Abstract
The body sizes of individuals of the freshwater prawn Macrobrachium nipponense at sexual maturity (1year-old females) were investigated in 23 local populations residing in diverse environments from estuaries to inland fresh waters. The body size of females at maturity was significantly smaller in populations inhabiting fresh waters (37.5 mm in modal body length), owing to the slower growth of individuals, than that found in populations inhabiting brackish-water lakes (57.5 mm) or estuaries (52.5 mm). In the laboratory, however, females from an estuarine population spawned at a body size much smaller (37.5 mm) than observed in nature, and this body size was very close to that observed in freshwater populations under both natural and experimental conditions. The great phenotypic plasticity of body size at maturity in estuarine populations would have played an important role in the colonization, by this species, of inland fresh waters.
