ABSTRACT
Control of the body water budget is an important aspect of the physiology of all organisms but is particularly important in forms inhabiting fluctuating salinities. The most commonly used marker for measurement of the permeability of whole organisms is tritiated water (THO). Animals are immersed in tritiated water long enough for sufficient THO to be taken up, preferably so that they are loaded to a steady state. Animals experiencing salinity changes would benefit from the ability to restrict the passage of water and ions when large osmotic gradients are present betweeen the body fluids and the external medium. The measurement of water permeability in various organisms has proved an extremely useful tool in interpreting the “whole animal” physiology, especially as related to the ecology of the species in question. The techniques involve assumptions and are open to some criticism, however it is felt that for invertebrate organisms the methods offer an important methodology which should not be ignored.
