two species based on a different study. Otolith analyses showed no sharp increases in otolith increment width or sharp decrease of otolith Sr:Ca ratio in either species, which are the characteristic changes corresponding to the onset of metamorphosis in many anguillid species including A. rostrata collected in North America and A. anguilla in Europe. The mean age at recruitment determined for the glass eels in Iceland were similar between the two species (336.6 ± 41.7 and 319.3 ± 36.0 days for A. anguilla and A. rostrata, respectively), as were their total lengths (range 58.0–78.5 mm and 58.5–73.0 mm). In addition, mean age at metamorphosis (278.0 ± 36.8 and 254.0 ± 47.7 days) and total age (372.3 ± 50.8 and 352.9 ± 42.6 days) were also similar between the two species. However, these ages of A. rostrata in Iceland were older than those in North America, and those of A. anguilla collected in Iceland were roughly intermediate between the rest of Europe and North Africa. These findings support the hypothesis that the timing of metamorphosis is a key factor for determining the place of recruitment of glass eels and maintaining the geographic separation between the two species.

The sagittal otolith morphology and microchemistry of reared juvenile ayu, Plecoglossus altivelis, were examined to describe the occurrence and microchemical characteristics of the abnormal otoliths in this species. Juvenile ayu (N = 31) were collected in June 2004 at three different locations, Wakayama, Kumamoto, and Biwa Lake in Japan, where they were being reared in freshwater aquaculture ponds after having been collected in the wild as larvae. Otolith abnormality was found in the sagittae of 26% (N = 8) of the individuals examined, of which five fish had abnormal otoliths only on one side, while the otolith on the other side was normal. Abnormal otoliths were more transparent and crystalline in appearance with irregular shapes compared to normal ones that were more opaque and less irregular. Abnormal otoliths were divided into two types, semi-abnormal (Type 1) with a normal part in the center, and fully-abnormal (Type 2) that were completely crystalline in appearance. The line transects and whole otolith concentration maps showed that the contents of Sr, Na and K were lower in the abnormal otolith regions compared to the normal ones, while those of Ca and S were almost constant in both. The appearance and microchemical properties of the abnormal ayu otoliths were similar to the abnormal otoliths in other species in which vaterite replaces the aragonite. Abnormal formation of otoliths occurred in ayu from Biwa Lake (30%) and Kumamoto (45%), while the Wakayama samples had no abnormality. The microchemical analyses of the normal and abnormal otoliths indicated that some abnormal otoliths had formed before the fish were captured and transferred to the hatchery, so the possible causes of otolith abnormality in ayu are discussed.

A sampling survey for leptocephali was conducted in the eastern Indian Ocean to the west of Sumatra from 5 to 20 June 2003 in an overlapping area with the historical survey in September to November 1929 during the Danish Round the World Expedition. The age and larval growth rate of 34 Anguilla bicolor bicolor collected in 2003 were estimated using their otolith microstructure to obtain new information about the early life history of this sub-species. The total lengths and ages of these leptocephali were 44·1–55·5 mm and 114–158 days, respectively. Their hatching dates ranged from 3 January to 20 February 2003. Combining these age data and the estimated age of leptocephali collected by others in the same area, this sub-species was estimated to have a wide range of spawning periods. Individual growth rates of the leptocephali in 2003 ranged from 0·32 to 0·39 mm day−1 with a mean ±s.d. of 0·35 ± 0·02 mm day−1. These values were lower than the growth rates of leptocephali of other tropical eels such as Anguilla celebesensis or Anguilla borneensis, suggesting that A. b. bicolor from the eastern Indian Ocean have a longer leptocephalus period of oceanic migration.

A research cruise was conducted in the eastern Indian Ocean off west Sumatra, Indonesia, in June 2003 to learn about the spawning and larval ecology of the tropical freshwater eels of the genus Anguilla in the region. A total of 43 anguillid leptocephali were collected during the cruise and they were genetically identified as 41 Anguilla bicolor bicolor, 1 Anguilla marmorata, and 1 Anguilla interioris. A. bicolor bicolor leptocephali were 44.1–55.5 mm TL and most of them were at the fully grown stage. Reexamination of the historical data of Jespersen (1942) also suggested a relatively low abundance of small size leptocephali (

In order to improve understanding of the larval migration and early life history charac- teristics of 4 tropical eels, Anguilla marmorata, A. bicolor pacifica, A. celebesensis and A. borneensis, the leptocephali, metamorphosing leptocephali and oceanic glass eels collected during 8 cruises in the western Pacific and Indonesian Seas from 1991 to 2002 were analyzed. The leptocephali of A. celebesensis and A. borneensis were collected only in close proximity to their relatively small spe- cies ranges in the Indonesian Archipelago and were found to have faster growth than the other 2 species with small-scale local migrations. The more widely distributed species A. marmorata and A. bicolor pacifica were collected in most sampling areas. Small leptocephali of A. marmorata were collected only to the west of the Mariana Islands, and only larger specimens, metamorphosing leptocephali, or oceanic glass eels of both species were collected in the Indonesian Seas. These distributions suggested that the 2 species have intermediate-scale migrations compared to other anguillid eels. The leptocephali of all 4 species appeared to reach a fully grown size of around 50 mm, which is considerably smaller than the maximum size of temperate anguillid leptocephali, and their growth was predominantly faster than that of temperate species. These data and recently derived molecular phylogenetic relationships among all anguillid species in the world suggest that the long spawning migrations of temperate eels evolved from much shorter migrations of tropical species, whose larval growth was faster and whose maximum larval sizes were smaller.

To clarify the role of thyroid function during metamorphosis from leptocephalus to glass eel in the Japanese eel, we examined the histology of the thyroid gland and measured whole-body concentrations of thyroid hormones, thyroxine (T4) and triiodothyronine (T3), and thyroid stimulating hormone β-subunit TSH (TSHβ) mRNA expression levels in five stages of artificially hatched eels (leptocephalus, early-metamorphosis, late-metamorphosis, glass eel, and elver). During metamorphosis, the inner colloid of thyroid follicles showed positive immunoreactivity for T4, and both T4 and T3 levels were significantly increased, whereas a small peak of TSHβ mRNA level was observed at the early-metamorphosis stage. Similarly, TSHβ mRNA levels were highest in the glass eel stage, and then decreased markedly in the elver stage. In contrast to TSHβ mRNA expression, thyroid hormones (both T4 and T3) increased further from the glass eel to elver stages. These results indicated that thyroid function in the Japanese eel was active both during and after metamorphosis. Therefore, the thyrotropic axis may play important roles not only in metamorphosis but also in subsequent inshore or upstream migrations.