Relationships between RNA content and egg production rate in Acartia bifilosa (Copepoda, Calanoida) of different spatial and temporal origin

To evaluate factors regulating RNA content (RNA, mu g RNA female(-1)) -egg production rate (EPR, eggs female(-1) day(-1)) relationship and to develop a model for in situ egg production rate estimates for Acartia bifilosa, we (1) measured EPR and RNA in females sampled at geographically distant areas at varying temperature (T, degrees C), (2) determined environmental (station, season, and T), endogenous (prosome length (PL), mm and EPR) variables that influence RNA levels, and (3) explored a set of multiple regression models to predict EPR from RNA, PL, station, season, and T. Egg production experiments were carried out in spring and summer 2005 in the Gulf of Finland and in the western part of the northern Baltic proper. We found that up to 88% of the RNA variation could be explained by variations in PL, EPR, and season/T. In explaining the RNA variability, PL played a major role followed in order of importance by EPR and season/T. Further, PL, RNA, and season/T explained up to 53% of the variation in EPR, nearly half of which is explained by RNA alone. The effect of spatial origin was never significant, suggesting that the derived relationships are general for A. bifilosa inhabiting northern Baltic proper.