BOOTH, C.E.; HENRY, R.P.; Eastern Connecticut State Univ., Willimantic; Auburn University, Alabama: The role of branchial H ion excretion in acid-base regulation in euryhaline and stenohaline brachyuran crabs.

Many aquatic animals are thought to use electroneutral ion exchanges (e.g., Na/H , Cl/ bicarbonate) across the gills or skin to regulate internal acid-base balance. Our understanding of this process in decapod crustaceans is based on animals ionoregulating in FW or dilute SW. Little is known about mechanisms of acid-base regulation via the gills in SW-acclimated decapods in which active Na and Cl ion uptake is not required. We examined the abilities of five crab species of varying ionoregulatory capacity to excrete H ions during and after 20 min. of enforced exercise in 33-35 ppt SW. H ion excretion rate was highest during exercise in the strongly euryhaline Callinectes sapidus (4.1 meq/kg/h ) and Carcinus maenas (2.2 meq/kg/h), but peaked early in recovery in the moderately euryhaline Cancer irroratus (1.7 meq/kg/h). In contrast, the stenohaline Libinia emarginata and Chionoecetes opilio did not excrete H ions during exercise, but did excrete H ions (0.5-1.0 meq/kg./h) during the recovery period. Preliminary data from C. opilio suggest that the urine is not an important route for H ion excretion. Thus, stenohaline crabs with limited or no capacity for ionoregulation are capable of branchial H ion excretion. There was no clear correlation between hemolymph lactate loads and H ion excretion rates among the five species. However, H ion excretion was correlated with a hemolymph metabolic H ion load that was smaller than the lactate load, indicating the removal of metabolic H ions from the hemolymph. It is not known if these species use the same mechanism(s) for excreting H ions. However, it appears that the regulation of H ion excretion during exercise and recovery differs between stenohaline and euryhaline species.