Morphological and molecular examination of the ciliate family Lagynusidae (Protista, Ciliophora, Prostomatea) with descriptions of two new genera and two new species from China

Ciliates in the class Prostomatea play an important role in the global microbial loop due to their significant abundances and broad feeding strategies at the foundation of food webs. Despite their importance in ecosystems, the taxonomy and systematics of this group of ciliates has long been poorly understood, with this being especially true for members of the family Lagynusidae. Here we examine four lagynusids collected from sandy beaches in China, using silver-staining and 18S rRNA gene sequencing techniques. These investigations revealed two new genera and two new species and provided details for two little known forms: Penardella marina gen. nov., sp. nov., Apolagynus cucumis (as reported by Penard. Études sur les infusoires d'eau douce. Georg and Cie, Genève, 1922) gen. nov., comb. nov., Lagynus minutus sp. nov., and Lagynus elegans (Engelmann in Z Wiss Zool 11:347–393, 1862) Quennerstedt (Acta Univ Lund 4:1–48, 1867). Penardella gen. nov. can be morphologically distinguished by having more than three dikinetidal perioral kineties. Apolagynus gen. nov. differs from the closely related genus Lagynus in the absence of a conspicuous neck-like region. The ciliature of Apolagynus cucumis is revealed here for the first time, which demonstrates the classification of this species within Lagynusidae. Furthermore, Apolagynus binucleatus (Jiang et al., 2021) comb. nov. is established according to the new finding. The results of our phylogenetic analyses based on the 18S rRNA gene support the establishment of two new genera and indicate that Lagynusidae is monophyletic, which further strengthens its valid taxonomic status.