Reproductive strategies in the intercrossing corals Acropora donei and A. tenuis to prevent hybridization

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 did not spawn vigorously at Sesoko Island. Histological observations indicated a decrease in the number of mature sperm and eggs in the weakly spawning colonies of A. donei at Sesoko Island. From the population density, as assessed by field surveys, we predicted that the encounter rate with heterospecific gametes for A. donei around Sesoko Island was higher than at the other locations. Thus, one predictable reason for the suppression of their gonadal development may be the avoidance of hybridization. Although synchronous spawning of A. tenuis and A. donei was observed at Aka Island and Oku, hybridization is likely infrequent, because sperm choice experiments demonstrated that the eggs preferred fertilization by conspecific sperm. Moreover, microsatellite analyses suggested that hybridization rarely occurred even at synchronous spawning sites, as the population structures of the two species at both Aka Island and Oku were not mixed. Collectively, the findings of this study highlight several strategies, tactics, and mechanisms of avoiding hybridization that are likely to be active in other species that hybridize in vitro.