OSMOTIC REGULATION and THE FAUNAS OF INLAND WATERS
Authors: BEADLE L. C. 
Published: January 2008

In order to colonize fresh waters, marine animals must maintain a total concentration of the body fluids above that of the external medium. This review is concerned with the probable course by which this independence was achieved during the evolutionary history of fresh‐water animals, so far as can be judged from ecological and physiological research. Many marine animals can endure a considerable dilution of the sea water without regulating their body fluids, the tissues being capable of normal function so long as the rate is not too great at which the body‐fluid concentration drops. Nereis diversicolor shows the beginnings of osmoregulation: a slight body fluid hypertonicity can be maintained when the external medium is dilute, and, when that medium fluctuates, the changes are sufficiently damped before they reach the tissues for the functioning of the latter to be unimpaired. By these means such animals have invaded brackish waters, and penetration of fresh waters may have been accomplished by some other groups through the immediate lowering of the blood concentration and subsequent development of a regulatory mechanism. On the other hand, some Crustacea (Erio‐cheir sinensis and Telphusa fluviatilis) have been able to invade brackish water thanks to a different regulating mechanism which prevents much diminution in body‐fluid concentration. Active absorption of salt from the medium through some part of the body surface (e.g. the gills) is an important component of this mechanism, though uptake from food via the gut must also contribute. The excretory organs, however, do not play a part, since the urine is isotonic with the blood. Most fresh‐water animals have a low blood concentration and the regulatory mechanism is assisted by renal reabsorption. Many are capable of actively absorbing ions from fresh water when the blood is artificially diluted, but it is not certain how far this is an important component of the mechanism in nature. On the other hand, several have remarkable powers of retaining salts, due no doubt to the high impermeability of most of the body surface. But it is possible that the organs concerned in absorption may also, by the same action, prevent loss of salt in very dilute media. Some animals however, are continuously dependent upon food for maintaining the salt level of the blood. The lower the blood concentration of a fresh‐water animal the lower the concentration of brackish water to which it can be adapted. This is no doubt partly due to irreversible adaptation of the tissues to a dilute body fluid, but the upper salinity tolerance limit may also be determined in part by the inability to obtain enough water for excretion from anything but a hypotonic medium. In the laboratory some marine animals have been adapted to fresh water and some fresh‐water species to sea water by altering the concentration of the medium extremely slowly. It seems that under such treatment the extension of the tolerance range is due to the development of a new mechanism not originally functional. Inland saline waters of low concentration have been invaded by some freshwater animals which cannot maintain their blood hypotonic to the medium, and, where the geographical situation permits, by some brackish water species. Adaptation to highly saline waters, however, requires a mechanism for hypotonic regulation, and all animals living in such waters are of fresh‐water origin. They can maintain their blood concentration at a level typical of fresh‐water animals. Some brackish water species are also capable of hypotonic regulation, but have not invaded waters of high salinity, probably because, unlike most fresh‐water animals, they do not produce stages resistant to the drought which is of common occurrence with inland saline waters.