368 Relations between Species 



sponges, coelenterates, mollusks, and worms. Some of these uni- 

 cellular forms are green algae known as zoochlorellae; others are 

 brown or yellow cells, believed to be flagellates, and are termed zoo- 

 xanthellae. Partnerships called plant-animals by Keeble (1910) are 

 formed by a turbellarian worm {Convolttta roscofensis) and large 

 numbers of zoochlorellae. The algal cells, often growing in the tissues 

 of the worm in such profusion as to give it a greenish appearance, 

 release oxygen during photosynthesis and produce nitrogen com- 

 pounds that are nutritionally beneficial to the host. In exchange the 

 algae obtain a suitable matrix for their growth and receive a supply of 

 nutrient material resulting from the animal's metabolic processes. 

 This balanced symbiotic relationship is thus able to persist indefi- 

 nitely, and the nourishment furnished by the algae enables the worm 

 to live and grow for long periods without taking in solid food. 



Zooxanthellae are found abundantly in the body wall of coral 

 polyps. These unicellular organisms serve the useful function of 

 removing nitrogenous wastes and carbon dioxide from the coral and 

 providing it with oxygen produced as a by-product of photosynthesis. 

 The zooxanthellae in turn benefit by the absorption of the metabolites 

 containing nitrogen and phosphorus, which are scarce in tropical 

 waters, as well as by the absorption of carbon dioxide resulting from 

 the catabolic processes of the coral animal. The fact that coral polyps 

 placed in the sea in sealed glass containers survived for 2 weeks is 

 evidence that the symbionts in this instance had approached a state of 

 balance, in regard to their respiratory exchanges at least, and perhaps 

 also to some extent in regard to other needs. 



The giant clam Tridacna (Fig. 7.11) grows on shallow coral reefs 

 with the opening between its shells directed upwards and the broad 

 edges of its mantle, containing vast numbers of zooxanthellae, spread 

 out horizontally where they receive intense radiation from the sun. 

 The mantle also contains great numbers of small lens-like organs 

 which probably serve to focus light into the tissue, making photo- 

 synthesis possible for the deeper-lying zooxanthellae. Since phagocy- 

 tic blood cells regularly engulf and digest large numbers of the zoo- 

 xanthellae, the giant clam may be though of as "farming" these sym- 

 biotic algae and deriving a considerable portion of its nutrition from 

 them (Yonge, 1944). 



Some symbionts reside in cavities of their hosts rather than in their 

 tissues. Ruminants and other animals living on a diet high in cellu- 

 lose are unable to digest this material without the enzymatic action 

 of cellulase produced by microorganisms in their intestines. Sym- 

 biotic bacteria fulfill this function in cattle and other grazing animals. 



