Feeding currents in calanoid copepods: two new hypotheses.

The interaction between planktonic herbivorous calanoid copepods and their food, planktonic algae, is investigated to increase our understanding of the physiological adaptations these small marine animals have acquired in the course of evolution. Emphasis is given to the centimetre -second scale where calanoids encounter algae, select and capture them, or reject them either passively or actively. Most calanoid copepods create feeding currents which can be subdivided into three cores: motion, viscous, and sensory cores. Algae contained in the sensory core are perceived and then re-routed towards the capture area. The perimeter encompassing all the points of these re-routings can be defined as the reactive field of awareness surrounding the calanoid. An analysis of typical biological oceanographic feeding experiments reveals that direct observations are necessary to understand the feeding behaviours and strategies of calanoid copepods. To facilitate further studies, a new experimental set-up has been described and two hypotheses have been formulated. The method allows direct observations, in all three dimensions, of free-swimming herbivorous calanoids and their food in a 6-litre vessel. The two hypotheses are based on the fact that calanoids create feeding currents and orient their bodies within the water column. The first hypothesis states that calanoid copepods create species-specific, and maybe even age-specific, feeding currents. The second one proposes that ambient water motions may act as a mechanism for niche separation in herbivorous calanoid copepods. This latter hypothesis is based on the inference that ambient water motions may interfere with the flow field of the feeding current thereby making it more difficult for calanoids to successfully re-route algae contained in the sensory core of the feeding current.