Abstract: For a year we investigated prey preference of a vermivorous opisthobranch, Pupa strigosa, at the mouth of the Inari River, Kagoshima Bay, Japan. Pupa strigosa were sampled twice a month from September, 1999, to October, 2000, at depths of 5 to 20 m. Gut contents were present in 253 of the 382 collected specimens, and the polychaete prey were identified from the setae and jaws found in the guts.The opisthobranchs fed on benthic polychaetes belonging to seven families. Their diet changed both with season and with increasing body size of the opisthobranchs. The shell size was small in autumn, when most individuals fed on dorvilleid polychaetes. Later they fed on glycerid, nereidid, spionid, pilargid, lumbrinerid, and sabellid polychaetes. The prey included chiefly free-living or crawling and carnivorous or omnivorous polychaetes, but rarely sessile forms.

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Abstract: Recruitment of naturally occurring Japanese scallops(Patinopecten(Mizuhopecten)yessoensis)into sowing-culture grounds has important implications for the population dynamics and aquaculture man agement because it can cause unfavorable lower growth due to high density and a highly variable size range, which can lead to increased costs for harvesting and selecting market-sized scallops. In the present study, the occurrence and magnitude of natural recruitment in a scallop population cultivated at Yubetsu seabed(northern Japan)was investigated. Assessment was done through a photocensus at 4 culture areas, A, B, C and D, which contained 1-, 2-, 3- and 4-year classes of seeded scallops, respectively. The magnitude of natural recruitment varied annually among the culture areas. In 1990 and 1991, natural recruits composed 20 and 32% of the scallops in area C and B, respectively. In 1992, few natural recruits were observed in area A(8%)and C(7%). The highest number of natural recruits occurred in 1993 in area A(≥ 41%). These results suggest that routine monitoring of density throughout the whole period of culture is indispensable. Transfer of scallops from high-density areas caused by natural recruitment to low-density areas should be considered.

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Abstract: The seasonal preferences for prey species and prey size by the dogwhelk Nucella freycineti(Deshayes)were investigated on an intertidal rocky shore in Hokkaido, northern Japan. Adult dogwhelks(>25 mm shell length)mainly preyed upon the mussels Septifer virgatus and Mytilus sp. in the field throughout the year. In the laboratory, they prefbrred Mytilus sp. over S. virgatus in June, and this preference increased in August. The prey values of the two mussels were compared using the formula(energy amount of one prey)/(drilling time). I assumed that the dry tissue weight of the prey reflected the energy content and that the valve thickness of the prey reflected the drilling time. Although the dry weights of Septifer and Mytilus were not significantly different in June, Mytilus sp. became heavier than the same size S. virgatus for all sizes in August. Indices of valve thickness(valve thickness-shell length ratio)athe point in mussel shells where dogwhelks preferred to make a hole before feeding were not significantly diffberent between the two mussel species, indicating that the drilling times of both prey types may be almost the same. These resultsuggest that Mytilus sp. and S. virgatus have almost the same prey value in June, but Mytilus sp. is a more valuable prey in August. Although the energy content of one prey and drilling effort required were used to estimate the prey value, this study suggests that other factors(e. g., morphological differences of prey, nutritional quality of prey)might be also important for seasonal prey selection. Dogwhelks selected significantly larger mussels in summer than in winter both in the laboratory and field. Hence, dogwhelks consume optimal species(more valuable species)and prey size(bigger sizes)in warm periods, buthe foraging behavior is restricted by environmental conditions in cold periods.

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Abstract: We examined whether exposure to predators affects the shell preference of the hermit crab Pagurus filholi. First, hermit crabs occupying the shells of three gastropods(Batillaria cumingi, Reticunassa fraterculus, and Homalopoma sangarense)were exposed to the predatory crab Gaetice depressus. B. cumingi shells provided the most protection, and H. sangarense shells provided the least protection against predation. We then used these two shell species for shell preference tests conducted in both the presence and absence of the predator. We placed 20 hermit crabs(10 in B. cumingi shells and 10 in H. sangarense shells)collected from the field into a plastic container that contained 100 empty shells of each species to allow them to select their preferred shell species and size. After this first shell selection, we divided the hermit crabs that occupied each shell species into two groups of approximately equal number. We then introduced two predatory crabs and a crushed piece of P. filholi everyday into one group but not the other. After 10 days we conducted a second shell selection experiment and compared the utilization frequencies of the 1st and 2nd shell selections for each group. Hermit crabs exposed to the predatory crabs increased their preference for B. cumingi shells over H. sangarense shells. On the other hand, crabs not exposed to predatory crabs increased their preference for H. sangarense shells, perhaps because they are lighter than B. cumingi shells. Our results suggest that shell preference of the hermit crab varies depending on environmental factors such as the threat of predation.

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Abstract: The distribution and spatio-temporal variation in population structure of the fluvial neritid gastropod Clithon retropictus(v. Martens, 1879)were studied over one year in the Saigo River, Fukuoka Prefecture, Japan. This species inhabits only the downstream reaches of the river, in the area that extends from freshwater to brackish water, and shows a marked spatio-temporal variation in population structure. The abundance of snails was greatest at the upstream limit of brackish water, where juveniles smaller than 8mm in shell width occurred throughout the year. Very few juveniles were found in the middle reaches of the brackish water and none was found in the freshwater area. Mean shell width of both males and females was smallest at the upstream limit of brackish water and tended to increase both upstream and downstream. Females were everywhere larger than males, and the sex ratio was always nearly one at the upstream limit of brackish water; however, there were many more females than males in the middle reaches of the brackish water area and throughout the freshwater reaches. These results suggest site-specific settlement of larvae at the upstream limit of brackish water, and growth-related movements of small and middle-sized snails both upstream and downstream. The mechanisms of downstream and upstream movement may be different, with the former being caused by passive displacement by floods or by density-dependent dispersal, whereas the latter probably involves active upstream migration.

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Abstract: The life history of the luminescent crustacean Vargula hilendorfii(Ostracoda: Myodocopida), which mainly inhabits sand/mud bottoms near the seashore, was investigated in Amakusa, Kyushu, Japan from April, 2000, to March, 2001. The species was collected from a beach monthly or semi-monthly using sledge nets. Ovigerous females occurred from April to ctober, and juveniles were collected from May to December. In winter, most V. hilgendorfii collected were adult females, which suggests that females that had copulated survive until spring. Brood size averaged 50-58 eggs in most months but was slightly smaller in April(41eggs). In the laboratory, juveniles that were released from the marsupium reached maturity in 50-55 days. We also investigated the occurrence of the crustacean ectoparasite Onisocryptus ovalis(Isopoda: Epicaridea)in V. hilgendorfii Male O. ovalis invaded female hosts, consumed host eggs, changed sex to be come female, produced eggs, and died after hatching of the eggs. The percentage of female hosts parasitized by O. ovalis was low(0-24%)from summer to autumn, but increased greatly(36-91%)from winter to spring, primarily because of a shortage of hosts. Usually one or two male parasites were found to occupy one host, and only one of them changed sex; from winter to spring, however, up to eight male parasites were found on a single host and a maximum of five changed ex.

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