Water flow around a fish mimic attracts a parasitic and deters a planktonic copepod

Fish typically evoke a flight response in copepods. The behaviour of infective stage parasitic copepods, Lepeophtheirus salmonis, and adult holoplanktonic copepods, Acartia spp., in response to the approach of a rubber fish mimic was compared. Responses were scored as attacks if the copepod moved closer to the head or escapes if it moved further from the head. The parasite attacked the mimic in 65% of its responses, whereas Acartia spp. escaped from the mimic in 87% of its responses. Lepeophtheirus salmonis escaped in sinuous routes that kept them closer to the fish, but Acartia spp. escaped in straight paths directed away from the mimic. Attacks by L. salmonis were equally frequent in the dark (68.9%) as in light (60.3%), and net-to-gross-displacement ratios under the two conditions were not significantly different. Copepod responses were evoked in regions of flow with linear strain rates greater than 0.5 s−1. The fish hydrodynamic signals thus serve as attractants that guide the parasite to the fish and produce an avoidance response of the holoplanktonic copepod. Thus, the same information, most likely received by the same sensors, produce ‘opposite’ reactions to the fish, suggesting the evolution of different behaviour patterns suited to the way of life of the copepod.