Water and chloride fluxes in estuarine nereid polychaetes

Abstract

1. 1. Fluxes of chloride and water in Nereis succinea and N. vexillosa have been studied by measuring changes in weight and body chloride concentration following the sudden osmotic stress of transfer from one salinity to another. 2. 2. When animals are transferred within the salinity range of osmotic conformity, there is an immediate change in the body chloride concentration due both to a rapid flux of water and to a rapid flux of chloride, in the directions of the imposed concentration gradients. In the range of osmotic regulation, chloride flux is delayed for over an hour. 3. 3. Using the radio-isotope tracer Cl 36, chloride exchange has been studied in N. limnicola, N. succinea and N. vexillosa adapted to various salinities. 4. 4. In N. succinea and N. vexillosa chloride exchange is directly proportional to the external chloride concentration over the entire salinity range. The total chloride exchanged is independent of the internal and external chloride concentrations in high salinities, but is much reduced in external concentrations lower than 200 mM Cl/l. At all salinities chloride exchange is lower in N. limnicola than in the other two species, and in low salinities there was no chloride exchange at all in 4 hr. 5. 5. These results do not support the hypothesis that active uptake of salts from the medium across the body surface is a major factor in the maintenance of hyperosmotic body fluids in low salinities. They indicate that there may be a reduction of body surface permeability to salts in low salinities, and that N. limnicola is either less permeable to salts than N. succinea and N. vexillosa, or that its permeable surface area is relatively less. 6. 6. These brackish-water nereids may approach semi-permeability as an adaptation to low salinities. 7. 7. It is suggested that the source of salts may be the food, that salts may be conserved by the production of a hypo-osmotic urine, and that the three species differ in their abilities to produce hypo-osmotic urine.