Hybridization is a controversial topic in coral reef ecology ^{1, 2} . While small numbers of hybrid embryos can be produced in a few species in vitro ³ , the evidence for hybrids in the field is often equivocal because the genetic techniques used for corals cannot distinguish between hybridization and incomplete lineage sorting ⁴ . In fact, only one of the over 1300 species in the order is generally accepted to be unequivocally of hybrid origin: Acropora prolifera ^{1, 5} . Nonetheless, hybridization is often invoked as a source of evolutionary novelty in the order Scleractinia ^{6, 7} .

Here, we report an incidental observation on the potential for hybridization between two closely related scleractinian corals species in the family Fungiidae, Ctenactis echinata and Ctenactis crassa. These species are sympatric, often dominating large multi-specific assemblages of fungiid corals throughout the central Indo-Pacific ⁸ . Colonies of these species are superficially very similar ( Figure 1A and B) but can readily be distinguished by the shape of the costal dentitions ⁹ . Both species are gonochoric, that is each colony is either male or female, and reproduce by broadcast spawning, releasing gametes into the water column for fertilization ⁸ ( Figure 1C and D). At our study site on Sesoko Island (26°38'13.00"N; 127°51'56.24"E), Okinawa, Japan, spawning occurs following the full moons from July to August ⁸ . Furthermore, both species release gametes at the same time ⁸ and consequently there is the potential for hybridization. In the days before the predicted date of spawning in July 2013, we collected four colonies of C. echinata and six colonies of C. crassa, to produce larvae for other experiments.

While the species are relatively easy to identify, determining the sex of each individual prior to spawning is impossible without destructive sampling to expose the gametes. Consequently, we placed each individual in a separate 20 L bucket containing sea water in the open air at approximately 20:00 h in order to sex each individual once gametes had been released. On the night of 27 July between 22:30 and 23:30 h three C. echinata and five C. crassa spawned revealing that the three spawning C. echinata were female, while four C. crassa were females and one was a male. The size of the eggs of each species at the time of release was distinct with a range in maximum diameter of 244–266 μm in C. echinata and 133–155 μm in C. crassa. In contrast to earlier work on C. echinata ¹⁰ , we saw no symbiotic algae in the eggs of either species. We collected approximately 5000 eggs from the three C. echinata females and mixed them with sperm from the C. crassa male. As a positive control we mixed eggs from the four C. crassa females with the C. crassa sperm. Approximately 100 eggs were observed under a stereo-dissecting microscope for cleavage, indicating fertilization, every 2 to 6 h over the next 24 h. At no point did we observe cleavage in the cross between species indicating that no hybrid embryos were produced. In contrast, over 90% of C. crassa eggs in the positive control were fertilized within 2 h. We conclude that despite synchrony in the time of gamete release between these two closely related sympatric species there appears to be strong pre-zygotic mechanism to avoid hybridization. While our observations are preliminary and in only one direction (i.e. we did not cross C. echinata males with C. crassa females) we predict that hybridization between these species is unlikely. This observation adds to a growing body of evidence indicating strong pre-zygotic barriers to hybridization in many scleractinian corals ^{11– 13} .