Bluestreak cleaner wrasse are unsuspecting parasite super-spreaders

Abstract

Personal hygiene matters, whether it's preening or taking a shower. But some creatures are unable to maintain their own cleanliness, so fish subcontract out their sanitation to smaller fish that nibble at lice on their skin. ‘Cleaner fish may clean throughout their entire lifespan, using cleaning as their primary way of acquiring food, or engage in cleaning interactions predominantly as juveniles or as a secondary source of food’, says Pauline Narvaez from James Cook University, Australia. However, Narvaez and Kate Hutson (James Cook University) wondered whether there was a darker side to the symbiotic hygiene story. Could cleaner fish unintentionally carry and transmit to unsuspecting clients some of the parasites that they usually vacuum up? Intrigued by the possibility that cleaner fish could be inadvertent parasite super-spreaders, Narvaez and Hutson joined forces with Renato Morais (James Cook University), David Vaughan (Central Queensland University, Australia) and Alexandra Grutter (The University of Queensland, St Lucia, Australia) to find out whether bluestreak cleaner wrasse (Labroides dimidiatus) are vulnerable to infection by the pests upon which they usually feast. Venturing out onto the Great Barrier Reef at Lizard Island, Australia, Narvaez and Morais went in search of bluestreak cleaner wrasse, armed with barrier- and hand-nets. ‘Once the cleaner fish were caught, we gently moved them into plastic bags until we got back to the research station’ says Narvaez. She then released five famished parasitic juvenile gnathiid isopod crustaceans (Gnathia aureamaculosa) into each tank with an individual wrasse to find out how succulent the cleaner fish were for the parasites. Back at James Cook University, Narvaez also isolated cleaner wrasse with juvenile parasitic protozoans (Cryptocaryon irritans), which cause marine white spot disease, and parasitic flatworm larvae (Neobenedenia girellae), which attach to their host's body, to find out how susceptible the fish were to other infections.Impressively, the cleaner wrasse were remarkably resistant to both white spot disease and the parasitic flatworms; however, the juvenile parasitic crustacean larvae were able to grab a hold on the cleaner wrasses’ skin. So bluestreak cleaner wrasse aren't completely immune to freely swimming juvenile parasites, although they seem to be less vulnerable than other similarly sized species. But how susceptible might the wrasse be to adult parasites, carried by their clients, while going about their day job?This time, Narvaez dextrously transferred adult flat worms to healthy bluestreak cleaner wrasse to find out how the parasitic flatworms fared on the cleaning professionals. Impressively, the flatworms managed to hold on to their cleaner wrasse rides for the first 24 h, although roughly half detached in the following 24 h; the adult parasites are able to cling on long enough to reach their next host. Fortunately, when Narvaez monitored the number of eggs released by the parasitic N. girellae hitch-hikers, she found they weren't as fertile as when catching a lift on their usual hosts, initially producing only a fraction (7%) of the eggs that they usually release, falling to virtually zero by the end of the week.‘Some parasites may have evolved to exploit cleaning symbiosis as a mechanism for transmission’, says Narvaez, explaining that the flexible flatworms, which shuffle about on their host's skin, could hop off their L. dimidiatus ride as the wrasse burrows about in the gills of another client to infect it. And, as bluestreak cleaner wrasse provide hygiene services to approximately 2300 fish each day, Narvaez and colleagues suspect that infected cleaners could be contributing unwittingly to the problem.