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Effects of NH3 concentration in sea water and pH of sea water on the motility of spermatozoa obtained from testes were examined in the Japanese pearl oyster. Percent motility at 30 s after dilution increased with increasing NH3 concentration in sea water from 0.75-2.0 mM. When spermatozoa were diluted with sea water containing 0.75 mM NH3, which is widely used as the insemination fluid in the hatchery of this species, the percent motility increased with time elapsed after dilution, and peaked at 5 min. For spermatozoa diluted with sea water containing 2.0 mM NH3, the percent motility increased rapidly and peaked at 30 s. The pH of sea water increased with increasing NH3 concentration from 8.2 (0 mM NH3) to 9.9 (5.0 mM NH3). When spermatozoa were diluted with artificial sea water at various pH (buffered without NH3 at 6.0-10.0), only spermatozoa diluted with artificial sea water of pH 10.0 were motile, and the percent was considerably lower than those in ammonical sea water. These results indicate that sea water containing 2.0 mM NH3 is a suitable solution for evaluating sperm motility, and that NH3 and/or ammonium ions may activate sperm motility in this species.KEY WORDS: ammonical seawater, artificial fertilization, Japanese pearl oyster, sperm motility.

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The present study dealt with seasonal and interannual variations in the abundance and biomass, and spatio-temporal distributions of the portunid crab Charybdis bimaculata dominant in Ise Bay, central Japan. The abundance and biomass of the crab decreased in summer when the oxygen-poor water developed in central or inner parts of the bay, and then increased through new recruits from autumn (October-November) to the following spring (March-May) when the oxygenpoor water disappeared. Berried females were collected mainly from spring to autumn. Recruits were collected in any season. Particularly in winter, most recruits were located in the innermost part of the bay. According to the cohort separation based on size frequency distribution in carapace width of the crab specimens, the cohorts that were derived from spawning in spring to summer largely contributed to establishing and maintaining the benthic populations for the following year in the bay, whereas those from other seasons failed to recruit because of serious damage caused by the oxygen-poor water. Most crab individuals one year post hatch contributed to spawning and then died by the winter of the same year.

Feeding habits of the two asteroids dominant in Ise Bay, Luidia quinaria and Astropecten scoparius, were examined as regards their temporal and spatial distributions and also the oxygen-poor water in the bay, based on megabenthos samples collected in the bay from November 1996 to November 1997. Luidia quinaria was distributed widely in the whole part of the bay while A. scoparius was mainly found in the southern part around the bay mouth area. Analysis of the stomach contents of these asteroids demonstrated that L. quinaria preferred to ingest echinoderm, particularly an ophiuroid, Ophiura kinbergi, that occupied approximately 60% of the percentage total food items; while the stomach contents of A. scoparius were dominated by molluscs, a minute gastropod Voorwindia paludinoides and a bivalve Alvenius ojianus, which occupied approximately 59% and 33% of the percentage of total food items, respectively. The differences in the feeding habits of these asteroids may be due to their distribution pattern, which may be determined by the differences in sediment characteristics. Distribution and feeding habits of L. quinaria in summer may be affected by the occurrence of oxygen-poor water.

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