Inter- and intraspecific morphological variation of four-shelled Actinomma taxa (Radiolaria) in polar and subpolar regions

Abstract

The taxonomy of polycystine radiolarians is important for biostratigraphic, paleoecological and paleoceanographical reconstructions. In this study we describe four morphogroups of Actinomma boreale from the North-Atlantic and Arctic oceans. We distinguish them from two other four-shelled Actinomma species from the Southern Ocean and North Pacific, namely Sphaeropyle langii and Prunopyle antarctica, which are both moved to the genus Actinomma. As Actinomma antarcticum is an already occupied name, P. antarctica is renamed as Actinomma friedrichdreyeri nomen novum. Quantitative and qualitative intraspecific morphological variation was studied by examining 922 specimens (8599 photographs) from 49 samples, of which 45 are from the surface sediment, two are from deeper sediments, and two are plankton samples. We found significant differences in shell diameters between North Atlantic and Arctic Atlantic samples, and between recent and downcore samples within A. boreale. We also found significant differences between populations of A. friedrichdreyeri and A. langii from the Southern Ocean versus the North Pacific. We do not recommend the naming of extant polycystine radiolarian species based on continuous characters without supporting phylogenetic information. Therefore, with this study we have provided directions for phylogenetic investigations on A. boreale, A. friedrichdreyeri and A. langii.