Achnanthidium ritae L.F.Costa, 2022

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 : 207-208

Achnanthidium ritae L.F.Costa sp. nov. ( Figs 194–214 )

Description:— LM: Valves linear-elliptic, with non-protracted, broadly rounded apices ( Figs 194–210 ); 5.3–10.3 µm long, 2.5–2.9 µm wide. Raphe valve: axial area narrow and linear, central area absent or small rounded, sometimes with one shortened stria on both sides of the valve ( Figs 194–201 ). Raphe filiform and straight. Transapical striae slightly radiate throughout the entire valve, more spaced in the central area ( Figs 194–201 ); 28–30 in 10 µm. Rapheless valve: axial area narrow and linear, slightly widening towards the central area; central area absent or narrow lanceolate ( Figs 202–210 ). Transapical striae slightly radiate throughout the entire valve, more spaced in the central area; 26–28 in 10 µm ( Figs 202–210 ).

SEM: Raphe prolonged, terminating after the last stria ( Fig. 211 ). Central raphe fissures close, straight and droplike expanded ( Fig. 211 ). Terminal raphe fissures straight ( Fig. 211 ). Striae mainly composed of 4–6 rounded to almost squared or elongated areolae ( Figs 211–214 ). Mantle with one row of slit-like areolae ( Figs 211–214 ).

Type:— BRAZIL. São Paulo state: São Paulo, Billings Complex, Taquacetuba branch, epilithon, 23° 48’ 44.1” S, 46° 37’ 51.24” W, S.Wengrat & D.C.Bicudo, 7th August 2009 (holotype SP-427904! = Fig. 205 , isotype: BR-4619) .

Etymology:— This species is dedicated to Rita Franco.

Taxonomical remarks:— Achnanthidium duriense Novais & Ector is the most similar species, however, it differs by having rapheless valves clearly lanceolate, and also by the higher stria density (35 in 10 µm, Novais et al. 2015).

Achnanthidium straubianum (Lange-Bertalot) Lange-Bertalot (1999: 273) presented wider valves in the protologue (3.5–4 µm wide, Lange-Bertalot & Metzeltin 1996), compared to the isotype that has narrower valves (2.6–3.5 µm, Hlúbiková et al. 2011). However, even in the isotype illustrations it is possible to observe wider and shorter valves (ratio length/width 2.0–2.6, measured from the Figs 73–103 in Hlúbiková et al. 2011) than the species described herein (2.2–3.9). Hlúbiková et al. (2011) stated the combination of length, width and striation pattern as a reliable criterion to separate A. straubianum from similar species.

Another taxon resembling A. ritae sp. nov. is A. nanum (F.Meister) Novais & Jüttner (in Novais et al. 2015: 121), a species described from Switzerland ( Meister 1935). Achnanthidium nanum presents a different valve outline in the illustrations of the protologue and type material reevaluated in Novais et al. (2015). It also has linear-lanceolate valves differing from the linear-elliptic valves of A. ritae sp. nov. Also, the species present different areolae shapes and the central area of our specimens is often absent while it is clearly visible in A. nanum .

Two species recently described from Korea, Achnanthidium ovale M.Miao & B.-H.Kim (in Miao et al. 2020: 5) and Achnanthidium cavitatum M.Miao & B.-H.Kim (in Miao et al. 2020: 7), also can be compared to the Brazilian species. Achnanthidium ovale has a more elliptic valve outline, wider valves (3.8–4.1 µm) and a greater stria density (30–35 in 10 µm). The ultrastructural differences, such as the terminal raphe fissures strongly bent to the same side in the Korean taxon, are another notable feature ( Miao et al. 2020). Achnanthidium cavitatum is similar in that it has linear-elliptic valves; however, the main difference from A. ritae sp. nov. is the wider axial area observed in the rapheless valve ( Miao et al. 2020). Achnanthidium cavitatum also has slightly wider valves (3.0–3.5 µm, Miao et al. 2020) than our population.

Furthermore, Achnanthidium rivulare Potapova & Ponader (2004: 36) and A. crassum (Hustedt) Ponader & Potapova (in Potapova & Ponader 2004: 38) differ from A. ritae sp. nov. in their linear-lanceolate axial area in the raphe valve, unilaterally deflected terminal raphe fissures and wider valves (2.6–4.4 µm and 3–4.5 µm respectively, Potapova & Ponader 2004).

Distribution and ecological information:— Achnanthidium ritae sp. nov. was found in 8 % of the counted samples, mainly occurring in periphytic samples (highest abundance: 14 % in SP-427904) with an ecological preference for alkaline (pH optimum of 7.9), high cond. (optimum of 182.4 μS∙ cm-1) and mesotrophic water conditions (TP optimum of 41.1 μg∙L- 1 and TN optimum 1306.2 μg∙L- 1). It also presented a wide tolerance and was found to occur in oligo- to eutrophic waters.

Figures

FIGURES 194–214. LM and SEM images of Achnanthidium ritae sp. nov.from the type (SP-427904) from Billings complex,Taquacetuba branch, São Paulo state. Figure 205 corresponds to the holotype. 194–201. LM views of raphe valves. 202–210. LM views of rapheless valves. 211. SEM external view of raphe valve showing striation pattern, axial and central areas. 212. SEM internal view of the raphe valve. 213. SEM external view of the rapheless valves showing striation pattern, axial and central areas. 214. 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.

FIGURES 100–116. LM and SEM images of Achnanthidium parvicapitatum sp. nov. from the type (SP-469524) from Santa Helena reservoir, São Paulo state. Figure 108 corresponds to the holotype. 100–105. LM views of raphe valves. 106–111. LM views of rapheless valves. 112. Girdle view of the new taxon. 113. SEM external view of raphe valve showing striation pattern, axial and central areas. 114. SEM internal view of the raphe and rapheless valves. 115. SEM external view of the rapheless valves showing striation pattern, axial and central areas, including the presence of two small punctuations on central area. 116. SEM internal view of the rapheless valve. LM scale bar = 10 μm. SEM scale bars indicated in each figure.