Client reef fish tolerate closer human approaches while being cleaned

On tropical reefs, cleaning interactions, in which a fish or shrimp cleaner removes ectoparasites from a client fish, are common. Such cleaning interactions have been shown to reduce physiological stress in the clients. We asked whether the process of cleaning by a cleaner wrasseThalassoma noronhanunmodified a fish client's risk assessment, hypothesizing that the benefits of being cleaned may modify the cost avoiding a potential disturbance or predator. We experimentally approached fish of two species (a parrotfishSparisoma amplumand a squirrelfishHolocentrus adscensionis) when they were being cleaned and when they were not being cleaned, and measured the client's flight initiation distance (FID) - a metric of risk assessment. Both client fish species tolerated similar to 30% closer approach when being cleaned. The body size of the client did not affect FID in either species, and the number of cleaners present did not influence FID of squirrelfish, but parrotfish who received cleaning from two cleaners had longer FID. These findings imply that fish being cleaned modify their risk assessment, and these results add to a list of potential costs fish clients face in this fascinating mutualism.