IS HATCHING SUCCESS OF COPEPOD EGGS DIATOM DENSITY-DEPENDENT

Two species of diatoms, Phaeodactylum tricornutum and Thalassiosira ro tula, were fed to Calanus helgolandicus females at concentrations vary ing between 10 and 10(6) cells ml(-1). Their effects on the fecundity and hatching success of eggs were compared with those of the dinoflage llate diet Prorocentrum minimum at 10(4) cells ml(-1) used as a contro l. Specific maximum fecundity was never reached with the diatom diets. The paradox is that fecundity and inhibition of hatching were diatom density-dependent, but varied in opposite directions in relation to co ncentration. At high diatom concentrations (ca greater than or equal t o 10(3) cells ml(-1)) fecundity increased, whereas hatching was totall y inhibited. At low concentrations (ca less than or equal to 10(3) cel ls ml(-1)), resembling in situ conditions, fecundity was depressed whi le hatching success values were generally below 70% or showed high amp litude variations. Inhibition of hatching was proportional to fecal pe llet production rate, reflecting the causal relation between ingestion , duration of feeding and accumulation of the inhibitor. The inhibitor y mechanism was best fitted by a logistic function of time, the parame ters of which were significantly modified for diatom concentrations gr eater than or equal to 10(3) 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 fecundi ty and hatching can be significantly lowered at low and high diatom co ncentrations, respectively.