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Bonefishes are a nearshore species targeted by non-commercial anglers and subsistence fishers in the Central and South Pacific islands. Among the bonefish species in the Indo-Pacific region, Albula glossodonta are known to have one of the widest geographic ranges, from the Red Sea to the Central Pacific, but it is unknown how dispersive A. glossodonta are between geographically isolated islands. Volunteer anglers collected A. glossodonta fin clips from the main Hawaiian Islands in the North Pacific, Anaa Atoll in the South Pacific, and intermediate Kiritimati Island (Line Islands) to assess the scale of dispersal and population structure within the Pacific Ocean. Population genomics was conducted based on 208 individuals and 7225 SNPs. Although adult A. glossodonta exhibit strong site fidelity, genomic results show no population differentiation between Oahu and Maui in Hawai'i. Bonefishes exhibit significant population structure between Anaa and Hawai'i (FST = 0.096), with intermediate Kiritimati comprising admixed fishes. A lengthy larval duration likely promotes connectivity between Pacific islands. Regional management regimes may be most appropriate for a species with this level of dispersal. [ABSTRACT FROM AUTHOR]

Context. Conger eels in temperate regions migrate offshore to reproduce in similar ways as anguillid eels do, but little is known about Conger life histories in the western South Pacific (WSP). Aims. To show the larval distribution and size, species composition, and early life history of WSP conger eels. Methods. Morphological and genetic species identification and otolith analysis were conducted using 71 Conger leptocephali individuals collected in five station-transects from 10 July to 3 October 2016 during the KH-16-4 cruise. Key results. We found C. cinereus, C. monganius, C. verreauxi and Conger sp. I leptocephali. Possible spawning areas of C. cinereus and C. verreauxi were discovered, and C. verreauxi appears to migrate offshore to reproduce. Growth rates of the four conger eels were higher than those of temperate Conger species, and of Anguilla leptocephali that were collected in the WSP. Conclusions. Aspects of the Conger species composition, spawning areas and larval distributions were shown. Conger leptocephali growfasterthan do Anguilla leptocephali in the WSP, probably because Conger larvae have larger maximum sizes. Implications. Our findings have improved understanding of the early life history of Conger species and highlighted need for further studies about life histories of Conger in the Indo-Pacific region. [ABSTRACT FROM AUTHOR]

A report of the first occurrence of species Dysomma bucephalus Alcock in the Arabian Sea. The species first recorded outside its type locality in the Bay of Bengal. Dysomma bucephalus is described from three adult specimens collected in the Arabian Sea off the Laccadives at a depth of 193 m. The larval form of the species D. bucephalus was first found in the Bay of Bengal near the Nicobar Islands and is also described herein. These findings expand the known range species westward. [ABSTRACT FROM AUTHOR]

Bonefishes are a nearshore species targeted by non-commercial anglers and subsistence fishers in the Central and South Pacific islands. Among the bonefish species in the Indo-Pacific region, Albula glossodonta are known to have one of the widest geographic ranges, from the Red Sea to the Central Pacific, but it is unknown how dispersive A. glossodonta are between geographically isolated islands. Volunteer anglers collected A. glossodonta fin clips from the main Hawaiian Islands in the North Pacific, Anaa Atoll in the South Pacific, and intermediate Kiritimati Island (Line Islands) to assess the scale of dispersal and population structure within the Pacific Ocean. Population genomics was conducted based on 208 individuals and 7225 SNPs. Although adult A. glossodonta exhibit strong site fidelity, genomic results show no population differentiation between Oahu and Maui in Hawai‘i. Bonefishes exhibit significant population structure between Anaa and Hawai‘i (FST = 0.096), with intermediate Kiritimati comprising admixed fishes. A lengthy larval duration likely promotes connectivity between Pacific islands. Regional management regimes may be most appropriate for a species with this level of dispersal.

Two types of ophichthid larvae collected from Japanese coastal waters around Kyushu and Shikoku were identified by DNA barcoding as Ophichthus celebicus (Bleeker 1856) and O. macrochir (Bleeker 1852). These two species have not been reported from Japan previously. The leptocephalus of O. celebicus was found to have melanophore spots or streaks on the eighth to last myosepta on the midline of the body. The metamorphic larva of O. macrochir was found to have subcutaneous melanophore distributed along myosepta in every first to fifth myomeres on the midline of the body. The larvae of those two species were distinguishable from other ophichthid larvae reported from Japan by these morphological features. The new standard Japanese names, "Torinoko-umihebi" and "Kuronaga-umihebi", are proposed for O. celebicus and O. macrochir, respectively. [ABSTRACT FROM AUTHOR]

This study was performed to investigate the effect of microbial supplementation diet on the survival rate and microbiota composition of artificially produced eel larvae. Microorganisms supplemented in the diet were isolated from wild glass eel intestines and identified as Bacillus sp. through 16S rRNA sequencing analysis. In vitro tests confirmed that the strain had no hemolytic activity and virulence genes. Microbial supplemental feeding significantly increased the survival rate of artificially produced eel larvae for 30 days post-hatchling compared with that of the control group. It also caused changes in the α-diversity, β-diversity, and relative abundance of the bacterial communities. Analysis via phylogenetic investigation of communities by reconstruction of unobserved states predicted that these microbial community changes would significantly increase the carbohydrate metabolism, membrane transport, and cellular community pathway of the microbial supplementation group. Therefore, microbial supplementation feeding for eel aquaculture could increase the viability of artificially produced eel larvae and alter the microbial composition to induce metabolic changes. [ABSTRACT FROM AUTHOR]

