Achnanthidium acutum L.F.Costa, C.E.Wetzel & Ector, 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 : 194-195

Achnanthidium acutum L.F.Costa, C.E.Wetzel & Ector sp. nov. ( Figs 2–25 )

Description:—LM: Valves narrowly linear-elliptic, with acutely rounded to very slightly rostrate apices ( Figs 2–20 ); 9.3–21.2 µm long, 2.2–3.2 µm wide. Raphe valve: axial area narrow and linear; central area rounded, sometimes forming a uni- or bilateral rectangular fascia, with no or one shortened stria on both sides of the valve ( Figs 2–12 ). Raphe filiform straight. Transapical striae slightly radiate throughout the entire valve, more spaced in the central area, becoming denser and more strongly radiate towards the apices; 27–30 in 10 µm ( Figs 2–12 ). Rapheless valve: axial area narrow and linear; central area absent, with 1–2 shortened striae on both sides of the valve ( Figs 13–20 ). Transapical striae slightly radiate throughout the entire valve, more spaced in the central area, and becoming denser and more strongly radiate towards the apices; 28–30 in 10 µm ( Figs 13–20 ).

SEM: Raphe terminating after the last stria ( Figs 21, 22 ). Central and terminal raphe fissures straight ( Figs 21, 22 ). Striae mainly composed of 3–4 or 4–5 rounded to elongated areolae ( Figs 21–25 ), sometimes slit-like near the valve margin ( Figs 21–23 ). Mantle with one row of slit-like areolae ( Figs 21–23 ).

Type:— BRAZIL. São Paulo state: São Paulo, Rio Grande reservoir, epiphyton ( Salvinia ), 23° 45’ 59.46” S, 46° 30’ 35.7” W, S. Wengrat & D. C.Bicudo, 5th August 2009 (holotype SP-427898! = Fig. 17 , isotype: BR-4614) .

Etymology:— The specific epithet refers to the acutely rounded apices format of the species.

Taxonomical remarks:— Concerning the characteristic linear valve shape, A. lineare resembles A. acutum sp. nov. However, A. lineare differs from Brazilian populations mostly by the almost parallel margins and broadly rounded apices in the raphe valves, whereas A. acutum sp. nov. shows narrowly linear-elliptic valves, with acutely rounded to very slightly rostrate apices. Also, the always-rectangular fascia in the central area of the raphe valve in A. lineare contrasts to the nearly rounded central area of A. acutum sp. nov. when observed in LM. Additional remarks include the density of areolae per striae in 10 µm, higher in A. acutum sp. nov. (up to 5) when compared to A. lineare (1– 3). This latter feature can be useful especially regarding the rapheless valve differentiation. The separation of both species was corroborated through morphological analysis between them. The results are presented and discussed in the ‘Morphological patterns in valve shape from EFD analysis’ section.

Van de Vijver et al. (2011) analyzed A. lineare type material and described two new species: A. pseudolineare Van de Vijver, Novais & Ector (in Van de Vijver et al. 2011: 186) and A. sublineare Van de Vijver, Jarlman & Ector (in Van de Vijver et al. 2011: 179). Both taxa are described as having a linear outline of the raphe valve. Furthermore, Achnanthidium pseudolineare clearly has a denser number of striae in 10 µm (35–40) compared to A. acutum sp. nov. (27–30). Similarly, A. sublineare has a higher number of striae in 10 µm (33–34), a wider and lanceolate axial area in the rapheless valve, 1–2 areolae per striae and narrower valves (1.5–2.1 µm wide). Besides, this taxon possesses the central area formed by a rectangular fascia in the raphe valve, comparable to those in A. lineare .

Achnanthidium petuniabuktianum Pinseel, Van de Vijver & Kopalová (2015: 68) also presents linear valves with rounded apices, but in SEM images, it shows a distinct wide axial area and striae with just 1–2 areolae, differing from A. acutum sp. nov. Also, the terminal raphe fissures are bent to the same side in A. petuniabuktianum while straight in our species.

In Brazilian waters, A. brasiliense sp. nov. can co-occurs with A. acutum sp. nov. but can be distinguished by its lanceolate valves with rostrate apices and the linear valves with acutely rounded apices of A. acutum sp. nov.

From the Antarctic region, two species closely resemble A. acutum sp. nov. but differ by possessing distinctly protracted and rostrate apices. The first one, A. maritimo-antarcticum Van de Vijver & Kopalová (2014: 6) , also has the central area of the rapheless valve in disagreement with A. acutum sp. nov. morphology: it is weakly elliptical, whereas in the Brazilian species a central area is absent. On the other hand, A. indistinctum Van de Vijver & Kopalová (2014: 4) received this name by presenting small frustules (length 8.5–13.0 μm, width 1.8–2.2 μm, Van de Vijver & Kopalová 2014), a feature that distinguish it from our species, since A. acutum sp. nov. has longer and wider valves. In addition, besides the higher stria density (30–35 in 10 μm) than Brazilian taxa, slit-like areolae are apparently never present in A. indistinctum , a feature observed in marginal areolae of some valves in our population.

Distribution and ecological information:— Achnanthidium acutum sp. nov. was found in all three habitats (i.e. periphyton, sediment and plankton) and present in 23 % of the quantified samples, being more abundant in periphytic samples (highest abundance: 12 % in SP427898). Due to its wide distribution, the ecological optimum could be calculated. The ecological preference of the species was for neutral pH (optimum of 6.8–7.1), medium to high cond. (optimum of 76.7–167.3 μS∙ cm-1), and meso- to eutrophic waters (TP optimum of 39.0–77.8 μg∙L- 1 and TN optimum of 647.7–1627.9 μg∙L- 1).

Figures

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.