Seven new species of the polyplacophoran genus Parachiton are described from the Northwest Pacific. This is the first record of the genus from that region.

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Batillaria flectosiphonata n. sp. is described from the Iriomote Island, the Nansei Islands, Japan. It is the fifth species known of the genus Batillaria (Gastropoda : Batillariidae) in the Indo-Western Pacific. It differs from all of the Japanese species of Batillaria, B. zonalis, B. multiformis and B. cummingi by having a distorted and deeply concave collumella and a siphonal canal which is reflected to the left. In the features of collumella and siphon, it resembles Batillaria solida (Gmelin), but differs from the latter species in having more slender and thinner shell with distinct axial ribs, and also by having radular ribbon with different-shaped lateral tooth.

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A species of Nassarius (Zeuxis) of the Nassariidae, collected from shallow-water submarine caves on the outer slopes of a coral reef in Ie and Shimoji Islets of Okinawa, Japan, and in Balicasag Islet of Bohol, the Philippines, is examined. This species is safely referable to Nassa cinnamomea A. Adams, 1852 described originally from Dumaguete, Negros Island of the Philippines, and has not been rediscovered since its first description and long synonymized with Nassarius (Zeuxis) comptus (A. Adams, 1852). The newly obtained material clearly shows that Nassa cinnamomea is a valid species as it is distinct from N. (Z.) comptus in its slenderer shell form, much finer axial ribs in the apical whorls, the presence of a subsutural collar in the whorl surface, a prominently variced outer lip, and consistently brown to dark-brown shell color. The species lives in gloomy to totally dark, shallow-water submarine caves, but has not been found in the nearby shallow-water bottoms outside of the caves. However, this species most probably lives in deepwaters more than 150 m deep around Balicasag Islet of Bohol, the Philippines.

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Imposex is a phenomenon of the development of additional male sex organs in female prosobranch gastropods. A survey of imposex in five prosobranch gastropods, i.e. Thais clavigera, Nassarius livescens, Babylonia areolata, B. formosae formosae, and Ficus ficus were conducted in the present study. Degrees of imposex expressed as relative penis size (RPS) values and percent imposex varied among species and the 21 collection sites. For example, F. ficus and N. livescens had lower RPS values (0 and 0 to 4.1%) than other species. RPS in T. clavigera were 0.4% in Tungkang and 71.8% in Shunsun. Among the 17 sites in the west coast of Taiwan, only one site (Anping) had no sign of imposex (expressed as percent imposex). Our results indicate that imposex is commonly observed in the intertidal and subtidal prosobranch gastropods around the west coast of Taiwan.

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Using larvae obtained by stimulating artificial spawning of Musculista senhousia and Limnoperna fortunei kikuchii in the laboratory, larval developoment was examined under different temperature conditions (15℃, 25℃, 30℃). At 25℃ and 30℃, larvae of L. fortunei kikuchii showed faster growth, shorter period from fertilization to settlement, and larger size at settlement than those of M. senhousia. At optimum growth temperature (30℃ for L. fortunei kikuchii, 25℃ for M. senhousia), size at settlement in larvae of the former species was larger by two times or more than the later species, and the days from fertilization to settlement were shorter by ca. 7 days. On the other hand, larvae of M. senhousia grew faster at 15℃ than that of L. fortunei kikuchii. However, this temperature hampered larval development of the above-mentioned two species, because the larvae did not grow to the pediveliger and did not settle on the bottom until our 25-day experiment ended.

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Vertical distribution and life cycle of the minute trochid snail Conotalopia mustelina (max. shell width : 3.2 mm) within intertidal mussel zones were studied at Shirahama, Wakayama Prefecture, Japan. At the study site, two mytilids Septifer virgatus and Hormomya mutabilis formed vertically contiguous mussel beds in the upper-mid and lower zones, respectively. Density of snails increased with decreasing shore height and was much greater in the H. mutabilis bed than in the S. virgatus bed throughout the year. Multiple linear regression analysis of snail density revealed that shore height was the only factor which showed a significant, negative correlation with density. Monthly change in size structure suggested that reproduction occurred in March-June, followed by abundant juvenile (1 mm>shell width) recruitment into algal mats within the H. mutabilis zone in April-June. As snails grew larger over 1.0 mm, they appeared within the H. mutabilis bed, increased in size until the next spring, and disappeared by mid summer. It was thus suggested that C. mustelina had one generation in a year with reproduction occurring chiefly in spring. Factors affecting vertical distribution and selective recruitment into the algal mats are discussed.

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The reproductive cycle of an ovoviviparous freshwater snail, Semisulcospira libertina, was studied in the Otani and Kohno populations in the Iga Basin with reference to changes in embryo stage composition in the brood pouch. The number of embryos per snail of 25-30 mm in shell height varied seasonally from 250 to 1000 in the Otani population and from 250 to 700 in the Kohno population, increasing in summer and decreasing in spring. Eggs were supplied to the brood pouch during the warm season from late April to mid October. Growth of embryos ceased in the winter. Eggs supplied before August developed into shelled embryos and were released from late June to October of the same year, whereas those supplied after late August were dormant over the winter and released from April to June of the next year. Duration of incubation was estimated to be 8 months for over wintered spring offspring, and about 2 months for summer-autumn offspring. Annual offspring productivity per snail was estimated to be about 1550-2100 individuals in the Otani and 1200 in the Kohno populations, if suppose the duration of incubation being uniformal through the warm season.

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Morphological features of adult and embryo shell and genetic variations of Semisulcospira libertina and S. reiniana were examined from 14 locations in Iga Basin. Two species were detected in 5 populations and only either one species in other 9 populations. In the populations which composed of either single species, adult shells were smooth in S. libertina, and usually ribbed in S. reiniana. The mother snails (25-30 mm in shell height) of S. libertina have 300-700 embryos per individual in winter and the embryo shells were mostly smooth and ranging 1.0-1.8 mm in maximum shell height. In S. reiniana, number of embryos per mother (25-30 mm in shell height) were 170 in winter. Embryo shells were mostly ribbed and larger than in S. libertina, ranging 2.3 mm in maximum shell height. To evaluate genetic variability between populations of these two closely related snail species, two allozymes were examined by starch gel electrophoresis. Marked difference in allele frequencies between the two species was observed at the MPI locus (Mannose phosphate isomerase). Three alleles were estimated at the Mpi^* locus. Those of S. libertina were fixed for the alleles ^*a and ^*a', while that of S. reiniana was for ^*b. The alleles at the locus of Pgm-1^* (Phosphoglucomutase) were highly variable in S. libertina, but less in those of S. reiniana. Population divergency in shell character in two species was correlated to genetic divergency in allopatric populations. By using MPI zymogram pattern, we could identify individuals of two species S. libertina, S. reiniana and their presumed hybrid in the Shourenji and Murou River where those had been difficult to be discriminated from each other by morphological character of adult or embryo shells. In the Tsuchiyahara population which composed of both species and hybrid type, adult snails have mostly smooth shells. Embryo characters exhibited an intermediate feature in MPI type of S. reiniana and MPI hybrid types, but, MPI type of S. libertina had the same embryo character as the allopatric population. While, the hybrid type was not observed in other two sympatric populations. In the Inagu population which included two species but not hybrid, most adult snails had smooth shells, but, embryos showed characters of respective species.

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