The influence of salinity on the distribution of two corixid species

Cenocorixa bifida (Hungerford) and Cenocorixa expleta (Hungerford) have different distributions which appear to be correlated with lake water sodium, chloride and osmotic pressure. C. bifida occurs in the lower and C. expleta in the higher salinities. An attempt was made to study the salinity relations of these two species. In low salinities both species maintain blood osmotic pressure, sodium, potassium and chloride well above those of the water but C. expleta tends to maintain lower levels than C. bifida. In high salinities both species are iso-osmotic to the water but C. expleta maintains blood sodium and potassium hypotonic to the water. C. bifida blood sodium and potassium are never hypotonic to the water. C. expleta contains more body water in all salinities. Both species appear to drink regularly in all salinities and mouth closure results in lowered blood sodium and chloride in hypo-osmotic media. The rostral ridges and third leg femoral-tibial intersegmental membranes are the only areas of cuticle which allowed penetration of silver ions. The abolishment of the osmotic gradient by adding sucrose to hypo-osmotic water appears to impair entry of water, sodium and chloride into C. bifida but not into C. expleta. C. bifida does not occur in water to which it is iso-osmotic as does C. expleta. This study suggests physiological adaptation of these species to different salinities.Science, Faculty ofZoology, Department ofGraduat