Chloride Regulation at Low Salinities by Nereis Diversicolor (Annelida, Polychaeta): II. Water Fluxes and Apparent Permeability to Water

Nereis diversicolor were collected and maintained as described in the first part of this study (Smith, 1970a). For determination of permeability to water D₂O (Bio-Rad Laboratories, Richmond, California, 95 moles%) was employed as a 5% solution in several dilutions of sea water (SW) made up with local pond water (PW). The use of D₂0 followed closely the method employed in previous studies on nereid polychaetes (Smith, 1964) and on the crabs Rhithropanopeus (Smith, 1967) and Carcinus (Smith, 1970c).

Worms used in this study were selected to give a range in weight from c. 100 mg to c. 500 mg, in an effort to permit calculation of the relationship of uptake to body weight. Only worms of normal body form were used; worms with extensive caudal amputations, even though fully healed, were discarded. N. diversicolor from two sources were used. One group of worms, adapted and tested in salinities ranging from PW to 50% SW, came from the Wansbeck estuary, representing a habitat of salinity below 50% SW and the same population that furnished material for the studies reported in the previous paper and by Oglesby (1970). A second group of worms, adapted and tested at salinities from 25 to 94% SW, was collected from the Black Middens, a fairly clean marine-dominated rocky and sandy area at the mouth of the River Tyne. Tests were carried out at 18–19°C, except for two groups of Wansbeck estuary worms that were tested at 22–23°C, and yielded slightly elevated water fluxes.

In several studies previously carried out on nereid polychaetes and crabs (Smith, 1964, 1967, 1970c), influx of D₂O had been found to be weight-specific. However, in the present study, despite the use of worms of a wide weight range (100–500 mg) the scatter did not permit very consistent values of (b – 1) to be obtained when the data were treated by the usual equation: Uptake = a weight(^b−1), as is evident from the points in Fig. 1. Since the mean weights of worms in the several groups were similar and the values for (b – 1) quite low, it seemed better not to extrapolate to unit weight, but simply to use the% concentration values directly.

Percentage concentration values are given in Fig. 2 and Table 1, together with standard deviations, standard errors, hourly water-exchange fractions, and the probabilities (by t-test) that adjacent mean values differ by chance. The overall picture is that permeability to water (as D₂O) is significantly less at low salinities, especially in PW and 1.8% SW. The trend is evident when worms from Wansbeck estuary and from Black Middens are considered separately, although no explanation can be offered for the apparent tendency of the latter group of worms to show lower permeability than their Wansbeck counterparts in 50% and 25% SW. The hourly water-exchange fraction (K) drops from 12·53 in 95·5% SW to 5·88–6·93 as PW is approached, a decrease of nearly 50%.

Two groups of Wansbeck worms had to be tested on a hot day when laboratory temperature rose to 22–23°C. There is a noticeable elevation of the% concentration attained in these groups of worms, indicating the need for caution when comparing water-exchange rates of animals tested under uncontrolled conditions.

