ASSOCIATIONS 



615 



genera are involved, Hippopus and Tridacna, of which the giant clam 

 T. derasa sometimes attains a length of nearly one and a half metres. 

 Immense numbers of zooxanthellae are present in these animals, and are 

 housed chiefly in the inner lobes of the mantle-edges on the dorsal side 

 of the body, but zooxanthellae also occur elsewhere in the connective 

 tissue into which light can penetrate. The animals literally "farm" their 

 algae, which synthesize starch and oil deposits. The zooxanthellae are 

 contained in amoeboid cells within blood sinuses and are carried from the 

 mantle to the digestive diverticula to be digested. Corculum cardissa, the 



yd)\ \i 



/ 



-' MSA Wmm JlflKifc 



"MR, ' 



fJ§MrejftHK? 



'^i^v.J: <y>^ Jt^ft' 



Fig. 14.21. Section through the Body Wall of Convoluta roscoffensis, 

 Showing Green Algal Cells in Mesenchyme Beneath the Epidermis 

 The algal cells contain pyrenoid bodies and nuclei. (From Keeble and Gamble (56).) 



Pacific heart shell, also contains abundant zooxanthellae in gills, mantle 

 and liver tissue (54, 107, 108, 110). 



Echinoderms. Several instances have been reported of green and blue- 

 green algae occurring in the tissues of echinoderms (sea-urchins, brittle 

 stars and starfish). These probably should be regarded as parasitic in- 

 fections, rare in occurrence, which may culminate in the death of the 

 infected animal (68, 69). 



Bryozoa. Filamentous green, brown and red algae are found under- 

 neath the cuticle of certain marine bryozoans, e.g. Flustra and Alcyoni- 

 dium (58). 



General Consideration of Symbiosis between Algae and Animals 



Symbiotic relationships between plants and animals are usually very 

 intimate, a particular species of alga being confined to a single host species. 

 Some symbiotic zooxanthellae appear to be no longer capable of living a 

 free existence, e.g. those occurring in many coelenterates ; sexual stages 

 and spore formation have been lost, and reproduction is accomplished 