Freshwater eel (Anguilla japonica) is the main aquaculture fish species in Korea, but artificial mass seed production has not been succeeded yet; eel aquaculture is therefore dependent on the seed of wild catches. Major problems in artificial production of eel seed are the low growth and survival rates of early larvae, likely may be caused by the feed used for the first and subsequent feedings. Thus, a physiological understanding of the digestive and absorptive capacities is necessary. This study was conducted for 50 days after hatching (DAH); we investigated the activities of digestive enzymes (trypsin, chymotrypsin, amylase, lipase), expression levels of digestive enzymes (trypsin (try), amylase (amy), pancreatic lipase (pnlip)), and expression levels of nutrient transporters (large neutral amino acids transporter small subunit 2 (slc7a8), sodium/glucose co-transporter member 1 (sglt1), Niemann-Pick C1-like 1 (npc1l1)). The try and slc7a8 expression increased with fluctuations until 50 DAH and showed a similar trend. The amy and sglt1 expression increased until 50 DAH and appeared to a similar trend. The pnlip and npc1l1 expression were relatively high until 15 DAH and decreased thereafter. And then, pnlip expression increased again from 30 DAH and npc1l1 expression maintained low until 50 DAH. Meanwhile, expression levels of digestive enzymes were highest in try, followed by amy and pnlip. And, the expression levels of nutrient transporters showed the highest values in the order of sglt1, slc7a8, and npc1l1. These findings suggested that protein and carbohydrates are more important than lipids in eel larvae. All digestive enzymes activities were low until 15 DAH and then rapidly increased to 30 DAH; these levels were maintained until 50 DAH. Eel larvae are considered to have insufficient and unstable digestive ability before 30 DAH. Therefore, in order to improve the growth and survival rate of eel larvae, this study is suggested that the composition of the current slurry-type diet could consist of easily digestible low-molecular substances before 30 DAH, increasing the protein and carbohydrate content and reducing the lipid content.

This study was performed to investigate the effect of microbial supplementation diet on the survival rate and microbiota composition of artificially produced eel larvae. Microorganisms supplemented in the diet were isolated from wild glass eel intestines and identified as Bacillus sp. through 16S rRNA sequencing analysis. In vitro tests confirmed that the strain had no hemolytic activity and virulence genes. Microbial supplemental feeding significantly increased the survival rate of artificially produced eel larvae for 30 days post-hatchling compared with that of the control group. It also caused changes in the α-diversity, β-diversity, and relative abundance of the bacterial communities. Analysis via phylogenetic investigation of communities by reconstruction of unobserved states predicted that these microbial community changes would significantly increase the carbohydrate metabolism, membrane transport, and cellular community pathway of the microbial supplementation group. Therefore, microbial supplementation feeding for eel aquaculture could increase the viability of artificially produced eel larvae and alter the microbial composition to induce metabolic changes.

Simple Summary: The freshwater eel Anguilla japonica is a commercially important in Northeast Asia. However, eels farming is entirely dependent on natural catches, and eel resources are declining. This study was conducted to provide baseline information on the development of mass seed production technology necessary for the conservation of species and the maintenance of aquaculture. This study was conducted for 200 days after hatching (DAH) and analyzed morphometry and allometry. In this study, cultured eel larvae stages were divided in size similar to wild eel larvae, but cultured eel differed from wild eel in growth rate and the number of preanal myomeres. In addition, as eel larvae rarely have mixed feeding periods, it is important to determine the optimal first feeding time. The eel larvae may need a change in diet type to prevent lower jaw deformity in the leptocephalus stage. As eel larvae changed to a willow leaf-like form, the relative growth pattern of the eel larval stages was unique. This growth pattern may reflect the early life history of long distance and diel vertical migration. Meanwhile, the inflection point in the body parts' growth patterns showed only before 30 DAH and was similar to the period of mass mortality. Therefore, future studies should focus on developing an optimal feeding and rearing protocol from the first feeding to 30 DAH. The freshwater eel Anguilla japonica is rapidly decreasing in number and has not yet been successfully mass produced. This may be at least partially attributable to the unique and long early life history of the eel. Therefore, we investigated its ontogeny of morphometry and growth pattern in larval stages to provide baseline information for understanding the early life history and improving seed rearing technology. This study was conducted for 200 days after hatching (DAH) and analyzed morphometry and allometry for eel larvae. The following cultured eel larval stages were identified: the yolk sac larvae stage (0–6 DAH, 3.23–6.85 mm total length (TL)), the pre-leptocephalus stage (7–30 DAH, 6.85–15.31 mm TL), and the leptocephalus stage (50–200 DAH, 15.31–60.06 mm TL). Cultured and wild eel larvae could be divided into characteristic larval stages at similar sizes. However, compared to wild eels, cultured eels had a slower growth rate and fewer preanal myomeres. Meanwhile, cultured eel larvae rarely had a mixed feeding period as the absorption of endogenous reserves was completed by 7 DAH. The lower jaw of eel larvae was significantly longer than the upper jaw from 50 DAH. In the pre-leptocephalus and leptocephalus stages, eel larvae showed continuous positive allometric growth at trunk height and tail muscle height with change to the willow leaf-like form. These growth characteristics may be the result of adaptation to the migration over long distances and to a diel vertical migration. The inflection point in the body parts growth patterns showed only before 30 DAH, and mass mortality appeared at this period. Therefore, to improve the growth and survival rates of cultured eel seed, it is necessary to focus on improving the feeding and rearing protocol until 30 DAH. [ABSTRACT FROM AUTHOR]

