Osmoregulation in two aquatic oligochaetes from habitats with different salinity and comparison to other annelids

The osmoregulatory capacity of two oligochaete species, Enchytraeus albidus Henle, 1837, and Heterochaeta costata (Claparede, 1863), was investigated by direct measurements of the osmolality of the coelomic fluid. Terrestrial and marine (28 parts per thousand S) populations of Enchytraeus albidus and a brackish water population (14 parts per thousand S) of H. costata were used in the study. The range of salinity acclimation investigated was 0-40 parts per thousand. The response to osmotic stress was measured (a) after a long-term maintenance(>14 days) in various salinities (E. albidus only), and (b) after a hyperosmotic shock as a short-term time-course sequence. The rate of water loss following a hyperosmotic shock was measured for E. albidus. Long-term acclimation. E. albidus maintained a hyperosmotic coelomic fluid over all salinities tested. In low salinities the osmolality of the coelomic fluid of the marine population was significantly higher than that of the terrestrial population. Possible genetic discrepancies or long-term acclimation may account for this difference. The coelomic fluid of H. costata was hyperosmotic at 15 parts per thousand S and isoosmotic at 30 parts per thousand S. Short-term acclimation (hyperosmotic shock). Both species investigated, kept at 15 parts per thousand S and then exposed to a salinity of 30 parts per thousand, showed fast responses: within the first two hours the internal concentrations were adjusted to the new external condition with only small subsequent changes. Regulation of the body-water content after an exposure to a hyperosmotic shock was much slower: individuals of terrestrial E. albidus, acclimated for two weeks to either 0 parts per thousand or 15 parts per thousand S, had the same water content; hence, they showed a 100% regulation. However, after exposure to 30 parts per thousand S, a 100% regulation was still not attained 4 days after the hyperosmotic shock. Enchytraeus albidus is capable of actively reducing water loss following the hyperosmotic shock: the observed loss of water was only 40% of that expected for a passive osmotic flow. The observed reactions are compared with those in other annelids. It seems that an active transport of ions combined with a changeable permeability of the body wall play a major role in the regulation of body fluids.