Ultrastructural comparison of two diatom genera: Falcula and Pseudofalcula

Scanning electron microscopy (SEM) was used to conduct morphological observations on diatoms of the genera Falcula and Pseudofalcula distributed along the coasts of South China and Australia. Falcula media Voigt was reported as a newly recorded diatom species for China, while the ultrastructural morphology of F. semiundulata Voigt was studied for the first time. Morphological differences between Falcula and Pseudofalcula were further defined by comparing the former two species with P. hyalina (Takano) Gomez, Wang, & Lin. A key distinguishing feature between the two genera was the structure of the apical pore fields, comprising several slits in Falcula, and presenting as an ocellulimbus with a grid-like structure set into the valve mantle in Pseudofalcula. Falcula species were mainly epiphytic diatoms attached to seaweeds, while Pseudofalcula species were typically epizoic and attached to marine copepods. This study expands the geographic distribution of F. media, F. semiundulata, and P. hyalina and clarifies the basis of the morphological classification of the Falcula and Pseudofalcula genera.