We investigated osmoregulatory ability of Pagurus minutus from three sites on the Waka River estuary (upper reach, river mouth, and middle). The sites have different salinity regimes and we tested whether crabs were adapted to their site locally or whether they showed phenotypic plasticity. The upper reach had a low salinity (∼16) during low tide, unlike the other two sites (∼28–31). The exposure of crabs from each site to seven different salinity levels (0–52.5) showed that P. minutus is a hyper-hypo-osmoregulator at all sites and is able to survive in 8.75–52.5 salinity levels for 24 h. Crabs from the upper reach, however, maintained greater hemolymph osmolarity in diluted media, and most of them were able to survive in even 0 salinity medium for 24 h. Crabs from the other two sites were not able to tolerate this condition and all died. Another experiment, involving a 1-wk acclimation to 5 decreasing salinity media (35 to 0), showed that the survival was about 1.3 times greater for crabs from the upper reach than from the other two sites. Because the crab density of the upper reach was significantly less than that of the other two sites, the upper reach is a marginal habitat established by asymmetrical larval dispersal from central saline habitats. Nevertheless, no crab from the saline sites survived exposure to the salinity of zero medium. These results suggest that the high osmoregulatory ability of crabs from the upper reach of the estuary does not represent a local adaptation; crabs acquire it after dispersal through developmental plasticity.