Sodium absorption coupled to ammonia excretion in osmoconforming marine invertebrates

Evidence was sought for the presence of amiloride-sensitive Na-NH4 and NaH exchange systems in four species of marine osmoconformers. When the crabs Cancer antennarius and Petrolisthes cinctipes were in seawater (SW), amiloride (10(-4) M) reduced NH3 efflux by approximately 33 and 60%. Inhibition was reversible on removing the amiloride. In dilute (60%) SW, inhibition of NH3 output by C. antennarius was even greater than in SW. Na+ uptake by P. cinctipes was reduced by approximately 20% in the presence of amiloride (the measurement was not made on C. antennarius). Amiloride had no effect on proton efflux in either crab. The data suggest that Na-NH4 exchange occurs in these animals but that Na-H exchange does not occur. The inhibitor had no effect on NH+4 excretion by the polychaete worm Nephtys caecoides and the mussel Mytilus californianus; it was also without effect on proton output by the worm. The data suggest that the exchange systems are absent from these animals. Implications of these observations for the evolution of such cation exchange systems are discussed.