Feeding rates and selectivity among nauplii, copepodites and adult females of Calanus finmarchicus and Calanus helgolandicus

This study focuses on selective feeding by developmental stages of two oceanic copepods, Calanus finmarchicus and Calanus helgolandicus from nauplii to adults. A mixture of four algal species of different biochemical composition, Prorocentrum nanum (dinoflagellate), Thalassiosira minima (diatom), Rhodomonas baltica (cryptophyte) and Dunaliella tertiolecta (chlorophyte), added in an equal biovolume, was used in three different experimental set-ups. In set-up 1 the algal species were present as single cells of similar size (14 µm). In set-up 2 the diatom T. minima was present in chains of two or three cells and was therefore larger than the other algae, while the biovolume of all species remained the same. In set-up 3, the diatom T. minima was excluded from the mixture. Feeding selectivity of the copepods was assessed in relation to the quality of the algal species expressed in terms of carbon and nitrogen content, fatty acid composition, and chain length of the diatom. The results show that younger stages and adult females of C. finmarchicus and C. helgolandicus did not show a preference for an algal species when the algae were of similar size. In the feeding experiments where the diatoms were offered as chains, both copepod species showed a selective behaviour only on the basis of algal size. Individual ingestion rates increased from 0.4 to 0.7 µg C day–¹ for nauplii of both species to 5 µg C day–¹ for adult females of C. helgolandicus to 12 µg C day–¹ for C. finmarchicus. Individual filtration rates ranged from 5 ml day–¹ for C. finmarchicus nauplii to 70–98 ml day–¹ for adult females, and from 3 ml day–¹ for C. helgolandicus nauplii to 35–46 ml day–¹ for adult females. Ingestion and filtration rates per unit body carbon decreased gradually in both copepod species with increasing body carbon. The daily ingested amount of food decreased for C. finmarchicus from 124–134% of the body carbon for nauplii to 19% of the body carbon for adult females, and for C. helgolandicus from 117–137% of the body carbon for nauplii to 13–26% of the body carbon of adult females.