During a survey of the aerophilic diatom flora on some localities in Papua New Guinea (Karkar Island, Boïsa Island), several morphologically distinguishable Orthoseira taxa have been observed by means of light and scanning electron microscopy. The genus Orthoseira is characterized by its typical central carinoportulae, the rather deep mantle and the well-developed girdle structure, composed of several broad copulae. Most species are typically found in aerophilic habitats such as the spray zone of waterfalls, moss carpets on tree trunks and soils.
At least four different taxa have been observed. They can be separated from each other based on the shape and structure of their linking spines, the structure of the carinoportulae, the valve surface structure composed of ridges, spines, rimmed areolae or simply large hyaline zones, and the girdle structure. The latter is often a neglected feature in the separation of the different morphological groups within Orthoseira. The ornamentation of the copulae varies in number and size of perforations and superficial warts and papillae. Additionally, the structure of the valvocopula also plays a distinct role in the characterization of the different taxa.
At present, none of the different observed taxa could be identified based on the currently available literature. Recently, the structure of the type of Orthoseira roeseana (Rabenhorst) Pfitzer was analysed showing that most records of the latter should be revised given its unique morphology (Kochman-Kędziora et al. 2023).
The present poster illustrates the different Orthoseira taxa on Papua New Guinea and compares their morphology with that of the type of O. roeseana.

Kochman-Kędziora N., Kusber W.-H., Kociolek J.P. & Van de Vijver B. (2023). Observations on the type material of Melosira roeseana Rabenhorst and Orthoseira spinosa W.Smith (Orthoseiraceae, Bacillariophyta). Notulae Algarum 274: 1–10.