RESPONSES OF AN ESTUARINE POPULATION OF THE POLYCHAETE NEREIS LIMNICOLA TO OSMOTIC STRESS

Abstract

1. The ecology of an estuarine population of the euryhaline nereid polychaete Nereis limnicola is briefly described. Seasonally this population is exposed to salinities varying from nearly fresh water to at least 85% sea water. Tidal variation is minimal.2. The pattern of chloride regulation in the body fluids is similar to that described for four other nereid species: osmotic and ionic conformity at salinities above 30% sea water; and hyperosmotic regulation in lower salinities, down to a critical low salinity at which hyperosmotic regulation begins to break down (about 1-3% sea water). There is little difference in this pattern between fresh-water and estuarine populations of N. limnicola.3. After a transfer from one salinity to another, both salinities within the range of osmotic conformity, there is an immediate change in the total body chloride of the worms, due both to a movement of water and to a movement of chloride, in the directions of the imposed concentration gradients. When worms are adapted to low salinities in the range of hyperosmotic regulation, rates of chloride movements are much reduced after a transfer.4. Complete adaptation to a new salinity after a transfer may take more than two days.5. Rates of chloride and water movement are lower in N. limnicola than in several less euryhaline species.6. The results suggest that N. limnicola is less permeable to salts, and perhaps also to water, than the less euryhaline species, and that permeability may be further reduced after adaptation to low salinities. These features are considered to be of significance in the physiological adaptation of N.limnicola to life in low salinities and fresh water, in that diffusional salt loss is much reduced.7. The results do not support the hypothesis that active transport of salts from the medium across the body surface is important in the maintenance of hyperosmotic body fluids.8. The fresh-water population of N. limnicola previously studied may have: (a) A slightly higher level of hyperosmotic regulation. (b) A slightly lower critical low salinity. (c) A somewhat lower permeability of the body surface to salts. However, differences in osmotic responses among the several brackish-and fresh-water populations of N. limnicola are slight when compared with the osmotic responses of less euryhaline species.