Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species Author Costa, Lívia F. 0000-0001-7148-9964 Instituto de Pesquisas Ambientais (IPA), Department of Biodiversity Conservation, Av. Miguel Estéfano 3687, 04301 - 012 São Paulo, SP, Brazil. & livia. fcosta @ hotmail. com; https: // orcid. org / 0000 - 0001 - 7148 - 9964 livia.fcosta@hotmail.com Author Wetzel, Carlos E. 0000-0001-5330-0494 Luxembourg Institute of Science and Technology (LIST), Environmental Research and Innovation Department (ERIN), Observatory for Climate, Environment and Biodiversity (OCEB), 41 rue du Brill, 4422 Belvaux, Luxembourg. & carlos. wetzel @ list. lu; https: // orcid. org / 0000 - 0001 - 5330 - 0494 carlos.wetzel@list.lu Author Maquardt, Gisele C. 0000-0002-2721-1487 Instituto de Pesquisas Ambientais (IPA), Department of Biodiversity Conservation, Av. Miguel Estéfano 3687, 04301 - 012 São Paulo, SP, Brazil. & Universidade Guarulhos, Centro de Pós-Graduação, Pesquisa e Extensão CEPPE, Praça Tereza Cristina 88, 07023070 Guarulhos, SP, Brazil. & giselecmarquardt @ gmail. com; https: // orcid. org / 0000 - 0002 - 2721 - 1487 giselecmarquardt@gmail.com Author Zanon, Jaques E. 0000-0002-9619-4393 Instituto de Pesquisas Ambientais (IPA), Department of Biodiversity Conservation, Av. Miguel Estéfano 3687, 04301 - 012 São Paulo, SP, Brazil. & jaques. zanon @ gmail. com; https: // orcid. org / 0000 - 0002 - 9619 - 4393 jaques.zanon@gmail.com Author Ector, Luc 0000-0002-4573-9445 Luxembourg Institute of Science and Technology (LIST), Environmental Research and Innovation Department (ERIN), Observatory for Climate, Environment and Biodiversity (OCEB), 41 rue du Brill, 4422 Belvaux, Luxembourg. & https: // orcid. org / 0000 - 0002 - 4573 - 9445 Author Bicudo, Denise C. 0000-0002-4248-3261 Instituto de Pesquisas Ambientais (IPA), Department of Biodiversity Conservation, Av. Miguel Estéfano 3687, 04301 - 012 São Paulo, SP, Brazil. & denisecbicudo @ gmail. com; https: // orcid. org / 0000 - 0002 - 4248 - 3261 denisecbicudo@gmail.com text Phytotaxa 2022 2022-12-13 575 3 187 223 http://dx.doi.org/10.11646/phytotaxa.575.3.1 journal article 10.11646/phytotaxa.575.3.1 1179-3163 7431242 Achnanthidium vandevijveri L.F.Costa, Ector & C.E.Wetzel sp. nov. ( Figs 276–316 ) Description:— LM: Valves 7.8–16.5 µm long, 3.0–3.8 µm wide. Raphe valve: valves linear-lanceolate to rhombiclanceolate, with subrostrate to rostrate apices ( Figs 276–291 ). Axial area narrow and linear, central area absent or small, rounded ( Figs 276–291 ). Raphe filiform and 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; 27–34 in 10 µm ( Figs 276–291 ). Rapheless valve: valves linear-lanceolate, with rostrate to subcapitate apices ( Figs 292–306 ). Axial area narrow and linear, slightly widening towards the central area; central area absent to very small rounded ( Figs 292–306 ). Transapical striae slightly radiate throughout the entire valve, becoming denser towards the apices; 30–32 in 10 µm ( Figs 292–306 ). FIGURES 238–259. LM and SEM images of Achnanthidium tropicocatenatum from Cachoeira do França reservoir, São Paulo state (type locality, SP-469430). 238–243. LM views of raphe valves. 244–251. LM views of rapheless valves. 252, 253. Girdle views of the taxon. 254. SEM external view of raphe valve showing striation pattern, axial and central areas. 255. SEM internal view of the raphe valve. 256. SEM internal detail of the proximal raphe end. 257, 258. SEM external view of the rapheless valves showing striation pattern, axial and central areas. 