Ruditapes philippinarum is a popular edible clam that occurs densely on the sandy tidal flats on the Japanese coast. However, on the tidal flats in Ariake Bay in Kumamoto Prefecture, western Japan, the clam suffered from extremely high mortality just after settlement on the sediment and this mortality seriously affected the population persistence of the clam. In this study, we focused on the negative impact of manganese in the sediment on the juvenile clam. We surveyed manganese ion concentration in the interstitial water of the sediment on three different sandy tidal flats facing Ariake Bay in Kumamoto Prefecture and one in the west coast of Korea. We conducted laboratory experiments exposing juvenile clam just after settlement to different contents of manganese dioxide in the sediment or manganese ion in the water: with sand containing high content of manganese dioxide (2,300 mg kg⁻¹ dry sediment; henceforth Experiment 1); with seawater containing 5.4 mg L⁻¹ of manganese ion (Experiment 2); and with natural sand and seawater containing 5.4 mg L⁻¹ of manganese ion (Experiment 3). Significant mortality was not noted in Experiment 1 or in Experiment 2, while high mortality was seen in Experiment 3. In the field survey, we detected manganese ion concentration in the interstitial water at only 2.78 mg L⁻¹ on the Arao Tidal Flat. However, we confirmed that manganese ion concentration in the interstitial water became more than 40 mg L⁻¹ in the extraction test with the sediment on the Arao Tidal Flat. These results indicate that manganese ion liquated from the sediment into the interstitial water, due to reduction of manganese oxide, may influence seriously the physiology of juvenile clams on the tidal flats.

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The neustonic calanoid copepod Pontella karachiensis, previously recorded only in the coastal waters of Pakistan and recently in the Arabian Gulf, is fully redescribed from the northern Red Sea because the previous description is insufficient to identify the species. This is the first record of its occurrence in the Red Sea and confirms that this copepod is a subtropical Indian Ocean species. The species belongs to the newly established karachiensis group because of closer similarity to P. minocerami, the other member of the group, than any species of the known species groups. Gut content analysis revealed that P. karachiensis is carnivorous, mainly feeding on planktonic copepods.

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The planktonic food web structure in the water off Cape Esan, southwestern Hokkaido, Japan was investigated at approximately one or two-month intervals from May 1997 through June 1999, based on biomass data of pico- (0.2–2 μm), nano- (2–10 μm), micro- (10–200 μm) and mesoplankton (200–20,000 μm). In spring (March–April), the water column was stratified and a spring diatom bloom occurred. In addition, microzooplankton and copepod biomass was high among the heterotrophic plankton in spring. In other seasons (summer, fall and winter), pico- and nanoplankton assemblages were dominant within the plankton community. These results indicate that the grazing food chain (micro-size phytoplankton – copepods) might be important and the indirect route (micro-size phytoplankton – microzooplankton – copepods) probably prevails in spring, while the microbial food web might be the predominant carbon flow route in other seasons.

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The vertical distribution of dominant euphausiid species was investigated in the offshore waters of northeastern Japan in April and September 1998 and November 1999 using MOCNESS-I net samples. In total, 32 euphausiid species in eight genera were identified in the study area. Subarctic Euphausia pacifica and Thysanoessa inspinata occurred over a wide range, although mainly in the Oyashio and cooler transitional areas, and the former made a relative contribution of 78% to the total numerical abundance of euphausiids. At night, these species tended to stay near the surface in the Oyashio area and in deeper layers in warmer areas. During the daytime, they tended to stay in deeper layers in the warmer areas, although E. pacifica stayed in deeper layers even in the Oyashio area where the chlorophyll a concentration was low. T. inspinata did not perform clear diel vertical migration. Except for these subarctic species, five warm-water species of Euphausia were abundant among the epipelagic euphausiids. At night, these species tended to stay at a wide range of depths from the surface to subsurface layers in the warmer Kuroshio area, but were concentrated near the surface in the transitional area. During the daytime, they stayed in deeper layers in the Kuroshio area. Diel vertical migration of euphausiids seems to be strongly affected by water temperature, but other factors such as phytoplankton abundance may also be important. The possible effects of diel vertical migration of euphausiids on euphausiid fisheries are discussed.

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A unique community of Acharax japonica has been found in the reductive bottom sediments in two natural seawater sinks, where a number of the clams can easily be collected and successfully maintained for over a year in laboratory conditions. Stable isotopic signature of their soft body parts shows typical range for chemosynthesis-based bivalves, indicating that the clams rely on sulfur-oxidizing bacteria in their gill. δ¹⁵N values of substrate ammonia have been measured together with those of the clam soft body parts. These results confirm that the characteristic negative δ¹⁵N values of the clam soft-body parts were caused by a large isotope fractionation during ammonia assimilation as previously documented for those from various locations. In addition, sulfur isotopic ratios of each organ show significant variation. Such trend is observed for some thiotrophic chemosynthesis-based bivalves inhabited on a cold-seep environment, suggesting that it is a common feature for thiotrophic bivalve communities depended on bacterial derived sulfide. Heterogeneity of sulfur isotopic signature among different organs from one specimen was considered due to primary heterogeneity of δ³⁴S values for source sulfide in their habitat. In addition a different turnover rate of the sulfur nutrition in each organ was conspicuous for the sulfur isotopic variation. This was confirmed by long-term maintenance using an external sulfur budget under laboratory conditions. The estimated turnover rates among three organs were the highest for gill (shorter than three months), the lowest for viscera (longer than a year), and intermediate for foot.

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The distribution of sulfate-reducing bacteria (SRB) was investigated in the sediments below fish cages used for torafugu (Takifugu rubripes) aquaculture along the coast of southern Fukui Prefecture, Japan. A quantitative competitive PCR targeting the gene coding for a portion of the α-subunit of the dissimilatory (bi)sulfite reductase (dsrA) gene was used to determine the distribution of SRB in the sediments. In unpolluted fish farm sediments, SRB were detected at levels of 0.5–8.4×10⁸ cells g⁻¹ dry sediment; whereas higher densities of SRB were found in sediments rich in organic matter and sulfide at levels of 0.8–4.4×10⁹ cells g⁻¹ dry sediment as determined by competitive PCR. We detected differences in SRB densities in fish farm sediments with different pollution levels, or organic contamination and sulfide content, suggesting SRB cell density may be used as an indicator of pollution levels in fish farm sediments as an adjunct to chemical analyses such as measurements of chemical oxygen demand (COD) or chemically determined total sulfide (TS).

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We investigated the molting of Euphausia pacifica in relation to reproductive and feeding activities off northeastern Japan. The percentage of molting individuals was lower (7%) in the gravid females than in other maturity stages (12–22%), which suggests that gravid females molt less frequently, about 2-fold less, than other stages of females and males. Feeding activity of molting individuals, assessed by the stomach fullness, was reduced compared with non-molting individuals. However, even molting individuals showed some feeding rhythm, with higher stomach fullness in the nighttime than in the daytime. Molting of this species proved to be related to reproductive and feeding activities.

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An easy polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis was designed to identify the three morphologically similar nereidid polychaete species Hediste atoka, Hediste diadroma and Hediste japonica which often dominate in estuarine macrobenthic fauna in eastern Asia. Restriction digestion analysis of the PCR products of the partial mitochondrial 16S rRNA gene (approximately 600 bp) of these three species, using EcoRV and TspEI endonucleases, generated species-specific restriction patterns. The PCR-RFLP method enables easy and accurate identification of the three Hediste species.

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