Morphological Constraint Shapes Diversification in a Hyperdiverse Lineage of Neotropical Tetras: A Geometric Morphometric Analysis of Three Subfamilies of Acestrorhamphidae (Teleostei: Characiformes).

The order Characiformes (Teleostei: Ostariophysi) represents one of the most diverse groups within the Neotropical freshwater ichthyofauna. Recently, a phylogeny with unprecedented resolution redefined its internal relationships and delimited Acestrorhamphidae as the most species-rich family within the group. Despite this high diversity, its members exhibit a notably homogeneous body morphology, raising questions about the evolutionary process governing phenotypic evolution in a group known for its ecological and morphological diversity. The present study investigates three subfamilies of Acestrorhamphidae, Pristellinae, Stethaprioninae, and an unnamed subfamily, using geometric morphometrics, phylogenetic comparative methods, and multivariate statistical approaches within a time-calibrated phylogenetic framework. The results revealed weak modularity and a strong integration signal among three recovered anatomical partitions (cranial, postcranial, and caudal), with morphological variation concentrated along a single dominant axis (74.93%) and significant anisotropy (p < 0.05). Morphological variation was largely organized around two contrasting phenotypes: elongated forms, with low body depth and long standard length, and robust forms, with greater body depth and shorter body length. The results suggest that phenotypic diversification in this acestrorhamphid clade has occurred within a restricted region of morphospace, consistent with a scenario in which strong integration channels morphological evolution along a limited set of viable body-shape trajectories.