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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.

To assist in detection of offshore spawning activities of the Japanese eel Anguilla japonica and facilitate interpretation of results of environmental DNA (eDNA) analysis in their spawning area, we examined the eDNA concentration released by each life history stage of artificially reared Japanese eels in the laboratory using quantitative real-time PCR (qPCR). We also compared eDNA concentrations between before and after artificially induced spawning activities. eDNA was not detected from three 30 L seawater tanks containing each single fertilized egg, but eDNA was found from other tanks each containing single individuals of larval stages (preleptocephalus and leptocephalus), juvenile stages (glass eel, elver and yellow eel) or adult stage (silver eel). The eDNA concentrations increased in the life history stages, showed a significant difference among all stages, and were positively correlated with the total length and wet weight. Moreover, the eDNA concentration after spawning was 10–200 times higher than that before spawning, which indicated that the spawning events in the ocean would produce relatively high eDNA concentration. These results in the laboratory suggested that eDNA analysis appears to be an effective method for assisting oceanic surveys to estimate the presence and spawning events of the Japanese eel in the spawning area.

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.

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.

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.

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.

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.

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.

The Japanese eel Anguilla japonica is an important aquaculture fish species in the East Asian countries of Japan, China, Korea, and Taiwan. All glass eel fry are captured from the wild and understanding the recruitment patterns of the glass eel is important. The larvae of A. japonica are passively transported to the East Asian Continental Shelf by the North Equatorial Current, the Kuroshio, the Kuroshio intrusion currents, and coastal currents. In each location, recruitment time is diverse: It is November in Taiwan and April in the Yalu River. How the glass eels reach recruitment areas remains poorly understood. Here, we combine information from larval ages based on otolith increments, simulated drifting paths on the East Asian Continental Shelf, and main fishing seasons in each location of East Asia. We identify five main recruitment blocks: (1) The main Kuroshio, (2) The Taiwan Strait Warm Current, (3) The Taiwan Warm Current, (4) The Yellow Sea Warm Current and (5) The branch of Yellow Sea Warm Current. The counted age of the glass eels is significantly underestimated for the later recruits, possibly due to the cessation of the otolith edge growth under low water temperatures. This study clarifies the eel&rsquo, s larval characteristics and transport mechanisms in the East Asia Continental Shelf, providing important information for its recruitment dynamics in the marine stage.

Usual diets for rearing leptocephalus larvae of Japanese eel Anguilla japonica include eggs of the endangered spiny dogfish Squalus acanthias (SE). We investigated the effects of alternative food materials, hen egg yolk (HEY) and exoskeleton-free (skinned) Antarctic krill (SAK), on the growth and survival of eel larvae. We found that feed comprising whole krill including exoskeleton (WAK) containing higher levels of fluoride (37.89 mg kg−1) was acutely toxic to eel larvae exposed to this alone. In contrast, extract from SAK containing lower concentrations of fluoride (4.25 mg kg−1) showed no apparent adverse effects. Growth of larvae fed a mixture of SE and SAK in a feed trial of 58 days [mean body weight (BW), 6.0 mg] was about twofold higher than that of larvae fed a mixture of SE and WAK (3.2 mg) (P

Development of the thyroid gland of the Japanese eel (Anguilla japonica) was studied with the use of tank-reared fish. A single thyroid follicle was first found in larvae at 29 days post hatching (dph), total length (TL) 12 mm. Until reaching 25 mm in TL (100 dph), leptocephali had one or two follicles per individual. The inner colloid of the follicles was weakly immuno-positive against the anti-thyroxine (T4) antibody. The number of thyroid follicles and the immunoreactivity later increased as the larvae grew. Thyroid hormones (TH) T4 and triiodothyronine (T3) were not detected in premetamorphic larvae by radioimmunoassays, but became detectable during metamorphosis. The maximum level of T4 was seen in fish at the end of metamorphosis and in just-metamorphosed juveniles, whereas T3 reached the highest level during metamorphosis and declined toward the end of metamorphosis. The results indicated that the thyroid gland first became active during metamorphosis in the development of eel.