Organogenesis of Japanese eels (Anguilla japonica) was investigated histologically from the late leptocephalus to the yellow eel stages. Early organogenesis, such as the formation of inner ears and the appearance of round blood cells that might be larval erythrocytes, had already begun at the late leptocephalus stage. During the first developmental phase (M1–M3 stages) of metamorphosing into early glass eels (G1 stage), the formation of gills and lateral muscles progressed conspicuously with a drastic body shape change from leaf-like to eel-like. In contrast, obvious regression in oesophageal muscle and pancreas occurred during metamorphosis. Formation of lateral line canals advanced continuously until the yellow eel stage. When the second developmental phase was initiated at the G1 stage, cone photoreceptor cells appeared, and the formation of oesophageal, stomach and intestinal muscles was initiated. Differentiation of gastric glands began at 1 week after metamorphosis. Erythrocytes increased continuously in density in glass eels and elvers (G1–E2 stages), and the morphological features of cone cells and olfactory epidermal cells became clearer with stage progression. In early elvers (E1 stage), the swimbladder initiated inflation, the stomach fully expanded and the rectal longitudinal fold changed to a circle. Swimbladder gas glands appeared in late elvers (E2 stage). In the yellow eels (juvenile stage), almost all organ structures were formed. These observations indicate that the organogenesis of A. japonica is ongoing after metamorphosis into glass eels, and the M1–E2 stages are considered to be a homologous phase to first metamorphosis, which is a transformation from the larval to the juvenile stages in other teleosts. In comparison to conger eels, the completion of the body shape change to eel-like occurs at the G1 stage, when organogenesis is still in progress, being followed by a prolonged duration of the G1–E2 stages before reaching the yellow eel juvenile stage, which may be a unique characteristic that is related to the early migratory life history of A. japonica. [ABSTRACT FROM AUTHOR]

The paper reports the first record of Nemichthys scolopaceus leptocephalus in the Adriatic Sea, including a detailed description of the specimen with morphometric measurements and meristic counts. A single leptocephalus specimen was caught on 18th September 2016 in south Adriatic with a small mid-water trawl at a depth of approx. 120 meters. Further research is needed to reveal more information about the distribution and potential spawning and nursery areas of slender snipe eel as well as the mechanism of retention and dispersion of their larvae in the Adriatic. [ABSTRACT FROM AUTHOR]

Little is known about early development of the near‐threatened bonefish (Albula vulpes), a member of superorder Elopomorpha. Members of Elopomorpha are partially defined by their synapomorphic leptocephalus larval stage, for which the nutritional requirements are not well understood. Characterizing the nutritional profile, including major nutrients (such as lipids) used for energetic processes, can help to gain a better understanding of the nutritional requirements for leptocephalus larvae. A total of 24 settlement stage A. vulpes leptocephalus larvae were collected at Long Caye Island, Belize. Samples were used to determine various biochemical characteristics including lipid class, fatty acid and glycosaminoglycan compositions. Each of these biochemical components plays a role in early developmental processes such as cellular membrane formation and is crucial for healthy development. Biochemical characteristics of settlement stage A. vulpes leptocephalus are presented in this study for the first time. The dominant lipid classes and fatty acids detected in these samples were consistent with prior studies using closely related species like the Japanese eel, indicating possible similarities in diets at this stage. In the future, similar analyses can be applied to other species that share the leptocephalus life stage to determine if nutritional requirements at this stage of development are unique to this species. The findings in this study will also help to facilitate the establishment of adequate aquaculture systems for captive bonefish, ultimately leading to improved management strategies for wild bonefish habitats. [ABSTRACT FROM AUTHOR]

The freshwater eel Anguilla japonica is rapidly decreasing in number and has not yet been successfully mass produced. This may be at least partially attributable to the unique and long early life history of the eel. Therefore, we investigated its ontogeny of morphometry and growth pattern in larval stages to provide baseline information for understanding the early life history and improving seed rearing technology. This study was conducted for 200 days after hatching (DAH) and analyzed morphometry and allometry for eel larvae. The following cultured eel larval stages were identified: the yolk sac larvae stage (0–6 DAH, 3.23–6.85 mm total length (TL)), the pre-leptocephalus stage (7–30 DAH, 6.85–15.31 mm TL), and the leptocephalus stage (50–200 DAH, 15.31–60.06 mm TL). Cultured and wild eel larvae could be divided into characteristic larval stages at similar sizes. However, compared to wild eels, cultured eels had a slower growth rate and fewer preanal myomeres. Meanwhile, cultured eel larvae rarely had a mixed feeding period as the absorption of endogenous reserves was completed by 7 DAH. The lower jaw of eel larvae was significantly longer than the upper jaw from 50 DAH. In the pre-leptocephalus and leptocephalus stages, eel larvae showed continuous positive allometric growth at trunk height and tail muscle height with change to the willow leaf-like form. These growth characteristics may be the result of adaptation to the migration over long distances and to a diel vertical migration. The inflection point in the body parts growth patterns showed only before 30 DAH, and mass mortality appeared at this period. Therefore, to improve the growth and survival rates of cultured eel seed, it is necessary to focus on improving the feeding and rearing protocol until 30 DAH.

