A PHYLOGENOMIC EXAMINATION OF PALMYRA ATOLL’S CORALLIMORPHARIAN INVADER

Abstract

The reefs at Palmyra Atoll, one of the northern Line Islands in the equatorial Pacific, have been undergoing a phase shift from scleractinian corals to a corallimorph-dominated benthos. It has been unclear whether there has been cryptic speciation or morphological plasticity leading to different ecotypes of Rhodactis howesii. Here, I use mitochondrial genomic analysis to assess species validation and underlying cause of morphological variation across the atoll. I mapped sequenced reads to Rhodactis indosinensis, R. howesii’s closest recorded genomic taxon. In additon to one individual from American Sāmoa, I assessed phylogenetic relationships of published corallimorph genomes with those from Palmyra. There was no identifiable population structure within Palmyra, and available dinoflagellate symbiont communities were consistent among the sequenced individuals. There were notable differences in symbiont communities between Palmyra and American Sāmoa individuals, as well as six fixed nucleotide differences. I conclude that the lack of taxonomically validated genetic reference material together with vague species descriptions, morphological plasticity and overlap among morphological characters, combine to raise doubts about the validity of the currently accepted species name, R. howesii. Comparison of my results to all currently available genetic data for corallimorpharians suggests that the species at Palmyra is most closely related to an unidentified species of Rhodactis from Okinawa. However, taxonomically confirmed R. howesii is absent from genetic databases so no firm conclusions about species identification can yet be drawn. It seems clear that this group is in need of additional taxonomic work and a broad phylogenetic survey of taxa and geographic distribution would further my understanding of marine biodiversity, conservation, and invasion dynamics of this understudied group.