Energetic cost of gonad development in Calanus finmarchicus and C-helgolandicus

The energetic cost of moulting and gonad maturation from Stage CV to adult has been estimated in the copepods Calanus helgolandicus and C. finmarchicus. For both species, this was done by following 2 laboratory-reared cohorts, one reared at a high food concentration (Cohort H) and the other at a comparatively lower food concentration (Cohort L). The 2 cohorts of each species were maintained at a constant temperature (15degreesC for C. helgolandicus and similar to8degreesC for C. finmarchicus). The wax ester (WE) content was estimated in late CV and new moulted females, on the day when approximately 40 to 50% of the individuals of the cohort had moulted to adulthood. Copepodite Stage CV in Cohort H had accumulated much more WE than those in Cohort L (2.2 and 1.4 times more in the case of C. helgolandicus and C. finmarchicus, respectively). However, for both species, the amount of WE catabolized during the moulting of the CV stage from Cohort H is very similar to that of CV from Cohort L. In both cohorts, it is equivalent to similar to20 mugC for C. helgo-landicus and similar to40 mugC for C. finmarchicus. Because of the difficulty of properly estimating the WE content at the end of CV, we consider that our values are underestimates, and a subsequent analysis, which considers the upper range of the WE content estimated at the end of CV, suggests values up to -30 jig C for C. helgolandicus and similar to70 mugC for C. finmarchicus. This WE catabolism is mainly due to gonad maturation, which occurred at this time, although part of it is also due to the loss of the exoskeleton and the energy required for the moulting process. We propose that the values obtained in this study can be used as a threshold to determine those animals that can descend for overwintering and those that have to remain at the surface, because the CVs with a WE storage below the threshold value will not be able to support the additional energetic costs linked to the overwintering strategy. Therefore, in the field, the percentage of copepods going into overwintering would be a consequence of the growth conditions. The WE threshold is likely to vary with temperature (with a lower value at higher temperature) since metabolism is more rapid at higher temperature.