Orthoseira is a small genus of centric diatoms, typically found in bryophytic communities.
The most widespread Orthoseira species apparently is O. roeseana (Rabenhorst) Pfister, showing a worldwide distribution with recently published records from all continents, including the Antarctic. A recent study on the taxonomic history of the genus Orthoseira (Danz et al. 2022) indicated that the generally accepted idea of the genus is not supported by the historic material and that its taxonomic identity is controversial due to incorrect interpretation and lack of proper analysis of historic (type) material. As a part of a complete taxonomic revision of the genus Orthoseira, the original Rabenhorst sample of Melosira roeseana and several other samples and slides from nineteenth century historic material, deposited in the Van Heurck collection (BR, Meise Botanic Garden) have been investigated using light and scanning electron microscopy observations. In combination with the review of all historical (nineteenth century) literature on this species, this allowed for a step-by-step tracing of the correct identity of Orthoseira roeseana and Orthoseira spinosa, generally regarded as a younger synonym. The results revealed that the two species, O. roeseana and O. spinosa, represent in fact two species with different morphological characteristics (Kochman-Kędziora et al. 2023). Most likely, force-fitting Orthoseira populations to Orthoseira roeseana obscure a lot of hidden diversity in this genus, therefore a better knowledge of original and historic material is crucial for further studies on taxa morphologically similar to Orthoseira roeseana.