WATER, SALTS AND MINERALS 63 



ion concentration is tolerated by tissues or organs. The lobster heart, 

 which has a pH optimum of 7-4, continues to beat normally over the range 

 pH 7-0-8-0, but values beyond these limits affect tonus, amplitude and 

 frequency. Cardiac and smooth muscles of other animals show similar 

 sensitivity. Herring eggs develop normally in sea water ranging from pH 

 6-7-8-7, but development is retarded in sea water with lower pH values (58). 



Ionic Regulation 



The ionic differences which exist between body fluids and the surround- 

 ing sea water could result from passive physical agencies or depend upon 

 active regulation by the animals' tissues. The body fluids of more primitive 

 groups, namely coelenterates, polychaetes and echinoderms, usually con- 

 tain very little protein, but in more active crustaceans and cephalopods 

 blood protein attains high levels, up to 100 g 1. in Loligo and Eledone {see 

 Tables 2.12 and 2.13). The presence of protein affects ionic diffusion and 

 concentrations in several ways. Proteins form undissociated complexes 

 with calcium and retain calcium at high levels. Other cations may be bound 

 by negatively charged protein molecules which are prevented from diffus- 

 ing across bounding membranes because of their large size. According to 

 the Donnan equilibrium, the product of diffusible cations and anions 

 inside must equal the product of diffusible cations and anions outside, 

 e.g. Na^ x CL; = Na X Cl . Since some of the cation is held by protein 

 the actual situation is— 



inside ^ T ' l 



Na^Pr 



outside Na Cl 



therefore, Na^ > Na , and C\ t < Cl . 



For the Donnan equilibrium to be operating under these circumstances 

 without endosmosis implies impermeability to certain ions, or an internal 

 hydrostatic pressure equal to or greater than the colloidal osmotic pressure. 



The magnitude of the ionic differences resulting from the undiffusibility 

 of proteins and protein calcium complexes across gills and other bounding 

 membranes can be estimated by dialysing the body fluids against sea 

 water and comparing the analyses of undialysed and dialysed samples. 

 Data obtained by Robertson and Webb for a series of marine animals are 

 shown in the accompanying tables (2.12 and 2.13). These investigations 

 show that the ionic differences between body fluids and sea water become 

 greatly reduced following dialysis, and reveal the small part played by 

 protein-binding in determining the concentration levels of ions in blood 

 and other internal media. Most of the disparity between internal and 

 external media is due to controlled regulation of ionic levels by the animal. 

 The extent to which different animals regulate separate ions can be appreci- 

 ated from an examination of Table 2.10 (relative ionic composition of 

 body fluids). 



The ability to regulate ionically is a universal characteristic of marine 

 animals, differing only in magnitude in various groups. In more primitive 



