FEEDING 255 



above a certain point (which is difficult to estabhsh precisely), the organs 

 can no longer deal with essential metabolic processes. In any case, the 

 food supply will probably have a set limit to further growth long before 

 this crucial point is reached. 



True, large animals need proportionally less food than their smaller 

 i-elatives (see Chapter 11), but even so, a large animal must eat more than 

 a small one, and, in fact, food supply faces whales with problems 

 already. 



Everyone knows that whales feed on plankton, though few can tell you 

 what plankton really is. Plankton is a Greek word meaning 'what drifts', 

 and has come to be applied to all living matter floating in the water which 

 cannot move across large distances by its own exertion, i.e. countless 

 millions of vegetable and animal organisms of all shapes and sizes. Thus, 

 while a large part of plankton is microscopically small, plankton also 

 contains worms, snails, shrimps and the still larger jellyfish. In what 

 follows, we shall deal only with planktonic organisms of up to three inches. 



One of the most striking characteristics of this kind of plankton is that, 

 in certain circumstances, it can multiply incredibly quickly, and cover 

 vast areas. In summer, some of our own ditches and pools may become so 

 choked up with it that they look like a mass of thick broth, and aquarists 

 the world over know how difficult it is to keep their aquaria free from this 

 scourge. Because of its abundance, quick growth, and multiplication, 

 plankton forms the ideal food for very large animals, and may even be 

 said to be the only food for them, since every other source would soon 

 become exhausted. This is borne out by the fact that the largest fish, i.e. 

 the Basking Shark and the Whale Shark, which may grow to a size of 

 forty and fifty feet respectively, also feed on plankton and particularly on 

 small crustaceans, and so does the biggest ray - the Manta. Even the 

 larger species of two groups of extinct reptiles were found to have had a 

 significant reduction in the size and number of their teeth, and there- 

 fore probably fed on plankton as well. 



The whale's chief diet in the Antarctic and to a somewhat lesser extent 

 in the Arctic is made up of a particular kind of plankton, called krill, which 

 consists of small crustaceans. In the Antarctic krill consists largely of 

 the species Euphausia superba Dana, crustaceans with a maximum length 

 of three inches (Fig. 132). In cold waters, where food is particularly 

 abundant, krill multiplies with fantastic rapidity, and a single female is 

 capable of laying 11,000 eggs. No wonder that krill occurs in such pro- 

 fusion that over vast areas the sea looks like red-brown soup. By using 

 special plankton nets, scientists discovered that, while krill occurs down 

 to more than 500 fathoms, the biggest concentrations are found up to 

 five fathoms down. Hence whales do not have to look for their prey as 