In order to encourage the survival of both captive and wild populations of bonefish (Albula vulpes), a deeper understanding of the species' early developmental characteristics is necessary. During ontogenesis, bonefish utilize lipids as a source of energy before the start of exogenous feeding. The goal of this study is to gain insight into the energetic needs of bonefish leptocephalus larvae in the post‐hatch larval stage. Broodstock were collected in the wild and hormone induced. Spawning yielded eggs that were fertilized and were then incubated until hatching. Larval development was monitored throughout the duration of the trial until all larvae perished. Samples of larval tissue were taken to the lab for lipid analysis and composition was compared at different developmental stages. Larval lipid composition was significantly different across sample groups showing a change in lipid content related to development. After hatching, larvae gradually depleted wax esters‐sterol ester (WE‐SE) reserves over a period of 4 days, while simultaneously increasing hydrocarbon (HC). The role of WE‐SE is seemingly tied to both buoyancy and energy reserves due to its high abundance immediately post‐hatch and selective catabolism. As larvae weaned off of the nutrition provided by the yolk, exogenous feeding began to diversify lipid composition and overall lipid reserves were depleted. Future directions included the development of optimal larval feeds based on this analysis in order to gain more insight into the nutritional needs and requirements during the critical leptocephalus stages. [ABSTRACT FROM AUTHOR]

Leptocephalus larvae of elopomorph fishes are a cryptic component of fish diversity in nearshore and oceanic habitats. However, identifying those leptocephali can be important in illuminating species richness in a region. Since the Deepwater Horizon oil spill in 2010, sampling of offshore fishes in the epi-, meso-, and upper bathypelagic depth strata of the northern Gulf of Mexico resulted in 8989 identifiable specimens of leptocephalus larvae or transforming juveniles, in 118 taxa representing 83 recognized and established species and an additional 35 distinctive leptocephalus morphotypes not yet linked to a known described species. Leptocephali account for ∼13% of the total species richness of fishes collected in the offshore region. A new morphotype of Muraenidae leptocephalus is also described. We compare this study with other leptocephalus diversity studies in the western Atlantic.

This study assessed the effects of diets supplemented with different types of chitin hydrolysate on larval growth of the Japanese eel Anguilla japonica. Larvae were reared for 26 days on artificial diets containing 2% of purified monomeric N-acetylglucosamine (NAG1), a mixture of mono-, di- and trimers (NAG3), a mixture of mono-, di-, tri-, tetra-, penta- and hexamers (NAG6), or a control diet. The larvae fed NAG3 and NAG6 grew significantly more rapidly than the control larvae (p < 0.05). However, the survival rate of the NAG6-fed larvae was low (8%) for unknown reasons, whereas the survival rates of the NAG1 and NAG3-fed larvae were similar to that of control larvae, about 46% (p > 0.9). These results indicate that NAG3 is the most effective supplement for eel larvae. A dose–response analysis showed that the optimal level of NAG3 in the diet was around 1%. Although the biological activity of chitin hydrolysates in eel larvae remains unclear, part of these hydrolysates are likely utilized by eel larvae as components of glycosaminoglycan, a major constituent of their bodies. Alternatively, the hydrolysates may act as a bioactive substance, influencing fish health, or in the effective utilization of energy in eel larvae, enhancing their growth. [ABSTRACT FROM AUTHOR]

Abstract. The oceanic early life histories of three species of tropical anguillid glass eels, namely Anguilla marmorata , Anguilla megastoma and Anguilla obscura , recruiting to Viti Levu, Fiji Islands, in the western South Pacific, were studied using otolith microstructures of 94 individuals. A. megastoma had the shortest leptocephalus duration, age at recruitment and metamorphosis stage (mean ± s.d. 104.7 ± 7.2, 141.3 ± 8.7 and 36.6 ± 8.5 days respectively), followed by A. marmorata (September–October recruitment peak: 113.0 ± 11.9, 115.1 ± 8.3 and 39.2 ± 11.1 days respectively; April recruitment peak: 152.2 ± 13.5, 154.0 ± 12.5 and 38.9 ± 10.1 days respectively) and A. obscura (118.0 ± 10.2, 168.4 ± 16.5 and 49.6 ± 12.5 days respectively). A. obscura also had the shortest oceanic glass eel duration of (12.2 ± 3.4 days), followed by both peaks of A. marmorata (23.3 ± 3.2 and 20.2 ± 3.3 days) and A. megastoma (20.7 ± 6.7 days). Otolith increment rates during the leptocephalus stage did not differ significantly among the three species, but there were significant differences between the leptocephalus duration and metamorphosis stage of A. megastoma compared with A. marmorata and A. obscura , as well as in the oceanic glass eel duration of A. obscura compared with A. marmorata and A. megastoma. There were also significant differences in the age at recruitment among all three species. Back-calculated hatch dates suggest separate peak seasonal spawning periods for each of the three species. Lay summary. To gather information on oceanic life histories and possible differences in spawning times and locations of freshwater eels in the western South Pacific, otoliths of three species occurring in Fiji were studied. Significant differences between the life histories of all three species were observed. Furthermore, separate peak seasonal spawning periods for each of the three species was revealed. [ABSTRACT FROM AUTHOR]

