ATCase activity, RNA:DNA ratio, gonad development stage, and egg production in the female copepod Calanus helgolandicus
Résumé
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 ®eld. 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 signi®cantly with increasing gonad maturation. In addition, ATCase activity in females responded immediately to changing food conditions, whilst changes in egg production lagged behind by 9 24 h. Shifting the ATCase data by 24 h revealed a signi®cant 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 ®eld, ATCase activity was signi®cantly 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 ®ndings 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 re¯ect copepod egg-production with any certainty.

