Abstract
Convolutriloba longifissura is a marine acoel flatworm that harbors endosymbiotic green algae to fill the energy requirement. C. longifissura is known to display a unique behavior refer as to "sunning" that allows endosymbiont to conduct photosynthesis more efficiently. A small area of lit surface can attract hundreds of acoel flatworm competing for the best position for photosynthesis. In contrast, evasive action of the animal from a high-light area can be also observed. It is of particular interest whether the symbiont algae control this characteristic host animal behavior. Here we demonstrate that the evasive behavior protects algal photosynthesis well as the host flatworms from high-light stress. We also show strong evidence that hydrogen peroxide produced under high-light conditions causes a lethal damage to the host animal, a mutual reason for the unique behavior of the flatworm-algae symbiotic system.