Natural stocks of Japanese eel (Anguilla japonica) have decreased drastically because of overfishing, habitat destruction, and changes in the ocean environment over the past few decades. However, to date, artificial mass production of glass eels is far from reality because of the lack of appropriate feed for the eel larvae. In this study, wild glass eel, leptocephali, preleptocephali, and embryos were collected to conduct RNA-seq. Approximately 279 million reads were generated and assembled into 224,043 transcripts. The transcript levels of genes coding for digestive enzymes and nutrient transporters were investigated to estimate the capacities for nutrient digestion and absorption during early development. The results showed that the transcript levels of protein digestion enzymes were higher than those of carbohydrate and lipid digestion enzymes in the preleptocephali and leptocephali, and the transcript levels of amino acid transporters were also higher than those of glucose and fructose transporters and the cholesterol transporter. In addition, the transcript levels of glucose and fructose transporters were significantly raising in the leptocephali. Moreover, the transcript levels of protein, carbohydrate, and lipid digestion enzymes were balanced in the glass eel, but the transcript levels of amino acid transporters were higher than those of glucose and cholesterol transporters. These findings implied that preleptocephali and leptocephali prefer high-protein food, and the nutritional requirements of monosaccharides and lipids for the eel larvae vary with growth. An online database (http://molas.iis.sinica.edu.tw/jpeel/) that will provide the sequences and the annotated results of assembled transcripts was established for the eel research community.

The development of gill chloride cells was examined in premetamorphic larvae (leptocephali) and juveniles (glass eels) of the Japanese eel, Anguilla japonica. Branchial chloride cells were detected by immunocytochemistry using an antiserum specific for Na+,K+-ATPase. The specificity and availability of the antiserum for the detection of Japanese eel chloride cells were confirmed by Western blot analysis. The chloride cells first appeared on the developing gill filaments in a mid larval stage of leptocephalus (32.2 mm). Both immunoreactivity and the number of chloride cells gradually increased as the fish grew to a late stage of leptocephalus over 54 mm. In glass eels just after metamorphosis, gill lamellae developed from the gill filaments, and a rich population of chloride cells was observed in the gill filaments. In glass eels collected at a coastal area, chloride cells were extensively distributed in the gill filaments. The chloride cell size decreased progressively in glass eels transferred from seawater (SW) to freshwater (FW), whereas there was no difference in cell number. In contrast, some Na+,K+-ATPase immunoreaction distinct from typical chloride cells was observed in the gill lamellae throughout FW-transferred fish, but disappeared in control fish maintained in SW for 14 days. These findings indicate that the gill and gill chloride cells developed slowly during the extremely long larval stage, followed by rapid differentiation during a short period of metamorphosis. The excellent euryhalinity of glass eels may be due to the presence of the filament chloride cells and lamellar Na+,K+-ATPase-immunoreaction, presumably being responsible for SW and FW adaptation, respectively.

Summary The age and migratory history of the Japanese eel, Anguilla japonica Temminck & Schlegel, collected in Miyako Bay along the Sanriku coast of Japan, was examined using the otolith microstructure and analysis of strontium (Sr) and calcium (Ca) concentrations conducted with wavelength dispersive X-ray spectrometry by an electron microprobe. The line analysis of Sr : Ca ratios along the life history transect of each otolith showed a peak (ca. 15–17 × 10−3) which corresponded with the period of their leptocephalus and early glass eel stages in the ocean. The mean Sr : Ca ratios from the elver mark to the otolith edge indicated that there were eels with several general categories of migratory history, including sea eels that never entered freshwater (average Sr : Ca ratios, ≥6.0 × 10−3), and others that entered freshwater for brief periods but returned to the estuary or bay. This evidence of the occurrence of sea eels in this northern area indicates that Japanese eels of the Sanriku coast do not necessarily migrate into freshwater rivers during recruitment as do glass eels at the beginning of their growth phase; even those that do enter freshwater may later return to the marine environment. Thus, anguillid eel migrations into freshwater are clearly not an obligatory migratory pathway, but rather a facultative catadromy with seawater or estuarine residents as an ecophenotype.

The migratory history of the Japanese eel Anguilla japonica collected in Otsuchi Bay and the adjacent stream along the Sanriku Coast of Japan was examined using otolith microchemical analysis of strontium (Sr) and calcium (Ca) concentrations. X-ray intensity maps of Sr concentration in the otoliths showed that all specimens generally had a high Sr core at the center of the otoliths, which corresponded to the period of leptocephalus and early glass eel stages in the ocean, while line analysis of Sr : Ca ratios along the life-history transect of each otolith showed a peak (≈ 13–20 × 10−3) between the core and elver mark. The mean Sr : Ca ratios from the elver mark to the otolith edge indicated eels of several general categories of migratory history, including sea eels that never entered freshwater (average Sr : Ca ratios > 6.0 × 10−3) and estuarine eels that entered freshwater for brief periods, but returned to the estuary or bay (average Sr : Ca ratios 2.5–6.0 × 10−3). This evidence of the occurrence of several migratory patterns in this northern area indicates that A. japonica has a flexible pattern of migration, with an ability to adapt to various habitats and salinities. Thus, migrations of anguillid eel into freshwater are facultative and not obligatory.

