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Although many sympatric species of the coral Acropora spawns on the same day, hybridization is partly prevented by their species-specific fertilization manner. On the other hand, several species show bidirectional intercrossing, but their hybridization is prevented by slight time differences in their spawning and fertilization preference with conspecific sperm. However, it is scarcely described how the intercrossing species avoid hybridization. Here we demonstrate that hybridization of the intercrossing species Acropora tenuis and A. donei is likely prevented by differences in the timing of spawning, gametogenesis (differences in production numbers of matured sperm and eggs), population density, and fertilization preferences. We investigated three locations in Okinawa, Japan: Aka Island, Sesoko Island, and Oku. The timing of gamete release in the two species overlapped at Aka Island and Oku, but A. donei …