259. SEM internal view of the rapheless valve. LM scale bar = 10 μm. SEM scale bars indicated in each figure. SEM: Raphe prolonged, terminating after the last stria on the border between the valve face and mantle ( Figs 307, 308 , 311–313 ). Central raphe fissures close and straight ( Figs 307, 308 , 311–313 ). Terminal raphe fissures short, weakly deflected ( Figs 307, 308 , 311–313 ). Striae mainly composed of 3–5 rounded to elongated areolae, sometimes slit-like near the valve margin ( Figs 307–316 ). Mantle with one row of slit-like areolae ( Figs 307, 309 , 314, 315 ). FIGURES 260–275. LM and SEM images of Achnanthidium tropicocatenatum from Santa Helena reservoir, São Paulo state (SP-469524). 260–267. LM views of raphe valves. 268–273. LM views of rapheless valves. 274. SEM external view of raphe valve showing striation pattern, axial and central areas. 275. SEM external view of the rapheless valve showing striation pattern, axial and central areas. LM scale bar = 10 μm. SEM scale bars indicated in each figure. Type:— BRAZIL . São Paulo state : São Paulo , Garças Lake , periphyton, 23° 38’ 44.4” S , 46° 37’ 29.9” W , P.D.Almeida & D.C.Bicudo , 7th July 2014 ( holotype SP-469485! = Fig. 282 , isotype: BR-4620) . Etymology: —We dedicate this species to Prof. Dr. Bart Van de Vijver (Meise Botanic Garden, Meise, Belgium ). Taxonomical remarks: — The different valve outline between the raphe and rapheless valves make the taxon more easily distinguishable from others. The rhombic-lanceolate outline of the raphe valve resembles the taxon to A. eutrophilum . Besides the different rapheless valve outline, A. eutrophilum presents wider valves and slightly lower stria density (3.2–5.8 µm wide, stria density 25–30/10 µm, Hlúbiková et al. 2011 ). Using SEM it is possible to see other differences such as the prolonged terminal raphe ends after the last stria and the almost never slit-like areolae shape. In contrast, in A. vandevijver sp. nov. , the terminal raphe fissures terminate right after the last stria, and the areolae are often slit-like near the margins of the valve. Despite the general similarity, Achnanthidium dolomiticum Cantonati & Lange-Bertalot (2006: 1185) presents its raphe valve with some singularities when compared with A. vandevijveri sp. nov . The Italian species has the central area forming a bilateral narrow fascia, a feature never present in the Brazilian one. In addition, the stria density is slightly higher ( 33–35 in 10 µm, Cantonati & Lange-Bertalot 2006 ) than our new species A. vandevijveri sp. nov . From our study area, the rapheless valve of A. vandevijveri sp. nov. is similar to valves of A. lusitanicum . The latter taxon differs mainly by the apices form in the raphe valve. While the rapheless valve has overlapping metric features, they can be distinguished by the visibly rounded central area in A. lusitanicum and the absence or presence of very small, rounded ones in A. vandevijveri sp. nov . Distribution and ecological information:— Achnanthidium vandevijveri sp. nov. was restricted to samples SP- 469483 and SP-469485 from Garças Lake, a eutrophic urban lake ( Bicudo et al. 2007 , 2020 ) in the metropolitan area of São Paulo . It occurred in plankton, but mainly in periphyton (highest abundance: 15 %). The limnological conditions of the sampling site varied from acid to alkaline (pH 5.8–8.6), cond. was high (307–378 μS∙ cm-1 ), as were the nutrient concentrations in the environment (TP 97–107.1 μg∙L- 1 , TN 1734.2–2757.9 μg∙L- 1 ).