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

Abstract

A method is presented for measuring and comparing the total salt losses from different species of brackish-water polychaetes acutely exposed to a standardized osmotic emergency. Nereis (Neanthes) succinea shows a higher salt loss rate than Nereis diversicolor and N. limnicola. These rates are lower in proportion to the degree of salinity lowering tolerated by tnese species in nature. In N. diversicolor, the rate of salt loss is proportional to the concentration gradient between body fluid and medium. 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 possible result of a higher level of salt regulation and a more effective volume regulation by urinary output. The slopes of tne 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 so departs from the "surface rule." N. limnicola (and, to a lesser extent, N. diversicolor) does not seem to change its shape during growth and follows the "surface rule" in respect to rate of salt loss.