Osmotic regulation in shrimp

Osmoregulatory capabilities were compared in the stenohaline shrimps, Sicyonia brevirostris Stimpson and S_. dorsalis Kingsley, the freshwater shrimps, Macrobrachium ohione Smith and M. rosenbergii de Man, and the euryhaline shrimps, Penaeus aztecus, Ives, P. duorarum, Burkenroad, P. setiferus (L), P. stylirostris Stimpson, and P. vannamei Boone. The sicyonid shrimp are osmotic conformers. The freshwater shrimp are capable of hyperosmotic regulation, but hypoosmotic regulation is either limited or absent. Penaeid shrimp are capable of both hyperosmotic and hypo- osmotic regulation. Urine in P. setiferus and P. stylirostris is isosmotic to the hemolymph. The primary mechanisms of osmotic regulation in penaeid shrimp are sodium and chloride uptake at low salinities and sodium and chloride excretion at high salinities. Measurements of electrical potentials and fluxes of sodium and chloride are consistent with the hypothesis that, in dilute solutions, the electrogenic active transport of chloride produces a negative (relative to the external solution) electrical gradient that passively maintains sodium balance, and that, in concentrated solutions, electrogenic sodium efflux produces a negative electrical gradient that passively maintains chloride balance.