290 THE BIOLOGY OF MARINE ANIMALS 



TMO than species from the North Sea, and seasonal changes have been 

 noted in the herring (46). 



The burden of palaeontological and physiological evidence indicates 

 that teleosts have made a secondary return to the sea. The ancestors of 

 extant marine species were originally freshwater inhabitants, and we find 

 that all marine teleosts today still retain a blood osmotic pressure which 

 is much less than that of sea water. This condition imposes a strong os- 

 motic gradient which the fish must combat to maintain physiological 

 homeostasis, and it is reasonable to suppose that the synthesis of TMO, 

 by substituting a highly soluble non-toxic substance for ammonia, is an 

 adaptation linked with the existence of osmotic stress and the necessity of 

 conserving water. Part of the trimethylamine oxide possibly arises from 

 detoxication of NH 3 , but a certain amount is exogenous in origin, originat- 

 ing in food. Chinook salmon (Oncorhynchus tshawytscha), for example, 

 lack TMO when in fresh water, although it is present in sea-run adults. 

 Salt water does not influence deposition of TMO but feeding Pecten 

 muscle (which contains TMO) causes rapid accumulation of the sub- 

 stance in salmon flesh. It has also been suggested that the trimethylamine 

 oxide occurring in the blood and tissues of marine teleosts may have 

 osmoregulatory significance, but the small quantities present in the blood 

 and the ease with which it crosses the gill membranes show that its osmotic 

 effect must be small (4, 35). 



Elasmobranchs are unique in that they combine a high level of uraemia 

 with ureotelism. Urea levels in the blood of selachians lie around 2-2-5% 

 (Table 2.7), and this substance forms the principal nitrogenous excretory 

 product (80-90% of the total nitrogen excreted). The gills are relatively 

 impermeable to urea and the kidneys regulate the amount of urea ex- 

 creted so as to maintain the blood concentration at a high level. As a 

 consequence of the high urea content, the blood of marine elasmobranchs 

 is slightly hypertonic to sea water although the salt content is actually 

 less than that of the outside medium (Chapter 2). It has been argued, 

 therefore, that modern elasmobranchs, like teleosts, are derived from 

 freshwater ancestors, and that by acquiring external membranes imper- 

 meable to urea they have been able to turn it to use in an osmoregulatory role. 



The regulation of osmotic pressure during development in selachians is 

 achieved in one of two ways, depending on the method of reproduction 

 in the species. Oviparous forms lay cleidoic eggs which are impermeable to 

 urea. The ammonia which is produced by the embryo during develop- 

 ment is converted into urea, and its accumulation provides a source of 

 osmoregulatory material for the young fish on hatching as well as solving 

 its major excretory problem. In viviparous forms, on the other hand, 

 the urea requirements of the embryo are met by the mother fish which 

 provides the necessary quantities of urea. 



In addition to urea, considerable quantities of trimethylamine oxide 

 occur in the blood and tissues of elasmobranchs, ranging from 0-25-1-43% 

 (Table 7.3). It has been reported that the concentration of TMO in the 