Possible causes of declines in recruitment of European, American and Japanese eels to continental waters are reviewed. Negative correlations between the Den Oever glass eel recruitment index (DOI) and the North Atlantic Oscillation Index since 1938 are discussed, together with older anecdotal evidence. Correlations are established between the DOI and sea surface temperature anomalies at 100-250 m between 1952 and 1995 in the Sargasso Sea/Sub-Tropical Gyre (STG) spawning area. It is hypothesised that, associated with global warming trends, STG warming inhibits spring thermocline mixing and nutrient circulation, with negative impacts on productivity and hence food for leptocephalus larvae. Concurrent gyre spin-up also affects major currents and slowing of oceanic migration has probably enhanced starvation and predation losses. Local factors, such as unfavourable wind-driven currents, can also affect recruitment of glass eels on continental shelves. In contrast, evidence is discussed that indicates fishing mortality and continental climate change appear to have had lesser impacts. Similar starvation-advection explanations for declines in Japanese eel recruitment are proposed. Predictions for the future are made and multidisciplinary and integrated monitoring and research are recommended for managing eel stocks and fisheries.

Weekly injections of salmon pituitary extracts (SPE) were administered to female Japanese eel, Anguilla japonica at a dose of 20 mg/fish. This induced vitellogenesis and caused oocytes to reach the migratory nucleus stage. Later, a majority of the females that received an injection of SPE at a priming dose, followed 24 h later by 17,20β-dihydroxy-4-pregnen-3-one (DHP), ovulated 15 to 18 h after the final injection. In cultivated males, repeated injections of human chorionic gonadotropin (hCG) at a dose of 1 IU/g BW/week induced spermatogenesis and spermiation. Since potassium ions were revealed to be an essential constituent for the maintenance of motility in the eel spermatozoa, artificial seminal plasma containing KCl was designed as a diluent of milt, and enabled the preservation of milt for several weeks in refrigeration. As a result, artificial fertilization performed immediately after ovulation with pre-diluted and stocked milt consistently resulted in the production of high-quality gametes. Recently, a slurry-type diet made from shark egg yolk has been found to be a suitable feed for captive-bred eel larvae. Although preleptocephalus larvae can be reared with this diet beyond the depletion of their yolk and oil droplet stores, it remains inadequate because larvae reared under this way cannot be raised to the following stage. Therefore, the diet was improved by supplements of krill hydrolysate, soybean peptide, vitamins and minerals. Larvae fed on this new diet have grown to 50 to 60 mm in total length (TL), and have begun to metamorphose into glass eels approximately 250 days after hatching.

Despite intensive research on wild and captive eels, no resource has so far provided access to all life cycle stages of the Japanese eel Anguilla japonica. The transition from the preleptocephalus (newly hatched larva) to the leptocephalus stage (typical leaf-like eel larva) has, therefore, remained the missing link in the eel life cycle. We recently found that a slurry-type diet made from shark egg powder is suitable feed for captive-bred eel larvae. The larvae were successfully reared with this diet in aquaria for 100 days and raised to 22.8 mm in total length (TL). Age, TL, and body proportions of the reared specimens overlapped with those of wild leptocephali. We revealed for the first time the transition from the preleptocephalus to the leptocephalus stage of the eel.

SUMMARY: Pituitary, thyroid gland and gonads in leptocephali of Japanese eel Anguilla japonica (19.8–32.6 mm in total length), A. obscura (45.0 mm), and A. bicolor pacifica (49.5 mm) and those in glass eels of the Japanese eel were histologically and immunohistochemically examined in order to observe the developmental changes of these endocrine organs in the Anguillidae. The pituitary, consisting of adenohypophysis and neurohypophysis in Japanese eel leptocephali over 22.5 mm, did not contain thyroid stimulating hormone (TSH) immunoreactive cells. Such cells were, however, detectable in the more developed pituitaries of leptocephali of A. obscura and A. bicolor pacifica and in those of glass eels. Conversely, thyroxine (T4)-immunoreactive thyroid follicles could be detected in all specimens, both leptocephalic and glass eel. Only in glass eels, gonads were found in the body cavity, and these gonads harbored one or two primordial germ cells (PGC) per cross-section. Our results indicate that thyroid hormones (TH) production started prior to TSH production, and that TSH and TH are both secreted during the metamorphosis from leptocephalus to glass eel. Therefore, it is plausible that the TSH–TH axis is involved in the metamorphosis from leptocephalus to glass eel, but not in the early growth from preleptocephalus to leptocephalus.

