Re-discovery and novel contributions to morphology and multigene phylogeny of Protospirella mazurica (Raabe, 1968) Aescht, 2001 (Ciliophora: Pleuronematida), an obligate symbiont of the river nerite Theodoxus fluviatilis Linnaeus, 1758 (Mollusca: Gastropoda)

Findings on three endocommensal scuticociliates (Protista, Ciliophora) from freshwater mollusks, including their morphology and molecular phylogeny with descriptions of two new species

“…Phylogenetic analysis strongly rejected the inclusion of Myxophyllum in the order Philasterida, and fully supported Myxophyllum as belonging to the order Pleuronematida. Furthermore, Zhang and Vďačný ( 2021 , 2023b ) proposed to transfer all members of the order Thigmotrichida into the order Pleuronematida, and proposed possible reasons for the divergence of Myxophyllum from typical pleuronematids, i.e., that the symbiotic lifestyle of Myxophyllum is likely responsible for the dramatic remodelling of its oral apparatus and ciliature (e.g., the transfer of the mouth pocket to the posterior body region, the strong reduction of the paroral membrane and adoral organelles, and the formation of vestibular kineties). The general topology of the 18S rRNA gene phylogenetic tree in the present study is consistent with previously published phylogenetic studies (Antipa et al 2016 , 2020 ; Gao et al 2013 ; Poláková et al 2021 ; Rataj and Vďačný 2018 , 2022 ; Zhang and Vďačný 2024 ), and the phylogenetic position of Myxophyllum is consistent with Zhang and Vďačný ( 2021 ), that is Myxophyllum belongs to the family Thigmophryidae, in the order Pleuronematida.…”

“…While the majority of ciliates are free-living in aquatic and terrestrial habitats (de Puytorac et al 1994 ; Hu et al 2019 ; Song et al 2009 ; Wang et al 2022 ), a substantial portion, encompassing at least 2600 species, are symbionts (Corliss 2000 ). Symbiotic ciliates are present in a broad spectrum of invertebrate and vertebrate hosts, including mollusks, which are one of the most broadly distributed and diverse invertebrate groups (Irwin and Lynn 2015 ; Lu et al 2023 ; Lynn et al 2018 ; Mayén-Estrada et al 2021 ; Prosser et al 2018 ; Raabe 1967 , 1970 , 1971 , 1972 ; Song et al 2003 ; Souidenne et al 2019 ; Van As and Basson 2004 ; Zhang and Vďačný 2021 , 2022 , 2023a , b , 2024 ).…”