Poststarvation feeding and swimming activity in Calanus pacificus and Metridia pacifica

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Abstract
Gut fluorescence analysis of Calanus pacificus and Metridia pacifica is used to describe variation in food intake rate at time scales of one to several hours. The animals are starved before each experiment, then exposed continuously to phytoplankton cultures. Variability of gut fullness is large (order 10‐fold) between individual copepods captured at the same time and is about proportional to the within‐time‐period average. The gut fullness curve shows a strong initial peak 1– 2 h after food is supplied and drops to about 20% of the peak level after 7–9 h of exposure to high food levels. For Calanus gut fullness and swimming activity at the time of capture are strongly correlated. Active animals have on average 2–5 times as much pigment in their guts as inactive animals captured at the same time. However, inactive animals resume swimming and feeding activity in 1–3 h. The changes in gut fullness appear to involve episodic on‐off switching of feeding activity as well as regulation of maximum instantaneous feeding rate and of either or both satiation and hunger thresholds.