Achnanthidium sp.

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 : 214-215

Achnanthidium sp. ( Figs 317–327 )

LM observations:— Valves linear-elliptic to linear-lanceolate, with broadly rounded apices to subrostrate ( Figs 317– 327 ); 7.4–11.6 µm long, 3.5–4.1 µm wide. Raphe valve: axial area narrow and linear, central area small rounded, with 1–2 shortened striae on both sides of the valve ( Figs 317–321 ). Raphe filiform straight. Transapical striae slightly radiate throughout the entire valve, becoming denser and strongly radiate towards the apices; 24–28 in 10 µm ( Figs 317–321 ). Rapheless valve: axial area narrow and linear, widening towards the central area, central area absent or lanceolate ( Figs 322–327 ). Transapical striae radiate throughout the entire valve, becoming denser and strongly radiate towards the apices; 23–27 in 10 µm ( Figs 322–327 ).

Taxonomical remarks:— Achnanthidium modestiformis (Lange-Bertalot) Van de Vijver (in Van de Vijver et al. 2002: 17) presents similar morphometric features (12–17 µm long, 3.5–4 µm wide, stria density 24–25/10 µm, as Achnanthes modesta Manguin (in Bourrelly & Manguin 1954: 22). However, the Brazilian population has broadly rounded to rostrate apices, whereas they are more capitate in A. modestiformis . Moreover, Achnanthidium sp. has shorter valves.

Another similar specimen cited in Brazilian literature is Achnanthes acares M.H. Hohn & Hellerman (1963: 273) ( Schneck et al. 2008). The illustrations presented by Hohn & Hellerman (1963) concerning this taxon show a different central area, elliptical in the raphe valve and coarse striae resembling Planothidium . Krammer & Lange-Bertalot (1991) illustrated the holotype of this species using LM, however, they only showed the raphe valves of the specimens, which probably do not correspond to a monoraphid taxon.

Additionally, another Brazilian population resembled ours, but it was not identified at the species level ( Santos et al. 2011, Figs 21–24 , as “ Achnanthidium sp. ”). More studies are necessary concerning the identity of this taxon.

Distribution and ecological information:— Achnanthidium sp. was rare (occurrence of <5 %), found in the three habitats in low abundance (≤ 0.6 %). In the surface sediment, it occurred in 10 % of the samples, showing ecological preferences for neutral pH (optimum of 7.2), medium cond. (optimum of 82.4 μS∙ cm-1), and hypereutrophic environments (TP optimum of 420.8 μg∙L- 1 and TN optimum of 8377.9 μg∙L- 1). The taxon presented the highest range of tolerance, occurring from oligotrophic to hypereutrophic water conditions.

Figures

FIGURES 317–327. LM images of Achnanthidium sp. from Serraria reservoir, São Paulo state (SP-469204). 317–321. LM views of raphe valves. 322–327. LM views of the rapheless valves. Scale bar = 10 μm.

FIGURES 2–25. LM and SEM images of Achnanthidium acutum sp. nov. from type material (SP-427898), Rio Grande reservoir, São Paulo state. Figure 17 corresponds to the holotype. 2–12. LM views of raphe valves. 13–20. LM views of rapheless valves. 21. SEM external view of raphe valves showing striation pattern, axial and central areas. 22. SEM external detail of the distal raphe end. 23. SEM external view of the rapheless valve showing striation pattern, axial and central areas. 24. SEM internal view of the raphe valve. 25. SEM internal view of the rapheless valve. LM scale bar = 10 μm. SEM scale bars indicated in each figure.