ATCase activity, RNA:DNA ratio, gonad development stage, and egg production in the female copepod Calanus helgolandicus

Some investigators have proposed the measurement of aspartate transcarbamylase (ATCase) activity as a suitable technique for estimating zooplankton productivity. However, this technique has never been comprehensively evaluated under controlled laboratory conditions nor compared extensively with other productivity techniques in the field. This paper describes such an evaluation, using the reproduction of the calanoid copepod Calanus helgolandicus as an index of germinal growth, an important aspect of zooplankton productivity. ATCase activity and RNA:DNA ratio both increased significantly with increasing gonad maturation. In addition, ATCase activity in females responded immediately to changing food conditions, whilst changes in egg production lagged behind by similar or equal to 24 h. Shifting the ATCase data by 24 h revealed a significant and positive correlation of ATCase activity with egg production. However, in well-fed females, ATCase activity assayed at constant temperature was apparently independent of environmental temperature conditions, whilst egg production was temperature-dependent. In the field, ATCase activity was significantly correlated with egg production only in autumn and winter; when a complete seasonal cycle was considered, no correlation was found between the two measurements. These findings suggest that ATCase is involved partially in either germinal growth and/or in other biosynthetic processes of female C. helgolandicus; thus, ATCase activity does not reflect copepod egg-production with any certainty.