Algal symbionts increase oxidative damage and death in coral larvae at high temperatures

Mutualisms are often viewed as reciprocal exploitations that nonetheless provide net benefits to each partner. While the benefits of symbiosis with dinoflagellates of the genus Symbio- dinium (zooxanthellae) for corals are well understood, the costs of the association, particularly when under stress, remain a focus of much research. One of the primary impediments to exploring the costs of symbiosis in zooxanthellate corals is that it is impossible to examine the animal host in isolation. Evidence for a cost of symbiosis with zooxanthellae includes the fact that direct transmission of zoox- anthellae between generations is rare, particularly in broadcast spawning corals. Fortuitously, the absence of zooxanthellae in oocytes of many species and the ability to readily infect larvae with zoox- anthellae provide an opportunity to compare individuals with and without these symbionts. Here, we use this larval model to show that individuals with zooxanthellae have lower survival than those lack- ing zooxanthellae when exposed to high temperature. Higher activity of anti-oxidant defenses and higher levels of oxidative cellular damage in larvae with zooxanthellae suggest that oxidative stress originating in the symbionts is a cause of tissue damage in the host under heat stress. We hypothesize that this may be one reason for the absence of direct transmission of zooxanthellae in most broadcast spawning corals whose propagules must spend at least 1 d on the ocean surface.