Cation balance in crustacean haemolymph: relationship to cell membrane potentials and membrane surface charge

Data were compiled on K+, Na+, Mg2+ and Ca2+ in haemolymph of 74 species of Malacostraca and three freshwater species of Anostraca. Mean [K+] is positively correlated with both mean [Na+] and mean [Mg2+]. Mean [Mg2+] and [Na+] are themselves positively correlated. Mean [Ca2+] shows little correlation with the others, except that concentrations of all four cations are low in the Anostraca. The data suggest that haemolymph composition has evolved in such a way as to preserve the transmembrane potential across particular areas of cell membrane, this being determined by constant transport and permeability properties for K+, Na+ and Ca2+, and by superficial fixed negative charges of constant density (about 1 e/nm2 on the outer surface)