Abstract
Stentor, 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 18 S 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. Here, 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. This work expands the current understanding of the ecological distribution of and behavioral features present within genus Stentor.
Data availability
Image and video files analyzed for phototaxis and habituation experiments are available upon request. 18 S SSU DNA sequences are included in the published work and are available on GenBank with the following accession numbers: Sequence 1 ( *S. coeruleus* ; wild strain) PX056129, Sequence 2 ( *S. muelleri* ; wild strain) PX056130, Sequence 3 ( *S. pyriformis* ; wild strain) PX056131, Sequence 4 ( *S. stipatus* ; spec. nov., wild strain) PX056132, Sequence 5 ( *S. stipatus* ; spec. nov., wild strain) PX056133, Sequence 6 ( *S. stipatus* ; spec. nov., wild strain) PX056134, Sequence 7 ( *S. stipatus* spec. nov. wild strain) PX393983. *S. stipatus* spec. nov. as described in this work, has been registered on ZooBank, LSID: urn: lsid: zoobank.org: act: D381C528-8486-424E-9C69-CBE1612C4295; with this work also registered under LSID: urn: lsid: zoobank.org: pub:0C20B702-EE37-47C9-8DDE-417C2B39CBC4.
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Acknowledgements
Initial discovery and collection of this novel Stentor species was made possible through funding by the Grass Foundation in 2023. Initial culturing and most of the phototaxis experiments were performed at the Marine Biological Laboratory (MBL) in the summers of 2023 and 2024. We also thank Mark Slabodnick for guidance on phylogenetic analysis.
Funding
Initial discovery of S. stipatus spec. nov. was supported by Grass Fellowship funding to DC from the Grass Foundation. Molecular analysis was performed in the CCC Summer Course supported by NSF grant DBI-1548297 and NIH 5K12GM081266. Experiments on S. stipatus spec. nov. habituation were supported by NIH grant R35GM130327 and work on the phylogeny of wild Stentor species was supported by a grant from the Moore Foundation.
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DHR performed the habituation experiments as well as the phylogenetic analysis and drafted the manuscript. AA, ET, AM, CV and BL all contributed to single cell 18 S SSU sequencing. BL also contributed to phylogenetic tree assembly. WFM supervised DHR and provided support and guidance for experimental design as well as feedback and edits on the manuscript. KR performed phototaxis and action spectrum experiments. DBC performed phototaxis and action spectrum experiments, did the imaging for morphological analysis, supervised KR, and drafted and edited the manuscript. All authors read and approved of the final manuscript.
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Rajan, D.H., Lee, B., Albright, A. et al. Stentor stipatus is a new unicellular species that demonstrates habituation and unique phototaxis. Sci Rep (2026). https://doi.org/10.1038/s41598-026-40277-0
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DOI: https://doi.org/10.1038/s41598-026-40277-0
