Ultrastructure of three species of Tryblionella (Bacillariophyceae) found in freshwater habitats in Hunan Province, China, including descriptions of two new species.

Two diatom samples collected from two freshwater habitats in Hunan Province, China, were studied using light microscopy and scanning electron microscopy. Three Tryblionella species were found; one was identified as T. calida (Grunow) Stroganov et al., and the other two species are established here as new species: T. dongtingensis sp. nov. and T. hunanensis sp. nov. These two new species are morphologically allied to T. calida because they possess the following unique combination of characters: 1) a more or less undulate valve face from side to side; 2) transapical ribs; 3) uniseriate striae with rounded, rimmed inner openings of areolae; 4) a longitudinal row of bar-like ribs connecting the raphe canal wall on both the valve side and the distal mantle side; 5) a marginal ridge; and 6) squat, sometimes apically elongated fibulae. This group around T. calida is different from the group around T. apiculata W. Gregory, which possesses an axial sternum and different stria and areola structures. Additional details of the complex marginal ridges in Tryblionella are revealed and discussed in this paper. Whether the group around T. calida is a monophyletic group and possesses a synapomorphy cannot currently be resolved, and further investigations are needed.