Species boundaries in the absence of morphological, ecological or geographical differentiation in the Red Sea octocoral genus Ovabunda (Alcyonacea: Xeniidae)

Abstract

The development of coalescent-based and other multilocus methods for species delimitation has facilitated the identification of cryptic species complexes across the tree of life. A recent taxonomic revision of the ecologically important soft coral genus Ovabunda validated 11morphospecies, all with type localities and overlapping geographic ranges in the Red Sea. A subsequent molecular phylogenetic analysis using mitochondrial and 28S nrDNA genes divided the genus into just two clades, with no apparent genetic distinctions among morphospecies. To further explore species boundaries among morphospecies of Ovabunda we sequenced three additional nuclear genes (ITS, ATPSα, ATPSβ), and obtained data for 1332 unlinked SNPs from restriction-site associated DNA sequencing. Both coalescent-based and allele-sharing species delimitation analyses supported four species of Ovabunda, each of which included multiple morphotypes encompassing the full range of morphological variation observed within the genus. All four species occurred over the same depth range of 5–41m, and were sympatric at sites separated by 1100km in the Red Sea. The only characters that have been found to distinguish three of the four species are diagnostic substitutions in the nuclear genome; the fourth differs by exhibiting polyp pulsation, a behavioral trait that can be assessed only in live colonies. The lack of any obvious morphological, life history, ecological or geographical differences among these four species begs the question of what drove the evolution and maintenance of reproductive isolating mechanisms in this cryptic species complex.