A COMPARISON OF SALT LOSS RATE IN THREE SPECIES OF BRACKISH-WATER NEREID POLYCHAETES

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

1. A standardized method is presented for measuring and comparing the total salt loss rates of different species of brackish-water polychaetes acutely exposed to an osmotic emergency situation.

2. Nereis (Neanthes) succinea shows a high salt loss rate in comparison to Nereis diversicolor and its close relative N. limnicola. These rates decrease in proportion to the degree of salinity lowering tolerated by these species in their natural habitats.

3. In N. diversicolor, the rate of salt loss is proportional (under the standard conditions) to the concentration gradient between body fluid and outside medium.

4. In N. limnicola, the tendency of the fresh-water population to have a higher salt loss rate than an estuarine population is seen as a result of a higher level of salt regulation and a more effective volume regulation by urinary output.

5. The slopes of the salt loss curves suggest that N. succinea changes shape during growth in such a way as to lessen the expected decrease in its surface/volume ratio, and thus departs from the "surface rule." N. limnicola (and, to a lesser extent, N. diversicolor) does not seem to change shape during growth and follows the "surface rule" in respect to rate of salt loss.