Reproductive cycle and cohort formation of Venus antiqua (Bivalvia: Veneridae) in the intertidal zone of southern Chile Ciclo reproductivo y formaci6n de una cohorte de Venus antiqua (Bivalvia: Veneridae) en el intermareal del sur de Chile

The reproductive cycle and generation of a new cohort of the clam Venus antiqua is described in relation to the annual food availability and temperature cycle at Yaldad Bay, Chiloe Island, southern Chile. The spawning period followed a seasonal pattern and was restricted to spring and summer (November 1990 to February 1991 ). whereas during autumn and winter the population was reproductively quiescent. First settlers were detected in January 1991, approximately thirty days after the main spawning event. Mean shell length at settlement was 0.26 mm; growth rate of this cohort was rapid till the end of summer (0.0021 to 0.0085 mm·d- 1 ). reaching an average length of I mm at the end of March, whereas during autumn and winter (May to September) almost no shell growth was observed. The cohort recommenced growth in spring 1991 (October) and the clams quickly attained an average length of 6 mm (0.0248 to 0.0510 mm·d- 1 ) at the end of the first year (January 1992). Both the reproductive cycle and growth of this cohort at Yaldad Bay followed a seasonal pattern which was coupled to water temperature and food supply (chlorophyll a and organic seston).