Achnanthidium peruvianum E.Morales & Ector in Morales et al. (2011: 121) (Figs 166–193)

Synonym: Achnanthes kryophiloides M.H.Hohn in Patrick et al. (1966), p. 468, pl. 1, Figs 5, 6

Morphometric features:— 8.0–20.2 µm long; 2.3–4.3 µm wide; 28–34 striae in 10 µm; striae mainly composed of 4–5 areolae.

Taxonomical remarks:— The species presents similar morphological features to another South American species called Achnanthidium sehuencoensis, which has almost the same valve outline. However, SEM analysis separated the two due to the deflected terminal raphe fissures, the non-expanded central raphe fissures, and lower stria density (20–24/10 µm, Morales et al. 2009) compared to A. peruvianum .

Achnanthidium petersenii (Hustedt) C.E.Wetzel, Ector & Jüttner (in Jüttner et al. 2019: 66) has wider fascia on the raphe valve. However, on the rapheless valves, the axial area is moderately broad, and the central area broadly rhombic-lanceolate (Jüttner et al. 2019). In A. peruvianum, the axial area is narrow and linear, while the central area is undifferentiated with short striae (Morales et al. 2011).

Distribution and ecological information:— Achnanthidium peruvianum was described from the Rondos River in Peru, South America (Hohn in Patrick et al. 1966). In our samples, the species was rare (abundance <3 %), occurring in less than 5 % of the counted samples in planktonic and surface sediment assemblages. In the surface sediment, it showed ecological preferences for neutral (pH optimum of 6.9), electrolyte-poor (optimum of 30.8 μS∙ cm-1), and oligotrophic environmental water conditions (TP optimum of 18.4 μg∙L- 1 and TN optimum of 372.5 μg∙L- 1).