Critical salinity, sensitivity, and commitment of salinity-mediated carbonic anhydrase induction in the gills of two euryhaline species of decapod crustaceans.

Two euryhaline species of decapod crustaceans, Carcinus maenas and Callinectes sapidus, were subjected to a series of acute low-salinity challenges, and changes in carbonic anhydrase (CA) activity in the gills were monitored in order to characterize the nature of salinity-sensitive CA induction. CA activity is uniformly low in all gills of both species at high salinity, but at a critical salinity of 27 ppt, CA induction occurs in the posterior, ion-transporting gills, with CA activity approximately doubling. This salinity occurs right at, or slightly above, the point at which these species make the transition from osmoconformity to osmoregulation. The regulatory mechanism that controls the levels of CA expression after the initial induction has occurred is also very sensitive. Changes in CA activity occur in response to changes in salinity as small as 20 milliosmoles. CA induction only occurs after a critical minimum amount of time of exposure to low salinity (48-72 hr in C. maenas and 12 hr in C. sapidus), but once induction is begun, it continues regardless of subsequent salinity changes. The timing is most likely due to the time it takes for changes in gene expression and resultant increases in CA mRNA to occur in response to low-salinity exposure, and the delay in CA induction could be an adaptation to avoid making metabolically expensive responses to potentially short-term environmental changes. Copyright 2005 Wiley-Liss, Inc.