TRANSACTIONS OF SECTION D. 421 



3. Momentum in Evolution. By Professor Arthur Dendy, F.R.S. 



See Reports, p. 277. 



4. The Food- supply of Aquatic Animals. By Dr. W. J. Dakin. 



In 1907 an extremely interesting paper appeared by August Putter of C46t- 

 tingen. The author pointed out that a very general assumption had been made 

 by biologists and physiologists to the effect that aquatic animals fed, like most 

 terrestrial animals, on solid food. The minute algae of the phytoplankton were, 

 for example, considered to be the ultimate source of food, and the animals of 

 the zooplankton fed directly upon these or upon microscopic animals, which in 

 their turn' fed upon the plants. Fixed animals like sponges and zoophytes, or 

 creatures like lamellibranchs, were dependent upon the organisms filtered or 

 removed from currents of sea-water. One reason for this unquestioned assump- 

 tion was that no other source of food-material was apparent. If it could be 

 proved, therefore, that large quantities of organic food-material exist in the sea 

 in some other form, it would be necessary thoroughly to revise our conceptions 

 of the biology of aquatic organisms. 



The first experiments made by Putter were for the purpose of determining 

 whether or no an organic carbon compound occurred in the fluid in which aquatic 

 animals lived. The result was startling, for he found that sea-water contained 

 24,000 times more carbon per litre than was present in the organisms (plankton) 

 of the same volume of water. This was his starting-point, and he has en- 

 deavoured to show that the sea or fresh water is more or less a nutrient fluid, 

 which is more important altogether than solid food in the physiology of aquatic 

 organisms. 



Since the publication of the first paper it has been shown that Putter's 

 analyses were not quite correct, and, further, that many mixed planktonic forms 

 were not considered in early determinations, for they were too small to be 

 captured by apparatus then used. 



Putter has been able, however, with the latest determinations, to show that 

 there is still more organic carbon present in solution than in the plankton. 



There is no reason whatever against aquatic animals using food in solution, 

 and it is important, therefore, to find out whether they do use it. 



The first and most important facts supporting the theory are the results of 

 experiments made to determine the amount of carbon and oxygen required by 

 certain animals per day to cover the loss due to metabolism. A specimen of the 

 sponge Suberites, of 60 gr. weight, requires as food 22 m.g. carbon per day. To 

 provide this amount the sponge would need to capture 



1,480,000,000 Scehtonema costatum, or 

 7,400,000,000 Thalassiosira nana. 



It would have to filter several thousand times its own volume of water per 

 hour to obtain enough food — an altogether unthinkable piece of work. 



If the food in solution in the sea-water was used, a much more rational 

 quantity of water would suffice and all the conditions would be more favourable. 



The same experiments have been carried out on Cucumaria and other animals 

 with the same results, and if calculations are made to determine the food 

 requirements of aquatic organisms, the general result is to find that the large 

 quantity of food needed is beyond the powers of the animals concerned if it can 

 only be solid food. For example, according to Winterstein, a Rhizostoma of 

 232 c.c. volume uses per day, at 26°, 122 m.g. of oxygen. This corresponds to a 

 food-supply of 



24,000 Temorella or 

 490,000 Nauplii. 



The larger Rhizostoma frequently caught would require 



10,600 Calanus. 

 392,000 Temorella. 

 2,630,000 Orthona. 

 7,820,000 Nauplii per day. 



