Phylogeny, morphology, and behavior of the new ciliate species Stentor stipatus

BackgroundStentor, the genus of large trumpet-shaped ciliates, is well-known for its complex morphology and striking behaviors. Members of this genus are distributed throughout the world in a wide and diverse pool of freshwater ecosystems. Recently, the molecular phylogeny of Stentor has been explored through comparison of 18S small subunit (SSU) ribosomal DNA (rDNA) sequences, clarifying several previously mischaracterized species and species complexes. However, despite their wide distribution, to-date, only about a dozen species of Stentor have been described and verified by phylogenetic means. ResultsHere, we introduce the discovery of a new species within genus Stentor: Stentor stipatus spec. nov., so named for their distinctive cytosolic dark pigmented granules which surround the macronucleus and are also present cortically alongside cortically-distributed green microalgae. We present morphological, phylogenetic, ecological, and behavioral characterizations of these cells. Phylogenetic analysis of S. stipatus spec. nov. by comparison of SSU rDNA sequence suggests it is a distinct species from its closest relative, S. amethystinus. We demonstrate that S. stipatus spec. nov. is capable of habituation in response to repeated mechanical stimulation. Further, S. stipatus spec. nov. exhibits strongly directed positive phototaxis, like its relative S. pyriformis, but with a distinct action spectrum from both S. coeruleus and S. pyriformis. Finally, S. stipatus phototaxis response strength varies in a consistent pattern throughout the day, providing evidence of potential circadian regulation. ConclusionsThis work expands the current understanding of the ecological distribution of and behavioral features present within genus Stentor.