“Phylo-allometric analyses showcase the interplay between life history patterns and phenotypic convergence in cleaner wrasses”

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Vikram B. Baliga and Rita S. Mehta (May 2018)

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Ontogenetic trajectories of cleaner fishes are shaped by life history patterns of feeding ecology

A cleared and double-stained specimen of Labroides bicolor, the bicolor cleaner wrasse. The specimen was borrowed from the Smithsonian National Museum of Natural History (catalog: USNM 407845) and was part of the study.
(Credit: Vikram B. Baliga)

Juvenile animals often show stark differences in ecology from their adult counterparts; capitalizing on specific resources may only be favorable in a certain phase of life. This raises the question of how the process of development may adapt to meet shifting ecological demands. Identifying how ecology is intertwined with the evolution of ontogeny has long relied on between-species comparisons of how traits scale with body size. But, because morphological evolution occurs through modification of ancestral developmental patterns, many aspects of a species’ ontogeny may be due to retained ancestral recipes. This influence can constrain how traits evolve or cloud the signal of new adaptations. Therefore, how ontogeny evolves with ecology is an exciting arena of study for phylogenetic comparative methods, which can unravel historical vs. novel signals in evolutionary patterns.

Vikram Baliga and Rita Mehta use the repeated evolution of cleaning behavior in the reef fish family Labridae (wrasses, parrotfishes) to understand how species can penetrate new ecological niches through adaptive changes to ontogeny. Cleaning is a mutualism wherein an individual (“cleaner”) feeds on ectoparasites living on other organisms. Remarkably, three-fourths of wrasses that clean do so predominately as juveniles, growing out of it (so to speak) as adults. The evolution of cleaning therefore provides a powerful opportunity to determine how ontogenetic shifts in feeding ecology can influence the evolution of scaling of musculoskeletal traits.

Baliga and Mehta examine the ontogenetic scaling patterns of 33 labrid species and develop an approach in which the ontogenetic trajectories of taxa can be ordinated while accounting for shared ancestry. They show that the recurring evolution of juvenile-only cleaning behavior in the Labridae yields similar changes to developmental patterning. This research explicitly shows that scaling trajectories are evolutionarily labile and can adapt to changing ecological pressures over ontogeny, which expands our fundamental understanding of how phenotypic diversity is generated and maintained.


Abstract

Phenotypic convergence is a macroevolutionary pattern that need not be consistent across life history. Ontogenetic transitions in dietary specialization clearly illustrate the dynamics of ecological selection as organisms grow. The extent of phenotypic convergence among taxa that share a similar ecological niche may therefore vary ontogenetically. Because ontogenetic processes have been shown to evolve, phylogenetic comparative methods can be useful in examining how the scaling of traits relates to ecology. Cleaning, a behavior in which taxa consume ectoparasites off clientele, is well-represented among wrasses (Labridae). Nearly three-fourths of labrids that clean do so predominately as juveniles, transitioning away as adults. We examine the scaling patterns of 33 labrid species to understand how life history patterns of cleaning relate to ontogenetic patterns of phenotypic convergence. We find that as juveniles, cleaners exhibit convergence in body and cranial traits that enhance ectoparasitivory. We then find that taxa that transition away from cleaning exhibit ontogenetic trajectories that are distinct from those of other wrasses. Obligate and facultative species that continue to clean over ontogeny, however, maintain characteristics that are conducive to cleaning. Collectively, we find that life history patterns of cleaning behavior are concordant with ontogenetic patterns in phenotype in wrasses.