Achnanthidium tropicocatenatum G.C.Marquardt, C.E.Wetzel & Ector (Figs 238–275)

Morphometric features:— 6.7–18.1 µm long; 2.3–3.1 µm wide; 36–42 striae in 10 µm; striae mainly composed of 4–6 areolae.

Taxonomical remarks:— Achnanthidium tropicocatenatum and A. catenatum are similar. In the protologue of the first species, many characters were taken into account to provide the differences between them (e.g. SEM, LM, geometric morphometric analysis, and ecological preferences). Nevertheless, it is possible to distinguish both taxa by the less inflated central portion, fewer capitate apices and higher stria density in A. tropicocatenatum (28–30 striae/10 µm in A. catenatum, Bílý & Marvan 1959). Also, in A. catenatum, the girdle view is distinct, with the valve being strongly curved with the apices sharply pointed. In contrast, in A. tropicocatenatum, the valve is less arched with apices slightly curved (Marquardt et al. 2017).

Marquardt et al. (2017) commented about the confusion made with two different populations in Novais et al. (2015), probably joining two different species in one taxon ( A. lusitanicum in Novais et al. 2015). The first population from Janeiro de Baixo, Zêzere River (Tejo basin, Portugal), is considered the type material of the species and differs from A. tropicatenatum by its rostrate apices, or broadly rounded ones in smaller specimens.

Distribution and ecological information:— The taxon was recently described despite its presence in Brazilian literature being misidentified as A. catenatum . Indeed, in our samples, the species A. tropicocatenatum had the highest frequency of occurrence (61 %). Its ecological optimum was calculated for all habitats. The species showed an ecological preference for slightly acid to neutral (pH optimum of 6.7–6.9), low to medium cond. (optimum of 53.7–94.3 μS∙ cm-1) and mesotrophic waters (TP optimum of 19.8–37.3 μg∙L- 1 and TN optimum of 504.2–665.8 μg∙L- 1). Achnanthidium tropicocatenatum was previously described from oligotrophic conditions in São Paulo state (Marquardt et al. 2017); however, it presented greater tolerance in our study area and was identified from oligo- to eutrophic conditions.