Two distinct lineages in the radiolarian Order Spumellaria having different ecological preferences

The radiolarian group Spumellaria compose a diverse assemblage of marine microzooplankton with widespread geographic and vertical distributions. Spumellarians are useful paleoceanographic indicators, because their siliceous shells are preserved in the sediment at water depths deeper than the carbonate compensation depth. However, lack of detailed biologic knowledge has slowed the establishment of definitive taxonomic criteria, particularly for higher-level taxa. Recent studies on molecular phylogeny based on small subunit ribosomal DNA (SSU rDNA) confirmed spumellarian taxonomic position within radiolarians. However, these sequences were obtained from spumellarian specimens taken from subtropical to tropical surface waters. In order to clarify the taxonomy of widely distributed spumellarians, we obtained seven novel SSU rDNA sequences of specimens collected from the subarctic region. Our phylogenetic analyses revealed two distinct spumellarian groups (Spumellaria I and II) and six subgroups, which mostly correspond with family level taxonomy inferred from the inner shell structures. The Family Spongodiscidae is however separated into two lineages associated with Pyloniidae (Spumellaria I) and Coccodiscidae (Spumellaria II) in our phylogeny. Notably, two groups (Spumellaria I and II) are composed of ecologically different species: Spumellaria I consists of temperate to cold water species and one group of Spumellaria II consists of warm water species. These two distinct groups have probably evolved in cold and warm environments, respectively.