Swimming and feeding behaviour of the planktonic copepod Clausocalanus furcatus

The objective of this study was to quantify motion and feeding behaviour of the small calanoid Clausocalanus furcatus, which is a common and abundant species in oligotrophic environments. Adult females were video recorded at 60 fields s(-1) using video equipment which allowed us to follow each free- swimming individual continuously in 3 l vessels. At 20 degrees C under dark conditions, C.furcatus moved continuously along convoluted small loops at a mean speed of similar to 10 mm s(-1) (corresponding to 10 body lengths s( -1)). This motion was occasionally interrupted by sudden somersaulting perf ormed at very high speed (up to 17 mm s(-1)). The copepods only occasionall y sank. There was no evidence that C.furcatus created feeding currents. Din oflagellate cells offered as food appeared to be perceived by direct encoun ter at high speed, being instantaneously captured and ingested or rejected. The capture rates were correlated with the frequency of somersaulting. By moving fast and by associating a high turning rate with a high frequency of crossing the previous tracks, C.furcatus searched 22-26% of the explored v olumes. The motion and feeding behaviour of C.furcatus show that the foragi ng tactic of this species is to explore small volumes of water rapidly. Thi s strategy appears, so far, to be unique among small planktonic copepods.