The search finds an end: the morphologically chimeric hysterocinetids belong to the subclass Hymenostomatia (Ciliophora: Oligohymenophorea)

Abstract

The digestive tract of oligochaetes harbours a diverse ciliate community embracing plagiotomids, nyctotherids, astomes, and hysterocinetids. Although several hundred intestinal ciliate species are known, only two hysterocinetids have been reported from the intestine of Holarctic lumbricid earthworms hitherto. The present study is focused on the taxonomy, morphology, and molecular phylogeny of two lumbricid-dwelling taxa, the insufficiently known Protoptychostomum simplex and Hysterocineta bellerophon sp. nov., detected in Central Europe. Diagnoses of the genera Protoptychostomum and Hysterocineta were improved taking into account also features of the oral ciliature for the first time. In the past, the proper classification of hysterocinetids in the class Oligohymenophorea was hampered by the dramatic remodelling of the anterior body end into a thigmotactic sucker, as well as by the helicalization and transfer of the oral apparatus to the posterior body pole. The present phylogenetic analyses of nuclear and mitochondrial genes robustly reject the classification of hysterocinetids in subclass Scuticociliatia or as a distinct subclass, Hysterocinetia. However, hysterocinetids were consistently and robustly nested deep in subclass Hymenostomatia in a sister-position to the free-living Tetrahymena paravorax. The hymenostome affinity of hysterocinetids is also corroborated by the parakinetal stomatogenesis and the stichodyad paroral membrane accompanied by three regularly organized membranelles.