Three dimensional observation of salinity-induced changes in the swimming behavior of the estuarine calanoid copepod Pseudodiaptomus annandalei

Abstract

Salinity is a major parameter in the ecology of estuarine copepods. However, its effects on swimming behavior are relatively unclear. The study of copepod swimming behavior requires a sufficient number of long and simultaneous trajectories to form a reliable basis for statistical analysis. Using a three-dimensional particle tracking technique, we observed changes in Pseudodiaptomus annandalei swimming behavior in response to a realistic salinity variation. The three adult states i.e. males, females and ovigerous females were actively swimming over the experimental range of salinity. However, an increase in salinity from 5 to 25 caused a decrease in ovigerous female speed and activity. In males and females, swimming speeds were higher at medium salinities and lower at salinities 5 and 25. Slower or faster swimming speeds were explained by variations in frequencies of resting periods (below 1mm/s), slow swimming (between 1 and 8mm/s) and fast swimming (above 8mm/s). Swimming state residence times varied in concordance. Longer durations of resting periods were observed at slow swimming speeds, together with shorter durations of periods of slow and fast swimming. The degree of space occupation of trajectories was estimated through their fractal dimension and was unaffected by salinity. The relative importance of extreme events i.e. large amplitudes of velocity and acceleration indicated intermittency in behavior. Overall, our results show an optimal salinity for swimming activity and highlight differences between adult states. Results add insight into how salinity may influence copepod swimming behavior. Redirection of energy from swimming to osmoregulation when salinity diverges from the optimal range of the species is a possible explanation. Further tests on total metabolism are warranted to confirm this hypothesis.