Phylogenetics and evolutionary patterns of cleaning
With at least 58 species that exhibit cleaning behavior, the Labridae (wrasses, parrotfishes, and the lesser-known weed whitings) show a tremendous diversity of fishes that engage in this behavior. This diversity over 4 times higher than that seen the next highest group: the Gobiidae (14 cleaner species). The diversity of cleaner fishes within a family does not simply scale with a family’s size; for example, the Labridae contain ~631 species, while the Gobiidae have nearly 2000 members.
The species richness of cleaners in the Labridae holds plenty of evolutionary intrigue. Cleaning itself appears to involve a variety of behavioral and morphological adaptations. Species that clean have the capability to target and acquire ectoparasites off the bodies of their 'clients', which often involves acquiescence from these clients. Therefore, one might expect the evolution of cleaning to have relatively few independent origins. In a study (Baliga and Law, 2016), we sought to understand how and when cleaning arose in this group.
We used maximum likelihood and Bayesian methods to infer the relationships between 320 species of labrids. Then, using stochastic character mapping, we simulated character histories across the trees we had generated. Our analyses incorporated information for 50 of the 58 species of known cleaners (100% coverage of genera).
From these reconstructions, we inferred that cleaning evolved at least 26 times within the Labridae, and that the earliest evolution of cleaning likely occurred 17-21 million years ago.
Cleaning appears to be very young: the vast majority of cleaning likely evolved within the last 10 million years.
The majority of these events involved transitions from non-cleaners to the juvenile cleaner state. Cleaning in the juvenile phase appears to be a crucial state that bridges other forms of cleaning (facultative and obligate). This underscores the importance of examining juvenile morphology and ontogenetic patterns of morphological change in this rich and diverse system.
Additional inferences of how and when cleaning evolved in other groups are forthcoming. Stay tuned!