Chasmagnathus granulatus is a hyper-hyporegulating crab that inhabits changing habitats of salinity in
Brazil, Uruguay and Argentina. Since the gills are the main sites for active ion transport in crabs, the
adaptive changes in the gill epithelium occurring under different conditions of salinity were studied by means
of morphological and morphometric analysis, and immunohistochemical identification of cell proliferation
(BrdU technique). In anterior (1–3) gills the epithelium thickness from crabs acclimatised to 12, 34 and
44 g/l ranged from 1.27 to 2.46 μm, with no significant change during acclimatisation, thus denoting a
respiratory function. Medial (4–5) gill epithelium was slightly thicker in extreme salinities, but these
differences were not statistically significant. In contrast, epithelial thickness of the posterior (6–8) gills
increased significantly up to 8.10 μm (dorsal zone of gill 8) both in hyper- and hyposaline media compared
with seawater. The dark areas measured in gill 8 treated with AgNO3 revealed putative ion transporting
tissue, especially at 12 and 44 g/l, corresponding to the zones of higher epithelial thickness. Hence these
areas seem to participate both in hyper- and hyporegulation. Proliferating cells labelled with BrdU almost
never occurred in the gills/salinity combinations studied during the initial 48 h of transfer from seawater to
hyperconcentrated or diluted media, thus suggesting an increase in cell size rather than cell proliferation.