Shedding light on the origin of Acanthocystidae: Ricksol blepharistes gen. n., sp. n. (Ricksolidae fam. n., Panacanthocystida, Centroplasthelida), with notes on the evolution of the genera Acanthocystis, Ozanamia gen. n. (Ozanamiidae fam. n.), and “Heterophrys-like organisms”

A new family, genus and species of centrohelid heliozoans, Ricksol blepharistes gen. n., sp. n. (Ricksolidae fam. n.), from the 6‰ water of White Sea, was studied with light and electron microscopy and SSU rRNA gene sequencing. R. blepharistes has two types of siliceous scales. Internal (plate) scales are guitar-shaped and flat, with an axial rib and thin margin. External (spine) scales are long with a long curved shaft and petite drop-shaped laterally winged basal plate. R. blepharistes is the only centohelid having external scales of this type. Molecular phylogenetic analysis places R. blepharistes in Panacanthocystida distantly from any identifiable families. This position is confirmed with the elongated sequences in the panacanthocystid increased regions (PINs) 1–4, 6, and 7. This pattern of elongated PINs correlates with the molecular phylogenetic place in the Acanthocystida subclade of Panacanthocystida. The morphology of the new genus has some similarities with the genera Meringosphaera, Ozanamia gen. n., and the strain SRT127 isolated from Tokyo Bay (Japan) in 2011 by Azumi Fukuda. For this strain, the molecular phylogenetic position in Acanthocystida was also reported. These similarities are discussed (Ozanamia symna gen. n., comb. n., Ozanamia curvata comb. n., and Ozanamiidae fam. n. are described) as well as the status of the taxa possibly related to R. blepharistes. The probable life cycles with interchanging scale- and spicule-bearing cells in early branches of Panacanthocystida are discussed, too. Last but not the least, we discuss the origin of the genus Acanthocystis from ancestors similar to R. blepharistes gen. n., sp. n. and/or SRT127 strain.