A new marine urostylid ciliate, Arcuseries minima sp. nov., was discovered in South Korea. Morphological observations and molecular phylogenetic analyses based on small subunit ribosomal DNA (18S rDNA) sequences were used to describe the new species. Arcuseries minima is most similar to A. scutellum among all Arcuseries species, but differs in the following main characters: number of adoral membranelles (13–16 vs. 17 or 18), cortical granules (yellowish, clustered around cirri and dorsal bristles vs. colorless, irregularly scattered), number of macronuclear nodules (20–27 vs. 42–90), number of midventral cirri (5–10 vs. 12–14), and number f transverse cirri (5 or 6 vs. 8). The new species and A. scutellum differ from A. petzi and A. warreni in having smaller body size (≤80 μm vs. ≥80 μm) and fewer cirri: left marginal (≤ 17 vs. ≥ 18) and transverse (≤ 8 vs. ≥8) cirri. This relationship was supported by the phylogenetic tree, where these two groups were separated into two branches.
