Individual-level variability in the behavioral responses of female Oithona davisae (Copepoda: Cyclopoida) to hydromechanical stimuli

Abstract

Planktonic copepods can detect potential preys and predators through mechanorecep-tion. Sensing a certain level of deformation rate of ambient water, they escape from the source of stimulus. Quantification of the deformation rates that evoke the escape behavior of copepods may thus help understand their living strategies. The term "zooplankton" generally refers to assemblages of individual zooplankters, and "zooplankton" has been usually studied by ignoring inter-individual differences. We here observed and quantified individually the behaviors of female Oithona davisae under spatially changing deformation rates produced with a suction flow system. Female 0. davisae typically escaped after being drawn to areas with deformation rates ranging 0.1-1.9 (0.54 ± 0.45) s-1. To escape, they jumped towards lower-deformation conditions with higher speed and longer distance than without stimulus, showing that they can detect not only the strength but also the directional information of flow fields. Moreover, significant inter-individual differences in the behavior were observed, indicating that copepods are a group of organisms with different individual characteristics. Our results also suggest that female 0. davisae prefers to stay in a quiescent environment where local deformation rate is smaller than 0.1 s"1. Because female 0. davisae ambushes prey by detecting weak hydromechanical signals, staying in environments with lower deformation rates may be beneficial to detect prey.