Functional diversity among coral reef fishes as consumers of ectoparasites

Nearly all organisms must cope with parasitic infestation, and most research on parasite ecology in marine systems has focused on the variety of both direct and indirect impacts that parasites can have on hosts. In coral reef ecosystems, gnathiid isopods are one of the most common ectoparasites of fishes. For individuals infested with gnathiids, there is a heavy reliance on intraspecific mutualisms where specialist species groom or “clean” parasites off host fishes. However, gnathiids spend most of their time free-living and not associated with a host and are thus susceptible to consumption by non-cleaner fish species. Here, we investigated if non-cleaner and facultative (less specialized) cleaner fish consume ectoparasitic gnathiid isopods as often or in similar quantities as dedicated (highly specialized) cleaners. To do so, we sampled the gut contents of 797 fishes, representing 61 species and including both cleaner and non-cleaner fishes, to compare their consumption of gnathiid isopods. We found that although there is some site level variation, cleaner fishes eat significantly more gnathiids, and eat them more frequently, compared to non-cleaner fishes. Our results highlight the importance of both dedicated and facultative cleaners as consumers of ectoparasites and show that their role is unlikely to be supplanted by non-cleaner consumers. Furthermore, we suggest that different cleaner species act as complementary rather than redundant specialists.