Behavioural responses of the copepods Calanus helgolandicus and Temora longicornis to dinoflagellate diets

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Abstract
Activity patterns of the first maxillae of Calanus helgolandicus and Temora longicornis, restrained in a flow-through system, were investigated using a micro-impedance electrode technique. Copepods were exposed to four dinoflagellate species of similar cell size with filtered sea water and the diatom Thalassiosira weissfloggi as controls. In short-term transition experiments the beat frequency of the maxilla changed rapidly with the introduction and removal of cells from the surrounding medium. Similarly the introduction of cell-free algal homogenate, filtered through a 0·2 µm membrane, produced elevated beat frequencies, evidence consistent with chemosensory food detection. During 24 h exposure experiments frequencies for Calanus on all dinoflagellate diets were lower than for the diatom, and in the case of Scrippsiella trochoidea did not differ significantly from the seawater controls. In Temora, frequencies when feeding on dinoflagellates, except Prorocentrum micans, were lower than on the diatom. With Scrippsiella trochoidea and Gonyaulax tamarensis beat frequencies were the same as those for the sea-water controls. Significant depression of frequency below that observed in filtered sea water was shown for both copepods when presented with the bloom-forming, Gyrodinium aureolum. Differences in beat frequency in response to dinoflagellate diets are mirrored by other aspects of feeding activity, gut contents, egg production and mortality.