Speciation processes in the sea are expected to differ from those in the terrestrial environment, especially in the case of sessile marine invertebrates. As compared to most terrestrial animals, these organisms display several distinct characteristics in life history and demography that are likely to affect their evolution. For example, they tend to have high dispersal capabilities, large population sizes, wide distributions, extreme longevities – and therefore largely overlapping generations – and high fecundities (Hughes et al. 1992). Moreover, many sessile marine invertebrates release their gametes into the water column where fertilization takes place. This creates unparalleled opportunities for interspecific hybridization and introgression in species that discharge their eggs and sperm in synchrony with sympatric congeners, as has been documented for many species of reef corals. Experimental breeding trials with scleractinian corals confirm that reticulate pathways are likely to have contributed to the evolution of modern Indo-Pacific coral species including the genus Acropora.
Acropora is one of the world’s most widespread scleractinian coral genera, spanning the Indian and Pacific Oceans and the Caribbean Sea. It is also extremely speciose; it is the largest extant reef-building coral genus. Recent revisions of the genus recognise 113 or approximately 180 Acropora species. Some species have very restricted distributions, whereas others are found throughout large parts of the tropics, and up to 70 Acropora species can be found in sympatry. An enormous amount of intraspecific morphological variability exists, while at the same time similarities between species are striking; for example, intraspecific geographic differences in morphology can be as large as differences between species. The fossil record shows that the genus probably originated during the Paleocene or Eocene, and became widely distributed in the early Miocene. Acropora thus provides an ideal model system for examining speciation and evolution of scleractinian reef coral species in general, on both temporal and spatial scales.