Achnanthidium lusitanicum Novais & M.Morais

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 : 198-200

Achnanthidium lusitanicum Novais & M.Morais in Novais et al. (2015: 129) ( Figs 45–99 )

Morphometric features:— 6.5–16.5 µm long; 2.6–3.4 µm wide; 30–34 striae in 10 µm; striae mainly composed of 4–6 areolae.

Taxonomical remarks:— Achnanthidium lusitanicum shares morphological similarities with A. eutrophilum (Lange-Bertalot) Lange-Bertalot (1999: 271) but our species has slender valves narrower than the latter taxon, which has rhombic-lanceolate to rhombic-elliptical frustules in small specimens and valve apices bluntly rounded to cuneate. Also, A. lusitanicum have a higher stria density ( A. eutrophilum : 3.2–5.8 µm wide, striae 25–30/10 µm, Hlúbiková et al. 2011).

Achnanthidium caravelense Novais & Ector (in Novais et al. 2011: 142) is similar to A. lusitanicum when analyzed by LM and SEM. However, it differs from A. lusitanicum mainly by the elliptic raphe valve with almost linear margins. The SEM images show that A. caravelense presents a lower number of areolae per striae (3–4, Novais et al. 2011).

The valve outline of Achnanthidium palmeti Gassiole, Le Cohu & M.Coste (2013: 22) is very similar to A. lusitanicum , and the cell metrics overlap. However, some A. palmeti specimens have slightly wider valves (2.7–4.6 μm, Gassiole et al. 2013). The main character differing both species is the frustules that are arched with slightly recurved apices in A. palmeti and its terminal raphe fissures of the raphid valve are deflected towards the same side. Indeed, our species has straight raphe fissures and an almost flat valve face.

In São Paulo state, Achnanthidium saprophilum (H.Kobayasi & Mayama) Round & Bukhtiyarova (1996: 349) shows valves with no inflation in the central portion as A. lusitanicum , and it also presents lower stria density (26–30 in 10 μm). Besides that, A. saprophilum showed presence of areolae with different shapes, a feature not observed in A. lusitanicum . In addition, a similar Brazilian population was cited as A. jackii by Bartozek et al. (2018). However, the cited species had linear-lanceolate valves with broadly rounded and rostrate apices, a central area always forming a fascia and lower stria density (28–30/10 µm, Van de Vijver et al. 2018). Notably, the central area of A. lusitanicum rarely produces a unilateral fascia, being often absent or small rounded with some shortened striae.

Distribution and ecological information:— The taxon was found in all three habitats (occurrence of 10 %), but it was more often found in surface sediments from reservoirs. In the sediment assemblages, the ecological preferences of the taxa were for slightly alkaline (pH optimum of 7.3), medium cond. (optimum of 100.3 μS∙ cm-1) and eutrophic waters (TP optimum of 56.7 μg∙L- 1 and TN optimum of 1016.5 μg∙L- 1). It is worth mentioning that it also occurred in oligo- and mesotrophic waters.

Figures

FIGURES 45–71. LM and SEM images of Achnanthidium lusitanicum from Garças Lake, São Paulo state (SP-469485). 45–56.LM views of raphe valves. 57–65. LM views of rapheless valves. 66, 67. SEM external view of raphe valves showing striation pattern, axial and central areas. 68. SEM internal view of the raphe valve. 69, 70. SEM external view of the rapheless valves showing striation pattern, axial and central areas. 71. SEM internal view of the rapheless valve. LM scale bar = 10 μm. SEM scale bars indicated in each figure.

FIGURES 72–99. LM and SEM images of Achnanthidium lusitanicum from Guarapiranga reservoir, São Paulo state (SP-428513). 72–83. LM views of raphe valves. 84–95. LM views of rapheless valves. 96, 97. SEM external view of raphe valves showing striation pattern, axial and central areas. 98. SEM external view of the rapheless valve showing striation pattern, axial and central areas. 99. SEM internal view of the rapheless valve. LM scale bar = 10 μm. SEM scale bars indicated in each figure.