Raphidiophrys heterophryoidea sp. nov. (Centrohelida: Raphidiophryidae), the first heliozoan species with a combination of siliceous and organic skeletal elements.

Centrohelid heliozoans form a monophyletic group with uncertain affinities to other groups of protists. Except for a number of naked species, they have either siliceous scales or organic spicules covering the cell. According to the phylogenetic analysis of the 18S rDNA gene, it is hypothesized that scales are the ancestral form of cell coverings, while spicules are derived structures. The present paper describes a new species of centrohelids, Raphidiophrys heterophryoidea, isolated from the inner lakes of Valamo Island (North-Western Russia). This species has tangential scales, consisting of two plates, connected with radial, sometimes branched septa. Raphidiophrys heterophryoidea also has radially oriented spindle-shaped spicules. The organic nature of spicules and the siliceous nature of scales were verified by energy-dispersive X-ray microanalysis. Such a combination of organic and siliceous skeletal elements in one heliozoan is novel for any heliozoan species. The complex bipartite structure of scales suggested placing this new species in the genus Raphidiophrys. Consequently, the diagnoses of the genus Raphidiophrys and the family Raphidiophryidae were amended taking into account the optional presence of spicules. Raphidiophrys heterophryoidea presumably represents a stage of shift from scales to spicules, which, according to the molecular phylogenetic data, occurred at least twice in centrohelid evolution.