The geographical range of many marine species is strongly influenced by the dispersal potential of propagules such as eggs and larvae. Here, we investigate morphological diversity and the effect of body shape on geographical range of leptocephali, the unique, laterally compressed larvae of eels (order Anguilliformes). We used phylogenetically informed analyses to examine the morphological variation of larvae for 17 Eastern Pacific eel species from three adult habitats. We also investigated whether morphological traits of leptocephali could predict larval latitudinal range, hypothesizing that body shape may influence passive dispersal via currents. We found that no two species shared the same multivariate growth trajectories, with the size and scaling of pectoral fin length and snout-to-anus length being particularly variable. Larvae with longer relative predorsal and snout-to-anus lengths at median sizes exhibited wider larval geographical ranges. Body aspect ratio and maximum body length at metamorphosis, two traits we hypothesized to be important for passive transport, were not significant predictors of maximal larval range. We discovered an increase in phylogenetic signal over larval development as eels approach metamorphosis, potentially due to similar selective pressures between related species (such as juvenile habitat or adult morphology). Lastly, we conclude that larval body shape is probably influenced by adult habitat and adult morphology. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The efficiency of a new liquid-type diet for long-term rearing of Japanese eel larvae until metamorphosis was examined, as was the effect of diet viscosity on diet intake and on the survival and growth of early larvae. The highest intake of the experimental diet by 6- and 9-day post-hatch (dph) larvae occurred at viscosities of 20–50 mPa·s, much lower than the viscosity of the currently used slurry-type diet (ca. 2900 mPa·s). Long-term feeding trials for 259 days (n = 4) showed that overall survival rates of larvae (37–59%) fed a liquid-type diet with lower viscosity (40–680 mPa·s) were 2 to 3.4 times higher than the survival rates of larvae fed the slurry-type diet (11–25%). Because nutrients were diluted in the liquid-type diet, the growth of larvae fed this diet was slower after about 200 days and metamorphosis was delayed. However, the yield of glass eels was 1.1 to 3.2 times higher in larvae fed the liquid than the slurry diet. These findings suggest that feeding the liquid-type diet can result in the mass production of glass eels by ensuring high growth, survival and metamorphosis rates. [ABSTRACT FROM AUTHOR]

Development of artificial production of glass eels still presents many problems to be solved. What and how to feed larvae is one of the most crucial problems for commercial mass production of glass eels. We evaluated a new liquid-type diet and feeding method against the conventional slurry-type diet by comparing the growth and survival of larvae in triplicate for 27 days after hatching. Larvae were fed by immersion in a liquid-type diet that was a 1.5 times dilution of the conventional slurry-type diet based on dogfish yolk spread over the bottom of the rearing tank. Larvae took more and denser liquid-type food in the intestine than the conventional slurry-type diet. Survival rates at 3 weeks after first feeding were about 90% in most experimental groups (5 groups) except one of three slurry-type diet groups (about 70%). Growth rates were 1.35 times higher in liquid-type diet groups (0.27 mm/day at 27 dph) than in conventional slurry-type diet groups (0.20 mm/day). These results suggest that the immersion feeding method in conjunction with the liquid-type diet has the potential to enable large scale production of glass eels by ensuring high growth and survival. [ABSTRACT FROM AUTHOR]

In August 2007, October 2008 and September–October 2010, 241 Tucker trawl and plankton net tows were conducted at the surface to depths of 1377 m at six locations in the northern and eastern Gulf of Mexico (GOM) to document leptocephalus diversity and determine how assemblage structure, larval size, abundance and isotopic signatures differ across the region and with depth. Overall, 2696 leptocephali representing 59 distinct taxa from 10 families were collected. Five families accounted for 96% of the total catch with Congridae and Ophichthidae being the most abundant. The top four most abundant species composed 59% of the total catch and included: Ariosoma balearicum, Paraconger caudilimbatus, Rhynchoconger flavus and Ophichthus gomesii. Four anguilliform species not previously documented in the GOM as adults or leptocephali were collected in this study, including Monopenchelys acuta, Quassiremus ascensionis, Saurenchelys stylura and one leptocephalus only known from its larval stage, Leptocephalus proboscideus. Leptocephalus catches were significantly greater at night than during the day. Catches at night were concentrated in the upper 200 m of the water column and significantly declined with increasing depth. Leptocephali abundances and assemblages were significantly different between sites on the upper continental slope (c. 500 m depth) and sites on the middle to lower continental slope (c. 1500–2300 m). Sites on the lower continental slope had a mixture of deep‐sea demersal, bathypelagic and coastal species, whereas upper‐slope sites contained several numerically dominant species (e.g., A. balearicum, P. caudilimbatus) that probably spawn over the continental shelf and upper slope of the GOM. Standard lengths of the four dominant species differed between sites and years, indicating heterochronic reproduction and potential larval source pools within and outside of the GOM. Stable‐isotope analyses (δ13C and δ15N) conducted on 185 specimens from six families revealed that leptocephali had a wide range of isotopic values at the family and size‐class levels. Species in the families Muraenidae, Congridae and Ophichthidae had similar δ15N values compared with the broad range of δ15N values seen in the deep‐sea families Nemichthyidae, Nettastomatidae and Synaphobranchidae. Stable‐isotope values were variably related to length, with δ15N values being positively size correlated in ophichthids and δ13C values being negatively size correlated in A. balearicum and P. caudilimbatus. Results suggest that leptocephali feed in various water depths and masses, and on different components of POM, which could lead to niche partitioning. Ecological aspects of these important members of the plankton community provide insight into larval connectivity in the GOM as well as the early life history of Anguilliformes. [ABSTRACT FROM AUTHOR]

Hyaluronan (HA) is a main component of the highly hydrated extracellular body matrix of the larval stage of eels (leptocephalus). To determine the physiological and nutritional significance of HA in artificially reared larvae of the Japanese eel Anguilla japonica , HA and water content were measured at various larval stages in captivity. HA accumulated in the larval body with growth and water content also increased in parallel with HA. Almost all the accumulated HA was consumed during metamorphosis, suggesting HA plays a significant role in eel growth and metamorphosis. However, the amounts of HA and water were significantly lower in reared than in wild-caught larvae at the young stage, indicating a poorer nutritional condition of reared larvae, probably due to the deficient performance of currently used artificial diets. HA accumulation in reared larvae, however, eventually attained a level sufficient for metamorphosis by 250 days or more, and the glass eels of reared larvae had the same nutritional condition as that of wild glass eels. These findings are discussed with respect to the development of more effective artificial diets for rearing eel larvae. [ABSTRACT FROM AUTHOR]

