The Osmotic and Ionic Regulation of Asellus Aquaticus (l.) By

The isopod, Asellus, has been reported from a wide range of conditions in fresh and brackish water. One species, A. aquaticus, has a natural salinity range from fresh water (0-15 m-equiv./l. Na; Moon, 1957) to the Baltic (103 mM./l. NaCl; Valikangas, 1933). Thienemann (1913) has also found it occurring in Westphalian brine pools at a concentration of 25-3 % salt (equivalent to 434 mM./l. NaCl). A natural salinity range as extensive as this is unusual in freshwater Crustacea. Heuts (1943) has investigated the salinity tolerance of Asellus and finds that the highest concentration of diluted sea water in which the animal survives for as long as 24 hr. is equivalent to 300 mM./l. NaCl. This level is considerably below that of the Westphalian brine pools, and the present experiments were undertaken to determine whether the animals could be acclimatized to higher concentrations by gradually raising the concentration of the medium over a long period. Details are also given of the permeability of Asellus to salts and water, and the regulation of the concentration of ions in the haemolymph has been studied under various conditions.