The Osmotic and Ionic Regulation of Asellus Aquaticus (L.)

The isopod, Asellus, has been reported from a wide range of conditions in fresh and brackish water. One species, A. aquaticus, has a natural salinity range from fresh water (0·15 m-equiv./l. Na; Moon, 1957) to the Baltic (103 mM./l. NaCl; Välikangas, 1933). Thienemann (1913) has also found it occurring in Westphalian brine pools at a concentration of 25·3% salt (equivalent to 434 mM./l. NaCl). A natural salinity range as extensive as this is unusual in freshwater Crustacea. Heuts (1943) has investigated the salinity tolerance of Asellus and finds that the highest concentration of diluted sea water in which the animal survives for as long as 24 hr. is equivalent to 300 mM./l. NaCl. This level is considerably below that of the Westphalian brine pools, and the present experiments were undertaken to determine whether the animals could be acclimatized to higher concentrations by gradually raising the concentration of the medium over a long period. Details are also given of the permeability of Asellus to salts and water, and the regulation of the concentration of ions in the haemolymph has been studied under various conditions.

A. aquaticus was collected from a deep drainage ditch on Coe Fen, Cambridge. The concentration of sodium in the water of this ditch varied from 1 to 2 m-equiv./l. over a period of 3 years. In the laboratory the animals were kept either in their natural medium or in tap water, and were fed on alder or willow leaves. They survived well and were apparently normal under these conditions.

Haemolymph samples were obtained from medium to large (8-15 mm.) males by the following procedure. The animal was first dried carefully with filter-paper. A fine Pyrex pipette was then introduced into the rear of the heart by pushing it forward through the intersegmental membrane separating the tergum of the last thoracic segment from the abdomen. Haemolymph flowed readily into the pipette and could be assisted if necessary by gentle suction. 2-5 μl. of haemolymph could be obtained from a single specimen. The haemolymph was blown out of the pipette under the surface of liquid paraffin in a laquered watch-glass. The haemolymph does not clot nor does the osmotic pressure alter appreciably over a period of 48 hr. at room temperature (15-20° C.). However, all determinations of osmotic pressure were carried out as soon as possible after sampling, though in a few cases sodium and chloride were not determined until the following day. Where possible all measurements were made on the haemolymph of a single individual, but in most cases the haemolymph of two or three had to be pooled.

Measurements of the osmotic pressure of both haemolymph and media were made by the micro-cryoscopic method of Ramsay & Brown (1955). Samples were measured in duplicate or triplicate and observations were repeatable to within 0-01° C. Results are expressed in terms of the concentration of NaCl (in mM./l.) which would give the same depression of freezing-point making use of the relationship A 1° C. = 285 mM./l. NaCl (Ramsay, 1949). In the range of concentrations used A varies directly as the concentration.

Sodium concentration was estimated by an E.E.L. flame photometer. Glass micro-pipettes were used to deliver a volume of about 1-2 μl. of haemolymph into 2 ml. of de-ionized water in a Polythene container. The sample was mixed by a stream of air bubbles and the reading given by the photometer was compared with that of a similarly treated volume of a known concentration of NaCl. The standard deviation of a number of similar samples was ±2%.

Chloride was determined electrometrically by the first method described by Ramsay, Brown & Croghan (1955). The same pipettes were used for chloride as for sodium determinations and again the values obtained for the haemolymph samples were compared with those obtained for the same volume of a known NaCl standard. Results given by a series of similar samples had a standard deviation of ± 1 %.

The proportion of heavy water in haemolymph samples was estimated cryoscopically after microdistillation.

The freezing-point elevation of D₂O and H₂O mixtures was found to vary directly as the concentration of D₂O.

Samples were measured cryoscopically as already described for osmotic-pressure determinations. The sensitivity of the method is less than for density methods, only 1 part in 200 being detectable, but was adequate for the present determinations.

Animals were placed in NaCl solutions of various concentrations for 7 days. The greatest concentration which an individual could survive for this period ranged from 200 to 270 mM./l. Higher concentrations could be tolerated for shorter times. Since these concentrations do not approach those at which Thienemann (1913) found Asellus occurring naturally, the concentration was increased slowly over a period of several months to see if the animals could be acclimatized to higher salinities.

The animals were placed in diluted sea water (osmotic pressure equivalent to 34 mM./l. NaCl) and provided with food. The concentration of the medium was slowly raised by evaporation and the addition of concentrated sea water. No animals survived a concentration greater than 220 mM./l. A repetition of this experiment gave similar results, the last of the experimental animals dying when the concentration reached 200 mM./l. Many of the young released from the brood pouch of a female during the course of this experiment survived, however, until the concentration reached 250 mM./l.

