SEASONAL REPRODUCTIVE-BIOLOGY OF THE SMALL CYCLOPOID COPEPOD OITHONA DAVISAE IN A TEMPERATE EUTROPHIC INLET

The seasonal variation in reproductive rate of the small cyclopoid Oithona davisae was investigated in a eutrophic inlet of the Inland Sea of Japan. The densities of its adults and copepodites were low in late winter and spring, and high in early summer and fall. A decline of the population in mid summer coincided with the occurrence of the lobate ctenophore Bolinopsis mikado. Adult sex composition was highly skewed for females (annual mean: 85%), particularly in winter (96%). Female prosome length changed inversely with temperature, while egg diameter did not change throughout the year. The variation in clutch size of ovigerous females was bicyclical, with peaks in early summer and fall. The duration of egg development was temperature dependent, while the time from hatching of egg sacs to production of new ones was uniform (0.75 d) and independent of temperature. The egg production rate of breeding O. davisae was high (mean: 11.6 eggs female(-1) d(-1), or a carbon weight-specific rate of 0.39 d(-1)) in early summer, and low (mean: 2.6 eggs female(-1) d(-1), or 0.08 d(-1)) in winter. Since there were considerable numbers of non-breeding females in the population, the egg production rate on a population average was lower than the above-mentioned rate. The specific egg production rate of reproductive females increased linearly with increasing temperature up to ca 22 degrees CC, but at temperatures >22 degrees C, the rate did not change or was even reduced. The reduction in the rate was remarkable in fall, indicating females' physiological stress in this season.