Location, size and age at onset of metamorphosis in the Japanese eel Anguilla japonica.

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.

Temporal and spatial variations of 134Cs and 137Cs levels in the Sea of Japan and Pacific coastal region: Implications for dispersion of FDNPP-derived radiocesium.

To investigate the dispersion of Fukushima Dai-ichi Nuclear Power Plant (FDNPP)-derived radiocesium in the Sea of Japan and western Pacific coastal region and determine the sources of radiocesium in these areas, we examined the temporal and spatial variations of 134Cs and 137Cs concentrations (activities) during 2011-2016 in seawaters around the western Japanese Archipelago, particularly in the Sea of Japan. In May 2013, the surface concentration of 134Cs was ∼0.5 mBq/L (decay-corrected to March 11, 2011), and that of 137Cs exceeded the pre-accident level in this study area, where the effects of radiocesium depositions just after the FDNPP accident disappeared in surface waters in October 2011. Subsequently, radiocesium concentrations gradually increased during 2013-2016 (∼0.5-1 mBq/L for 134Cs), exhibiting approximately homogeneous distributions in each year. The temporal and spatial variations of 134Cs and 137Cs concentrations indicated that FDNPP-derived radiocesium around the western Japanese Archipelago, including the Sea of Japan, has been supported by the Kuroshio Current and its branch, Tsushima Warm Current, during 2013-2016. However, in the Sea of Japan, the penetration of 134Cs was limited to depths of less than ∼200 m during three years following the re-delivery of FDNPP-derived radiocesium.

Reproductive isolation between sympatric Anguilla japonica and Anguilla marmorata.

Species-specific restriction fragment length polymorphism in the intron of the androgen receptor gene (ar5) was found in glass to silver-stage individuals of Anguilla japonica (n = 51) and A. marmorata (n = 21). The sequence analysis of 16S rDNA from 328 anguillid leptocephali collected in the North Equatorial Current of the western North Pacific Ocean revealed the specimens to be A. japonica (n = 194), A. marmorata (n = 128), A. bicolor pacifica (n = 5) and A. luzonensis (n = 1). All leptocephali of A. japonica and A. marmorata were monomorphic and did not share an allele at the ar5 locus, indicating that the two species are reproductively isolated.