Determinations were made of the osmotic pressure (O.P.I) of the haemolymph of both freshly caught animals and animals which had been kept in Coe Fen water under as natural conditions as possible in the laboratory. The mean value of O.P.I obtained was 150·5 mM./l. NaCl (n = 121). Most of the determinations were made in the months September-May inclusive. This figure may be compared with other mean values for O.P.I of Asellus given by Parry (1953) and Widmann (1935) of 141 and 251 mM./l. NaCl respectively. Heuts (1943) has criticized Widmann’s results and his own values for O.P.I are about half those given by Widmann.

O.P.1 has also been determined over the salinity range tolerated in nature by Asellus to determine to what extent the haemolymph concentration is regulated. Excluding the special case of its occurrence in the Westphalian brine pools, the natural salinity range is 0·15 m-equiv./l. Na to 103 mM./l. NaCl. O.P.1 near the lower limit of this range was determined by washing the animals with distilled water for a period sufficient to decrease the haemolymph concentration markedly (to 65 % in 48 hr.), and then allowing them to take up salts from dilute solutions of NaCl for a period of 7 days. Table 1 shows that O.P.I is close to the normal freshwater value in a medium with a concentration of 0·06 mM./l. NaCl and at the normal value in solutions containing more than 0·088 mM./l. NaCl. This latter concentration is well below the minimum natural lower limit of 0·15 mM./l., at which therefore a haemolymph concentration of 150 mM./l. is to be expected, O.P.I at 103 mM./l. was determined after acclimatizing animals to this concentration of NaCl for 7 days. After this time O.P.I was close to 170 mM./l.

The rise in the haemolymph concentration over the normal 700-fold range of medium concentration is thus only from 150 to 170 mM./l. or some 15 %. It seems clear that the active uptake mechanisms display very considerable sensitivity in regulating the concentration of the haemolymph.

The relationship between O.P.I and the NaCl concentration of the medium was investigated during acclimatization of the animals to saline solutions.

For short-term acclimatizations animals were placed in NaCl solutions of different concentrations for 6–8 days and the haemolymph then sampled. (It will be shown in a later section that the haemolymph concentration reaches equilibrium in hypertonic media within 24-48 hr.) 0.P.1 rose as the medium concentration was increased. It approached isotonicity with the medium at high concentrations but was still hypertonic at the highest medium concentration compatible with life (Fig. 1). Heuts (1943) obtained similar results.

During the long-term salinity acclimatization experiments, described above, animals were removed at intervals and their haemolymph sampled. These animals had a very slightly lower O.P.I at any given medium concentration than animals only 1 week in media of the same concentration (Fig. 1).

The concentrations of three of the major ionic constituents of the haemolymph are compared in relation to the total osmotic pressure of normal animals in Table 2. Sodium and potassium together account for some 96% of the O.P.I due to cations. Calcium and magnesium have not been determined, but by analogy with other freshwater Crustacea such as Astacus may be expected to make up a considerable proportion of the remaining 4%. The un-ionized organic contribution to O.P.I must therefore be small.

As has been mentioned, it is possible to vary the animal’s total haemolymph concentration by placing it in distilled water or various concentrations of saline. The relationship between Nai, Ch and O.P.1 over the viable range is summarized in Fig. 2. The figure shows that over the whole range most of O.P.1 is due to sodium and chloride. As would be expected if there is a proportion of large organic anions in the haemolymph which are not readily lost from the body, the Ch/o.p.1 ratio shows some slight fall at low values of O.P.1. The ratio Na,/o.p.1 remains constant (Table 3).

Large animals were placed in a 50% solution of D₂O at 18° C. After 25 min. samples of haemolymph were taken and the percentage of D₂O was determined in three individuals. The percentage of haemolymph water exchanged in this time had a mean value of 53%. This value is intermediate between some other comparable values obtained on Crustacea. Ussing (in Krogh, 1939) found an 80% exchange within 2 min. for Daphnia and approximately 25 % exchange in $112$ hr. for Artemia. It is difficult to interpret D₂O exchange experiments in terms of net rate of water entry, but the fact that it is possible for D₂O to exchange to this extent with the haemolymph water in so short a time indicates that some portion of the body is permeable to water. Osmotic uptake of water may therefore be expected to occur when the animal is in its normal fresh-water environment. This water is presumably excreted as urine.

