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

The energetic cost of moulting and gonad matu- ration from Stage CV to adult has been estimated in the cope- pods 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 (15°C for C. helgolandicus and ~8°C for C. finmarchicus). The wax ester (WE) content was esti- mated 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. fin- marchicus ,r espectively). 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 ~20 µg C for C. helgo- landicus and ~40 µg C for C. finmarchicus. Because of the dif- ficulty of properly estimating the WE content at the end of CV, we consider that our values are underestimates, and a sub- sequent analysis, which considers the upper range of the WE content estimated at the end of CV, suggests values up to ~30 µgC for C. helgolandicus and ~70 µg C for C. finmarchi- cus. 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 addi- tional 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 condi- tions. The WE threshold is likely to vary with temperature (with a lower value at higher temperature) since metabolism is more rapid at higher temperature.

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