Relation of behavior of copepod juveniles to potential predation by omnivorous copepods: an empirical-modeling study
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An empirical-modeling study was carried out to examine the motion behavior of copepod nauplii in relation to their potential predation by omnivorous copepods. First, video observations were taken on the encounters of Centropages velificatus, Paracalanus aculeatus, Paracalanus quasimodo and Temora stylifera nauplii with free-swimming adult females of C. velificatus. By examining each female’s behavior, the observed encounters were arranged into 3 categories: (1) quasi-steady, (2) unsteady, and (3) unsteady with body rotation. We found that a wide spectrum of motion behaviors contributes to the generation of hydrodynamic signals. A hydrodynamic model was developed to analyze 4 encounters under the quasi-steady category. The model takes into account the effects of the finite-sized body morphology and the no-slip boundary condition imposed at the surface of the nauplius. Therefore, the model is able to calculate the tip-base velocity differences, as well as the tip-base shear rates over the length of the setae of the A1 (first antennae) of the nauplii, resulting from the flow disturbances generated by a steadily approaching C. velificatus female. When the tip-base shear rates around its A1 setae reached the range of 3.0 to 4.8 s(-1), a C. velificatus nauplius, moving only occasionally, detected the approaching female copepod. By contrast, a P. quasimodo nauplius and a T stylifera nauplius, both moving continuously by beating their cephalic appendages, did not detect the female, even when the tip-base shear rates reached the range of 5.2 to 6.2 s(-1) for the former and 8.4 to 11.4 s(-1) for the latter. The modeling results also reveal the cause of the difference in the sensitivity to hydrodynamic signals. Nauplii moving only occasionally have a much weaker noise field (originating from the naupliar self-generated flow) around their A1 setae than those moving their cephalic appendages continuously to achieve a smooth and continuous motion pattern, and therefore are better suited to perceiving nearby predators. This model provides a mechanistic understanding as to why continuously moving nauplii are preyed upon at a higher rate than those moving intermittently.