The lessened osmotic uptake of water reported for Nereis diversicolor by Jorgensen & Dales (1957) has thus been shown to have a physical basis in a lowered diffusional permeability to water (as D₂O). However, it would be better to use the term ‘apparent permeability’ in this context, since there is no evidence to differentiate a true perm-ability change from a change in the exchange rate of D₂O brought about by, e.g. circulatory changes in the animal. With this reservation in mind, the K values presented here should permit a preliminary estimate of the net uptake of water entering by diffusion and available for release as urine, at different salinities, provided that certain assumptions may be accepted for purposes of the calculations. Unfortunately data on the osmotic concentration of coelomic fluid were not obtained in the present study, but an approximation may be made by correcting the data available for chloride concentration of N. diversicolor of the Wansbeck estuary population (Oglesby 1970; Smith 1970a). Oglesby (1969), in summarizing several studies by different authors, concluded that osmotic concentration of coelomic fluid in N. diversicolor, if expressed as equivalent NaCl, is higher at all salinities than the measured molarities of Na⁺ and Cl⁻ would indicate. Hohendorf (1963) found that inorganic ions accounted for all but 7% of the total osmotic concentration of the coelomic fluid of N. diversicolor. In a study subsequent to the present one, it was found (Smith, 1970b) in a different population of this species, that the chloride molarity of coelomic fluid, reckoned as NaCl, accounts for 93·3% of the osmotic concentration as equivalent NaCl. Thus, for purposes of calculation, the osmotic concentration of the coelomic fluid of N. diversicolor is taken as equivalent to the measured chloride molarity × 2 × (100/93). The resultant osmolarity of coelomic fluid is expressed in Table 2 (line 6) as equivalent SW, with 100% SW considered to have an osmolarity of 1·0. Since this arbitrary correction of chloride concentrations of coelomic fluid gives values slightly below those of 75 and 94·5% SW, and since Hohendorf (1963) has shown that the body fluid of N. diversicolor is always hyperosmotic to the medium, even in SW, the values of coelomic fluid in 75 and 94·5% SW have been further raised to o-oi osmole above the medium. Hourly water exchange fractions used in Table 2 (line 7) were obtained in this study, using average values where the curves from Wansbeck worms and Black Middens worms overlap. A calculation, e.g. for a worm in 50% SW, may be made on the assumptions stated and yields the data for net water uptake as% of body weight/h (Table 2, line 14). The calculation is made as follows: The osmolarity of 50% SW is 0·50, the mole fraction of water is 55·56/(55·56+ 0·50) = 0·9911 (line 8); for a worm in 50% SW, the osmolarity of coelomic fluid is estimated as 0-52 (see above), the mole fraction of water in coelomic fluid is 55·56/(55·56+ 0·52) = 0·9907 (line 9); the mole fraction difference is 0·0004 (line 10). This difference accounts for the net flux of water (excess of water diffusing in over water diffusing out), and this net flux represents 0·0004/0·9911 = 0·0404% of the influx in any given time (line 13). Taking the water content of N. diversicolor in 50% SW as 85·8% (Oglesby, 1970), and assuming that all water is exchangeable, the water influx is K × 85·8 % = 909·5 % of body weight/h (line 12). Net flux is 0·0404% of 909·5 = 0·37% of body weight/h (line 14). This is the net amount of water entering by diffusion and available for release as urine. The results of this and similar calculations are given in Table 2 and the potential urine volumes are plotted in Fig. 3.

Table 2 indicates that net loss of water as urine has a maximum (2·78% of body weight/h) in 10% SW, and is reduced (2·10% of body weight) in nearly fresh water. As expected, urine volumes are very low in 50% SW and above, in which the worm is nearly iso-osmotic with the medium. The overall results support the conclusion that urine volume in FW is reduced by about 25% below the peak volume in 10% SW, and that this volume reduction is a consequence of a lowered apparent permeability to water.

Jørgensen & Dales (loc. cit.) give estimates of urine volumes in N. diversicolor based on osmotic experiments and weight changes: in 21% SW, 5·4% of body weight/h; in 12% SW, 9·3%/h; in FW, c. 4%/h. These estimates are over twice the present estimates based on exchange of D₂O, but may be said to be in general agreement.

A similar discrepancy exists between direct estimates of the urine volume of the crab Carcinus and the net flux of water based on exchange of D₂O (Smith, 1970c). It would not be surprising if the true urine volume of N. diversicolor proves to be above the present estimates, and by direct observation (Smith, 1970b) the urine flow of N. diversicolor may be seen to be copious.

When the volumes of urine as estimated in the present study are compared with the calculated losses of chloride via the urine (Smith, 1970a), it may reasonably be concluded that the urine is hypotonic at low salinities. Thus, if we calculate the loss of chloride that would occur in urine isotonic to coelomic fluid (Table 2, line 15), and compare these values with the chloride efflux via the urine, calculated from exchange of ³⁶C1 (line 16), we see that chloride loss in isotonic urine agrees well with calculated chloride efflux via the urine in 50% SW and higher, where urine is undoubtedly isotonic. But in the osmoregulatory range, in 25% SW and below, isotonic urine would account for far more than the calculated chloride efflux, indeed, for over half the total efflux (line 17). Hence, it is concluded that the urine, in salinities below 25% SW, is probably hypotonic in chloride to the coelomic fluid.

Confirmation or refutation of the last hypothesis would require determination at least of the chloride concentration of urine. This has not yet been done, but in the following paper (Smith, 1970b) the hypo-osmoticity of the urine will be directly demonstrated. It thus appears evident that reduction of urine volume and hypotonicity of urine are both features of the chloride regulation of N. diversicolor.

1. By studies of D₂0 exchange the apparent permeability of Nereis diversicolor to water has been shown to vary in response to changes of salinity.

2. The hourly water-exchange fraction drops from over 12 in SW nearly to 6 in fresh water.

3. Calculation of the net flux of water indicates potential urine volumes of 0·3% of body weight/h in SW, a maximum of 2·8% in 10% SW, and a reduction to 2·1% in fresh water.

4. It is calculated that, if such urine volumes are produced, the urine is probably hypotonic in chloride to the coelomic fluid at salinities of 25% SW and lower.