Evidence of morphological differences between Astyanax bimaculatus (Actinopterygii: Characidae) from reaches above and below dams on a tropical river

Abstract

Environmental alterations following river damming can cause body shape changes in freshwater fish species. However, little information is available on morphological responses of fishes to new environments after dam construction. We examined morphological differentiation of a small bodied characin fish collected from three pairs of reservoir and river habitats with distinct hydrological connectivity level (i.e., the degree to which fish movement is facilitated or impaired across reservoir and downriver habitats): (i) without, (ii) intermittent, and (iii) permanent connectivity. We predicted that river damming causes shape changes in fish species and share a common pattern of morphological differentiation, with individuals from lotic habitats having a more hydrodynamic body shape and being poorly adapted for maneuverability compared with those from lentic habitats. Geometric morphometric analysis of the body shape revealed that individuals inhabiting reservoirs were deeper-bodied and had longer dorsal and anal-fin bases compared with individuals from the rivers, features associated with body shape adaptation to lentic environments. We also showed that connectivity between reservoir and river habitats appears to have constrained morphological differentiation, presumably due to population mixing rates. Local characteristics associated with each impoundment (e.g., migration routes and environmental homogeneity) seem to play a critical role in Astyanax bimaculatus body shape differentiation. The observed patterns were consistent with functional morphological principles suggesting adaptive divergence.