Copepod feeding currents: Food capture at low Reynolds number1

High-speed motion pictures of dye streams around feeding Calanoid copepods revealed that these important planktonic herbivores do not strain algae out of the water as previously described. Rather, a copepod flaps four pairs of feeding appendages to propel water past itself and uses its second maxillae to actively capture parcels of that water containing food particles. The feeding appendages of Eucalanus pile&us operate at Reynolds numbers of only 1O-2 to 10-l. In the viscous world of a feeding copepod, water flow is laminar, bristled appendages behave as solid paddles rather then open rakes, particles can neither be scooped up nor left behind because appendages have thick layers of water adhering to them, and water and particle movement stops immediately when an animal stops beating its appendages. Calanoid copepods are abundant planktonic crustaceans that play a major role in the transfer of energy through marine food chains. Copepods are selective feeders, exhibit a plasticity of feeding behavior (e.g. Poulet 1974; Richman et al. 1977, 1980; Cowles 1979; Donaghay and Small 1979; Runge 1980; Skiver 1980), and can markedly influence the composition of phytoplankton populations (e.g. Porter 1973; Poulet 1973; McCauley and Briand 1979). In spite of the ecological importance of copepod feeding, the mechanisms by which these animals capture particles (such as diatoms and flagellates) have been poorly understood due to the technical difficulties involved in observing feeding appendages only fractions of a millimeter long that are moving at rates of 20-80 Hz. Until now, descriptions of copepod feeding have been based on careful microscope observations of currents produced by copepods in drops of water (e.g. Cannon 1928; Starch 1929; Marshall and Orr 1955). The “textbook description” l This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada, the National Science Foundation (OCE76-01142), and the University of California, Berkeley (Committee on Research). (e.g. Russell-Hunter 1979; Barnes 1980) of copepod feeding based on such observations is basically as follows: The beating of the feeding appendages (labeled in Fig. 1A) pushes water postero-laterally, forming a large swirl on each side of the animal (Fig. 1B). Some of this swirling water is sucked antero-medially by the outward swing of the maxillipeds. The inward swing of the maxillipeds then pushes water between the setae and setules (bristles on the setae) of the second maxillae, which sieve particles out of the water. The filtered water is then expelled anteriorly by the first maxillae, and the captured food is transferred to the mouth by the endites of the first maxillae. We suspected that the recirculating swirls were artifacts of the small volume of water in which the observed copepods were immersed. Furthermore, we were puzzled by water flowing between the closely spaced setae and setules of the stationary second maxillae rather than flowing around them along the paths of least resistance. Attempts to analyze the feeding behavior of Calanoid copepods have been based on the concept that they feed by sieving, as described above. For example, several analyses of size-selective feeding by copepods have focused on the spacing of the setules on the setae of the second