Resisting flow–laboratory study of rheotaxis of the estuarine copepod Pseudodiaptomus annandalei

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Rheotaxis is a ubiquitous phenomenon among aquatic animals and thought to be an adaptation to maintain populations in flowing waters. While many estuarine copepods can retain their populations in estuaries with net seaward flow, rheotaxis of individual copepods has not been reported before. In this study, the behavior of a calanoid copepod Pseudodiaptomus annandalei in flow was examined in a recirculating laboratory flume. This estuarine copepod displayed different responses to ambient flow fields while swimming in the water column or attaching to the flume bed (walls). Copepods in the water column showed vigorous countercurrent swimming by occasional bounding when flow velocity was increased up to 2.1 cm s−1, but none of the individuals in the water column were retained in the flume when flow speeds were higher than 4 cm s−1. This indicates P. annandalei profits little from rheotaxis to withstand flow when they were swimming in the water column. Instead, more individuals attempted sinking downwards to the slow flow region near the flume bed (walls) and showed active substrate attachment to avoid being flushed out by the high-velocity channel flow. The results suggest that P. annandalei benefits from rheotaxis and association with the substrate which allows them to hold position well at ambient flow velocities up to 3 cm s−1. These adaptive responses might be important for population maintenance.