Even though reared leptocephalus larvae of the Japanese eel Anguilla japonica have a high incidence of notochord deformities (>60%), the cause is unknown. We performed histological examinations of the notochord and associated organs in reared larvae to better understand the process causing notochord deformation in eel larvae. In deformed larvae, unknown tissue thickening was discovered near the notochord sheath. Azan staining revealed that these tissue thickenings are most likely collagen fibers within fibrous connective tissue. This was almost identical to the connective tissue found in the primordium of the vertebral body around the notochord sheath in properly metamorphosing larvae. Furthermore, the amount of the thyroid hormone triiodothyronine (T3) was significantly higher in deformed larvae than in normal larvae, indicating that notochord deformity is probably linked to metamorphosis despite the immature stage of growth. We suggest that the aberrant growth of connective tissue surrounding the notochord sheath induced by incomplete metamorphosis causes deformities in eel larvae. The reason why deformed larvae have greater thyroid hormone levels is still unknown. It is important to assess how environmental and dietary factors affect the thyroid hormone levels of eel larvae raised in captivity.

Abstract: The abundance and distribution of leptocephalus larvae of the European eel (Anguilla anguilla, Anguillidae) were examined using ten historic and recent Sargasso Sea expeditions that were selected on the basis of having the largest number of sampling stations and highest catches. The surveys cover the period 1920–2014. Station data were recalculated to the same unit of larval density per unit area, and the irregular station positions were transformed to a regular spatial grid to allow calculation of comparable measures of abundance of the youngest (0+) leptocephalus cohort. The result is that the mean and maximum densities of 0+ leptocephali after 2007 on average have decreased by 70%–80% from the densities during the period before the drastic decrease in glass eel recruitment, which started in the 1980s. This is of the same magnitude as the change in spawning stock, if the total continental commercial landings are used as a proxy. In the same period, the glass eel recruitment in Europe has decreased by more than 95%. The conclusion is that a major cause for the recruitment decrease may be an increased leptocephalus mortality during the oceanic phase or a large geographic shift in glass eel arrival. Combining the survey data, the spatial distribution of 0+ leptocephali was concentrated south of the northernmost front in the Subtropical Convergence Zone, but high densities were also found far south of the front in the western part of the distribution area and leptocephali were present also north of the average frontal position. [ABSTRACT FROM AUTHOR]

This study clarifies the location, size and age at the onset of metamorphosis in Japanese eels Anguilla japonica through oceanic surveys, rearing experiments and analyses of the morphology and otoliths of leptocephali and glass eels. Twenty‐eight metamorphosing leptocephali were collected in the mesoscale eddy region to the east of Taiwan during research expeditions in 2004. Rearing experiments showed that the total length (LT) of leptocephali decreased by an average of 12·5% during metamorphosis and 13·9% during the 2–12 h after death. Thus, the mean back‐calculated LT at the onset of metamorphosis for 630 glass eels from Taiwan and Japan was estimated at 67·8 ± 2·7 mm (mean ± S.D.). The estimated mean ante‐mortem size of the fully grown pre‐metamorphic leptocephali collected in 2004 was 64·6 ± 3·4 mm, which was consistent with the LT estimate for glass eels. Otolith analysis showed that the mean age at the onset of metamorphosis was 137 ± 15 days and indicated that Japanese eels may have a recruitment route through the mesoscale eddies to the east of Taiwan in addition to the direct transfer route from the North Equatorial Current to the Kuroshio Current. [ABSTRACT FROM AUTHOR]

The high mortality rate of reared Japanese eel Anguilla japonica larvae is largely due to lower growth rate and the higher rate of deformed larvae. To establish an effective rearing protocol for this species, we examined the effects of water temperature and feeding regimes on their growth and notochord kyphosis. Larvae at 165 days post hatching were reared for 28 days at mean temperatures of 24, 25 and 27 °C, and were fed 4 or 6 times per day. Larval growth rate was significantly higher in larvae reared at 24-25 °C and fed 6 times per day. However, growth rate was significantly reduced at 27 °C, suggesting a shortage of metabolic energy due to an elevated cost of the higher basal metabolic rate at higher temperatures and low nutritional performance of currently used artificial diet. Notochord kyphosis was promoted by elevated water temperature, and two-way ANOVA showed that water temperature and feeding frequency had combined effects on the deformity. These findings suggest the importance of concurrently manipulating both environmental and nutritional factors to produce healthy eel larvae in captivity. [ABSTRACT FROM AUTHOR]

Anguillid eels are a high value species for aquaculture in East Asian countries, but natural stocks of eels have been decreasing. Mass production of glass eels is one of the most effective ways to solve this problem. Genetic improvements will become important to establish mass production of glass eels in the near future. However, there is a lack of basic information, such as parentage assignments, for implementing an eel breeding program. In this study, we aimed to understand the parentage assignment of Japanese eel ( Anguilla japonica ) using a multiple breeding method and microsatellites from eight loci. Five female and 15 male eels were used for mass spawning, and the progeny were collected at the pre-leptocephalus (2 days post-hatch [dph]) and the leptocephalus stage (30 dph). Almost all progeny (297/301 = 98.7%) were assigned to a parental couple. Only two female eels (F86 and F88) contributed to the next generation, whereas 13 of 15 males contributed to the next generation. Although there were no statistical differences in the paternity ratio between stages, some males changed the ratio in both females. These data are not directly useful for a breeding program, but are useful to maintain pedigree information, which is essential for implementing a breeding program. [ABSTRACT FROM AUTHOR]

