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Functional niches of cleanerfish species are mediated by habitat use, cleaning intensity, and client selectivity.

An animal's functional niche is a complex, multidimensional construct, mediated by an individual's morphology, physiology, and behaviour. Behavioural aspects of the niche can be difficult to quantify, as their expression is often subtle and tailored to an infinite number of different situations that involve sophisticated mechanisms such as mutualisms, species dominance, or fear effects. The extreme diversity of tropical fish assemblages has led to extensive debate over the extent to which species differ in their resource use and functional role. Ectoparasite removal by cleanerfish species is considered a behaviourally complex interspecific interaction in vertebrates, but differences in the services rendered by various species of cleanerfish, and potential consequences for the range of clients (i.e., resources) they attract, have rarely been examined. Here, we quantify differences among three coexisting species of morphologically similar cleaner wrasses (Labroides bicolor, L. dimidiatus, and L. pectoralis) in the global centre of marine biodiversity, the Coral Triangle. We found no clear taxonomic partitioning of clients among cleanerfishes. However, the three cleanerfish species exhibited distinct habitat preferences, and differed in their cleaning intensity: L. bicolor serviced the fewest species and clients, while L. pectoralis serviced the most clients and spent the most time cleaning. Accordingly, L. pectoralis showed no preference for clients based on client size or abundance, while both L. bicolor and L. dimidiatus had a higher likelihood of interacting with clients based on their size (larger client species in L. bicolor, smaller client species in L. dimidiatus) and abundance (more abundant client species for both). Our results suggest that the services rendered by the three species of cleanerfishes differ in their spatial availability, quality, and selectivity, thus permitting the coexistence of these species despite their ecological similarity. This, in turn, creates a complex seascape of species-specific cleaning services that underpins crucial biotic interactions in the ocean's most diverse ecosystem.