ECOLOGY OF TADPOLES 



49 



is used in two entirely different ways. In the physical sense, it is no 

 more than a region where the surface energy relations are different 

 from those in the depths of the water. Goldacre (1949) has shown that 

 on natural waters, there is a surface film, consisting of a protein or 

 Hpo-protein, and so far from this being a physical fiction, it is definitely 

 food. Goldacre calculated that if a tadpole continued to eat it at the 

 same rate as it did when under observation it could eat an amount 

 equal to its own dry weight every day. A tadpole that turns itself 

 upside down and grazes at the film is not wasting its time. 



The tadpole has no gastric mill, and we must now pass to consider 

 what apparatus it has for effecting even a Hmited amount of breaking 

 of the algal cells, and also the remainder of the digestive anatomy, so 

 far as it affects the ecology and life history of the animal. 



Digestive Anatomy 



The beak of the tadpole is a pair of shears, so effective that it can 

 sever cleanly the roots of duckweed, which in relation to the size of 

 the animal, are quite large objects. The beak can also be brought to 

 bear on flat surfaces, and it is no doubt in this way that the tadpole 

 eats the sessile algae. A more puzzling feature is to be found in the 

 rows of cuticular teeth that surround the mouth. A high speed cinema 

 film lent some support to the idea that these teeth are scarifying 

 devices, serving to break open some of the algal cells before the beak 

 detaches them for, just before the beak closes, the small teeth can be 

 seen to sweep over the surface of the glass through wliich the film 

 was taken. 



It seems surprising that, with all this apparatus for cutting and 

 scraping food, the tadpoles in the end seem to utihze so little of the 

 algae they eat. But they are not alone in what appears to us to be such 

 a wasteful method of feeding. In the Mollusca, Fretter (1936) has shown 



Fig. 16. The Lungs of Three Species of Tadpoles 



(a) An undescribed species (C98) from Papua, found in temporary water 

 in a wheel-rut. 



(b) R. temporaria, also at times an inhabitant of small, badly oxygenated 

 ponds. Lung development less than C98, but much more than that of 

 B. bufo. 



(c) B. bufo, an inhabitant of large clean ponds. No functional lungs, so 

 that the animal is entirely dependent on a sufficient supply of dissolved 

 oxygen. The lungs are so small that it was necessary to draw the dissection 

 from the ventral side in order to show them at all. 



