The taxonomic position of an armored dinoflagellate Prorocentrum donghaiense Lu, 2001, which is one of the causative species of large red tides occurring off Changjiang River mouth was examined and compared to a similar species P. shikokuense. In both species, the following morphological features were shared; a weakly concave cell with a tiny apical spine around the periflagellar area, one side of the cell being slightly extended on the anterior end, a rounded posterior end, a rounded nucleus located in the posterior region and similar size of cells. DNA sequences of small subunit rDNA and internal transcribed spacer 1 (ITS1)-5.8S rDNA-ITS2 regions of these two species were largely the same. Morphological and molecular data suggest that P. donghaiense and P. shikokuense are synonymous and P. shikokuense has priority over P. donghaiense.

View full abstract

The signal crayfish Pacifastacus leniusculus, native to northwestern North America, is among the most problematic invaders in freshwater systems of Europe and Japan. To provide an effective tool to analyze population genetics of this alien species, we have developed five polymorphic microsatellite DNA markers from the genome of this species. The number of alleles and expected heterozygosity in each locus ranged between 4–33 and 0.304–0.941, respectively, indicating the utility of the markers in population analysis. We analyzed 212 individuals of P. leniusculus from seven locations in Hokkaido and Honshu, Japan using these markers. When we used a Bayesian clustering analysis, the three clusters, Akashina, Tankai and Hokkaido, were discriminated from each other with different allele frequencies. In addition, a recently found population in Tone River in central Honshu appeared to have originated from Hokkaido, consistent with a previous study using ectosymbiont worms. Thus, the new microsatellite markers are useful in identifying the population structure and genetic connectivity of P. leniusculus in Japan. We discuss potential applications of microsatellite analysis in tracking dispersal pathways and defining eradication units for this invasive crayfish.

View full abstract

Temporal variation in the wintertime diatom community structure in Tokyo Bay was investigated from 1981 to 2000 with respect to possible environmental influences. A previous study found that rapid industrialization around the 1960s altered the diatom community. We found that a similar eutrophic-type community with low species diversity dominated by Skeletonema costatum sensu lato (s.l.) still prevailed in 2000, despite recent relaxation of eutrophic conditions. When the cell density and carbon biomass of S. costatum s.l. declined from the mid 1980s to the late 1990s, diatom communities dominated by various species occurred sequentially and species diversity increased. Principal components analysis on environmental factors revealed that S. costatum s.l. favored typical wintertime conditions in the Tokyo district, sunny (less rain) and cold. In contrast, cloudy and warm conditions from the mid 1980s to the mid 1990s were responsible for the decline in S. costatum s.l., allowing other diatom species to grow. The second most important species, Eucampia zodiacus, favored windy years, presumably with strong advection and vertical mixing. The interannual variation of S. costatum s.l. and E. zodiacus populations was out of phase, suggesting that these species had responded to different climatic forcing at different temporal scales.

View full abstract

The abundance and species composition of viable resting stage cells of diatoms were investigated in deep waters (200, 500 and 1,000 m depths) collected at neighboring stations in Kumano-Nada, central part of Japan, in April, August and October 2006. Viable resting stage cells were enumerated by the modified extinction dilution method [most probable number (MPN) method] based on incubation. Resting stage cells were detected from all samples, except at 500 m depth in August, in a range of abundances between 200–4,415 cells L⁻¹. The resting stage cells of diatoms observed belonged to four genera of the Centrales, Chaetoceros spp. (C. curvisetus, C. socialis and Chaetoceros sp.), Leptocylindrus danicus, Skeletonema spp. and Thalassiosira spp., and two of the Pennales, Cylidrotheca closterium and Navicula sp., along with one centric and three pennate unidentified diatoms, in total. It is indicated, in this study, that the abundances enumerated by the MPN method could represent those of disassembled resting stage cells originally aggregated in marine snow, of which the sinking rate is reportedly great (e.g. 68 m day⁻¹). This, eventually, suggests that the abundance and species composition of resting stage cells in the deep waters reflected the states of blooms occurring just before our samplings in the surface water of the Kumano-Nada region. In this region, coastal upwelling occasionally occurs, implying that such resting stage cells sinking from the euphotic layer to deeper depths might have some chance to return to the surface and be able to act as a “seeding population” for further blooms.

View full abstract

The diurnal vertical distribution of zooplankton have been compared on a sunny and on a cloudy day in two shallow, oligotrophic and fishless alpine lakes, exploring the role of cloud cover in shaping zooplankton movements. UV ray avoidance is considered the ultimate reason for zooplankton vertical migration in the lakes, which are naturally devoid of zooplanktivorous fish. Thus, as a thick cloud cover reduces the visible and UV radiation, a shallower diurnal distribution was expected on cloudy days, since migrant zooplankton should balance the amplitude of migration with the light intensity. However, we found very little field evidence that zooplankton change their vertical distribution according to daytime light conditions.

View full abstract