Fresh caught Clibanarius vittatus [SW, 31‰ salinity (S)] were acclimated to a dilute medium (15‰ S) for 10 days, employing silver staining to locate gill ion transporting tissue, immunofluorescence to localize the Na/K-ATPase α-subunit in the lamellae, and electron microscopy to portray ultrastructural changes in the gill epithelia. Na/K-ATPase activity was characterized kinetically in a gill microsomal fraction, including synergistic stimulation by NH plus K. Silver staining revealed that all 26 phyllobranchiate arthro- and pleurobranchiae participate in ion transport. Na/K-ATPase α-subunit staining was weak in SW crabs and distributed exclusively and irregularly within the intralamellar septal cells, particularly at the septal-pillar cell body junctions, and septal cell cytoplasm facing the hemolymph space. In 15‰ S crabs, α-subunit localization was intense, occupying the entire thickened septum. Pillar cells and flanges did not stain. Mitochondria and membrane foldings increased in the pillar cell flanges and intralamellar septal cells, greatly amplifying surface area. Only a single ATP binding site (V =  130.8 ± 10.5 nmol min mg protein; K =  55.3 ± 1.7 μmol l) obeying Michaelis-Menten kinetics was disclosed. Na/K-ATPase activity was modulated by Mg, Na, and NH, exhibiting site-site interactions; K modulation showed Michaelis-Menten kinetics. K plus NH synergistically stimulated activity ≈ 1.7-fold. Ouabain inhibited total ATPase activity by ≈ 70% (K =  220-300 μmol l), revealing phosphohydrolytic activities other than the Na/K-ATPase. Despite ample phylogenetic separation, the phyllobranchiate lamellae of the Anomura and Caridea share many ultrastructural features, that is, an intralamellar septum and opposed abutting pillar cells, similar Na/K-ATPase distribution, and comparable kinetic characteristics. These findings suggest either convergent evolution at the structural and biochemical levels, or preservation of traits present in a remote common ancestor.

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