Chemical cues affect behaviour in fish lice ( Argulus spp.) and three‐spined stickleback hosts ( Gasterosteus aculeatus )

We found evidence of mutual recognition of chemical cues between fish lice ( Argulus spp.) and three‐spined sticklebacks ( Gasterosteus aculeatus ). Each manifested changes in movement in response to chemical cues from one another; Argulus increased speed while G. aculeatus decreased theirs. As Argulus use active searching behaviours to locate hosts, higher speeds indicate increased searching activity. Conversely, G. aculeatus may slow down to augment visual detection while also reducing mechanical cues which attract lice. This represents the first evidence that G. aculeatus may use chemical cues to mitigate the risk of parasitism from lice.