Comparison of in situ egg production rate in Calanus finmarchicus and Metridia longa: Discriminating between methodological and species-specific effects

Abstract

A comparison of the in situ egg production rate (EPR) in Calanus finmarchicus and Metridia longa was conducted using freshly collected adult females (C6f), individually incubated in 45 ml dishes with and without egg separators at 6°C for 24 h. Incubations with egg separators resulted in significantly greater clutch size and in situ EPR in M. longa than without egg separation, while no significant effect was measured in C. finmarchicus. The use of egg separators resulted in lower hatching success and naupliar viability (%), an effect more pronounced in M. longa. A highly significant effect of food on egg removal was observed in M. longa, with C6f removing up to 85 % of their eggs when the ambient algal biomass at capture was less than 50 mg chl a m -2, suggesting an effect of the feeding 'history' of C6f. In the absence of cannibalism on the eggs, the combined effect of smaller clutch size and longer spawning interval resulted in a lower EPR in M. longa by a factor of 4 to 5 relative to C. finmarchicus. Based on egg and C6f weight, the weight-specific EPR (growth) in M. longa was half that in C. finmarchicus. M. longa produced eggs at a rate similar to C. finmarchicus only when C6f were incubated at a higher temperature (+4°C). These results are discussed in the context of the paradox between the low EPR and high abundance in Metridia species and the different life history strategies developed by Metridia and Calanus species. © Inter-Research 2008.