Combined molecular and morphological evidence for phylogeny of pennatulacean and calcaxonian octocorals

Postulated by Bayer (1955), the calcaxonian gorgonians may have close affinity to the pennatulaceans based on the resemblance of axial structure observed under polarized light microscopy. To further confirm the phylogenetic relationship between the two groups of octocorals, this study applied both polarized light microscopy and scanning electron microscopy to examine the extinction patterns of sclerites and axial skeletons in macro- to ultrastructural scale from the total of thirteen genera of calcaxonian octocorals and nineteen genera of sea pens. The results show significant similarities between calcaxonian gorgonians and sea pens on scleritic crystal orientations and organization of axial structures, together with the inferred phylogeny of forty-six taxa (seventeen gorgonians and twenty-nine pennatulaceans) using ND2 mtDNA sequence data, all strongly support Bayer’s postulation and reveal the sister group relationship between the Ellisellid calcaxonian gorgonians and the pennatulaceans. The X-ray microanalysis was also carried out for inter-family comparison of the axial chemical composition between calcaxonian gorgonians and sea pens for the implication of phylogeny, however the results shown that the axial chemical composition is largely determined by ambient seawater instead of biologically intrinsic differences, therefore it did not provide much resolution to gain plausible phylogenetic implication in consequence.