OSMOTIC AND IONIC REGULATION IN AQUATIC 

 INVERTEBRATES 



James D. Robertson 



University of Glasgow 



The study of osmotic and ionic regulation in aquatic animals was fur- 

 thered by Krogh's (1939) discussion of the literature up to 1938. In 

 recent years, work in this general field has centered principally on prob- 

 lems of active transport of ions in tissues ; various reviews are available 

 (e.g., Brown and Danielli, 1954). This paper reviews recent work on 

 ionic regulation in marine invertebrates, and osmoregulation in inverte- 

 brates living in fresh and brajvish water. A short account is given of some 

 problems of salt and water balance in crustacean nerve and muscle. 



Marine Invertebrates 



Most marine invertebrates have body fluids in osmotic equilibrium 

 with sea water, the principal exceptions being some prawns and the grap- 

 soid crabs. All show to a varying extent ionic regulation, which may be 

 defined as the maintenance in a body fluid of concentrations of ions dift"er- 

 ing from those of a passive equilibrium with the external medium. Many 

 marine invertebrates have considerable quantities of protein in their blood 

 plasma, a feature which complicates the interpretation of concentration 

 differences between plasma and sea water, owing to the Donnan effects 

 produced by the indiffusibility of the proteins and the formation of cal- 

 cium-protein complexes. Although protein concentrations may reach 80 

 gm. /liter in the decapod Crustacea and exceed 100 gm. /liter in the cephal- 

 opod Mollusca (Robertson, 1949, 1953), the Donnan ratio does not seem 

 to exceed 1.03 and only 10-20% of the calcium is in the form of a calcium 

 proteinate. 



By comparing the composition of plasma or coelomic fluid as drawn 

 from an animal with the composition of a sample dialyzed against sea 

 water across a collodion membrane, a true idea is obtained of the extent 

 by which it differs from that resulting from a passive equilibrium. 



Table 1 gives a selection from data on nearly 40 invertebrates (Robert- 

 son, 1949, 1953, 1954). In general, ionic regulation of body fluids is slight 

 in the lower, less highly organized groups such as echinoderms, tunicates, 

 polychaetes, lamellibranchs, and gastropods, and in the mesogloea of the 

 jellyfish Aurelia. With the exception of the prosobranch gastropods, these 

 animals have very low protein concentrations in their body fluids, usually 

 below 1 gm. /liter. In the more highly organized decapod crustaceans and 



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