ASSOCIATIONS 



619 



degenerate. In various other forms the algae are killed and digested by the 

 host, namely, in later stages of the life-history of C. roscoffensis, in the 

 Xeniidae and the Tridacnidae. The majority of coelenterates, however, 

 do not digest their associated algae (22, 33, 107, 109, 1 12). 



It is possible to distinguish between associations in which the algal 

 symbionts are not essential to the life of the animal, namely Madreporaria, 

 Actiniaria, and those in which the animal is dependent upon the algae, from 

 which it derives all its nourishment, namely Convoluta roscoffensis and 



100 



*80 

 -c: 



60 



SO 



c: 

 20 



12 3 4 5 



D&ys 



Fig. 14.23. A Graph Showing the Percentage Changes in Phosphorus 

 Content of Sea Water Containing Different Species of Corals 



Animals in jars 2-5 1 capacity; original level of P, 2-036 g/m 3 of sea water, a. Psammo- 

 cora; b. Pontes; c. Favia; k. Control. (From Yonge (109).) 



tropical Alcyonaria such as the Xeniidae. Experimentally, in many animals 

 the zooxanthellae can be removed only with difficulty. Under conditions of 

 extreme starvation Convoluta convoluta eventually destroys and digests its 

 algae, but can be reinfected. Madreporarians, actiniarians and Scyphome- 

 dusae (Cassiopeia) containing zooxanthellae gradually lose the latter 

 when they are starved, kept in the dark or otherwise subjected to adverse 

 conditions, but the algae are extruded or excreted through the gastric 

 filaments or mesenteries, and are not digested. Madreporarian corals 

 possessing zooxanthellae show the effects of starvation as quickly as 

 those lacking zooxanthellae, thus demonstrating that the latter are not 

 consumed. 



