NUTRITION AND METHODS OF FEEDING 183 



in considerable amount. Other mineral salts utilised by 

 marine plants, such as the chlorides, sulphates, etc., of sodium, 

 potassium, magnesium, and iron, are present in ample 

 proportions. 



(6) Oxygen is necessary for the respiration of both plants 

 and animals. It is dissolved from the air and is given off 

 as a result of the photo-synthesis of plants. Oxygen occurs 

 in solution in varying quantities up to saturation, the latter 

 point being rarely reached except, perhaps, in the tidal area, 

 which is the most highly oxygenated area in the whole 

 sea. 



Since all of these substances : nitrogen compounds, 

 carbon dioxide, phosphoric acids, silica, and various mineral 

 salts, are essential to the life of plants, it follows that the 

 absence of any one of them will be sufficient to put an end 

 to growth entirely. Similarly, if one of the essential con- 

 stituents is present in minimal quantity growth will also be 

 minimal. Thus, " the growth of a plant is dependent upon 

 the amount of the foodstuff which is presented to it in 

 minimal quantity." This is Liebig's Law of the Minimum, 

 the application of which is of fundamental importance in 

 the sea, since on it, though indirectly, the quantity of animal 

 life also depends. 



Nutrition of Marine Animals. — None of the substances 

 which we have enumerated, however, are capable of serving 

 as food for marine animals. These, like land animals, can 

 utilise (if we are not to credit the theories of Piitter) only 

 substances such as proteins, carbohydrates, and fats, which 

 have been built up within the bodies of other organisms. 

 Both in the sea and on land, therefore, the plants are the 

 indispensable links between the ultimate foodstuffs and the 

 living animal. 



The immediate diet of a marine animal may consist 

 solely of flesh or of plants, or it may be composed of a mixture 

 of both. In actual fact the number of omnivorous marine 

 animals probably greatly exceeds that of terrestrial animals 

 with the same feeding habits. The most characteristic 

 way in which marine animals obtain their food is by straining 



