Osmotic Behaviour of the Fairy Shrimp Chirocephalus Diaphanus Prévost

The material was kindly collected for me from pools of fresh water on Roborough Down, near Plymouth, by Mr G. M. Spooner to whom my grateful thanks are due. A subsequent culture was made in the laboratory from mud taken from the bottom of jars in which the shrimps had lived and died. The osmotic pressure was determined correct to + 0 · 003 % NaCl by Baldes’s micromodification of the Hill vapour pressure method ; details of procedure adopted are given’ in another paper (Panikkar, 1941). For removing the blood, each animal was first placed on a dry filter paper to remove the adhering water and later, on a slide, under a binocular microscope, the blood being withdrawn from the dorsal blood vessel with a specially prepared glass cannula. The blood was transferred to the thermocouples almost immediately after withdrawal; one Chirocephalus provided enough blood for two estimations. The mean value for each sample is represented by each point on the graph (Fig. 1).

The blood showed an osmotic pressure equivalent to 0 · 44 to about 0 · 5 % NaCl in an external medium (natural habitat) of 0 · 002% NaCl. These values were obtained a day after collection and it was later found that there was little change even after the animals had been living in the laboratory for some days in the same water and mud brought from Roborough Down. There was no appreciable difference between the osmotic concentrations of males and egg-bearing females.

When Chirocephalus was kept without food in filtered tap water, there was no appreciable decline in osmotic pressure, but the animals were not able to live for long in this medium (Fig. 1 A, B). In most of the experiments all individuals died within the first 50 hr. ; in one experiment, however, two specimens lived for 3 days, after which they became inactive. Osmotic pressure of one of them was determined in this Condition and showed a very slight fall from the normal. The other specimen died within a few hours.

In glass-distilled water, Chirocephalus could live only for a comparatively short period. Most of the experimental specimens died within 36 hr. The osmotic pressure of these animals gradually fell from the normal (0 · 44 − 0 · 5 % NaCl) to about 0 · 3% NaCl, which is probably the minimum dilution of blood which it could survive (Fig. 1 C). Plymouth tap water contains a trace of chloride and this would appear to be helpful to the animals in maintaining their normal concentration in tap water in the absence of food. It is interesting to note that two individuals, which were in glass-distilled water for 32 hr. and had become inactive and sunk to the bottom of the jar, slowly recovered when taken to tap water, though they died in the new medium after a lapse of a further 30 hr.

The above results suggest that Chirocephalus is able to assimilate ions from tap water. To prove the occurrence of ion absorption conclusively, a batch of specimens was transferred to very dilute sea water having an osmotic pressure of 0 · 351% NaCl. Blood from experimental animals measured at intervals showed a rise in value from the normal 0·4 − 0· 5 to about 0 · 55 % NaCl, within 15 hr. The osmotic pressure returned to normal afterwards as shown in the graph (D). The increase in value in this experiment cannot be due to osmotic phenomena since the external medium was definitely hypotonic, but can only be due to the active absorption of salts against the osmotic gradient. It would appear from the initial rise in osmotic pressure that the normal process of ion absorption was continued at the same rate for some time, even after transfer to a medium richer in salts than the former one and that the value was brought to normal by later regulation. All the animals in this medium died after 3 days.

When transferred directly to sea water from fresh water, the specimens died within an hour or two. They became inactive in less than half an hour, but recovered when immediately taken back to tap water. Acclimatization experiments with very dilute sea water of osmotic pressures 0 · 210 and 0 · 298% NaCl failed, since in both media all animals died within 3 days.

In all experiments where salt water was employed, a change in the appearance of the bracts became evident after some hours. In the normal animal the bract, which is the proximal exite of the thoracic limb and the only part of the limb without spines or setae, is transparent like the rest of the appendage. In salt water these structures slowly turn opaque; the higher the salt concentration of the medium the sooner this happens. This change is seen before the animal shows any sign of distress or inactivity. The bracts of Chirocephalus are homologous with the gill-sacs of Cladocera and are believed to be respiratory in function. The cuticle covering them is extremely thin as compared with that of other regions of the animal. In a paper published before the concept of ion absorption in animals was introduced by Koch and Krogh, Dejdar (1930) with the technique developed at Prague by Gicklhorn and Keller, investigated a number of branchiopods, including Chirocephalus-, he showed the selective reaction of the bracts to vital staining with dyes and dilute solutions of silver salts and other substances. From the similarity in behaviour of the bracts and the dorsal organ (Nackenschild), especially during the early stages, Dejdar concluded that the latter is a respiratory structure that functions during larval life. Koch (1934) has explained these vital staining phenomena on the basis of active absorption of ions which is now well known from Krogh’s work. I have obtained results similar to those of Dejdar by adding drops of millimolar silver nitrate to jars containing Chirocephalus in glass-distilled water. When the silver salt is reduced with 0· 5% KMnO4 in strong light, the bracts show the characteristic black appearance whereas the remaining parts of the appendage remain unaffected.

