Why Do the Tridacna Clam and Soft Coral Have High Saturation Points?
There are several possible answers to this question. First, the saturation point varies among zooxanthellae clades, and ‘A’ clades (such as found in many Tridacna species) are known to be tolerant of high light. Second, tissues of soft corals and clam mantles offer a relatively thick cross section (as compared to many SPS corals), and self-shading from other zooxanthellae or by coral/clam tissues is almost certain to occur. Finally, tridacnid (and some other photosynthetic clams’) mantle tissues are layered and light is refracted. This explains why viewing clams from above gives the most spectacular view.
