Abstract
Carbonic anhydrase (CA) activity was measured in the gills, branchiostegite, and cuticle of 5 species of decapod crustaceans displaying fully aquatic, semi‐terrestrial, and fully terrestrial lifestyles. Density‐gradient and differential ultracentrifugation were used to separate 2 subcellular fractions from these tissues, basal‐lateral membranes, and cytoplasm, in which CA activity was determined. Callinectes sapidus, a euryhaline aquatic species, had high levels of CA activity in both the cytoplasmic and membrane fractions of the gills. These pools of CA are believed to function in ion transport and CO~2~ excretion, respectively. The branchiostegite and cuticle, which do not play an apparent role in either process in the blue crab, possess much lower levels of CA activity in both fractions. In contrast, a stenohaline, stenothermal species, Chaceon fenneri, which has a low metabolic rate, has low levels of CA activity in the cytoplasm and membrane fractions of the gills. As decapod crustaceans invade the terrestrial habitat and make the transition to breathing air, gill surface area is reduced and the branchiostegite develops into a primitive lung. In Cardisoma guanhumi, a semi‐terrestrial crab, branchiostegal tissue has very little CA activity in either fraction, and enzyme activity is not different from that in the general cuticle; the gills still possess the bulk of CA activity in the animal. In fully terrestrial species (Gecarcinus lateralis and Birgus latro), however, the membrane fraction of the branchiostegite has the same level of CA activity as that of the gills, regardless of species, while CA in the cuticle remains low. It appears, according to the distribution of membrane‐associated CA, that while the gills of aquatic crustaceans are the primary route of CO~2~ excretion, the branchiostegite can supplement branchial CO~2~ elimination in terrestrial species.