Responses of light-harvesting and photoprotective pigments to the length of dark storage and silicate availability were determined for the marine diatom Thalassiosira weissflogii during dark survival and recovery. During 14-day's darkness, no significant changes in the concentration of silicate were observed, whereas the cell density increased by 34% for 8 days and decreased afterward. All cellular pigment contents decreased sharply between days 3 and 5 and stayed at relatively constant levels on days 11 and 14. Once the cells were exposed to a light:dark cycle on days 3, 8, and 14 of the dark storage experiments, there was a significant loss of all cellular pigment contents during the first 24 h of the light:dark cycle, which corresponded to the log-phase of growth. Cellular photoprotective pigment levels then started to recover to levels similar to those observed on days 11 and 14 in the dark storage experiments. When the dark-acclimated cells were exposed to light:dark cycle conditions, cell growth was immediately restored. The growth rate decreased significantly with increasing length of dark storage but not with silicate availability. This observation may suggest that T. weissflogii trapped in ballast water are able to survive in total darkness and to restore cell division once they are released into ambient coastal waters within two weeks.
We examined feeding preference in the threadsail filefish Stephanolepis cirrhifer on jellyfish and lobworm. Juvenile filefish were fed moon jellyfish Aurelia sp. and/or lobworm Perinereis nuntia vallata for 30 min in tanks with or without sand substrate, and the onset of foraging behavior, response latency (RL) and time spent foraging (total time spent searching for food and feeding) were analyzed using video recordings and measurements of food consumption. Organic matter consumption per unit of foraging time was also estimated for each prey item. In tanks with a sand substrate, RL was significantly shorter, foraging time was significantly longer, and organic matter consumption per unit foraging time was higher for jellyfish than for lobworm; however, the opposite trend was observed in tanks without a sand substrate. Filefish in all treatments consumed an average of 99±25% of their body weight in jellyfish in 30 min. This study revealed that although filefish prefer lobworm to jellyfish when both are readily available, filefish will select jellyfish when only jellyfish are visible. These findings imply that frequent jellyfish blooms increase the feeding opportunities of filefish on jellyfish.
Brackish-water calanoid copepods, defined as those predominantly inhabiting oligo- and mesohaline waters, were investigated at 45 rivers in western Japan during 2006–2009 to review their faunas and, for comparison, 4 rivers of Korea in 2006. Eight species (Acartia ohtsukai, A. sinjiensis, A. tsuensis, Pseudodiaptomus inopinus, P. ishigakiensis, Sinocalanus sinensis, S. tenellus and Tortanus derjugini) were identified as brackish-water calanoid copepods from Japan. Among them, P. inopinus was most often collected; it occurred in 38 rivers. Acartia tsuensis and A. sinjiensis were also frequent species in Japan. Pseudodiaptomus ishigakiensis discovered from Kyushu Island is new to the mainland of Japan. From the rivers of Korea six species were identified (A. ohtsukai, A. sp., P. inopinus, P. sp., S. tenellus and T. dextrilobatus). Based on the present and previous studies, a total of 13 brackish-water calanoid copepods have been recorded from Japan and Korea, of which eight species, including three continental relicts (A. ohtsukai, S. sinensis and T. derjugini) in Kyushu Island, are common to both regions. Among them, only two species (P. inopinus and S. tenellus) are distributed widely on both Japanese and Korean coasts of the Tsushima (Korea) Strait. This indicates that the faunas of brackish-water calanoid copepods are very different between Japan and Korea despite the short geographical distance (about 170 km) and that the Tsushima Strait has played the role of an effective barrier for their dispersion.
Long-term variations of the harmful diatom Eucampia zodiacus, a causative organism for bleaching of aquacultured “nori” (Porphyra yezoensis), had been monitored at 19 sampling stations in Harima-Nada, eastern Seto Inland Sea, Japan for 35 years from 1974 to 2008. Over the study period, E. zodiacus cells were detected every year, and seasonal cell densities tended to be higher from January to April. The proportion of E. zodiacus to the total phytoplankton cell density has increased in recent years, because the abundances of E. zodiacus started to increase in the mid 1990s, whereas the total phytoplankton cell densities in the 1980s and thereafter appeared to be lower than those of the 1970s. During the 35-year period, there were two significant long-term changes, i.e. an increase in winter water temperatures and a decrease in the concentration of dissolved inorganic nitrogen. The present results suggest that the shift of environmental conditions is more advantageous to the growth of E. zodiacus, which contributed to the domination of E. zodiacus abundance in recent years.
