Chloride regulation at low salinities by Nereis diversicolor (Annelida, Polychaeta). I. Uptake and exchanges of chloride.

The ability of the common brackish water polychaete worm Nereis diversicolor O. F. Miiller to regulate osmotically and ionically has been well established (Schlieper, 1929; Beadle, 1937; Smith, 1955; Hohendorf, 1963; Oglesby, 1970; and others). Jergensen & Dales (1957) demonstrated an uptake of chloride against the concentration gradient at low salinities, with a maximum rate of exchange under steady-state conditions at external chloride concentrations of 65-120 min/l, and a drop in uptake rate to about one-third of that maximum in fresh water (Cl = i^min/l). Fretter (1955) demonstrated a comparable uptake of sodium against the concentration gradient in this species, but without the maximum shown in the chloride exchanges. Evidence that N. diversicolor has a considerably lower permeability to chloride than has the more marine Nereis virens was given by Jergensen & Dales (loc. cit.) while Fretter (loc cit.) demonstrated that iV. diversicolor is comparably lower in permeability to sodium than the marine Perinereis cultrifera. It therefore appears that a lowering of integumental permeability to ions is part of the adaptation of N. diversicolor for life in low salinities. However, whether permeability to water is similarly lowered is a matter of less certainty. Smith (1963) showed that the tendency to lose salts in an acute short exposure to distilled water is markedly less in N. diversicolor than in the more marine Nereis sucdnea. Nereis limnicola, a species perhaps more tolerant of low salinities than N. diversicolor, showed even less tendency to lose salts in the above acute test. But a study of the uptake of DSO (Smith, 1964) showed that N. sucdnea and N. limnicola had essentially the same permeability to inward diffusion of water, indicating that permeability to ions may vary independently of that to water. Jargensen & Dales (1957) presented evidence that N. diversicolor in fresh water decreases its permeability to water to about one-third (< 40 %) of that characteristic of higher salinities, and they considered it 'likely' that a comparable decrease in permeability to chloride occurred. On the basis of the observed decrease in permeability to water and the assumed decrease in permeability to chloride, these authors considered that the observed reduction of chloride exchanges in fresh water was consistent with the view that iV. diversicolor produced a urine isotonic to the body fluid. However, Potts & Parry (1964, pp. 145-155), reviewing the above paper, considered that the evidence