DIE OSMOREGULATION WASSERLEBENDER TIERE
Authors: SCHLIEPER CARL 
Published: January 2008

The body surfaces and the gills of aquatic animals which use dissolved oxygen for respiration are, in general, permeable to water. It is, therefore, necessary to assume mechanisms for controlling the water content of all those forms in which the body fluids have a molecular concentration differing from that of the external medium. This is the case in all freshwater animals and in marine teleosts. These organisms are homoiosmotic, that is, they are independent of the molecular concentration of the external medium. In contrast to them most stenohaline marine invertebrates are poikilosmotic: their body fluids have an osmotic pressure which is the same as that of the external medium. The beginnings of active osmotic control are, however, apparently present even in this group of animals, since the body fluids of various molluscs and decapod crustaceans and of tunicates are frequently somewhat hypertonic to sea water. Mechanisms for osmotic control are undoubtedly present in those euryhaline marine invertebrates (Carcinus maenas, etc.), which are actively homoiosmotic in dilute sea water (Schlieper).–Marine teleosts, in contrast to the last two groups of animals, have a lower molecular concentration in their blood than that of the surrounding sea water. It follows that their osmotic control mechanisms must work in the opposite sense. Something of the same nature exists also in certain invertebrates (Noctiluca, Artemia, Pachygrapsus), that is to say, the internal medium is hypotonic to the external.

It has generally been supposed up to the present, since Overton wrote in 1904, that freshwater animals continuously take in water osmotically through the skin, and then pass it out with the aid of their excretory organs (contractile vacuoles, excretory canal systems of flatworms and rotifers, antennary glands, kidneys, etc.). This view is supported by the fact that the urine of the frog (Rana esculenta, according to Bottazzi) and that of a river crab (Potamobius astacus, according to Herrmann) is hypertonic to the blood. Newer investigations show, nevertheless, that osmotic control in freshwater animals frequently takes place in quite a different manner from this.

Experiments by Adolph (1926) with Amoeba proteus demonstrate that the amount of water passed out by the contractile vacuole is independent of the concentration of the external medium. This makes it probable that the animal actively controls the passage of water through its protoplasm. Certain observations of Gruber (1899) and Herfs (1922), according to which Protozoa without contractile vacuoles (the marine form of Actinophris, Opalina) can be kept alive for some time in pure fresh water, are pertinent in this connection. An undoubted proof that the excretory organs are not concerned in osmotic control is furnished in those cases where the urine (in Telphusa and Eriocheir) is found to be isotonic with the blood (Schlieper). It has also been shown that the antennary glands (kidneys) of Carcinus maenas, an animal that is homoiosmotic in dilute sea water, always produce urine isotonic with the blood (Schlieper, 1929c). The existence in fresh water of certain animals such as sponges and Hydrozoa, which have no special excretory organs, is in harmony with this fact that animals whose excretory organs are not concerned in osmotic control can nevertheless live in a hypotonic medium (fresh and brackish water).

The kidneys of marine bony fishes are not concerned in osmotic control either, for the urine which they produce is always slightly hypotonic to the blood. It is at present unknown how the molecular concentration of the body fluids is maintained in marine and in freshwater teleosts, although an indication is given by certain experiments of Duval (1925), proving that in the eel (Anguilla) the mucus covering of the body has an osmotically protective function. Gueylard (1923) treads quite a new path in her investigations of the viability of the stickleback (Gasterosteus) in sea water. According to this worker the proportion of cholesterol to fatty acids in the blood and tissues is concerned in the capacity of this small freshwater teleost to live in sea water. The spleen, which controls the cholesterol content of the organism in Gasterosteus according to Gueylard, is supposed to be the actual organ of osmotic control. Further investigations on this line are needed.

In Amphibia the water content of the body is regulated actively by the skin and by the kidneys according to recent investigations by Parnas (1921), Przylecki (1922–4), Wertheimer (1923–4), Bauer (1925) and Adolph (1925–7). The skin continuously transports water and dissolved salts from outside to inside, while the kidneys selectively excrete superfluous water and salts. The osmotic intake of water appears to be of lesser importance than this actively controlled water current through the body.

Numerous biological observations and experiments show that the oxygen requirements and oxygen consumption increase in euryhaline invertebrates as the salt concentration of the external medium decreases. It is assumed that this increased respiration is required for the work done against an osmotic intake of water from the outside (Schlieper, 1929a).