In western Japan, Heterocapsa circularisquama blooms occur almost every year. Two distinct viruses infectious to H. circularisquama, a large double-stranded DNA (dsDNA) virus, ‘HcV’, and a small single-stranded RNA (ssRNA) virus, ‘HcRNAV’, were found and characterized. Here we report the co-occurrence of the two viruses in Hiroshima Bay, Japan, during 2006. Viruses isolated using H. circularisquama strains HU9433-P and MZ2 (the hosts) were large dsDNA viruses (ca. 0.2 μm in diameter) that accumulated in the host cytoplasm. Pulsed-field gel electrophoresis analysis showed their genome sizes were ca. 388 kbp, comparable to the previously reported HcV strain. Small ssRNA viruses were also isolated (using the host strain HCLG-1) at ca. 30 nm in diameter and formed crystalline arrays and/or were randomly accumulated in the host cytoplasm, and their partial genome sequences were highly similar to a previously reported HcRNAV strain. This is the first report of the co-occurrence of HcV and HcRNAV in natural water and their successful isolation. The data suggest the two distinct viruses may cooperatively affect the dynamics of H. circularisquama blooms.
The horizontal and vertical distributions of larvae and reproductive timing of euphausiids were investigated in the Ross Sea and its adjacent waters during austral summer 2004–2005. Occurrences of larvae of Euphausia frigida and E. triacantha were confined to the northern oceanic area where the Upper Circumpolar Deep Water prevailed, although their juvenile and adult stages extended the distribution further southward to the area where cooler Lower Circumpolar Deep Water prevailed. Larvae of Thysanoessa spp. were widely distributed within the oceanic to slope areas but did not occur on the shelf as juvenile or adult stages. Eggs and larvae of E. superba occurred with gravid females along the slope, but no juveniles occurred concurrently. Thus the slope does not appear to be a nursery ground for this species. The distribution of E. crystallorophias larvae was mostly confined to the shelf in the juvenile and adult stages. The onset of deepening was from early and later frucilia stages onward for E. triacantha and E. frigida, respectively. However, Thysanoessa spp. were concentrated increasingly within the surface layers from furcilia I onward. The onset of recruitment to calyptopis I appeared to be earlier in the more northern species with the exception of E. crystallorophias, which recruited before E. superba. The intense spawning of E. crystallorophias and E. superba coincided with a period of development of a coastal polynya. Generally, the onset of spawning of euphausiids appeared to be related to the latitudinal distribution and timing of sea-ice melting. Relationships between surface temperatures and development and recruitment of larvae of euphausiids are discussed.
The reduction of planktivorous fish populations in lakes through biomanipulation often results in increased abundance of the large cladoceran Daphnia and invertebrate predators, such as cyclopoid copepods, and also in decreased numbers of small zooplankters, such as the cladoceran Bosmina longirostris and rotifers. To investigate the factors causing reduced abundance of small zooplankton species in lakes, we performed a mesocosm experiment where the abundance of Daphnia and cyclopoid copepods (Mesocyclops sp.) were controlled. Bosmina populations declined substantially in mesocosms with abundant copepods, implying that predation by cyclopoid copepods is a major factor controlling Bosmina populations in lakes. In contrast, rotifer densities were reduced in the mesocosms with Daphnia, even though excess food (Chlorella vulgaris) was provided. This suggests that rotifer populations are effectively controlled by interference competition with Daphnia.
Size-fractionated (0.25, 0.5, 1, 2, and 4 mm mesh size) wet mass (WM) and dry mass (DM) determinations and optical plankton counter (OPC) measurements were carried out on zooplankton samples collected at 15 stations in the northern North Pacific Ocean, Bering Sea and Chukchi Sea during July–August 2007. The total sample WM and DM estimated from OPC data corresponded closely to those of measured values by a factor of 0.970–1.098. However when the sample was portioned into different size groups, estimates of size-fractionated WM and DM by OPC data varied from measured masses by a factor of between 0.202 and 1.768. The high variability was caused by an underestimation of sizes of the large sized (>4 mm) fraction, or an overestimation of the number of the small size fraction (2–4 mm). The underestimation in the >4 mm and overestimation in the 2–4 mm respectively were caused by the dominance of transparent hydromedusae, and slender-shaped euphausiids in the >4 mm fraction. This study suggests that OPC analysis could be susceptible to errors in zooplankton biomass estimates in the large size fraction (>4 mm) especially when euphausiids and hydromedusae dominate the population. On the other hand, OPC based estimates of DM within 0.25–4 mm size fraction are more robust, which may be due to the dominance of large copepods, and low detritus content in the samples from the oceanic subarctic Pacific, in summer 2007.
Density and adult ratio of the symbiotic harpacticoid copepod, Idomene purpurocincta, living in the colony of the compound ascidian host, Aplidium yamazii, were investigated on the specimens taken from a boulder shore of Tosa Bay, western Japan, to know how these ecological parameters of the symbiont were related to the host size. Field collection of the host ascidian was carried out during the low tide in every spring tide from December 2004 to August 2006. All of the copepods isolated from the hosts using menthol were enumerated according to ontogenetic stages. The ascidian host colonies appeared from January to June 2005 and from December 2005 to August 2006. The symbiotic copepod was found in all of the hosts with few exceptions even at the initial appearance of the hosts in each season. These facts suggest that the copepod entered the host immediately after the colony formation. The density of the copepod in a host was usually >103 individuals per gram of host in dry weight. An exponential negative correlation was seen between the copepod density and the host size, and the density varied less as the host size increased. The adult ratio of the copepod also greatly varied in small hosts and tended to converge to about 25% with increase of the host size, and was negatively correlated with the copepod density. The reason for these host size-related copepod density and adult ratio may be explained by recruitment of nauplii in a host and migration of adults among hosts.
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