Here is the list separated into BENEFITS and COSTS for the zooxanthellae. All seem reasonable except perhaps for "photosynthates used by host". Does this imply that zooxanthellae that live separate from a host are less "leaky" and keep their photosynthetic products more to themselves? Also, if mutant zooxanthellea were to appear with less leaky membranes, would the coral cell sense this and expel them? More importantly, if the photsynthates were NOT available to the host, then how could the mutualism have evolved?