Reproductive response of Calanus helgolandicus .2. In situ inhibition of embryonic development

Seasonal variations in the viability of Calanus helgolandicus eggs are reported in coastal waters off Roscoff, in the western English Channel. In situ hatching success was highly variable with >50% amplitude between minimum and maximum values. Highest production of abnormal embryos and nauplii was recorded during spring and midsummer. Examination of the faecal pellet contents produced by wild females revealed that they were feeding on extremely diversified prey, depending on the lime of year. During spring-summer phytoplankton blooms, diatoms were always the dominant food, the remains of which were found in the faeces. During the rest of the year, non-diatom diets constituted the most important fraction of the prey. Inhibition of embryonic development was attributed to the ingestion of diatoms by wild females, based on laboratory experiments demonstrating similar low hatching success and anomalies when females were fed dense diatom cultures. The failure to statistically establish a straightforward correlation between the concentration of chlorophyll a and variations of hatching success in the field is discussed with reference to experimental data.