One may conclude from this fact that the site of the inward passage of ions (and possibly their outward passage in Artemia) is in the bracts. Whatever other functions they may have, the bracts would thus seem to have a role in the osmoregulation of branchiopods. If this conclusion is correct, it is likely that the dorsal organ has a similar function during the larval stages of many branchiopods and in the adults of forms like Leptodora which are without bracts but retain the dorsal organ.

The general behaviour of Chirocephalus appears from these experiments to be similar to that of Branchipus and Lepidurus, examined by Krogh (1939), in its inability to survive in media without food except for short periods. Fall in osmotic pressure of animals kept in glass-distilled water implies the continuous loss of chloride and this Krogh found in the two genera he examined. These three phyllopods differ from Daphnia and possibly other Cladocera which can live in pure distilled water for several days without food (Naumann, 1933). It is reasonable to assume that Daphnia is able to control loss of salts more efficiently than these phyllopods or that ion absorption is so efficient that the chloride lost is immediately reabsorbed.

In a recent note, Beadle & Cragg (1940) have questioned the universal importance of active ion absorption in the osmoregulation of fresh-water animals. They found that Gammarus pulex and Asellus aquaticus (both fresh-water forms) can live in distilled water for at least 8 days without food. While the fresh-water variety of Gammarus duebeni could live in distilled water for at least 4 days, the brackish water variety of the same species rapidly lost chloride and eventually died. Though active ion absorption does take place in these animals, they conclude that the essential part of the osmoregulatory mechanism is the power of retention-of salts and not the power of absorbing salts. Salt retention may thus vary in closely related species and even in physiological races of the same species.

Judging from the results obtained with Chirocephalus, it appears to me that ion absorption plays a more important role in this animal than in fresh-water gammarids or even Daphnia. Only this could explain the longer period of its survival in tap water without serious fall in osmotic pressure, as compared with its behaviour in distilled water. Absorption of ions seems to be important at least in those fresh-water animals with feeble powers of salt retention. In animals where the loss of salts is small, uptake of salts under normal conditions is probably less significant. In the latter, not only the loss of salts, but also the entry of water into the animals, has to be minimized and, I think, the permeability of the integument is a vital factor. If the amount of water entering osmotically into the animal is small the loss of salts by way of the urine could also be brought to a minimum. It is worth mentioning that, as in euryhaline crustacea like Carcinus (videWebb, 1940), ion absorption seems to play a significant role even in the homoiosmotic crustacean Palaemonetes varions, which can live in media varying from water that is nearly fresh to concentrated sea water equivalent to 5 · 0% NaCl (Panikkar, 1939, 1941). This prawn is unable to live in distilled water for more than a few hours, but lives for about a day in tap water. The period of survival without food could, however, be prolonged for many weeks if the chloride concentration of the medium is raised to about 0 · 010% NaCl. Successful adaptation of an animal to fresh water would depend upon its ability to maintain a favourable balance between the amount of salts lost into the medium and the amount absorbed against the osmotic gradient. It would seem that ability to survive for long periods without food in distilled water and in inert media like glycerol would only be observed in animals where the loss of ions from the body is so small that the osmotic concentration of the blood is maintained well above the minimum threshold.

The blood of Chirocephalus has a normal osmotic concentration equivalent to 0 · 44 − 0· 5 % NaCl in an external medium of 0· 002 % NaCl. The osmotic pressure falls rapidly when the animal is kept in glass-distilled water, but is fairly well maintained in tap water. There is an initial rise and a later return to normal even in hypotonic saline media, and this indicates active absorption of ions. The bracts are presumably the organs concerned in the salt absorption. The animal is unable to live in tap water or distilled water for more than 2 − 3 days without food and its general behaviour is thus markedly different from that of Daphnia.

I wish to thank the Director and the staff of the Plymouth Laboratory for their interest and helpful advice in this work.