The different aspects in motion of the three reproductive stages of Pseudodiaptomus annandalei (Copepoda, Calanoida)

Individual variation in morphology is linked to organism displacement in numerous taxa. This relationship among adult copepods has previously received little attention. In this study, we questioned whether the swimming behaviour of three morphologically different adult states of Pseudodiaptomus annandalei (i.e. males, non-ovigerous and ovigerous females) could be differentiated. Two-dimensional video observations revealed that some swimming patterns were common to all adult states, whereas others were more specific to one of the adult states. Males swim in a straight way, ovigerous females exhibit jerky swimming behaviour, and non-ovigerous females exhibit slow and steady gliding, interrupted occasionally by jumps. Direct measures of the swimming behaviour (i.e. mean, maximal and probability density of speed) for a large number of trajectories were considered first. Deeper comparisons of each sexual state were conducted using scale-dependant (i.e. net-to-gross displacement ratio), scale-independent (power spectrum, multi-fractal) metrics and symbolic analysis. The combined analysis emphasized the differences (e.g. male swimming patterns were slower and less complex, both ovigerous and non-ovigerous female swimming patterns were faster and more complex) and similarities (e.g. all swimming patterns were different from the Brownian motion) between adult swimming patterns. The multi-fractal approach identified differences between the two sexes suggesting differential swimming behaviour. In an ecological discussion, we relate these differences to mating and predator avoidance strategies.