Geometric morphometrics reveals morphological variation in the subterranean amphipod Potiberaba porakuara Fišer, Zagmajster and Ferreira (Amphipoda: Mesogammaridae) from Caatinga cave

Phenotypic variation represents an important component of biological diversity and may reflect organismal responses to contrasting environmental conditions. In subterranean aquatic ecosystems, however, the influence of hydrodynamic conditions on morphological variation remains poorly understood, particularly in semiarid regions. Here, we investigated morphological differentiation between populations of the subterranean amphipod Potiberaba porakuara inhabiting lotic and lentic caves in the Caatinga of northeastern Brazil using geometric morphometrics. Adult females were collected from two caves representing contrasting hydrodynamic regimes, and landmark-based analyses were performed on the cephalic region and the propodus of gnathopod 1. Geometric morphometric analyses revealed significant shape differences between populations for both anatomical structures (p < .001). Individuals from the lotic cave exhibited narrower, more elongated and hydrodynamically streamlined morphologies, whereas specimens from the lentic cave displayed broader and more robust conformations. Discriminant analyses correctly classified most individuals according to their habitat of origin, indicating that body shape reliably distinguishes populations from contrasting hydrodynamic environments. Allometric analyses showed that centroid size had little influence on shape variation, indicating that the observed differences were largely independent of body size. Previous molecular studies reported no significant genetic divergence between these populations, suggesting that the observed morphological differentiation is associated with local environmental conditions rather than evolutionary divergence. Our findings provide the first evidence of environmentally associated intraspecific morphological differentiation in a subterranean amphipod from the Brazilian semiarid region and highlight hydrodynamics as an important factor associated with phenotypic variation in subterranean aquatic ecosystems.