Differences in behavioral responses of Eurytemora affinis (Copepoda, Calanoida) reproductive stages to salinity variations

Estuarine copepods experience strong and frequent salinity variations caused by alternating tides. Salinity is known to be a major parameter in the ecology of copepods from brackish waters but its effects on small scale behavior are relatively unclear. In this study, we used two-dimensional laboratory filming techniques to record swimming paths of the three reproductive stages (i.e. male, non-ovigerous female and ovigerous female) of Eurytemora affinis from the Seine estuary, under different salinity conditions. We show that the probability density of both the instantaneous swimming speed and the duration of the two most frequent swimming states (i.e. break and slow swimming) followed a power-law trend regardless of the salinity. Increase in salinity (i.e. 5, 15, 25 and 30) steadily decreased the instantaneous speed of E. affinis reproductive stages. At the extreme salinity of 0.5, the mean speed decreased for non-ovigerous females and ovigerous females but increased for males, when compared with higher salinities. Maximum speed was observed at salinity 0.5 for males and at salinity 5 for non-ovigerous females and ovigerous females. Low swimming speeds were associated with break events of high frequency and long duration. High swimming speeds were exhibited with cruising states of high frequency and break events of short duration. This supports laboratory and field studies indicating a preference of E. affinis for low to medium salinities.