Osmoregulation was studied in the zoeal stages I to 111, the rnegalopa, juvenile crab stages
I and 11, and In adults of the grapsid crab Armases mjersii. The larvae hatch and develop in tropicrll
supratidal rock pools, where ample variations of salinity occur. To cope with this harsh environment,
the capacity for osrnoregulation is well developed at hatching, and becomes further accentuated in the
larvae and juveniles. All zoeal stages hyper-regulated at low salinity (5 to 26 PSU), but at higher salinity
(33 to 44 PSU), they were hyper-osmoconformers. The type of osmoregulation changed in the rnegalopa
stage to a hyper-hypo-regulation pattern While the hyper-osmoregulatory capacity increased
gradually throughout postembryonic development from hatching to adult, the hypo-osmoregulatory
capacity increased from the megalopa to the adult. The ontogenic acquisition of os~noregulat~on was
faster for hyper- than for hypo-regulation: 85 and 41 '% of the adult capacity, respectively, was acquired
in the crab I stage. These findings confirm that important physiological changes occur at metal~iorphosis.
A correlation is established between the osmoregulatoi-y ability of each developmental stage and
its salinity tolerance. The ecological implications and the adaptive and evolutionary significance of
osmoregulation in early life-history stages of A. miersii and other aquatic crustaceans are discussed