Achnanthidium saprophilum (H.Kobayashi & Mayama) Round & Bukhtiyarova, 1996

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 : 208-210

Achnanthidium saprophilum (H.Kobayashi & Mayama) Round & Bukhtiyarova ( Figs 215–237 )

Morphometric features:— 6.0–15.3 µm long; 2.5–3.2 µm wide; 26–32 striae in 10 µm; striae mainly composed of 4–5 areolae.

Taxonomical remarks:— The species can be confused with smaller specimens of A. tropicocatenatum , however, they differ by the subcapitate to capitate apices and higher stria density (36–45 in 10 µm) in A. tropicocatenatum . Achnanthidium peetersianum presents small capitate apices and has narrower valves (2.0–2.7 µm) than A. saprophilum . Also, externally terminal raphe fissures are slightly bent to the same side in A. peetersianum , at the same level as the striae on the valve face, whereas in our species external terminal raphe fissures are always straight and terminating after the last striae. Despite displaying different raphid valves with linear-lanceolate to rhombic-lanceolate in A. vandevijveri sp. nov., the rapheless valves present almost the same valve outline as A. saprophilum . However, the valves outline in A. vandevijveri sp. nov. are slightly wider and longer.

Achnanthidium acsiae Wojtal, E.Morales, Van de Vijver & Ector (in Wojtal 2011: 226) has similar valve outline and metric features as A. saprophilum , however, the former has the central area of raphe valve often forming a bilateral fascia, a feature never observed in our population. Achnanthidium saprophilum rarely shows a unilateral fascia. Still, the stria density is higher than ours and they are composed of less areolae (28–36 striae in 10 µm, 3–4 areolae per stria, Wojtal et al. 2011).

Achnanthidium pseudolineare differs from A. saprophilum by the rapheless valve, presenting linear valves with parallel margins and broadly rounded apices. The stria density is also different between the species, being higher in A. pseudolineare (32–40 in 10 µm, Van de Vijver et al. 2011). Achnanthidium costei Pérès & Le Cohu (in Pérès et al. 2014: 390) also has linear valves with parallel margins, but this outline is characteristic of the raphe valves ( Pérès et al. 2014). Its rapheless valves shows another distinct feature: the axial area is wider and lanceolate. Achnanthidium saprophilum has both valves linear-lanceolate with convex margins, and the axial area straight on rapheless valves.

Concerning Achnanthidium saprophilum , despite the number of areolae per striae in the isotype being higher (4–7, Hlúbiková et al. 2011) than ours (4–5), the areolae morphology of the valves from our population is variable, including apically elongated areolae, as described for A. saprophilum ( Kobayasi & Mayama 1982) .

Distribution and ecological information:— The species occurred in planktonic, periphytic and surface sediment assemblages in low abundance (<5 %). In one surface sediment sample, it occurred with a high relative abundance (25 %). This environment is characterized by neutral waters (pH 7.1), with relatively high cond. 225.6 μS∙ cm-1, and mesotrophic conditions (TP 20 μg∙L- 1, TN 699.9 μg∙L- 1).

Figures

FIGURES 215–237. LM and SEM images of Achnanthidium saprophilum from Rio Grande reservoir, São Paulo state (SP427898). 215–222. LM views of raphe valves. 223–231. LM views of rapheless valves. 232–234. SEM external view of raphe valves showing striation pattern, axial and central areas. 235. SEM internal view of the raphe valve. 236. SEM external view of the rapheless valve showing striation pattern, axial and central areas. 237. SEM internal view of the rapheless valve. LM scale bar = 10 μm. SEM scale bars indicated in each figure.