Functional modulation of gill (Na, K)-ATPase K-phosphatase activity in mangrove crabs with different degrees of terrestriality.

Mangrove ecosystems are tidal wetlands with anaerobic, salt-saturated soils that support diverse fauna. Despite intensive studies on the osmoregulatory mechanisms of decapods inhabiting distinct osmotic niches, there is little information on the kinetic characteristics of the gill (Na, K)-ATPase of decapod crustaceans inhabiting mangroves. Here, we provide a comprehensive kinetic characterization of the K-phosphatase activity of the gill (Na, K)-ATPase in two semi-terrestrial brachyuran crabs, Goniopsis cruentata and Cardisoma guanhumi, which respectively inhabit intermittently flooded lower zones and drier upper zones along the vertical gradient of mangrove environments. This study revealed distinct kinetic profiles between the two species. Goniopsis cruentata shows twice as much K-phosphatase activity and a 3-fold greater affinity for p-nitrophenyl phosphate than Cardisoma guanhumi. Substrate hydrolysis by the Goniopsis cruentata enzyme follows cooperative kinetics, whereas Cardisoma guanhumi displays typical Michaelis-Menten behavior. In both species, stimulation by K ions shows cooperative kinetics. Notably, Cardisoma guanhumi also exhibits 25% higher K-phosphatase activity when both K and NH₄ are present. However, whether these differences arise from phylogenetic divergence or reflect physiological adaptation to different mangrove microhabitats remains unresolved. Given the paucity of kinetic studies on (Na, K)-ATPase in semi-/terrestrial crabs, our data contribute novel insights into the enzymatic basis of ion transport in decapods inhabiting variable salinity environments.