Hybridization as driving force for cryptic species diversity in the Caribbean coral genus Madracis

Species boundaries in scleractinian corals remain highly elusive due to conflicting patterns between morphological and molecular phylogenies, often caused by morphological plasticity, occurrence of cryptic species, incomplete lineage sorting or introgressive hybridization. Here, we use an integrated systematics approach, which combines reduced representation genome sequencing (nextRAD), micro-morphometric characterization, SEM analyses and compilation of life history traits, to infer phylogenetic relationships among closely related species in the Caribbean coral genus Madracis. In total, we analyzed 235 Madracis specimens from Curaçao and Bermuda collected from 10-90 m depth. Sequence- and SNP-based analyses for 115 samples generated unprecedented species resolution in Madracis, greatly supporting the morphology-based taxonomy of the current, accepted Caribbean species M. senaria, M. decactis, M. formosa, M. carmabi and M. mirabilis (M. auretenra). The exception was M. pharensis, in which we found evidence for three separate lineages, and for which we found signatures of admixture and introgression. These three M. pharensis lineages showed distinct depth distributions (thus classified as shallow, deep and very deep) and were partially distinguishable on the basis of fine microstructural elements of the collumella, septa and coenosteum. Further taxonomic comparisons are needed to formalize these putative cryptic species. Overall, our integrated systematics approach further resolves species relationships in the Caribbean genus Madracis, supports the morphological descriptions for most of the recognized species, but also reveals the existence of cryptic diversity in groups marked by high admixture, thus suggesting hybridization as a driving force in coral species diversity.