Corculum and Zooxanthellae — KAWAGUTI 
49 
The Tridacnidae have thick, heavy shells 
and usually rest on the hinge and umbo. 
Moreover, they culture immense numbers of 
zooxanthellae in the mantle edge which is 
exposed to the sunlight. Consequently the 
relation of the mantle and shell to the other 
organs is greatly different from in Corculum. 
The Tridacnidae may be divided into two 
groups according to their mode of life; one 
is a surface living species and the other a 
boring species. However, they both can attain 
immense size of shell in tropical seas where 
nutritive plankton is rather rare. 
Yonge (loc. cit.) considered that the pres- 
ence of associated algae may enable the Tri- 
dacnidae to exceed the limits normally set to 
the size of a plankton-feeder and thus be 
responsible for the immense size attained by 
T. derasa. 
It would seem then that the presence of 
associated algae induces the immense size of 
shell in Tridacna but results in the thin and 
rather transparent shell of Corculum. How- 
ever, the presence of the algae may not be the 
primary factor in the formation of the large 
shell in Tridacna, but it may have served to 
allow the greater development of an evolu- 
tionary tendency. That is, in the Tridacnidae 
it perhaps accelerated a tendency to settling 
into the reef with a heavy shell, whereas 
in Corculum it would seem to have accel- 
erated a tendency toward expansion of a thin 
transparent shell. 