To evaluate the variations in the toxin profiles of Alexandrium tamarense (Dinophyceae) from Hokkaido and southern Sakhalin, paralytic shellfish poison of 103 culture strains obtained from six areas (Funka Bay, Tomakomai, Urakawa, Akkeshi, Okhotsk Sea and Aniva Bay) was analyzed using high performance liquid chromatography-fluorescent detection. All of the strains, except for two culture strains, were classified by a hierarchical cluster analysis into clusters in which the strains had almost the same toxin composition as found in previous studies (major toxins: C2, gonyautoxin 4, neosaxitoxin and gonyautoxin 3). On the other hand, cellular toxin contents of the culture strains varied from 1 to 1,128 fmol cell−1 and the cellular toxin content (y fmol cell−1) was inversely proportional to the cell density (x cells mL−1) (y=9942.2x−0.448, Spearman's rank correlation coefficient: rs=0.539, p<0.01). It can be concluded that the toxin compositions of A. tamarense are almost the same in Hokkaido and southern Sakhalin, and high-toxicity blooms of A. tamarense as usually found possibly occur in the natural environment at low cell densities (10−2–102 cells mL−1).
Genetic population structure of the Neptune whelk (Neptunea arthritica) in northern Japan was estimated from six samples collected from Hokkaido and one sample from Aomori, in northernmost Honshu, using five polymorphic microsatellite DNA loci. Pairwise FST estimates indicated a genetic cline from eastern and northeastern Hokkaido to southern Hokkaido and northernmost Honshu. The individual-based assignment method and analyses of molecular variance suggested three geographic groups within this cline. The observed population structure was most likely influenced by isolation-by-distance with restricted gene flow, as suggested by the significant correlation between genetic and geographic distance for the entire region examined. The inferred restriction of gene flow is likely due to the poor dispersal ability of this species, which has a benthic, sedentary life history and passive dispersal along the coasts. The observed genetic structure of N. arthritica will be useful for conservation and fisheries management of this species.
Giant clams are important species in tropical aquaculture. The larvae artificially hatched are often cleaned by running seawater containing the larvae through a coarse mesh, designed to catch contaminants larger than the larvae (e.g. fouling materials detached from adult clams); and then through a fine mesh, designed to catch the larvae and wash away smaller contaminants (e.g. bacteria). Such larval cleaning is assumed to improve the larval survival rate. We conducted experiments on how the cleaning improves the survival rate of larvae of the giant clam Tridacna crocea during the planktonic stage. The experiments revealed that larval cleaning improves the survival rate for larval densities of 0.3 to 9.8 indiv. mL−1. The experiments suggested that the survival rate at 0.5 indiv. mL−1, a typical larval density in hatcheries, would be 16.6% for the cleaned larvae, much higher than 1.4% for those not cleaned. Through larval cleaning, both bacteria and small T. crocea embryos showing maldevelopment were found to decrease in the water containing T. crocea larvae, which can explain in part the improved survival rate.
We measured stable carbon and nitrogen isotope ratios (13C/12C and 15N/14N) of organic matter sources and consumers, including fishery resources, along the Okhotsk coast of northern Hokkaido, Japan, in early spring to characterize the food web. Phytoplankton and benthic detritus, including ice algae, fecal pellets, and benthic microalgae, constituted the main food sources for primary consumers (zooplankton and small benthic invertebrates, respectively); seagrass and terrestrial plant debris served no trophic function for consumers. Planktivorous and benthivorous fish species could be differentiated using stable carbon isotope ratios. Differences in stable nitrogen isotope ratios revealed species-specific feeding habits of crustacean groups: copepods were separated into three groups as herbivorous, omnivorous, or omnivorous-carnivorous, and benthic amphipods were divided into two groups as either omnivorous or omnivorous-carnivorous. We distinguished four levels of both pelagic and benthic trophic linkages. The δ13C values were enriched by 0.9–1.3‰ at the linkage between each pelagic trophic level except between the 1st and 2nd levels; δ15N values were enriched by 2.2–2.9‰ at each level. For benthic trophic linkages, the enrichment for δ13C and δ15N values between each level ranged from 0.6‰ to 1.0‰ and from 1.6‰ to 2.6‰, respectively. Trophic positions of copepod, mysids, and sand lance appeared between pelagic and benthic linkages. This suggests that those species have fed on both pelagic and benthic diets.
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