The turbulent lives of copepods : how flow over a coral reef affects their ability to detect predators
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Calanoid copepods exhibit rapid escape behavior in response to hydrodynamic stimuli generated by their predators. Currents and turbulence in nature may cause copepods to become less responsive to these stimuli or may otherwise interfere with the detection of predators. Structurally complex environments, such as coral reefs, disrupt water flow and present a range of flow microhabitats varying in both mean and turbulence characteristics. The heterogeneous distribution of flow over the reef may play an important role in the interactions between copepods and planktivorous fish through effects on copepod escape behavior. The escape response and capture rates of the copepod Acartia tonsa were examined in flumes that created both unidirectional and oscillatory flow conditions similar to those found on coral reefs. Copepods were tested in unidirectional flow velocities up to 34.4 cm s-1, which mimicked water currents, and also in oscillatory flow which simulated wave periods of 2.1 seconds. Two turbulent regimes were produced in each flume: "smooth" flow was formed using a grid collimator and "rough" flow was generated by placing a branched coral skeleton upstream of the flume's working section. A predator flow field was simulated by a fixed siphon. Using video analysis, copepod detection of the siphon "predator" was measured as the distance from the siphon tip to where an escape response was initiated. This reactive distance remained the same in low flow conditions as in still water but was reduced 25% in flow speeds of 11.1 cm s-1, indicating a decline in copepods' ability to detect velocity gradients formed by the siphon. Rough turbulence regimes in moderate flow conditions intensified the effect of current speeds by decreasing copepod reactive distance an additional 27%. Capture rates of copepods increased with current speed, wave motion and in rough flow, while the capture rates of non-evasive prey, Artemia nauplii, did not vary with flume conditions. The differences in capture rates between evasive and non-evasive prey suggest that behavioral shifts in copepod escape thresholds may account for increases in predation by reef-dwelling fishes observed in hydrodynamically complex coral environments.