WATER, SALTS AND MINERALS 



53 



the blood are diminished. Threatened hydration of the tissues is to that 

 extent reduced and the water balance is conserved by the production of a 

 very dilute urine. 



The chimaeroids also contain large quantities of urea, and appear to 

 have an osmoregulatory system similar to that of selachians (127). 



Teleosts. Unlike the great majority of marine animals the blood of 

 marine teleosts is hypo-osmotic to sea water and the animals are con- 

 fronted with the functional problem of avoiding osmotic dehydration in 

 an aqueous medium. Blood osmotic pressures in these animals generally 

 lie between A 0-7 and A 1-0, well below that of the external medium (Figs. 

 2.16 and 2.17). Marine fish drink large quantities of sea water; water 



Animal Group 

 Delphinus dolphin 

 terrestrial mammal 



Fulmarus marine 

 terrestrial bird 



Caretta marine turtle 

 Emys freshwater turtle 



\Gadus marine cod 

 \Cyprmus freshwater carp 



Squalus marine dogfish 

 Pristis freshwater saw-fish 



(Myxine marine 

 Petromyzon freshwater 



Mytilus marine 

 Anodonta freshwater 



\Maia Marine crab 

 \Potamon freshwater crab 



A Blood or body fluids 



A Normal 

 sea water 



0-4 0-6 1-2 1-6 



Freezing point depression 



Fig. 2.17. Histograms Illustrating Osmotic Pressures of 



Blood and Body Fluids of Representative Marine and 



Freshwater Animals from Different Groups 



together with sodium, potassium and chloride ions is absorbed through the 

 gut wall, while most of the ionic magnesium and sulphate is rejected 

 (Angui/Ia, Myoxocephalus, Lophius). Water conservation is aided by the 

 production of only small quantities of urine, which is always isotonic or 

 slightly hypotonic to the blood and low in ionic sodium, potassium and 

 chloride. The constant salt uptake, which is a consequence of these pro- 

 cesses, is compensated by the secretion of salts across the gills (13, 126). 



In estuarine and freshwater environments the salt content of the blood 

 is lowered, but the blood is still strongly hyperosmotic to fresh water 

 (A 0-45-zl 0-60, see Table 2-6). Water is absorbed by endosmosis through 

 the exposed gill and oral membranes, but the rest of the body is relatively 

 impermeable because of the covering of scales and mucus. Fish living in 

 brackish or fresh water drink little water, and they get rid of the excess 

 quantities absorbed by excreting copious amounts of a hypotonic urine. 

 There is some loss of salts in the urine and in the faeces, which are parti- 



