Differences in spawning time drive cryptic speciation in the coral Acropora divaricata

Most of the reef-building Acropora corals around Okinawa Island spawn in May and/or June. We found that two morphotypes of Acropora divaricata spawned in August and/or September. The “robust” and “slender” morphotypes differed in branch width and in the diameter of axial corallites. Histological analyses showed that the onset of gamete production/maturation occurred from June onwards. Most of the other Acropora species spawned from late May to early June. A . florida , which spawned in May/June, initiated gamete production in early April. We assumed that the two morphotypes of A . divaricata were reproductively isolated from most of the other Acropora species because of differences in the spawning seasons. We found that the spawning seasons of the two morphotypes slightly overlapped in 2015 but not in 2019, and inter-morphotype gamete compatibility was high. However, population genetics analyses and a phylogeny of the mitochondrial d -loop region showed that they were genetically distinct and rarely hybridized. Thus, the broadcast-spawning coral A. divaricata might have speciated when there might have been only a low possibility of interspecific gamete interaction.