Hypo-osmotic urine in Nereis diversicolor.

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ABSTRACT Although Beadle as early as 1937 suggested that the urine of Nereis diversicolor O. F. Müller might be of lower concentration than the body fluids when this worm is hyperregulating in low salinities, there has been no direct measurement of urine concentration to support this reasonable idea. Jørgensen & Dales (1957) studied osmotic water uptake and loss in N. diversicolor and calculated that this animal reduces its permeability to water by over 60% when in fresh water. On the assumption that chloride permeability was comparably reduced, they calculated that the observed rate of chloride exchange and the level of chloride concentration in the coelomic fluid were compatible with the production of a small volume of urine isotonic in chloride to the body fluid. However, Potts & Parry (1964) in reviewing this matter regarded the evidence of Jørgensen & Dales for reduced permeability to water as not well founded and, without presenting new data but by making the assumption that no changes in permeability occurred, drew up a balance-sheet for chloride exchanges which led them to postulate a reduction of chloride loss in low salinities by production of an hypotonic urine. Although there is no incompatibility between reduction of chloride loss by reduction of permeability to water (and hence urine volume) and by resorption of chloride from a consequently hypotonic urine, the fact that different authors can postulate these as alternative mechanisms on the basis of the same data makes it evident that measurements of urine concentration and volume would be useful in further attempts at analysis of the regulation of chloride and water in N. diversicolor. In a preceding paper (Smith, 1970a) there has been shown a correlation between chloride loss and weight loss compatible with the production of hypotonic urine by this species at low salinities, but this evidence is not conclusive. It has further been shown (Smith, 1970b) that N. diversicolor reduces its apparent permeability to water (as D2O) in low salinities, which seems to confirm the view of Jørgensen & Dales (1957), but the urine volumes calculated were such as to make it probable that chloride resorption and production of a urine hypotonic in chloride to the body fluid can also take place.