The curious case of Hypotrichidium bichromum n. sp., a new polymorphic and cannibalistic spirofilid ciliate (Ciliophora, Hypotrichia)

Members of the genus Hypotrichidium are adapted to a planktonic lifestyle and are characterized among hypotrichs by their unique pyriform body with pointed tail and spiral cirral rows around the whole body. During a survey of Korean freshwater ciliates, we discovered a new Hypotrichidium species that changes its cortical granules color. Observations on the morphology and behavior of Hypotrichidium bichromum n. sp. revealed several other unusual features such as the polymorphic life cycle and cannibalistic behavior. When grown in a fresh culture, well-nourished specimens are characterized by a pyriform body, yellow cortical granules, and two macronuclear nodules. In declining cultures, two morphologically different morphs occur, both with ruby-red cortical granules: (1) a well-nourished morph with a pyriform body and 2–4 macronuclear nodules, which shows a cannibalistic behavior feeding on other well-nourished cells, encysting cells, and reorganizers; (2) a fast-swimming, non-feeding morph with slender body, two macronuclear nodules, and highly degenerated undulating membranes. The complete ontogenetic process of the new species is described to show the origin of each structure. Moreover, phylogenetic analyses based on 18S rRNA gene sequences show that the new species is placed in a clade containing sequences of three congeners. • A new species of Hypotrichidium with unique morphological characters and behavior is discovered and described in detail. • The changing color of the new species is an unusual feature that is present in only few ciliate species. • The polymorphic life cycles are found in only a few ciliate species and is a strong species character. • The detailed ontogenetic study shows more characters to consider when studying spirofilid ciliates.