Insights into Nitzschia amphibia Grunow (Bacillariophyta, Bacillariaceae): Lectotypification and a comparative study with N. amphibioides Hustedt and N. semirobusta Lange-Bertalot

The original description and illustrations of Nitzschia amphibia by Grunow in 1862 are insufficiently clear for the accurate identification of contemporary specimens, which may have contributed to misidentifications among N. amphibia, N. amphibioides, and N. semirobusta commonly found in the literature. This study aims to clarify the morphological differences between these three species by examining both historical and recent samples, along with the original description of N. amphibia. The species are compared morphologically, and a lectotype for N. amphibia is designated. This work also includes the first report of scanning electron microscopy images of N. amphibioides. While fibula morphology clearly distinguishes N. amphibia from N. semirobusta, measurements such as valve length, width, and the densities of striae and fibulae do not separate these two species. In contrast, N. amphibioides is distinctly different, characterized by longer, wider, and more robust valves with coarser striation. Additionally, this study reassesses various previously published records and revises some identifications based on newly gathered data.