Pre-metamorphic leptocephali of Pisodonophis sangjuensis ( n = 91, 10.4−90.2 mm in total length, TL) were collected in the East China Sea for the first time. Pre-metamorphic leptocephali of P. sangjuensis, which were identified using mitochondrial DNA cytochrome c oxidase subunit I gene (mtDNA COI), are characterized by various combinations of morphological characters: 8 moderate to pronounced gut loops with the kidney terminating on the 6-7th loops; and 8 subcutaneous pigment patches on the tail just ventral to the notochord. Pisodonophis sangjuensis leptocephali were more numerously collected offshore than inshore around Jeju Island. The smallest leptocephali (< 15.0 mm TL) were collected from the south far from Jeju Island, and the largest leptocephali (> 100.0 mm TL) were collected from around Jeju Island and the southern coast of Korea. Our findings indicate that P. sangjuensis spawns offshore south of Jeju Island that is an area associated with high water temperature, and then the hatched leptocephali are transported to Jeju Island or the southern coast of Korea by the Tsushima Warm Current. [ABSTRACT FROM AUTHOR]

Hatchery produced leptocephalus larvae of the Japanese eel (Anguilla japonica) were reared on a research vessel and fed natural particulate organic matter (POM) collected in theWestern North Pacific. A small net (36 cm diameter, 60 mm mesh) was vertically towed from 200m depth to surface in 2013, and POM-enriched water (POMEW) filtered through 350 mm mesh was fed to the leptocephali for 11 days. Although the swollen gut corroborated the active ingestion of POM by the leptocephali, low survival rate and heavily melanized gut followed by necrosis in the mid-hindgut region of the leptocephali were observed. A large net (1.14m diameter, 30 mm mesh) was used in 2014 and 2015, which was horizontally drifted subsurface (100-175 m). POMEWs filtered through 53 or 25 mm meshes were fed to the leptocephali for 5-18 days. Neither melanized gut nor necrosis occurred, but considerably low survival rate and little growth comparable with that in a starved condition were observed. No apparent shift in stable carbon and nitrogen isotope ratios was observed in the reared leptocephali. These indicated that the ingested POMs were not assimilated by the leptocephali and suggested that smaller particles may be important for the leptocephali. [ABSTRACT FROM AUTHOR]

The Japanese eel (Anguilla japonica) spends a long period as the leptocephalus larval form under current rearing conditions. The duration of the larval stage until metamorphosis is influenced by body size and growth; however, little knowledge exists of the regulatory mechanism of growth in eel larvae. The present study focused on growth hormone (GH), insulin-like growth factors (IGFs), and IGF binding protein (IGFBP) as the central regulators of growth in teleost fishes and transforming growth factor-beta 3 (TGF-β3) as a possible key modulator of muscle growth and body component synthesis. Japanese eel IGFBP-1a and TGF-β3, comprising 264 and 411 amino acids, respectively, were cloned. Short-term (5-day) fasting and refeeding experiments were performed to understand changes in growth-related genes affected by nutritional status. The relative expression of gh increased with fasting and subsequently decreased with refeeding to the basal levels of the fed control. Relative igf-1 and igf-2 expression levels were high in the fasted group. Relative igf-1 was reduced after 2-day refeeding, whereas igf-2 decreased to the basal level after 1-day refeeding, suggesting that IGF-1 and IGF-2 might be regulated independently and contribute to postnatal growth in eel larvae. Relative igfbp-1a expression was sharply increased by fasting, whereas tgf-β3 showed high and low values in the fed and fasted groups, respectively. Relative igfbp-1a and tgf-β3 levels were negatively and positively correlated with body size, respectively. These results suggest that igfbp-1a and tgf-β3 are potential indices of growth for exploring optimal rearing conditions to shorten the larval stage in Japanese eels. [Display omitted] • Fasting increased the expressions of gh , igfs , and igfbp-1a in early larvae. • Expression of IGFBP-1a is a useful indicator of growth retardation and stress. • TGF-β3 gene would involve in growth in the early larvae of Japanese eel. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The taxonomic status of three species of eels described from leptocephali larvae in 1960, is revised and elucidated: Leptocephalus anaelisae Tommasi, 1960, a member of Moringuidae, is confirmed as a junior synonym of Neoconger mucronatus Girard, 1858; Leptocephalus juliae Tommasi, 1960, a Muraenidae, is confirmed as a junior synonym of Uropterygius macularius (Lesueur, 1825); and Leptocephalus humbermariorum Tommasi, 1960, a Congridae, is now placed as species inquirenda in Rhynchoconger Jordan & Hubbs, 1925. Each of the three species was described based on a unique holotype, but none of the tree holotypes were found in any scientific collection investigated by us. Indeed, the three holotypes are considered to be permanently lost. [ABSTRACT FROM AUTHOR]

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 HO) was slightly lower than in those (344 mOsm/kg HO) 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. [ABSTRACT FROM AUTHOR]

