The molecular basis of differential morphology and bleaching thresholds in two morphs of the coral Pocillopora acuta

Processes of cnidarian evolution, including hybridization and phenotypic plasticity, have complicated the clear diagnosis of species boundaries within the phylum. Pocillopora acuta, a species of scleractinian coral that was recently split from the widespread Pocillopora damicornis species complex, occurs in at least two distinct morphs on the Great Barrier Reef. Contrasting morphology combined with evidence of differential bleaching thresholds among sympatrically distributed colonies suggest that the taxonomy of this recently described species is not fully resolved and may represent its own species complex. To examine the basis of sympatric differentiation between the two morphs, we combined analyses of micro- and macro-skeletal morphology with genome wide sequencing of the coral host, as well as ITS2 genotyping of the associated Symbiodinium communities. We found consistent differences between morphs on both the macro- and micro-skeletal scale. In addition, we identified 18 candidate functional genes that relate to skeletal formation and morphology that may explain how the two morphs regulate growth to achieve their distinct growth forms. With inconclusive results in endosymbiotic algal community diversity between the two morphs, we propose that colony morphology may be linked to bleaching susceptibility. We conclude that cryptic speciation may be in the early stages within the species P. acuta.