Achnanthidium parvicapitatum L.F.Costa, Ector & C.E.Wetzel, 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 : 201-202

Achnanthidium parvicapitatum L.F.Costa, Ector & C.E.Wetzel sp. nov. ( Figs 100–116 )

Description:— LM: Valves linear-lanceolate, with subtle shoulders and subcapitate to small capitate apices ( Figs 100– 111 ); 18.2–22.7 µm long, 2.7–3.0 µm wide. Raphe valve: axial area narrow and linear, central area rounded, with 2–4 shortened striae on both sides of the valve, rarely forming a rectangular fascia ( Figs 100–105 ). Raphe filiform straight. Transapical striae slightly radiate throughout the entire valve, more spaced in the central area, and becoming denser and more strongly radiate towards the apices ( Figs 100–105 ); 29–33 in 10 µm. Rapheless valve: axial area narrow and linear, widening towards the central area; central area absent or narrow lanceolate, with 1–2 shortened striae on both sides of the valve ( Figs 106–111 ). Transapical striae slightly radiate throughout the entire valve, more spaced in the central area, and becoming denser towards the apices; 28–32 in 10 µm. Girdle view rectangularly arched, with pointed apices recurved to the rapheless valve ( Fig. 112 ).

SEM: Raphe prolonged after the striae, terminating on the border between the valve face and mantle ( Fig. 113 ). Central and terminal raphe fissures straight ( Fig. 113 ). Striae mainly composed of 3–4 or 4–5 rounded to elongated areolae, sometimes slit-like near the valve margin ( Figs 113, 115 ). Mantle with one row of slit-like areolae ( Fig. 113 ). Presence of two small punctuations, spaced and apically arranged on central area of rapheless valve of some longer valves ( Fig. 115 ).

Type:— BRAZIL. São Paulo state: Votorantim, Santa Helena reservoir, epilithon, 23º 34’ 58.56” S, 47º 25’ 50.52” W, E.C. R. Bartozek & D.C.Bicudo, 27th February 2014 (holotype SP-469524! = Fig. 108 , isotype: BR-4616) .

Etymology:— The specific epithet refers to the shape of the apices, which are small and capitate.

Taxonomical remarks:— Achnanthidium sieminskae Witkowski, Kulikovskiy & Riaux-Gobin (2012: 65) described from marine environments of the Kerguelen Archipelago, Austral Islands ( Witkowski et al. 2012) resemble our population. The Antarctic taxon has linear to narrowly linear-lanceolate valves with almost parallel to slightly convex margins ( Van de Vijver & Kopalová 2014), whereas A. parvicapitatum sp. nov. has more lanceolate valves with clearly convex margins, with the central portion evidently larger than the constriction before the apices, and sometimes larger than the apices. The apices in A. sieminskae can also be rostrate ( Van de Vijver & Kopalová 2014), differing from our populations.

Achnanthidium caledonicum (Lange-Bertalot) Lange-Bertalot (1999: 277) can be confused with A. parvicapitatum sp. nov. The main difference between these two species is the broadly capitate apices in A. caledonicum , while the Brazilian species present small capitate to subcapitate specimens. Another difference is that the valves of A. caledonicum are linear with parallel margins or sometimes inflated just at the centre, while A. parvicapitatum sp. nov. is linear-lanceolate and narrower. Previously, one Austrian specimen, in a mixed population, closely resembling A. parvicapitatum sp. nov. was illustrated in Lange-Bertalot & Krammer (1989, pl. 55, Fig. 4 ) and identified as Achnanthes minutissima var. scotica (J.R.Carter) Lange-Bertalot (in Lange-Bertalot & Krammer 1989: 106; currently considered a synonym of Achnanthidium caledonicum ). Specimens incorrectly identified as A. caledonicum are very similar ( Krammer & Lange-Bertalot 2004, pl. 34, Figs 4–6 ) and an investigation of those specimens is currently being carried out. Furthermore, similar German specimens were also identified as Achnanthidium cf. caledonicum (pl. 24, Figs 58–60 in Lange-Bertalot et al. 2017).

Achnanthidium neomicrocephalum Lange-Bertalot & F.Staab (in Krammer & Lange-Bertalot 2004: 431) bears some similarities with A. parvicapitatum sp. nov., such as linear-lanceolate valves with capitate to subcapitate apices. Its slender valve shape with longer, narrower valves (22–38 µm long, 2.5–2.8 µm wide, Krammer & Lange-Bertalot 2004) allows their separation, even though there are some overlaps in size ranges reported (21.6–25.3 μm long, 2.5–2.8 μm wide, by Silva-Lehmkuhl et al. 2019). Another distinction between the taxa is the subtle shoulders observed in the Brazilian populations but not present in A. neomicrocephalum .

Recently described from China, Achnanthidium longissimum P.Yu, Q. -M.You & Kociolek (in Yu et al. 2018: 340) was compared in its protologue to A. caledonicum and A. neomicrocephalum . Despite the similar valve outline to A. parvicapitatum sp. nov., A. longissimum has capitate apices, and the raphe’s terminal fissures are strongly bent towards the same side of the valve. This feature is clearly visible in LM images. Achnanthidium longissimum also has larger valves and a reduced stria density (36–48 μm long, 4.0–4.5 μm wide, 22–25 striae in 10 μm, Yu et al. 2018) when compared to the new Brazilian taxon.

Another similar species to A. parvicapitatum sp. nov. is Achnanthidium digitatum Pinseel, Vanormelingen, P.B.Hamilton & Van de Vijver (in Pinseel et al. 2017: 271). However, A. digitatum has smaller, narrower valves and a higher stria density (8.6–19.1 μm long, 1.8–2.3 μm wide, 31–36 striae in 10 µm, Pinseel et al. 2017). Additionally, the valves of A. digitatum are more linear, with margins almost parallel, and the striae are mostly composed of two areolae, an observation that is different from the population described here.

In our study area, A. parvicapitatum sp. nov. co-occurred with two taxa that are sometimes difficult to separate using LM: A. brasiliense sp. nov. and A. tropicocatenatum . In general, smaller specimens are difficult to discern, however, A. parvicapitatum sp. nov. is usually slender and has less lanceolate valve margins than A. brasiliense sp. nov. They also have slightly distinct apices, while the latter taxon presents more rostrate than subcapitate apices. Concerning A. tropicocatenatum , it is smaller and presents visibly inflated valves in the median portion. The girdle view is also slightly arched, but the apices in A. tropicocatenatum are more strongly and sharply recurved to the rapheless valve.

Distribution and ecological information:— Achnanthidium parvicapitatum sp. nov. was distributed in all the habitats, occurring in 20 % of the counted samples (highest abundance: 36 % in SP-428935). In all the communities, the species showed ecological preferences for slightly acid (pH optimum of 6.6–6.8), low to medium cond. (optimum of 56–74 μS∙ cm-1), and oligo- to mesotrophic water conditions (TP optimum of 14.1–22.5 μg∙L- 1 and TN optimum of 321.4–468.2 μg∙L- 1).

Figures

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.

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.

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.