WATER, SALTS AND MINERALS 55 



the salinity rises above 35% present an analogous problem in the added 

 strain imposed in resisting dehydration. In the Bitter Lakes and Lake 

 Timsah of the Suez Canal Zone, salinities up to 53% have been reported. 

 Normal inhabitants of these waters are sole Solea solea and grey mullet 

 Mugil cephalus. In the desert brine pools bordering the Bitter Lakes a 

 cyprinodont Aphanius dispar and a gastropod Pirenella conica live in 

 waters where the salt concentration reaches 1407T (about 6m NaCl). 

 Osmotic stresses of this magnitude are most unusual (33). 



Reptiles, Birds and Mammals. The higher vertebrates which have re- 

 invaded the sea have to some extent been functionally pre-adapted for a 

 maritime life since their terrestrial ancestors have had to solve the problem 

 of water conservation. Marine reptiles, birds and mammals are all pro- 

 vided with an impermeable integument which prevents the passage of 

 water and salts. Since these animals are predominantly air-breathers, the 

 respiratory surfaces are shielded from sea water. 



Reptiles. The chief marine reptiles are turtles (Sphargidae and Chelonii- 

 dae), Gal&pagos marine lizard (Amblyrhynchus), estuarine and salt water 

 crocodiles (Crocodylus), and sea snakes (Hydrophiidae). Some of these 

 animals, on occasion, have access to fresh water, but others, such as the 

 leathery turtle Dermochelys coriacea and the pelagic sea snake Pelamis 

 platurus, are wholly marine (120, 129). 



In reptiles and birds the terminal portion of the cloaca serves to recover 

 water from the faeces and kidney excreta. The main end-product of nitro- 

 gen metabolism (in birds and many reptiles) is uric acid, which possesses 

 low solubility and is excreted in a nearly solid state. This is not the case 

 in aquatic turtles, however, which excrete mainly urea and ammonia. The 

 osmotic concentration of the blood of marine turtles is somewhat higher 

 than that of freshwater forms (Caretta, marine, A 0-76; Emys, fresh water, 

 A 0-44; see Fig. 2.17). The urine is reported to be hypotonic to the blood. 

 As far as we know, marine turtles are largely dependent upon food and 

 metabolic sources for water (62, 92, 126, 135). 



Birds. Inshore species of birds such as gulls, cormorants and steamer 

 ducks may periodically resort to fresh water, but many pelagic species 

 (petrels, albatrosses, penguins) never taste fresh water throughout their 

 lives. Maritime birds drink salt water and also derive water from their food 

 and metabolic processes. A considerable amount of water is lost by 

 evaporation from the lungs and air sacs (76, 77, 93, 147). 



It is a common physiological condition among vertebrates for the 

 marine representatives to have more concentrated blood than allied fresh- 

 water species (Fig. 2.17). There is some evidence that birds can produce a 

 slightly hypertonic urine. Water is conserved by absorption in the kidney 

 tubules and hind gut, and excess salt is excreted in the urine. The more 

 concentrated blood of marine species, by lowering the osmotic gradient, 

 reduces the amount of osmotic work that must be performed in the secre- 

 tion of salt and the production of a hypertonic urine (105). 



A peculiarity of petrels and their allies is the presence of a greatly 



