Morphology, Ultrastructure, and Phylogeny of Two Novel Species of Ventrifissura (V. oblonga n. sp. and V. velata n. sp., Thecofilosea, Cercozoa)

Ventrifissura is a group of poorly studied heterotrophic biflagellates in the phylum Cercozoa. Despite a phylogenetic placement with only weak support and a lack of ultrastructural data, Ventrifissura was assigned to Thecofilosea. In the presented study, we established cultures of two novel species of Ventrifissura (V. oblonga n. sp. and V. velata n. sp.) isolated from coastal marine environments in Japan, and performed light and electron microscopy observations and molecular phylogenetic analysis. Transmission electron microscopy revealed that V. oblonga shares several ultrastructural characteristics with thecofilosean flagellates, including permanently condensed chromosomes, a extracellular theca, and slender extrusomes. Molecular phylogenetic analysis could not resolve the phylogenetic position, but the possibility that Ventrifissura clusters into Ventrifilosa was supported by approximately unbiased tests. Based on both morphological and phylogenetic findings, we concluded that Ventrifissura is a basal lineage of Thecofilosea.