616 THE BIOLOGY OF MARINE ANIMALS 



by fission. Others still retain free-swimming sexual stages, capable of 

 infecting new host animals, e.g. the algal symbionts of Convoluta. Al- 

 though Convoluta is dependent upon its algal associates, the reverse is 

 not the case and it is not unlikely that many symbiotic algae are repre- 

 sented by free-living individuals as well (31). 



The possession of algal symbionts is obligatory in most animal species 

 in which this relationship exists, and it is more exceptional to find a 

 facultative association. Populations of Noctiluca miliaris in the Indian 

 Ocean contain zoochlorellae, but in the North Atlantic symbionts are 

 absent. Specimens of the ciliate Trichodina patellae are infected with 

 zooxanthellae on the coast of Normandy (Cap de la Havre), but not at 

 Wimereux (Pas de Calais). Yonge (109) notes that reef-building corals 

 are able to live with few or no zooxanthellae in dark places in nature, as 

 well as under experimental conditions. 



Structural Modifications of Animals in Conjunction with Algal Symbiosis 



Various genotypic modifications are encountered among animals in 

 which algal symbiosis is obligatory : examples are Turbellaria, Alcyonaria 

 and Lamellibranchia. Tropical Alcyonacea show a reduction of the ventral 

 mesenterial filaments in correlation with the number of zooxanthellae 

 present. These are the regions which secrete protease for digestion of 

 animal food. In Lobophytum there is little reduction of the filaments and 

 few algae are present; in Sclerophytum the filaments are very small, gland 

 cells are few and algae are abundant (Fig. 14.19). Xeniids show extreme 

 adaptation to a symbiotic existence: the stomodaeum is very short, the 

 tentacles do not react to animal food and the animal is entirely dependent 

 upon the symbiotic activities of its algal associates (33). 



In the reef clams (Tridacnidae) enormous numbers of zooxanthellae 

 are found in those portions of the mantle edge concerned with the formation 

 of the siphons (Fig. 14.22). Profound modifications in structure have oc- 

 curred in that the mantle and shell have rotated about 180° in relation to 

 the visceral mass and foot; consequently the siphonal tissues are trans- 

 ferred from the normal posterior to a dorsal position. This allows the 

 mantle edges to be exposed fully to the light, which is focused deeply into 

 the tissues by means of hyaline lens-like bodies, about which the zoo- 

 xanthellae are congregated in enormous numbers. These are always 

 contained in phagocytic cells and are carried to the digestive diverticula 

 to be digested intracellularly. Heart shells Corculam are also modified in 

 relation to algal symbiosis. The shell is very thin and transparent, allowing 

 light to reach the algal symbionts which are concentrated in the gills, 

 palps and lower mantle surface. Their occurrence in digestive diverticula 

 indicates they are consumed by the animal (54, 107, 110). 



Special Habits in Connexion with Symbiosis 



Owing to the photosynthetic activity of imprisoned algae, animals 

 containing symbionts tend to seek out or settle in well-illuminated 



