Achnanthidium peetersianum C.E.Wetzel, Jüttner & Ector

Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, Phytotaxa 575 (3), pp. 187-223 : 204-205

Achnanthidium peetersianum C.E.Wetzel, Jüttner & Ector in Wetzel et al. (2019: 345) ( Figs 144–165 )

Morphometric features:— 7.6–12.3 µm long; 2.0–2.7 µm wide; 29–34 striae in 10 µm; striae mainly composed of 3–5 areolae.

Taxonomical remarks:— Our population is in agreement with the type species, besides it has slightly narrower valves when compared to the French population (2.8–3.3 μm, Wetzel et al. 2019). This species has narrow valves with subcapitate to small capitate apices, similar to A. macrocephalum (Hustedt) Round & Bukhtiyarova (1996: 349) , although the latter taxon presents broadly capitate apices. Despite being difficult to separate, a reevaluation of A. macrocephalum type material was recently published in the same study as A. peetersianum ( Wetzel et al. 2019) . Using SEM micrographs, the authors showed that A. peetersianum presented predominantly rounded areolae and had a higher density per stria, while A. macrocephalum striae are composed of 1–2 elongated areolae of different lengths. Moreover, the axial area of the rapheless valve of the latter taxon is expanded, widening towards the central area. Recently, Yu et al. (2019) described two small species that resemble ours: A. subtilissimum P.Yu, Q. -M.You & Q.-X.Wang (in Yu et al. 2019: 161) and A. limosua P.Yu, Q. -M.You & Q.-X.Wang (in Yu et al. 2019: 158). The first species is likely a synonym of A. macrocephalum , and the second presents visibly larger valves (3.2–4.0 µm) than A. peetersianum .

Achnanthidium indistinctum also bears several similarities with A. peetersianum . They can be distinguished in girdle view by the apparently thickened striae in the first taxon, which is not visible in our specimens (see Figs 158, 159 ). Concerning the valve view, A. indistinctum presents rostrate apices and an almost nonexistent central area that never forms a fascia. On the other hand, A. peetersianum exhibits subcapitate to capitate apices and a small rounded central area that rarely forms a fascia. Notably, SEM analysis can also help separate both taxa due to the distal raphe ends straight in A. indistinctum and bent in our specimens. Despite the slightly narrower valves in the latter taxon (1.8–2.2 µm, Van de Vijver & Kopalová 2014), the metric features of these two species overlap.

Achnanthidium reimeri (Camburn) Ponader & Potapova (2007: 235) has a similar valve outline with our population, however, it presents longer and wider valves and a rhomboid central area on the rapheless valve ( Ponader & Potapova 2007).

Distribution and ecological information:— Recently described from Le Sauvigny River in France, it was one of the most well-distributed species found in this study (40 % of occurrence). It is plausible that this taxon was previously identified in Brazilian publications as A. macrocephalum . In this study, A. peetersianum occurred in all of the sampled habitats. Its ecological preference was for slightly acidic (pH optimum of 6.6–6.7), electrolytic-poor (optimum of 28.2–49 μS∙ cm-1) and oligo- to mesotrophic waters (TP optimum of 14.9–27.1 μg∙L- 1, TN optimum of 374.4–519.4 μg∙L- 1). We also observed this species in enriched waters (TP> 50 μg∙L- 1).

Figures

FIGURES 144–165. LM and SEM images of Achnanthidium peetersianum from Rifaína city, São Paulo state (SP371175). 144–154. LM views of raphe valves. 155–157. LM views of rapheless valves. 158, 159. Girdle views of the taxon. 160–162. SEM external view of raphe valves showing striation pattern, axial and central areas and the terminal raphe fissures deflected. 163. SEM internal view of the raphe valve. 164, 165. SEM external view of the rapheless valves showing striation pattern, axial and central areas. LM scale bar = 10 μm. SEM scale bars indicated in each figure.