Are cleaner fish clean?

Cleaner fish remove parasites from other organisms, called clients. While there is an extensive body of work on the positive role of cleaners for their clients and reef communities, remarkably, potential parasites hosted by specialised cleaner fishes themselves have not been explored. In this study, we surveyed the parasite community of the Indo-Pacific cleaner wrasse Labroides dimidiatus , and compared it to other wrasses from the same region. L. dimidiatus was found to be infected by eight parasite groups including ectoparasites (copepods, isopods, trichodinids, monogeneans and turbellarians) and endoparasites (myxozoans, trematodes and cestodes) representing at least 12 species. The abundance and prevalence of most parasite groups was comparable to other wrasses, with the exception of bucephalid trematodes, which are not known to infect any other tropical wrasses except for Labroides species. This adds to mounting evidence that some parasite species exhibit atypical life cycles that exploit cleaning symbiosis. Particularly noteworthy was the discovery of gnathiid isopods on L. dimidiatus , which are generally considered the cleaner’s primary food item. Our findings provide new evidence for a potential role of wild cleaner fish as vectors of parasites to new clients, which highlights potential costs associated with cleaning symbiosis.