One of the difficulties to rear Anguilla japonica larvae is the frequent occurrence of notochord deformities. We tested the effect of salinity on the occurrence of the deformities, because we have been using 50 % diluted seawater (50 % SW) for glass eel production, on the basis of the fact that intermediate salinity saves energy due to lower cost for osmoregulation and contributes higher survival and growth rates. We reared 6-day-old larvae in 50 and 100 % SW for 85 days and observed their morphology. The occurrence rate of deformed larvae, including kyphosis and scoliosis, was significantly higher in 50 % SW (35.8 %) than in 100 % SW (25.4 %), while survival rate was significantly higher in 50 % SW (69.8 %) than in 100 % SW (32.3 %) and growth in 50 % SW (mean body depth: 7.9 ± 5.3 mm) was better than in 100 % SW (6.8 ± 4.6 mm). We speculate that the most of severely deformed larvae could not survive in the tougher condition of 100 % SW, showing the lower occurrence of deformed larvae. Eventually, the yield of normal larvae after 85 days rearing was 1.9-fold higher in 50 % SW than in 100 % SW, implying that the advantage of 50 % SW for rearing eel larvae outweighs the risk of deformities. [ABSTRACT FROM AUTHOR]

Hatchery produced leptocephalus larvae of the Japanese eel (Anguilla japonica) were reared on a research vessel and fed natural particulate organic matter (POM) collected in the Western North Pacific. A small net (36 cm diameter, 60 µm mesh) was vertically towed from 200 m depth to surface in 2013, and POM-enriched water (POMEW) filtered through 350 µm mesh was fed to the leptocephali for 11 days. Although the swollen gut corroborated the active ingestion of POM by the leptocephali, low survival rate and heavily melanized gut followed by necrosis in the mid-hindgut region of the leptocephali were observed. A large net (1.14 m diameter, 30 µm mesh) was used in 2014 and 2015, which was horizontally drifted subsurface (100–175 m). POMEWs filtered through 53 or 25 µm meshes were fed to the leptocephali for 5–18 days. Neither melanized gut nor necrosis occurred, but considerably low survival rate and little growth comparable with that in a starved condition were observed. No apparent shift in stable carbon and nitrogen isotope ratios was observed in the reared leptocephali. These indicated that the ingested POMs were not assimilated by the leptocephali and suggested that smaller particles may be important for the leptocephali.

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. [ABSTRACT FROM AUTHOR]

To understand the transport and recruitment processes of the daggertooth pike conger, Muraenesox cinereus, in the marginal seas of East Asia, we investigated the distribution pattern, estimated spawning areas and periods, and recruitment mechanisms of M. cinereus, based on 51 individuals of leptocephali collected from Korean waters during 2010-2014. Back-calculated hatching dates, determined from the daily incremental growth rates of the otoliths, indicated that the spawning period for M. cinereus was during July-September. The size range of M. cinereus leptocephali collected offshore of Jeju Island and southeast of the Korea-Japan intermediate zone was 16.6-20.9 mm TL (age, 18-23 d). We hypothesize that one of the spawning grounds of M. cinereus is located offshore in the East China Sea. In Korean waters, the ages and body lengths of M. cinereus leptocephali increased northward from latitude 31°30′N to 34°40′N, with metamorphosis occurring at latitude 34°40′N. Therefore, we surmised that the hatched preleptocephali of M. cinereus were transported from offshore areas in the East China Sea to Jeju Island and the Korea Strait (KS) by the Kuroshio and Tsushima Warm Current. [ABSTRACT FROM AUTHOR]

Eel aquaculture, though thriving nowadays, is totally dependent on the successful capture of wild eel fry and glass eels for its seedlings. The declination of eel resources in recent years has resulted in an urgent need for technology development in artificial seedlings production on an aquaculture basis, in order to protect natural resources and to stabilize the eel supply in the farming industry. Since the life history of the eel holds many mysteries, artificial hatching and rearing of larvae has long been regarded as an extremely difficult task. However, in recent years, the spawning ground of the Japanese eel has finally been located after continuous effort with intensive marine surveys, in which wild parental eels were captured, followed by the collection of fertilized eggs and the harvest of newly hatched preleptocephali. Meanwhile, through the collaborative efforts of many researchers, progress has also been made in improving technologies for artificial maturation of parental eels, which do not mature naturally in captivity, as well as in the technology for artificial hatching. Moreover, a technology for producing feed-rearing eel hatchlings, the most challenging process of all, has advanced rapidly after suitable feed was developed in the 1990s. Then, in 2002, for the first time in the world, larvae were successfully reared up to the glass eel stage, and second generation artificial hatchlings were born in 2010. In this way, eel farming technology that is not reliant on natural resources has been developed. There are strong hopes now for a technology for stable mass production of glass eels to be developed in the near future. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

One hypothesis to explain the large decline in the recruitment of the European eel ( Anguilla anguilla) from the 1980s onwards is that a change in ocean circulation has influenced the drift of eel larvae, resulting in lower recruitment. To test this hypothesis, a simple Lagrangian model was constructed to simulate a passive drift from the spawning area in the Sargasso Sea to the European shelf. The simulation utilised the velocity data from a reanalysis of ocean climate, the Simple Ocean Data Assimilation ( SODA 2.1.6), for the period 1958-2008 covering the time of regime shift in eel recruitment. The average drift time and latitudinal distribution of the arrival of eel larvae were explored for a range of constant depth levels and instantaneous mortalities. The model showed that the proportion of eel larvae carried by the North- East Atlantic Current to northern latitudes of arrival was greater before 1970, whereas there was an increase in amount of larvae being entrained into the southbound current branches after this time. The overall success of drift from the spawning area to the East Atlantic was not affected and clearly contradicts that this could explain the observed recruitment decline. [ABSTRACT FROM AUTHOR]