In situ feeding rhythms of herbivorous copepods,and the effect of starvation

In situ diel variations in gut pigment contents of neritic (Acartia omorii andPseudocalanus minutus) and oceanic copepods (Calanus plumchrus andC. cristatus) were analyzed.A. omorii andP. minutus were sampled in Onagawa Bay on the east coast of Japan in May and August 1987, andC. plumchrus andC. cristatus were sampled in the Bering Sea in June 1986. Gut pigments were generally high at night, and bimodal feeding rhythms were observed in all species. The first peak of gut pigments occurred between sunset and midnight and was followed by a midnight decrease in gut pigment levels, resulting in eventual evacuation of the gut. The second peak was observed a few hours after sunrise. Incubation experiments indicated that ingestion rates of starved copepods were higher than those of acclimated copepods. This phenomenon was most notable at high food concentrations. Gut pigments of starved copepods rapidly increased after exposure to high concentrations of culturedThalassiosira decipiens. These findings suggest that in situ feeding behavior of herbivorous copepods includes periods of cessation or reduction in feeding during the night, and consequently, feeding activity is periodically enhanced with starvation. Starvation enhanced feeding behavior is most obvious in the large oceanic species,C. plumchrus andC. cristatus and is not distinct in small coastal species such asA. omorii.