Hemolymph composition, gene expressions in the gills, and thus the survival of euryhaline crabs are controlled by ambient minor cations according to osmotic condition-dependent manner.

Na and Cl are the most abundant dissolved ions in seawater, constituting ~ 85% of total ions. They significantly affect the osmolality of body fluids of marine invertebrates. Seawater also contains minor ions such as Mg, Ca, K, and SO , but their effects on marine organisms are unclear. This study analyzed the effects of Mg, Ca, and K (ambient minor cations) on survival, hemolymph ionic composition, and gene expression in the gills of three euryhaline crabs: , , and . Ambient minor cations were required for survival of and under isosmotic conditions with seawater. The ambient minor cations also affected the osmolality and ionic composition of hemolymph by regulating expressions of specific genes in the gills required for Na uptake, such as Na/K ATPase, cytoplasmic carbonic anhydrase, and Na/H exchanger. Administration of carbonic anhydrase and Na/H exchanger inhibitors increased the survival rate even if ambient minor cations did not exist. In contrast, under hypo-osmotic conditions, ambient minor cations had different effects on crabs, a lethal effect on , and an increase of the hemolymph K concentration in and . It is thus concluded that the effects of ambient minor cations are osmolality-dependent. In contrast, in , the hemolymph ionic composition and survival rate were hardly affected by ambient minor cations, probably reflecting the habitat of this species. These results strongly indicated that is less susceptive to ambient minor cations compared to and .