Intraspecific adaptation process in fishes from natural and antropogenic heterogeneous environments

We are interested in analyze how the environmental heterogeneity works as a factor in the adaptation process in fishes. To this propose, we are analyzing the species Jenynsia multidentata (Cyprinodontiformes: Anablepidae) that occurs both in marine, brackish and freshwater (natural heterogeneous environments). This species showed different size according salinity degree, with population from low salinity environment showing smallest body size than populations from high salinity environment. This condition was also found for other fish species, being associated with the predation pressure that these species are submitted. In freshwater, we can found more predator species (these species are small fishes, generally omnivorous, being prey of other piscivorous fishes). And with different predation pressures, the populations of freshwater and brackish environment could show different phenotypes associated with the ability of escape (explosive swimming). And to analyze the heterogeneous anthropogenic environment, we are studying the possible phenotypic differences in Bryconamericus iheringii from a stream that show a dam. In a personal communication (research partner), this species seems to show differences in the body shape among the environments lotic and lentic from this stream. This situation has also been showed for other species in similar condition of habitat alteration. The alteration caused by the dam construction generates a different kind of environment, which can acts as a new source of natural selection.