Evolutionary theory predicts that in the absence of outcrossing opportunities, simultaneously hermaphroditic organisms should eventually switch to self-fertilization as a form of reproductive assurance. Here we report the existence of facultative self-fertilization in the free-living flatworm Macrostomum hystrix, a species in which outcrossing occurs via hypodermic insemination of sperm into the parenchyma of the mating partner. First, we show that isolated individuals significantly delay the onset of reproduction compared to individuals with outcrossing opportunities (“delayed...