Background Early life history traits of the temperate eel Anguilla japonica and tropical eel Anguilla marmorata were examined to determine the possible reason why these two species have similar spawning areas and oceanic larval transport in the North Equatorial Current and yet are recruited to different but partly overlapping continental growth habitats in northern East Asia. To understand the segregative migration of these two sympatric eel species, their glass eels were collected from nine estuaries in the Philippines, Taiwan, Japan, and China. The age at metamorphosis from leptocephalus to glass eel (T m), the age at estuarine arrival (T t), the time between metamorphosis and estuarine arrival (T t−m), and the growth rate (G t) of glass eels were calculated from daily growth increments in their otoliths. Results Results indicated that the G t was faster and the T m was younger in A. marmorata than in A. japonica. On the other hand, fish length and the T t at estuarine arrival were larger in A. japonica than in A. marmorata, indicating that elvers of A. japonica experience a longer oceanic drift than those of A. marmorata. In addition, the T t−m also indicated that A. japonica experienced a longer coastal migration than A. marmorata. Conclusion This study validated that the T m and G t seem to play important roles in the segregative migration and latitudinal distribution of these two sympatric freshwater eel species in the northwestern Pacific.

To study the mechanism of Sr incorporation into otoliths of the Japanese eel, Anguilla japonica , a total of 100 elvers collected from an estuary were reared in the laboratory at salinities of 0, 10, 25 and 35%. for approximately seven months. The elvers grew from 56 mm TL to 100–300 mm TL. Twenty elvers were randomly selected and the Ca and Sr concentrations of their otoliths were analyzed from the primordium to the edge, using an electron microprobe equipped with a four-channel wavelength-dispersive spectrometer. Sr Ca ratios in the otoliths of eels reared in various salinities were much lower than the ratio of 15 × 10 −3 observed in elvers about one month before they arrived at the estuary. The irreversibility of Sr Ca ratios at 35%. salinity in this experiment indicated that the drastic change of the Sr Ca ratios in otoliths of elvers was not due to the reduction of salinity in the coastal waters, but more likely to the development from leptocephalus to glass eel. The mean Sr Ca ratios in the new increments of the otoliths of eels during the rearing period were highly correlated with salinity ( S ), [ Sr Ca ] × 10 3 = 3.797 + 0.14S ( n = 20, r = 0.77), which can be used to predict elver movements and habitat utilization rates.

Fishery sustainability and the extinction risk of the Japanese eel, Anguilla japonica, are of global concern. The landings of the Japanese eel in Japan comprise a large part of the landings in East Asia. This study provides a compiled dataset of the annual fisheries statistics of the Japanese eel in Japan for stock assessment. The Japanese government has been recording Japanese eel statistics annually since 1984 in five series of annual reports by conducting systematic questionnaire surveys of fisheries managers and associations; however, most of these data are stored in analog format. The key variables in the dataset include the harvest weight of eels, the harvest weight and number of seeds for aquaculture, the number of eels stocked, and the number of management entities engaged in the eel fishery. The levels of spatial aggregation of the variables include the site (river and lake), prefecture, inland and coastal waters, and total in Japan. We also incorporated location data (latitude and longitude) of the site and prefecture into the dataset. Eel harvest includes primarily yellow eels (late juvenile stage) and silver eels (mature stage). Seed harvest in inland waters includes glass eels (intermediate stage between leptocephalus and elver) and elvers (early juvenile stage). Seed harvest from coastal waters comprises glass eels. This dataset provides information to assess long-term trends in the Japanese eel population.

