Chloride regulation at low salinities by Nereis diversicolor (Annelida, Polychaeta). II. Water fluxes and apparent permeability to water.

Evaluation of the chloride exchanges of Nereis diversicolor in the steady state has shown that this worm 'cuts its losses', i.e. reduces the fluxes, of chloride at very low salinities (Smith, 1970a), and that this reduction of chloride loss takes place mainly in the urinary component of the efflux, as has also been suggested by previous authors. Jorgensen & Dales (1957) measured the rates of osmotic gain or loss of water, and concluded that at low salinities N. diversicolor could reduce its permeability to water by over 60%. Assuming a comparable reduction in permeability to chloride, they calculated that the observed reduction of chloride efflux could be accounted for by reduction of urine volume, without the necessity of postulating the production of a urine rendered hypotonic to the body fluids by recovery of solutes by the nephridia. Potts & Parry (1964), on the other hand, in their review of the previous authors' work, felt unable to accept the evidence for a reduction of permeability to water and, on the assumption of no permeability changes, drew up a balance sheet in support of the hypothesis that reduction of chloride loss was achieved in N. diversicolor by recovery of ions from a consequently hypotonic urine. These views are not mutually exclusive, and the problem is not to choose between them but to obtain evidence to indicate the extent to which each postulated mechanism may be operative. In the previous paper (Smith, 1970a), evidence was presented compatible with the concept that the urine of N. diversicolor may be hypotonic in chloride to the coelomic fluid at low salinities, although this evidence was indirect and not conclusive. In a subsequent paper (Smith, 1970&) direct evidence will be presented that the urine of N. diversicolor is hypoosmotic. In order fully to evaluate the hypothesis of Jorgensen & Dales it would be necessary to measure both permeability to water and urine volume. So far, it has not been possible to measure urine volume, but data on permeability to water have been obtained, and an estimate of water fluxes attempted.