282 THE BIOLOGY OF MARINE ANIMALS 



Survey of Nitrogenous End-products among Marine Animals 



Ammonia is formed by deamination of amino-acids, and sometimes of 

 purines. To avoid toxaemia ammonia must be excreted rapidly or converted 

 into a less toxic compound. Ammonia is very soluble in water, diffuses 

 rapidly and is eliminated readily by aquatic animals. Ammonia tolerance 

 varies among animals but the concentration in the blood is always low. 

 Some recorded values (as mg NH 3 -N per 100 c.c. or 100 g blood) are: 

 crustaceans, 0-4-2-5; cephalopods, 1-4-4-8; selachians, 1-4-2-5; and tele- 

 osts, 0-3-5-5 mg%) (Table 7.1). 



Animals which eliminate a high proportion of nitrogenous waste as 

 ammonia are termed ammonotelic (Needham). These include actinians, 

 polychaetes, sipunculoids, crustaceans, sublittoral gastropods, lamelli- 

 branchs, cephalopods and echinoderms. Teleost fishes also excrete much 

 ammonia (Table 7.2). 



Bacterial oxidation of ammonia in the sea is discussed by Spencer (50a). 



Urea is derived from amino compounds, namely amino-acids and purines. 

 Like ammonia it is very soluble and diffusible but much less toxic. Urea 

 forms a much smaller proportion of the total nitrogenous excretion than 

 ammonia in marine invertebrates. It is excreted in small and variable 

 amounts in all the major groups examined, namely coelenterates (actinians), 

 annelids, sipunculoids, molluscs, crustaceans and echinoderms (Table 

 7.2). Animals in which the principal excretory end-product is urea are 

 termed ureotelic and are found among vertebrates ; ureotelism seems never 

 to have been exploited by an invertebrate group. Marine elasmobranchs 

 are highly ureotelic and excrete more than four-fifths of non-protein 

 nitrogen as urea. 



Uric acid is a relatively non-toxic substance of low solubility, which can 

 be excreted in solid form. It is an important excretory product in birds, 

 reptiles, insects and possibly certain gastropods. In some of these animals 

 the ammonia produced by degradation of proteins is largely converted into 

 uric acid, and such forms are termed uricotelic. Uric acid is also formed by 

 the oxidative deamination of purines and is excreted in traces or small 

 amounts by many marine invertebrates and fishes. The occurrence of 

 ureotelism and uricotelism has special significance in conjunction with 

 environmental conditions and breeding habits, and is discussed in a later 

 section. 



Purines: when nucleic acids are hydrolysed in the organism, purine 

 and pyrimidine groups are liberated. The purine bases so formed are 

 adenine and guanine. These substances are excreted unaltered by some 

 animals, whereas others degrade them to a greater or lesser extent and 

 excrete them as uric acid, allantoin, allantoic acid, urea or ammonia, 

 depending on the degree of breakdown of the purines. Guanine or guanine- 

 like substances are utilized by certain animals (cephalopods, Crustacea, 

 fish) as reflecting agents in chromatophores, sometimes in association 

 with pterines (55). Little is known about the fate of pyrimidine bases. 



