Mechanisms of acid-base regulation in seawater-acclimated green crabs (Carcinus maenas).
The present study investigated acid-base regulatory mechanisms in seawater-acclimated green crabs (Carcinus maenas (L., 1758)). In full-strength seawater, green crabs are osmoconformers so that the majority of the observed responses were attributed to ion fluxes based on acid-base compensatory responses alone. Similar to observations in brackish-water-acclimated C. maenas, seawater-acclimated green crabs exposed to hypercapnia rapidly accumulated HCO3- in their hemolymph, compensating for the respiratory acidosis caused by excess hemolymph pCO2. A full recovery from the decreased hemolymph pH after 48 h, however, was not observed. Gill perfusion experiments on anterior gill No. 5 indicated the involvement of all investigated genes (i.e., bicarbonate transporters, V-(H+)-ATPase, Na+/K+-ATPase, K+-channels, Na+/H+-exchanger, and carbonic anhydrase) in the excretion of acid-base equivalents. The most significant effects were observed when targeting a potentially cytoplasmic and (or) basolaterally localized V-(H+)-ATPase, as well as potentially basolaterally localized bicarbonate transporter (likely a Na+/HCO3--cotransporter). In both cases, H+ accumulated in the hemolymph and CO2 excretion across the gill epithelium was significantly reduced or even reversed when blocking bicarbonate transporters. Based on the findings in this study, a working model for acid-base regulatory mechanisms and their link to ammonia excretion in the gill epithelium of C. maenas has been developed.