D₂O UPTAKE RATE IN TWO BRACKISH-WATER NEREID POLYCHAETES

¹ Department of Zoology, University of California, Berkeley 4, California

1. A simplified method for the estimation of heavy water (D₂O) uptake in polychaetes is presented.

2. It is shown that, although Nereis liminicola shows a smaller salt-loss and less net osmotic water uptake in a salt-free medium than does N. succinea, both species show an equal D₂O influx at a body weight of ca. 100 mg.

3. As in the case of salt-loss, N. limnicola follows the "surface rule" in its D₂O uptake; N. succinea does not.

4. No increased uptake of D₂O has been demonstrated in lower salinities in which there is presumed to be an osmotic inflow of water. This is consistent with the possibility that water-permeability is less at low environmental salinities.

5. A preliminary analysis of the osmotic and diffusional influxes of water in these two species suggests that their different osmoregulatory capabilities may be related to urine output (greater in N. limnicola), to a difference in integumental "pore size" (smaller in N. limnicola), or to some combination of these factors.