Evolution of the Functional Morphology of Feeding in Rasborine Fishes

Functional morphology plays a crucial role in shaping ecological diversification among fishes. This study investigates the evolution of feeding mechanics within the Rasborinae, a diverse subfamily of freshwater fishes in South and Southeast Asia, by quantifying interspecific variation in suction-feeding kinematics. We collected high-speed videos of eight Rasborinae species during prey capture and measured fourteen performance variables that describe jaw protrusion, gape, and hyoid depression dynamics. Our results reveal marked divergence in strike duration, timing, and extent of feeding motions among species, with Trigonostigma hengeli exhibiting extreme values for gape and hyoid motion. Conversely, species such as Trigonostigma espei and Boraras merah displayed shorter strike durations and smaller jaw protrusion distances. Phylogenetic mapping highlighted lineage-specific trait evolution, including rapid divergence in hyoid depression and jaw protrusion within the Trigonostigma clade, and possible convergence between distantly related species like Brevibora dorsiocellata and Rasbora rubrodorsalis. Comparative analyses with prior work on danionids indicate that the timing of jaw protrusion has evolved considerably within this lineage. Future work with the zebrafish model can be used to determine the developmental factors that have shaped trophic diversification within the Danioinidae.