Seasonal variation in reproduction and population structure of the bivalve Limopsis tajimae in the upper bathyal zone of Suruga Bay, Japan

Seasonal variation in reproduction and population size structure was investigated for the suspension-feeding bivalve Limopsis tajimae Sowerby inhabiting the upper bathyal zone (300 m deep) of Suruga Bay, central Japan. The bivalve was collected at 1- to 4-month intervals for a period of 22 months, and bottom environment was monitored concurrently to detect factors affecting seasonality in the bivalve. Bottom water temperature, organic carbon and nitrogen contents in the sediments did not exhibit seasonal variation. Size-adjusted soft-tissue weight varied slightly, but statistically significantly between stations and months. However, its seasonal pattern was not obvious, and the pattern of temporal variation was totally different between stations. The sex ratio did not deviate from 1:1, and there was no significant difference between shell lengths of females and males. Females possess both immature small oocytes and large developed oocytes in their ovaries throughout the year, suggesting that they can potentially undergo year-round continuous reproduction. The proportion of developed oocytes in each female varied greatly from month to month, although no seasonal cycle was obvious. Population size structure of L. tajimae was polymodal. A mode of the smallest size class occurred in most months, suggesting long periods of bivalve recruitment. These findings indicate that seasonal variation in reproduction of the bivalve was negligible, probably reflecting constant physical and nutritive conditions of the bottom environment. Received: 6 January 1998 / Accepted: 19 May 1998