Nauplii of Calanus helgolandicus were raised from eggs, laid within a 12 h period, to Copepodite Stage I (CI) on 5 different species of algae at high concentrations at 15°C. The diets used were Isochrysis galbana (5 ?m spherical diameter), Rhodomonas baltica (7 ?m), the coccolithophorid Pleurochrysis carterae (12 ?m), the diatom Thalassiosira weissflogii (14 ?m) and the dinoflagellate Prorocentrum micans (30 ?m). Each day a sample was taken and preserved for later cohort analysis. Growth was estimated from CHN samples collected almost daily, from which naupliar stages were also distinguished. Ingestion was measured for each naupliar feeding stage. The fastest development was obtained with I. galbana and P. micans. We found the highest value of carbon and nitrogen content of Naupliar Stages NV to CI for individuals reared on the smallest algae, I. galbana and R. baltica. However, ingestion rate in terms of carbon or nitrogen was lowest with these same (smallest) algae. Therefore, the gross growth efficiency was highest for the smallest algae. These results suggest the following: Firstly, that factors influencing development time and weight in stage are different; weight in stage is negatively related to algal size, whereas development time is independent of it. The quality of the algal biochemical components could be the factor influencing development. Secondly, that small algae are fully assimilated in the gut whereas larger cells, i.e. those with indigestible components around the cell (theca, frustule, calcium layer) are only partly assimilated
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