Bioacoustics of Rubrioxytricha

With the newly sequenced Rubrioxytricha guamensis included in the phylogenetic analysis, the monophyly of the genus Rubrioxytricha is corroborated, which is consistent with previous studies ( Chen et al . 2015 , 2017 ; Kumar et al . 2018a ). However, Luo et al . (2021) argued for the paraphyly of the genus because of the nesting of Pseudogastrostyla flava and Polystichotrix monilata , although it was not statistically supported by the ML (59) analysis. In the current study, the sister relationship between these two species and the genus Rubrioxytricha is fully supported (ML/BI, 100%/1.00). It can be explained by the high level of morphological similarity among them: they all have a flexible body, one marginal row on each side of the body, and brightly coloured cortical granules. However, Rubrioxytricha differs significantly from Polystichothrix and Pseudogastrostyla by having a typical oxytrichid pattern of 18 FVT cirri (vs more than 18 FVT cirri in short lines) ( Fan et al . 2015 ; Luo et al . 2017 , 2021 ). Furthermore, in terms of dorsal ciliature, the Rubrioxytricha –Polystichothrix– Pseudogastrostyla clade demonstrates great variation: (1) Pseudogastrostyla shares four DK with R. guamensis , R. haematoplasma and R. guangzhouensis ; (2) Polystichothrix shares five DK with R. ferruginea and R. indica ; (3) six DK are present in R. tsinlingensis ; (4) Pseudogastrostyla shares one caudal cirrus with R. guamensis , R. haematoplasma , R. indica and R. guangzhouensis ; (5) one or two caudal cirri occur in R. ferruginea and three caudal cirri are present in R. tsinlingensis ; (6) Polystichothrix lacks caudal cirri ( Berger 1999 ; Naqvi et al . 2006 ; Chen et al . 2015 ; Fan et al . 2015 ; Luo et al . 2017 ; Kumar et al . 2018a ). The branch consisting of species within the genus Rubrioxytricha and Polystichothrix– Pseudogastrostyla is closely related to Ponturostyla enigmatica and Pseudocyrtohymena koreana as shown in the previous reports ( Chen et al . 2017 ; Kumar et al . 2018a ; Luo et al . 2021 ). Morphologically, Ponturostyla differs significantly from the genus Rubrioxytricha in: (1) having multiple fragmentations (vs absence) of DK 3; (2) more than one (vs one) left and right marginal rows; and (3) absence (vs presence) of caudal cirri ( Song 2001 ). Pseudocyrtohymena differs from the genus Rubrioxytricha in: (1) the arrangement of the undulating membranes (in Cyrtohymena vs Australocirrus pattern); and (2) the slightly higher number of adoral membranelles (38–53 vs 21–48) ( Jung et al . 2015 ). Thus, the close grouping in phylogenetic analyses might be attributed to the lack of molecular sequences of related species. A detailed investigation of the morphology and the gene sequences of related and yet-tobe-discovered species will clarify further the phylogenetic positions of this group of ciliates.