Achnanthidium brasiliense L.F.Costa & G.C.Marquardt, 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 : 196

Achnanthidium brasiliense L.F.Costa & G.C.Marquardt sp. nov. ( Figs 26–44 )

Description:— LM: Valves linear-lanceolate, with rostrate to subcapitate apices ( Figs 26–38 ); 12.3–17.7 µm long, 2.5–2.9 µm wide. Raphe valve: axial area linear to linear-lanceolate, very narrow; central area small, rounded, almost absent, sometimes forming a rectangular fascia reaching one or both sides ( Figs 26–33 ). Raphe filiform, straight. Transapical striae slightly radiate throughout the entire valve, becoming denser and more strongly radiate towards the apices ( Figs 26–33 ); 26–30 in 10 µm. Rapheless valve: axial area narrow and linear, slightly widening towards the central area; central area absent or narrow lanceolate ( Figs 34–38 ). Transapical striae slightly radiate throughout the entire valve, more spaced in the central area and sometimes lacking one stria unilaterally ( Figs 34–38 ). Striae become denser towards the apices: 26–28 in 10 µm.

SEM: Raphe terminating after the last stria in the border between the valve face and mantle ( Figs 39, 40 ). Central and terminal raphe fissures straight and slightly expanded ( Figs 39, 40 ). Striae mainly composed of 2–4 rounded to elongated areolae, sometimes slit-like shaped near the valve margin and becoming denser and wider near the distal ends ( Figs 39–44 ). Mantle with one row of slit-like areolae ( Figs 39, 44 ).

Type:— BRAZIL. São Paulo state: São Paulo, Guarulhos, Tanque Grande reservoir, epiphython, 23° 22’ 29” S, 46° 27’ 31” W, S.A.Oliveira & D.C.Bicudo, 9th January 2014 (holotype SP-428935! = Fig. 34 , isotype: BR-4615) .

Etymology:— The specific epithet brasiliense sp. nov. refers to the country of the type locality.

Taxonomical remarks:— Achnanthidium brasiliense sp. nov. resembles A. minutissimum , however, an important feature in the separation of both taxa are the apices, which are always protracted in A. minutissimum . Observing the illustrations provided by studies of the type material ( Novais et al. 2015, Marquardt et al. 2017) it is possible to notice the stronger curved valves in apices of A. minutissimum , showed by the difficulty in focusing on the apices under LM. Also, in SEM micrographs of A. minutissimum ( Fig. 7B, C, E in Marquardt et al. 2017), the araphid valves present a strong curvature following the axial area, a feature not observed in our specimens. Regarding the morphometric features, A. brasiliense sp. nov. presents a lower stria density than the German A. minutissimum type material (30– 35/10 µm, Novais et al. 2015) and the type material population presented in Marquardt et al. (2017), which includes rapheless valves ranging from 30–34 striae in 10 µm in the middle. Although the number of striae in A. minutissimum is also denser near the distal ends, the appearance of areolae near the apices in the raphe valves of A. brasiliense sp. nov. change in size and shape, becoming larger.

Achnanthidium minutissimum is a widely used name for a complex of morphologically similar species worldwide. The morphometric characteristics of all the species that belong to this complex and were observed in this study are summarized in Table 4 . In addition, a quantitative shape analysis was carried out to separate A. brasiliense sp. nov. from A. minutisimum and its results are discussed below.

Still concerning the similarities among A. brasiliense sp. nov. and other species, Novais et al. (2015) reanalyzed the type materials of two species resembling ours: A. microcephalum Kützing (1844: 75) and A. neocryptocephalum (Grunow in Van Heurck) Novais & Van de Vijver (in Novais et al. 2015: 116) ≡ A. minutissima var. cryptocephala Grunow (in Van Heurck 1880: 27). Achnanthidium brasiliense sp. nov. presents slender valves than A. microcephalum and the stria density is also a distinguishing feature (30–32 in 10 µm, Novais et al. 2015). Moreover, external terminal raphe fissures in A. microcephalum are slightly deflected in the same direction but straight in A. brasiliense sp. nov. Although the striae are also denser near the apices in both taxa, areolae become wider in A. brasiliense sp. nov. Regarding A. neocryptocephalum , their representatives always present a rectangular fascia in the raphe valve, a rare feature in A. brasiliense sp. nov. Achnanthidium jackii Rabenhorst (1861: 1003) ( Rabenhorst 1861 –1862) described from “Quellwasser bei Salem” (Municipality of Baden-Württemberg state, southern Germany) was studied in full detail by Van de Vijver et al. (2018), has priority over A. neocryptocephalum , a new name for Achnanthes minutissima var. cryptocephala , thus is also a taxonomic synonym of A. jackii .

Achnanthidium barbei Le Cohu & Pérès (in Pérès et al. 2014: 388) also belongs to the A. minutissimum -complex and presents a similar morphology. It never forms a fascia in the central area of the raphe valve and has protracted apices ( Pérès et al. 2014).

Distribution and ecological information:— In our dataset, A. brasiliense sp. nov. was the most frequent species (63 % of occurrence) observed in periphytic, planktonic and surface sediment assemblages (highest abundance: 72 % in SP469485). In all these habitats, the ecological preference of the species was for neutral waters (pH optimum of 6.8– 7.2), medium to high cond. (optimum of 64.3–148.7 μS∙ cm-1) and mesotrophic conditions (TP optimum of 25.5–46.4 μg∙L- 1 and TN optimum of 596–1303.8 μg∙L- 1). However, it also occurred in enriched waters (TP> 50 μg∙L- 1).

Figures

FIGURES 26–44. LM and SEM images of Achnanthidium brasiliense sp. nov. from the type (SP-428935) from Tanque Grande reservoir, São Paulo state. Figure 34 corresponds to the holotype. 26–33. LM views of raphe valves. 34–38. LM views of rapheless valves. 39, 40. SEM external view of raphe valves showing striation pattern, axial and central areas. 41. SEM internal view of the raphe valve. 42, 43. SEM external view of the rapheless valves showing striation pattern, axial and central areas. 44. SEM internal view of the rapheless valve. LM scale bar = 10 μm. SEM scale bars indicated in each figure.

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.