CHAPTER 5 



NUTRITION AND FEEDING MECHANISMS 



For they shall suck of the abundance of the seas, and of treasures hid in 

 the sand. 



Deuteronomy 



INTRODUCTION 



The food of all animals in the sea is ultimately derived from marine plants 

 which synthesize carbohydrates, fats, proteins and other compounds such 

 as some vitamins. Plant cells are eaten by animals, or their dead bodies 

 provide a source of dissolved organic material which is utilized by bacteria. 

 Seaweeds become broken down into detritus, which provides food for 

 many shallow-water animals. Bacteria break down waste and dissolved 

 organic matter into forms which can be utilized as food by marine animals, 

 and bacteria are themselves consumed by many animals (119). 



The background conditions of nutrition in the sea have been subjected 

 to intensive study during the present century. These investigations have 

 been concerned with the spatial distribution of phyto- and zooplankton, 

 and with variations in abundance, both long-term and seasonal, of 

 planktonic organisms. No less important has been the study of physical 

 conditions — namely temperature, light and nutrient salts — which affect 

 the growth of floating plants and, indirectly, the animals dependent upon 

 them. A knowledge of these interrelated conditions is a necessary pre- 

 requisite for a fuller understanding of factors regulating the abundance 

 of marine organisms, the annual turnover of organic matter and the limits 

 of economic exploitation of marine resources. For an extended treatment 

 of this subject, the reader is referred to Harvey (43, 44) and Sverdrup, 

 Johnson and Fleming (97). 



Flagellates, including both green and colourless species, contain various 

 forms that bridge the gap between phototrophic and heterotrophic organ- 

 isms. Certain flagellates are strictly autotrophic and can subsist on in- 

 organic sources of nitrogen and carbon; others require one or more organic 

 substances which they take from solution. Apart from these flagellates all 

 animals are dependent upon a source of preformed carbohydrates, fats, 

 proteins and certain other essential substances, i.e. they are holozoic 

 (86, 90). 



The zooplankton is largely responsible for harvesting the phyto- 

 plankton crop in the sea, and in turn provides food for other organisms. 

 Herbivores predominate in the zooplankton and are generally of small 

 size. Chief among them are copepods, especially nauplii and juvenile 

 stages. In the southern oceans the large krill, Euphausia superba, sometimes 



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