Ontogeny, phylogeny, and ecology
Over 120 species of marine fishes across 18 families engage in cleaning behavior.
Curiously, the majority of these species (over two-thirds) clean predominately as juveniles, transitioning to other dietary strategies upon reaching adulthood. Figuring out why certain species clean as juveniles while others do not has been a focal point of my research.
I use the evolution of cleaning behavior in the Labridae to learn how the process of development may adapt to meet shifting ecological demands over ontogeny.
A case study: Ontogenetic trends in the genus thalassoma
Nearly all cleaners that are in the genus Thalassoma clean predominately as juveniles. In a case study (which became my first paper!!), we discovered that cleaners in Thalassoma, as juveniles, possess jaws with low mobility and exhibit low bite forces compared to non-cleaner congeners (Baliga and Mehta, 2014). Upon reaching adulthood, however, morphological and functional differences between cleaners and non-cleaners begin to blur. The sizes at which we first found overlap in feeding traits between cleaner and non-cleaner species approximately correspond with the body lengths around which these species shift from juvenile to adult coloration patterns. Thus, an important suite of ontogenetic changes accompanies the dietary shift in these species.
Vertebral shape and flexibility of the Body
In the aquatic environment, the shape of the body, fins, and the underlying axial skeleton reflect the ability of organisms to propel and maneuver through water. We predicted that size, shape, and flexibility of the axial skeleton are important traits for cleaning and that cleaners may be evolutionarily constrained to exhibit a particular phenotype. We found (Baliga and Mehta, 2016) that across wrasses, the ontogeny of body shape shows evolutionary concordance with the degree of flexibility of the vertebral column. A key driver of this relationship is that species that shift away from cleaning over ontogeny show stronger positive allometry for body depth and vertebral moment of inertia than do other taxa. Species that clean throughout their life histories show a relatively more elongate body and vertebral column, and tend to maintain the combination of these characteristics over ontogeny.
Putting it all together: phylogenetically-informed analyses of multivariate allometry
We developed an approach in which the ontogenetic trajectories of taxa can be ordinated while accounting for shared ancestry (Baliga and Mehta, in press). These analyses enabled me to show that the recurring evolution of juvenile-only cleaning behavior in the Labridae yielded similar adaptive 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.