Swimming and feeding behaviour of the planktonic copepod Clausocalanus furcatus

Abstract

The objective of this study was to quantify motion and feeding behaviour of the small calanoid <it>Clausocalanus furcatus</it>, 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 1 vessels. At 20<IMG SRC="/math/deg.gif">C under dark conditions, <it>C.furcatus</it> moved continuously along convoluted small loops at a mean speed of &bsim;10 mm s^-1 (corresponding to 10 body lengths s^-1). This motion was occasionally interrupted by sudden somersaulting performed at very high speed (up to 17 mm s^-1). The copepods only occasionally sank. There was no evidence that <it>C.furcatus</it> created feeding currents. Dinoflagellate cells offered as food appeared to be perceived by direct encounter 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,<it>C.furcatus</it> searched 22-26% of the explored volumes. The motion and feeding behaviour of <it>C.furcatus</it> show that the foraging tactic of this species is to explore small volumes of water rapidly. This strategy appears, so far, to be unique among small planktonic copepods.