A rediscovery of Cymbella mirabilis Hustedt, a rare endemic diatom, and description of Alveocymba gen. nov.

The first comprehensive morphological investigation of the rare, endemic species Cymbella mirabilis Hustedt from the Malili lake system (Indonesia) was prepared using recent samples. This species was originally described on the basis of a single valve. The species was assigned to Cymbella sensu lato on the basis of its asymmetry about the apical axis. SEM investigation shows that this species has a number of unique features relative to other freshwater cymbelloid diatoms, such as an axial plate and marginal laminae on either side of the valve and pseudoseptum at each end of the valve. In these features C. mirabilis resembles certain species in genera such as Gomphoneis Cleve, Afrocymbella Krammer, Gomphosinica Kociolek et al. and Gomphonema Ehrenberg. The unique combination of features in this taxon suggest it cannot be placed in any known genus within the Cymbellales. Thus, we describe a new diatom genus, Alveocymba gen. nov., however, the taxonomic placement of this new genus remains unclear and requires further investigation.