Widespread and homologous life-history traits within the Rhopalodiales (Bacillariophyceae)

Significant changes have been proposed for the order Rhopalodiales, including subsuming Rhopalodia O.Mll. into Epithemia Ktz. to maintain monophyly and recognition of the genus Tetralunata Hamsher et al. However, hesitancies about adopting these higher-level classifications remain. We report widespread and homologous internal valve and life history traits as evidence to guide higher-level classification within the Rhopalodiales. We provide the first reports of internal valve formations in five species and show examples from six species representing multiple branches in the Rhopalodiales: Epithemia (Rhopalodia) gibba (Ehrenb.) Ktz., Epithemia adnata (Ktz.) Brb., Epithemia turgida (Ehrenb.) Ktz., Epithemia argus (Ehrenb.) Ktz., Epithemia catenata Schvarcz, Stancheva et Steward, and Epithemia reicheltii Fricke. This observation, along with shared, unique cell development and life history traits including endosymbiosis and sexual reproduction, should be considered in phylogenetic revisions.