Is Montipora dilatata an endangered coral species or an ecotype? Genes and skeletal microstructure lump seven Hawaiian species into four groups

Image by J. E. Maragos. Foreground: M. capitata; above: M. dilatata; above, lower left: invasive algae (Kappaphycus/Eucheuma spp.) 2 Executive summary Montipora dilatata is considered to be one of the rarest corals known. Thought to be endemic to Hawaii, only a few colonies have ever been found despite extensive surveys. Endangered species status would have major conservation implications; however, coral species boundaries are poorly understood. In order to examine genetic and morphological variation in Hawaiian Montipora, a suite of molecular markers (mitochondrial: COI, CR, Cyt-B, 16S, ATP6; nuclear: ATPsβ, ITS), in addition to a suite of measurements on skeletal microstructure, were examined. The ITS region and mitochondrial markers revealed four distinct clades: I) M. patula/M. verilli, II) M. incrassata, III) M. capitata, IV) M. dilatata/M. flabellata/M. turgescens. The nuclear ATPsβ intron tree had several exceptions that are generally interpreted as resulting from recent hybridization between clades or incomplete lineage sorting. Since the multicopy nuclear ITS region was concordant with the mitochondrial data, incomplete lineage sorting of the ATPsβ intron is a more likely explanation. Principal components analysis (PCA) of microstructure measurements agreed with the genetic clades rather than the nominal taxa. These species groups therefore either represent recent or insipient (CA <1MYA) species or morphological variants of the same biological species. These clades are likely to occur outside of Hawaii according to mitochondrial control region haplotypes from previous studies. Common garden experiments were conducted on distinct morphotypes of M. capitata to test the hypothesis that micro-skeletal traits can be phenotypically plastic in this genus. Although the experiment suffered high mortality from parasitic flatworms, verrucae (rice-grain sized bumps) were documented to form on formerly smooth colonies, indicating plasticity. This study contributes towards understanding the relationship between genetic and morphological variation in this taxonomically challenging group, which is essential for effective conservation and the key to understanding the evolution and biodiversity of reef building corals. Management implications This study identified two closely related species complexes (M. dilatata/M. flabellata/M. turgescens and M. patula/M. verilli) that may occur outside of Hawaii. We found no significant genetic or micro-skeletal differences between nominal taxa within these species complexes; however, this study may not be capable of detecting recent or insipient species (CA <1MYA). Now that these species complexes have been identified, population genetic and/or reproductive studies are necessary to determine if the nominal species within each complex are actively interbreeding. The findings of rapid evolution and …