Morphological Variation of the Red-Bellied Piranha Pygocentrus nattereri Kner 1858 (Characiformes: Serrasalmidae) Across Bolivia: Insights From Historical and Ecological Factors.

The red-bellied piranha, Pygocentrus nattereri Kner 1858, is one of the most widespread and economically important freshwater fishes in the Amazon and De La Plata basins, yet the factors driving its morphological variation remain poorly understood. This study evaluated the influence of geographic structure and environmental heterogeneity on body shape variation across the Bolivian Amazon and De La Plata basins using geometric morphometrics. A total of 144 specimens from five river sub-basins (Beni, Mamoré, Iténez, Orthon [Amazon Basin], and Paraguay [De La Plata Basin]) were analyzed using 16 anatomical landmarks and 36 semilandmarks. Shape variation was assessed through Generalized Procrustes Analysis, Procrustes ANOVA, PCA, and Linear Discriminant Analysis. Geographic factors included sub-basin affiliation and the presence of major rapids, whereas environmental factors included water type (white vs. clear/black water), water origin (Andean, lowland, and Brazilian Shield), and habitat (lentic vs. lotic). Geographic subdivision among basins and sub-basins was the strongest determinant of body shape variation, explaining 7.7% of the total variation and having a stronger effect than all other factors. Water type (3.1%) and water origin (2.9%) also had significant effects, indicating that watershed-scale environmental characteristics contribute to morphological differentiation. In contrast, rapids exerted only a weak effect (1.5%), whereas habitat type did not significantly influence body shape. Eye morphology exhibited a different pattern, being associated primarily with geographic subdivision, rapids, and habitat, but not with water type or water origin. These findings demonstrate that body shape variation in P. nattereri is driven principally by historical and biogeographic processes associated with drainage configuration, while regional environmental gradients linked to water type and watershed origin provide additional sources of phenotypic differentiation. The contrasting responses of body and eye morphology suggest that different morphological traits may be influenced by distinct ecological and evolutionary processes. This study provides new insights into the relative importance of historical and environmental drivers of intraspecific morphological diversification in a widespread Neotropical freshwater fish.