Novel findings on two loricate hypotrichous ciliates (Protista: Ciliophora): integrating morphology, morphogenesis, ecology, and phylogeny, with description of a new species

While the overwhelming majority of hypotrichous ciliates are lorica-free benthic organisms, certain species like those in the genus Stichotricha Perty, 1849 exhibit dual lifestyles, i.e. residing in self-secreted loricae when attached to substrates, while adopting a free-swimming aloricate existence upon detachment. These chambered ciliates are rarely studied, and most species lack information on their infraciliature, morphogenetic processes, and molecular phylogeny. In this study, a new species, Stichotricha weishanensis sp. nov. and a new Chinese record, also type species, Stichotricha secunda Perty, 1849, were investigated. Morphogenetic information of S. weishanensis sp. nov., which constitutes the first detailed morphogenetic report of this genus, and the first SSU rDNA sequences for both species are provided. Phylogenetic analyses consistently placed Stichotricha within the family Chaetospiridae with full support and revealed a close relationship between Chaetospiridae and Gonostomatidae, supported by the shared Gonostomum-patterned adoral zone, arrangement of dorsal kineties, and certain morphogenetic features. Based on an analysis of the updated information, the diagnoses of Stichotricha and S. secunda are revised and a key to the identification of Stichotricha species is provided. In addition, loricae secretion behaviour, selective adaptive mechanisms and geographic distributions of Stichotricha species are discussed.