Changes in local free-living parasite populations in response to cleaner manipulation over 12 years

Predation on parasites is an important ecological process, but few experimental studies have examined the long-term impacts on the prey. Cleaner fish prey upon large numbers and selectively feed on the larger individuals of the ectoparasitic stage of gnathiid isopods. Removal of cleaner fish Labroides dimidiatus for 1.5-12.5 years negatively affects coral reef fishes, but the mechanism is unclear. A reduction in local parasite populations or the size of individual parasites would benefit all susceptible fishes. We tested whether cleaner presence reduces local gnathiid populations using 18 patch-reefs distributed between two sites (both at Lizard Island, Great Barrier Reef) which were maintained cleaner-free or undisturbed for 12 years. Using emergence traps (1 m²), free-living gnathiid stages were sampled before and after cleaner fish were removed during the day and night, up to 11 times over the course of the experiment. There were effects of the removal in the predicted direction, driven largely by the response at one site over the other involving 200% more gnathiids, but manifested only in the daytime sampling after 4 months. There was also a main effect (36%) for the shared sample dates at both sites after 12 years. Gnathiid size occasionally differed with cleaner presence, but in no consistent way over time. Contrary to our predictions, changes in free-living gnathiid population numbers and their size structure rarely reflected the changes in fish populations and individuals observed on cleaner-free reefs. Therefore, evidence that this predator alone regulates gnathiids remains limited, suggesting other contributing processes are involved.