This study aims to give an integrative description of the correlation of physiological param- eters of osmoregulation and the habitats of the four common Uca species in Taiwan. Uca arcuata inhabits areas close to fresh water in the upper beach. Uca formosensis is only found in the areas near the mean high water of spring tide where there is a clear dry-wet transition within a single semilunar cycle. Uca vocans is found in the lower intertidal zone. Uca lactea, the most widely distributed species, can easily be found on most muddy sand shores. The number of gills was observed and histological sectioning per- formed on each species. The range of salinity in which the fiddler crabs maintained their hemolymph osmo- lality without any significant change (i.e. osmoregulatory homeostasis) and the gill Na+, K+-ATPase activity were determined by transferring individuals to different salinity tanks. The results suggest that U. formosen- sis and U. lactea can sustain a wider range of salinity change through both modification in gill morphology and Na+, K+-ATPase activity. Uca arcuata can regulate in a hypo-osmotic condition and U. vocans tends to be a weak-osmoregulator.