The inward penetration of sodium into the body was shown by determining O.P.1 and Na₁ of animals placed in a solution of 1 % NaCl. The temperature was 15-18° C. O.P.1 and Na₁ rose rapidly in the first few hours, the haemolymph regaining hypertonicity to the medium with respect to sodium after about 8 hr. Na₁ continued to rise, however, until after 24 hr. it had reached a value of 197 m-equiv./l. at which it remained constant (Fig. 3). The concentration of the medium was checked at intervals and did not vary. Heuts (1943) found that hypertonicity with respect to chloride is regained 6 hr. after Asellus is placed in a sea-water medium equivalent to 222 mM./l. NaCl. Osmotic withdrawal of water might account for the rise in ion content of the haemolymph during the first few hours before O.P.I becomes hypertonic to the medium. Active movement of ions against the concentration gradient must account for the further increase in haemolymph ion concentration after this time.

Animals exposed to continuously flowing de-ionized water lose ions to the medium, sodium being lost at a rate sufficient to lower O.P.I and Na₁ to about 70% of normal in 24 hr. The further responses to a flow of de-ionized water are complex and will be discussed in detail elsewhere.

The permeability of the body to ions and water and the fairly rapid rate of salt loss in de-ionized water suggest that the hypertonicity of the haemolymph in fresh water must be maintained either by a high salt intake in the food or by active up take. Krogh (1939) has shown that starved Branchipus and Apus lose chloride to their medium. Pannikar (1941) has noted a similar effect of Chirocephalus, and Heuts claims that chloride is lost from starved Asellus.

The effect of starvation on Asellus has been re-investigated. Animals taken from their normal habitat were placed in filtered water at io° C. Haemolymph samples were taken at intervals of 24 hr. for a total period of 8 days. For each determination the haemolymph of 2 or 3 animals was pooled and 0.P.1, Na₁ and C1₁ measured. No significant change occurred in any of these factors (Fig. 4).

If sodium is normally lost from the animal at a rate similar to that initially observed in de-ionized water then, during an 8-day period, almost the whole of the sodium in the body should have been lost. In fact O.P.J, Na₁ and Cl₁ are observed to remain at their normal levels, within the range of individual variation. It seems clear that the loss of ions from the body is balanced by a simultaneous active uptake of ions from the medium. The maintenance of Na₁ and C1₁ in Asellus is therefore independent of a supply of sodium and chloride in the food.

The osmotic and ionic concentrations of the haemolymph of A. aquaticus are typical of those of other freshwater Crustacea.

Although Asellus tolerates a rise in the salt concentration of its medium up to values approximately the same as the normal haemolymph concentration, it cannot be regarded as being an animal well adapted to life at high salinities. Long-term acclimatization to a gradually increasing concentration has little effect in increasing the salinity tolerance. It thus differs from the only other freshwater isopods previously investigated. Both Caecospheroma virei (Derouet, 1952) and Mesidotea entomon vetterensis (Lockwood & Croghan, 1957) have been found to tolerate all salinities from fresh water to sea water (at least for the duration of the experiments) and these must therefore be regarded as being more truly euryhaline species than Asellus aquaticus. The occurrence of the last species in the highly saline Westphalian brine lakes (Thienemann, 1913) cannot be accounted for in terms of the salinity tolerance of other populations and is possibly to be regarded as the result of genetic selection over a long period.

The maintenance of Nai against a concentration gradient of approximately 100:1 must result from the replacement of ions from the medium at the same rate as they are lost from the body by diffusion and in urine. The continued maintenance of O.P.1, Na₁ and Cl₁ during 8 days of starvation shows that Asellus can, if necessary, replace NaCl lost from the body solely by active uptake from the medium. The intake of these ions is thus not dependent on the food supply.

Attention has been drawn to the inability of Asellus to tolerate high salinities. It may be noted, however, that the effective range of concentration tolerated by Asellus is considerable. Excluding the occurrence in the Westphalian brine lakes, the animal is known to occur over a 700-fold range of medium concentration. This range of concentration is 6 times the range tolerated by the highly euryhaline brine shrimp Artemia under experimental conditions (Croghan, 1958), though it is true that the osmotic problem to be overcome by the latter is very different. The sensitivity of the ion-regulating mechanisms of Asellus is shown by the fact that, under experimental conditions, the haemolymph concentration remains practically constant over approximately the natural range, O.P.₁ is only some 15% higher in media of 100 mM./l. than it is in media of 0 · 150 mM./l. NaCl.

I would like to thank Dr J. A. Ramsay, F.R.S., for all his interest and advice and also the Department of Scientific and Industrial Research for a maintenance grant. I am also indebted to Dr P. King who kindly translated the paper by M. J. Heuts.