Phylogenetic positions of “pico-sized” radiolarians from middle layer waters of the tropical Pacific

In order to determine whether truly pico-sized adult radiolarians exist, we compared spumellarian sequences from individual adult samples collected in the central Pacific, with filtered sea water samples of juvenile (5–42 μm) and gamete (0.2–5 μm) sized fractions to see whether the gene sequences are similar or different. Environmental spumellarian-affiliated sequences we sampled were mostly concentrated in samples from 250 to 400 m depth and only appeared in the RAD-III clade, which corresponds to the family Astrosphaeridae (including Arachnosphaera , Astrosphaera , and Cladococcus ). None of the same ITS (internal transcribed spacer) sequences were found in both filter membranes of the same sea water samples. Pairwise distances among these environmental spumellarian-affiliated sequences are within or slightly above the range of intra-morphospecific variations. We propose a model to explain our observations based on the hypothesis that the “pico-sized radiolarians” represent gametes of radiolarians of normal size, assuming different sinking speeds of parents and offspring.