Inter- and intraspecific morphological variation of three Hexacontium (Radiolaria) species in three southern Norwegian fjords: H. gigantheum, H. pachydermum and H. enthacantum

•Morphology of three Hexacontium species from southern Norwegian fjords•All H. gigantheum specimens distinguished from H. pachydermum and H. enthacanthum•Homogeneous intraspecific variation between fjord systems•Tetrapetaloid structure in 1st shell of three Hexacontium species•Genus Hexacontium in family Hexalonchidae and order Entactinaria

The taxonomy of polycystine radiolarians is important for biostratigraphic, paleoecological and paleoceanographical reconstructions. In this study we examine the morphological variation of three Hexacontium species, namely Hexacontium gigantheum, Hexacontium pachydermum and Hexacontium enthacanthum. Quantitative and qualitative intraspecific morphological variation was studied by examining 774 specimens (7428 photographs) from 16 sediment samples in three southern Norwegian fjord systems. We can distinguish all H. gigantheum specimens from H. pachydermum and H. enthacanthum, and most H. pachydermum and H. enthacanthum specimens from one another. Intraspecific variation is homogeneous in the three examined fjord systems. We have observed a tetrapetaloid structure in the first shell of all three Hexacontium species and this supports the classification of Hexacontium in the family of Hexalonchidae and order Entactinaria. Additional phylogenetic data is necessary to answer to what extent the examined species are related, whether they consist of clades of cryptic taxa, and whether H. enthacanthum is a juvenile stage of H. pachydermum.