Morphological, Molecular Phylogenetic, and Mitochondrial Genome Characterization of a Deep‐Branching Marine Flagellate, Katablepharis ulleungensis sp. nov. (Cryptista)

Katablepharids (or kathablepharids) are plastid-less, biflagellate protists widely distributed in marine and freshwater environments. Here, we report a new katablepharid species, Katablepharis ulleungensis sp. nov., isolated from a black sandy beach in Ulleung-do, South Korea. The stock strain was co-cultured with a small bicosoecid flagellate, serving as its prey. Cells were oblong, displayed two rows of Type I ejectisomes, and possessed two subapically inserted flagella with the posterior flagellum slightly longer than the anterior one. Rather than exhibiting linear or spiral locomotion, cells rotated in place using both flagella. Transmission and scanning electron microscopy revealed two types of ejectisomes, each consisting of two coiled ribbons. The feeding apparatus, situated at the cell apex, comprised a cytostome, 9-10 cytopharyngeal rings, numerous putative enzymatic vesicles, and inner microtubular arrays. Phylogenetic analyses using 18S rRNA gene placed the isolate within the genus Katablepharis; the phylogenetic placement together with distinct morphological characteristics supports its recognition as a new species. Using Illumina data, the complete mitochondrial genome (35,191 bp) was assembled and annotated to contain 60 genes, including 33 protein-coding genes, 25 tRNAs, and 2 rRNAs. This study expands our understanding of katablepharid diversity and contributes to mitochondrial genome resources for marine protists.