Particle rejection by Calanus pacificus: discrimination between similarly sized particles
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Particle rejection behavior was studied for late-stage copepodites and adult females of Calanus pacificus collected from La Jolla Bay, California (32oN; 117oW) in spring and summer of both 1979 and 1982. Several particles of approximately the same size were offered, both singly and in mixtures, to C. pacificus. In the first set of experiments the copepods were presented with choices between a diatom, Thalassiosira weissflogii (13.1 μm equivalent spherical diam), and polystyrene beads of two sizes (11.1 and 16.5 μm diam). In the second set of experiments they were presented with choices between one of two dinoflagellates, Gyrodinium dorsum (27 μm diam) or Peridinium trochoideum (22 μm diam), and polystyrene beads of three sizes (15, 20 and 25 μm diam). When offered mixtures of cells and beads, the filtration and ingestion rates were significantly greater on the cells than on the beads, irrespective of bead size. We also find that C. pacificus demonstrates a clear preference for G. dorsum relative to P. trochoideum. Our results cannot be explained by a model of passive, size-selective feeding.
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