KOBAYASHI, SATOSHI and SHUHEI MATSUURA (Department of Fisheries, Faculty of Agriculture, Kyushu University). 1992. Ecological Studies on the Japanese Mitten Crab Eriocheir japonicus DE HAAN -III. Relative Growth of the Chela and Soft-hair Distribution on the Chela. Benthos Research, 45 : 1-9.
The relative growth pattern of the chela and the soft-hair distribution on the chela of the Japanese mitten crab, Eriocheir japonicus were investigated. According to the distribution pattern of dots plotted between the chela propodus height (CPH) and carapace width (CW), we distinguished three growth phases for the male and two phases for the female, and we fitted a regression line to each phase. The first and the second phases were distinguishable by an inflection at about 30 mm≤CW≤40 mm in both sexes, and the third phase in the male could be noted by a discontinuity from the second phase between 50 mm≤CW≤65 mm. The slope and/or Y-intercept of the regression line for each phase of the male was significantly higher than that for the same phase of the female. The area occupied by soft hair on the chela increased with the CW in both sexes, with the increase being greater in the male than in the female. In the large male (60 mm≤CW), the surface of the chela was almost wholly covered with hair, whereas in the large female, both the outer and inner lateral sides were partially covered.

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IMABAYASHI, HIROMICHI, KAZUHIRO ISHIKAWA and AKIO GO (Faculty of Applied Biological Science, Hiroshima University). 1993. Larval Settlement of a Penaeid Shirimp Metapenaeopsis acclivis in Suo-Nada, the Seto Inland Sea. Benthos Reseach, 45 : 11-18.
Population of a penaied shrimp Metapenaeopsis acclivis in Suo-Nada, the Seto Inland Sea, seemed to settle at a depth of 10 m in September. The morphological characteristics, i, e., relative length of 6th abdominal somite, number of setae on pleopod and number of teeth on rostrum, suggested that the smallest benthic shrimps corresponded to megalopal larvae immediately after settlement. The carapace length of megalopal laevae exhibited a mode of 1.70 mm, with a 95% confidence interval of 1.69 to 1.71 mm. They fed mainly on small benthic organisms, e. g., Microsetella sp. and Coscinodiscus spp., which were slightly different from the food of juveniles.

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ITAOKA, MUTSUMI and KYOICHI TAMAI (Nansei National Fisheries Research Institute). 1993. Effect of eutrophication on the structure of meiobenthic communities in Hiroshima Bay. Benthos Research, 45 : 19-28.
Meiobenthos was surveyed to investigate the relationship between its community structure and eutrophication of the bottom sediment at three stations in Hiroshima Bay throughout the year. The organic content of sediments was high at all these stations, namely about 14% in ignition loss and 2.3-2.6 mg/g dry sediment in total nitrogen content. At the innermost station, total sulphide concentration was 1.6 mg/g dry sediment in average, which was 3.6-5.6 times higher than at the other stations. The oxygen concentration in the bottom water was also fairly low (range : 1.6-2.9 ppm) from June to September at the station. The abundance of meiobenthos was highest at the innermost station, being about twice as much as that at the outermost station. Except at the outermost station, the abundance increased from spring to summer and decreased in September. Nematodes comprised more than 85% of the total meiobenthos at all stations, with the highest abundance occurring at the innermost station. Harpacticoids were most abundant at the outermost station. The nematode/copepod ratio was highest at the innermost station, and lowest at the outermost station. Most harpacticoids occurred in the top centimeter of sediment. Nematodes inhabited at least down to 5 cm, but they tended to occur more in the surface layer at the innermost station. It is suggested that stressful bottom conditions such as low oxygen concentrations and high total sulphide caused change of meiobenthic community structure.

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KAWAI, KEI and SHIGERU NAKAO (Faculty of Fisheries, Hokkaido University). 1993. Reproductive cycle, copulating activity and dispersion pattern of the dogwhelk Nucella freycineti (DESHAYES). Benthos Research, 45 : 29-41.
The reproductive cycle of Nucella freycineti was studied based on samples collected at monthly intervals from September 1987 to December 1988 and from May 1990 to July 1991 in Usujiri, southern Hokkaido, Japan. The developmental processes of the gonads and the seminal vesicle were classified into four and three successive stages, respectively. The development of the gonads and the seminal vesicle exhibits a distinct annual cycle. In both sexes, remarkable gonadal development was observed during the warm season from June to October. More than 90% of the males were mature in October. However, oogenesis was observed through the winter until the next breeding season in April and May. The annual cycle of the ovary is not synchronous with that of the testis but with that of the seminal vesicle. Size at sexual maturity was 19+ mm and 15+ mm shell length for females and males, respectively. MORISITA index (I_δ) showed that these snails aggregated all year round. Strongest aggregation was observed during the colder and coldest seasons from November to May, when high copulation and breeding activities successively occurred. During warmer period from June to September, the index was relatively low but high foraging activity was observed.

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YAMAMOTO, TOMOKO (Department of Zoology, Kyoto University). 1993. Foraging rhythm of the whelk Muricodrupa fusca (KÜSTER) (Muricidae) in intertidal rockpools. Benthos Research, 45 : 43-48.
Foraging rhythm of predatory gastropod Muricodrupa fusca (KÜSTER) was investigated in some intertidal rockpools of mid-tide level at the south-west coast of Kii Peninsula. The foraging activity of M. fusca did not exhibit clear rhythms with day-night periodicity, high and low tide periodcity nor even semilunar periodicity. Comparison of these results with those from other studies about animals living out of pools evidently indicate that M. fusca in the pools do not have any foraging rhythmicity reported in the other studies. It was suggested that such a foraging pattern of the whelk was caused by its long foraging time and less changeable environmental conditions in rockpools. This non-rhythmicity of the foraging was considered not to be so disadvantageous to the living in rockpools.

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