In this study we explore the possible role of phenotypic plasticity in the process of adaptation and evolutionary
change in the African cichlid Pseudocrenilabrus multicolor victoriae. Parental fish were collected from a hypoxic swamp,
a lake ecotone, and a river in Uganda. Broods (F1) were split and grown under hypoxia or normoxia. We measured morphological
parameters of the gill apparatus, structural elements surrounding the gills, brain mass, and body shape. Most
traits showed substantial plasticity in response to the rearing environment. Population effects were evident for the gill apparatus,
surrounding elements, body shape, and brain size; however, brain size was the only trait to exhibit variation in
plasticity among populations; fish of swamp origin showed no plasticity and fish of river and lake origin exhibited smaller
brain size under hypoxia. We interpret these patterns as consistent with genetic assimilation via canalization (brain) or via a shift in the norm of reaction (other traits).