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.

To establish a simple and reliable index for determining silvering stages of the Japanese eel, Anguilla japonica, we observed the colorations of various body parts and biological characteristics of the eels collected in a coastal area of Japan (Mikawa Bay). The four silvering stages are characterized by the colorations of pectoral fins and ventral skin as follows: (1) Y1, yellow eel without a metallic hue at the base of pectoral fins, (2) Y2, late yellow eel with a metallic hue at the base of the pectoral fins but without melanization at the tip of pectoral fins, (3) S1, silver eel with complete melanization at the tip of pectoral fins but without full pigmented belly in black or dark brown, and (4) S2, late silver eel with black or dark brown belly. The body size, eye diameter and sexual maturity of each stage increased in the order of Y1, Y2, S1 and S2 stages, whereas the digestive tract degenerated in the same order, suggesting a sequential development of these ontogenetic stages identified in the study. The Y1, Y2 and S1 stages could be also distinguished by canonical discriminant function analysis using three internal (gonad-somatic index, GSI; hepato-somatic index, HIS; and gut index) and two morphometric (condition factor and eye index) parameters, supporting the significance of these stages. This method of staging for the silvering process of the Japanese eel appeared to be applicable to all specimens of this species, since this index used only simple external characteristics that would be easy to observe during field surveys.

We have determined the complete cDNA sequences of fresh water rod opsin gene (fwo) and deep-sea rod opsin gene (dso) from Japanese eel Anguilla japonica. The cDNA clones of fwo and dso consisted of 1437 and 1497 nucleotides, respectively. The predicted opsins of both genes consisted of 352 amino acid residues. Southern blot and PCR analyses of genomic DNA indicated that the Japanese eel genome contains only one fwo and one dso and they are intronless. Quantitative RT-PCR analyses revealed that the expression of fwo decreases with sexual maturation while that of dso increases.z 2000 Federation of European Biochemical Societies.

-Remarkable progress in techniques for maturation of adults and rearing of larvae has been achieved for the Japanese eel Anguilla japonica over the past 50 years, but recent efforts have not yet succeeded in the mass production of glass eels. This article reviews recent advances in techniques for artificial production of A. japonica glass eels. Successful new protocols for obtaining viable eggs and larvae of A. japonica are largely resulting from using artificially feminised eels instead of wild silver eels. The feminisation technique by administration of 17b-estradiol to glass eels not only provides an opportunity for using females as experimental broodstock throughout the year, but also accelerates their oocyte development or even increases the competence of females to respond to salmon pituitary extract. Induced spontaneous spawning with paired eels can improve the success rate for obtaining good-quality eggs compared with the traditional stripping-insemination method. As a result of the accumulation of field data on natural environmental conditions, such as an optimum temperature, where eel eggs and larvae were collected, the survival and growth rate of captive eel leptocephali have been much improved under these conditions. However, 50% diluted sea water (17.5 psu) that is far from the natural condition can result in better growth and survival performance in A. japonica leptocephali, possibly because it reduces the energy necessary for osmoregulation. Starvation can be a cue for triggering the onset of metamorphosis in A. japonica leptocephali, reducing the prolonged duration of the larval stage in captivity. To reach the mass production of glass eels at a commercial level, further improvement in quality of an artificial larval diet not containing eggs of the endangered shark Squalus acanthias is needed. The control of deformities in captive leptocephali and glass eels is also an important task. New techniques reducing the excessive use of exogenous hormones with parent eels should also be developed, to avoid public perception of an unhealthy product. If these techniques are completely established, we can exchange artificially produced glass eels with some part of the wild glass eels that are heavily exploited and provided to the eel aquaculture industry, and this can help to reduce the impact of eel consumption, which will contribute to the conservation of eel species worldwide.

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