A New Flagship Peritrich (Ciliophora,Peritrichida) from the River Rhine,Germany: Apocarchesium arndti n. sp.

ABSTRACT. We discovered a free‐living peritrich ciliate with outstanding features in the River Rhine. Its morphology and 18S rRNA gene sequence were studied with standard methods. Apocarchesium arndti n. sp. has several peculiarities. (i) There are ordinary zooids, macrozooids, and microzooids, which form a hemispherical rosette on a discoidal base, the stalk dish, locking the ～18 μm wide and up to 2 mm long, spirally contracting colony stalk. (ii) The stalk myoneme is connected only to the microzooids. (iii) A rosette contains up to 50 zooids not connected to each other but individually attached to the stalk dish with the scopula. (iv) The ordinary zooids are epistylidid, trumpet‐shaped (～6:1 length:width), about 180 × 30 μm in size, and have an ellipsoidal macronucleus subapically between oral cavity and dorsal side. (v) The myoneme system of the zooids, which can contract individually, forms a tube‐like structure in the narrow posterior half of the cell. (vi) The silverline pattern belongs to the transverse‐striate type. (vii) The oral apparatus is of usual structure, with kinety 1 of peniculus 3 distinctly shortened proximally. (viii) The 18S rRNA places A. arndti n. sp. as a distinct lineage near Vorticella and Carchesium. These data are used to provide an improved diagnosis of the genus Apocarchesium. Features (i)–(iii) and the molecular data indicate that Apocarchesium could be the type genus of a new peritrich family.