Pepsinogen is the precursor form of the gastric-specific digestive enzyme, pepsin. Ghrelin is a representative gastric hormone with multiple functions in vertebrates, including the regulation of growth hormone release, stimulation of food intake and gastrointestinal motility function. We investigated chronological changes in the distribution of pepsinogen-expressing cells by in situ hybridization and ghrelin-immunoreactive cells by immunohistochemistry in the Japanese eel (Anguilla japonica) during metamorphosis from the leptocephalus sage to the elver stage. The ghrelin-producing cells first appeared in the gastric cecum and pyloric portion of the stomach in the late phase of metamorphosing leptocephali, whereas the pepsinogen-producing cells were first detected in the early phase of the glass-eel stage. These suggest that endocrine cells differentiated earlier than exocrine cells in the eel stomach. Accompanying eel development, the distribution of ghrelin-producing cells spread to the esophagus and other regions of the stomach, but not to the intestine. These results may be related to the changes in dietary habits during metamorphosis in the Japanese eel.

In a previous study, we identified cDNAs encoding the growth hormone receptor (eGHR1) and eGHR1 homologue (eGHR2) in Japanese eel (Anguilla japonica). In the present study, changes in the developmental expression of growth hormone (GH), eGHR1 and eGHR2 were investigated in the Japanese eel eggs and preleptocephali by RT-PCR and immunohistochemical methods in an attempt to examine the involvement of these proteins in larval growth. The GH transcripts and the production of GH protein were not detected in the newly hatched larvae and preleptocephali at day 3 post-hatch, however, these were detected at day 6 post-hatch, and also detected at higher levels at day 10 post-hatch. In contrast, prolactin and somatolactin transcripts could not be detected in all preleptocephalus specimens (newly hatched larvae and preleptocephali at day 3, 6 and 10 post-hatch). eGHR1 and eGHR2 transcripts were detected in all preleptocephalus specimens. Therefore, it is plausible that the actions of GH during the preleptocephalus stage are mediated through the eGHRs. The present data suggest that GHR-mediated actions of GH begin at the same time as the initiation of GH production, and that GH plays important roles in larval growth and survival to the leptocephalus stage. eGHR1 mRNA, which is thought to be of maternal origin, was also detected in ovulated eggs. However, the role of eGHR1 mRNA in eggs is not clear.

Techniques for artificial breeding of the Japanese eel, Anguilla japonica, have been studied intensively since the 1960s. Yamamoto and Yamauchi (1974) first succeeded in obtaining fertilized eggs and larvae of the Japanese eel by hormone treatment, and preleptocephalus larvae were reared for 2 weeks (Yamauchi et aL.1976). Thereafter, many researchers succeeded in obtaining eel larvae (Satoh 1979; Wang et al. 1980; Yu et al. 1993), but suitable larval feeds were not identified. As a result, preleptocephalus larvae could not survive beyond the depletion of their yolk and oil droplet stores. In other eel species, for example, the European eel A. anguilla (Prokhorchik 1986) and the New Zealand freshwater eels A. dieffenbachii and A. australis (Lokman and Young 2000), experimentally produced larvae survived only for a few days and, like the Japanese eel, did not develop into leptocephali. Thus, the transition from preleptocephalus, just before first feeding, into the willow leafshaped leptocephalus stage caught in the wild has never been observed. Therefore, attempts to rear captive-bred eel larvae in aquaria to observe this transition have been intensively continued, which is of obvious benefit for both clarifying the eel life history and artificial propagation programs. Tanaka et al. (2001) recently discovered that a slurry-type diet made from shark-egg powder is a suitable feed for captive-bred eel larvae. Larvae were reared on this diet for more than 200 days and raised to 31 mm in total length (TL).

Deals with the relation between the physiological diel vertical-migration strategy of the Japanese eel (Anguilla japonica) and its transportation strategy which is related to the transfer from North Equatorial Current to the Kuroshio Current. The author investigates the diel vertical migration (DVM) of leptocephalus larvae through the transportation simulation with the genetic algorithm for understanding the various DVM strategies.

The eel has long been esteemed as an important food fish in the world, especially in Japan, and has been used as an experimental fish for many fields of fish physiology. However, the decreases in eel resources have been a serious concern in recent years. The catches of glass eels as seedlings for aquaculture have shown a long-term decrease in both Europe and East Asia. To increase eel resources, the development of techniques for artificial induction of maturation and spawning and rearing their larvae have been eagerly desired. Recent progress of reproductive physiology of fish, especially mechanisms of oocyte maturation and ovulation in female and of spermatozoa maturation in male, facilitate to establish techniques for hormonal induction of maturation and spawning in sexually immature eels. With persistent effort to development of rearing techniques of larvae, we have first succeeded to produce glass eel. These applied techniques are may contribute to understand the basic reproductive physiology of the eel.