Molecular phylogenetic position of Hexacontium pachydermum Jørgensen (Radiolaria)

The taxonomic affiliation of Hexacontium pachydermum Jørgensen, specifically whether it belongs to the order Spumellarida or the order Entactinarida, is a subject of ongoing debate. In this study, we sequenced the 18S rRNA gene of H. pachydermum and of three spherical spumellarians of Cladococcus viminalis Haeckel, Arachnosphaera myriacantha Haeckel, and Astrosphaera hexagonalis Haeckel. Our molecular phylogenetic analysis revealed that the spumellarian species of C. viminalis, A. myriacantha, and A. hexagonalis form a monophyletic group. Moreover, this clade occupies a sister position to the clade comprising the spongodiscid spumellarians, coccodiscid spumellarians, and H. pachydermum. This finding is contrary to the results of morphological studies based on internal spicular morphology, placing H. pachydermum in the order Entactinarida, which had been considered to have a common ancestor shared with the nassellarians.