The Regulation of the Internal Sodium Concentration of Asellus Aquaticus in the Absence of Sodium Chloride in the Medium

Abstract

1. The freshwater isopod Asellus aquaticus has been observed to survive in continuously flowing de-ionized water for up to 16 days. 2. The haemolymph osmotic pressure, and the concentrations of sodium and chloride fall rapidly to about 65 % of the normal value and then decrease very slowly until death. 3. Animals loaded to a steady state in ²²Na and then washed with de-ionized water lose tracer at a rate which only decreases slightly between the beginning of washing out and the death of the animal. 4. It is suggested that a movement of water from haemolymph to cells as the haemolymph concentration falls may partially account for the maintenance of the haemolymph concentration despite the steady ion loss. 5. Animals loaded for only a short period in tracer before washing out lose about 50-60% of their ²²Na after a few hours in de-ionized water and then show little further loss for a long period. 6. The differences in the tracer loss curves following long-term and short-term loading have been interpreted as indicating that a considerable proportion of the total sodium of Asellus is not in the haemolymph.