Effect of salinity on the swimming behaviour of the estuarine calanoid copepod Eurytemora affinis

The calanoid copepod, Eurytemora affinis, is the dominant zooplankton species in most of European and North American estuaries, and their population maintenance within an environment of net seaward flow has mainly been explained by endogenous rhythms of circatidal swimming activity. However, no attention has been paid to the potential link between the swimming behaviour of this species and salinity. The swimming behaviour of males, non-ovigerous females and ovigerous females from a continuous culture was investigated under different salinity conditions. Increase and decrease in salinity, respectively, increased and decreased the overall swimming activity of both males and non-ovigerous females. The complexity of the swimming paths of males and non-ovigerous females significantly increased with salinity. In contrast, ovigerous females were less motile and mainly sank. These observations suggest an endogenous behavioural adaptive strategy to salinity fluctuations and the intrinsic ability of E. affinis to undergo short-scale vertical migration triggered by changes in salinity. This supports field observations reporting increased abundance of E. affinis in the water column during flood tides and provides a behavioural basis for the maintenance of viable populations under net ouflow conditions.