Résumé
Two species of diatoms, Phaeodactylum tricornutum and Thalassiosira rotula, were fed to Calanus helgolandicus females at concentrations varying between 10 and 106 cells ml−1. Their effects on the fecundity and hatching success of eggs were compared with those of the dinoflagellate diet Prorocentrum minimum at 104 cells ml−1 used as a control. Specific maximum fecundity was never reached with the diatom diets. The paradox is that fecundity and inhibition of hatching were diatom densitydependent, but varied in opposite directions in relation to concentration. At high diatom concentrations (ca ≥103 cells ml−1) fecundity increased, whereas hatching was totally inhibited. At low concentrations (ca ≤103 cells ml−1), resembling in situ conditions, fecundity was depressed while hatching success values were generally below 70% or showed high amplitude variations. Inhibition of hatching was proportional to fecal pellet production rate, reflecting the causal relation between ingestion, duration of feeding and accumulation of the inhibitor. The inhibitory mechanism was best fitted by a logistic function of time, the parameters of which were significantly modified for diatom concentrations ≥103 cells ml−1. These results suggest that ingestion of diatoms might be considered as one of the causes limiting C. helgolandicus population recruitment in nature, since both fecundity and hatching can be significantly lowered at low and high diatom concentrations, respectively.