Zooplankton abundance and biomass along 165°E and 165°W in the North Pacific were estimated using an optical plankton counter on preserved samples collected with a plankton net of 335 μm mesh-size from 0–150 m at 89 stations from the subarctic to subtropical regions during the summers of 2003–2006; the stations were grouped into subarctic (SA), transitional (TR) and subtropical (ST) regions for latitudinal comparisons. The two-way ANOVA and a post hoc test showed that total abundance and biomass were significantly larger in SA and smaller in ST stations, respectively, than in the other regions, but were not significantly different between 165°E and 165°W. Total ranges of abundance and biomass were: 34×103–65×103 indiv. m−2 and 2.9–7.9 g dry mass m−2, respectively. The variation in abundance was mainly governed by the variation of 0.34–1.00 mm equivalent spherical diameter (ESD) size class zooplankton, but total biomass variations were due mainly to the 2.00–3.00 mm ESD size class, which corresponds to the calanoid copepod Neocalanus spp. copepodid stage 5 (C5) that had a greater abundance in SA and TR than in ST. Despite possibly higher abundances of Neocalanus C5 in TR than in SA, abundance and biomass of the 2.00–3.00 mm ESD size class were not significantly different between the two regions. Size reduction of individuals due to higher temperatures in TR than in SA may be a possible explanation.
The validity of the assumption that the specific growth rate of coccoid microorganisms can be expressed as the absolute value of the derivative of frequency of dividing cells (FDC) with respect to time was tested experimentally using a Microcystis aeruginosa strain. FDC was calculated based on the assumption that elongated cells, constricted cells or both may correspond to dividing cells. The specific growth rates that were calculated from the diel changes in FDC were compared with those calculated from the change in the cell density. The results revealed that constricted cells can serve as the optimal dividing cells, and the calculated specific growth rates were statistically identical to those obtained by the change in the cell density.
Distribution patterns and abundance of the epiplanktonic shrimp Lucifer were examined in the East China Sea (23°30′–33°N, 118°30′–128°E), in relation to temperature and salinity. A total of 443 samples were collected from four seasonal surveys conducted between 1997 and 2000. The yield density model was used to predict optimal temperature (OT) and optimal salinity (OS) of four Lucifer species: Lucifer typus, L. hanseni, L. intermedius and L. penicillifer. Thereafter, their distribution patterns were determined. The results indicated that these species are the most abundant in summer. Lucifer typus, with OT of 28.0°C and OS of 33.8, is considered to be an oceanic tropic water species. The species is mainly found in the northern waters off Taiwan in summer and autumn. Lucifer hanseni, L. intermedius and L. penicillifer, with OTs of 26.4, 28.0, 27.4°C and OSs of 33.6, 33.4, 33.2, are off-shore subtropical water species. They are mainly distributed in the south off-shore the East China Sea and north of Taiwan.
The roles of heterotrophic organisms (microzooplankton, mesozooplankton, bacteria and heterotrophic nanoflagellates) were examined during a nutrient enrichment experiment using a mesocosm in Saanich Inlet, British Columbia, Canada. Grazing rates of microzooplankton, copepods, and Noctiluca scintillans were respectively estimated by the dilution method, from the egg production, and the apparent growth rate. The primary production increased by about 11 times during the initial 3 days, and the grazing rate by zooplankton also increased by 7.4 times. The primary production exceeded the grazing rate during the initial 5 days, after that, almost balanced rates were observed. Biomass peaks of bacteria and HNFs (heterotrophic nanoflagellates) were observed after the decline of the phytoplankton bloom. Bacterial production and HNF bacterivory gradually increased from the beginning to the end of the experiment. Microzooplankton consistently removed about half of the primary production. The contribution of microzooplankton to grazing was largest during the initial 7 days. Heterotrophic dinoflagellates were the most dominant component of the microzooplankton, but oligotrich ciliates showed the fastest growth response to phytoplankton production. Noctiluca scintillans became an important grazer after the bloom. Overall, the contribution of microzooplankton grazing was the largest of the processes through which phytoplankton were lost. Cell sinking was a minor component contributing to loss of phytoplankton. Thus, oligotrich ciliates and heterotrophic dinoflagellates were the most plausible organisms contributing to the steady state of phytoplankton concentrations.
Chronic outbreaks of the coral eating crown-of-thorns starfish (Acanthaster planci) have devastated coral communities throughout the Ryukyu Islands in the past four decades. Since A. planci has high fecundity and long larval duration, knowledge of reproductive timing and larval dispersal are keys to understanding mechanisms of successive recruitment and consequent outbreaks of adults. As the first step to acquiring this knowledge, we monitored water temperature and peak spawning periods using gonad observation. A total of 379 A. planci specimens were collected from May to September in 2004 at five latitudinally different locales in the Ryukyu Islands. Peak spawning, at which starfish gonads rapidly shrunk in size, was first observed in late May in Ishigaki Island, the southernmost sampling site. Then spawning time was progressively delayed with increasing latitude, ie. from mid to late June in Miyako Is., late June to mid July in Kerama Is., late July in Okinawa Is. and Amami Ohshima Is. When we started the survey in May, some A. planci had already started spawning, though not at Amami Ohshima. The peak spawning time in every locality coincided with the time when the water temperature exceeded 28°C. Accordingly, we can predict the approximate peak for spawning by monitoring water temperatures throughout the Ryukyu Islands. The information will be useful for: (1) numerical simulations predicting larval dispersal pathways by particle tracking through provision of the input parameter for the timing of larval release, (2) selecting times for larval sampling in the field in empirical early life history studies.
The North American invasive signal crayfish Pacifastacus leniusculus was designated as an ‘Invasive Alien Species’ by the Ministry of the Environment of Japan and by the Ministry of Agriculture, Forestry and Fisheries of Japan on 1 February 2006. We report the first record of P. leniusculus from the Kanto region (Chiba Prefecture), central Japan. Adult specimens of P. leniusculus were collected from the Tone River basin between September 2009 and January 2010. Ectosymbiotic crayfish worms (Annelida, Clitellata, Branchiobdellidae) were found on the crayfish and all specimens were identified as Sathodrilus attenuatus, which outside of its home range in North America occurs only in the Hokkaido and Fukushima Prefecture populations of P. leniusculus; thus suggesting that P. leniusculus in the Tone River basin was introduced from Hokkaido or Fukushima Prefecture. Signal crayfish is considered a cool water species and has been reported mostly in Hokkaido, northern Japan, but our findings strongly indicate that P. leniusculus has become established in a warm water area in central Japan, where the maximum water temperature is above 30°C in summer. These results indicate that P. leniusculus can inhabit warm water areas as well as cool areas and would be able to extend its distribution over a wide range in Japan.