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The historic type material of Microneis gracillima, Navicula pyrenaica, Achnanthes amphicephala, Achnanthes thienemannii and Achnanthidium rostropyrenaicum was investigated together with additional European populations of these species using light and scanning electron microscopy. The morphology of the species was compared to similar taxa. Achnanthes amphicephala is transferred to the genus Achnanthidium. The species are distinguished from each other in the shape of their poles and in valve outline. There are currently very few photographic records of the species, but our results suggest that these species are typical in base-rich, oligotrophic waters, and might be important indicator species for these habitats.

We investigated several common, needle-shaped diatoms to better characterize the taxonomy of the genus Fragilaria in the Laurentian Great Lakes. We conducted diatom morphometric analysis facilitated by SEM and LM imaging on samples collected as a part of the USEPA's long-term biological monitoring program. We resolved several decades-long taxonomic problems in the Great Lakes. The results indicated that previous records of species formerly belonging to the genus Synedra, such as S. (Fragilaria) ostenfeldii, S. (Fragilaria) radians, and S. filiformis, should be corrected as these species likely do not occur in the Great Lakes. Valve morphology confirmed the presence of four previously undescribed species: Fragilaria andreseniana sp. nov., Fragilaria stoermeriana sp. nov., Fragilaria limnetica sp. nov., and Fragilaria michiga-nensis sp. nov. The morphology of several other Fragilaria taxa in the Great Lakes was examined, including a teratologic taxon (Fragilaria sp. 1), Fragilaria crotonensis, Fragilaria grunowii, and a taxon showing mor-phological affinity to Fragilaria lemanensis (reported here as Fragilaria cf. lemanensis). The spatial and sea-sonal distributions of these needle-shaped taxa were also analyzed and discussed.(c) 2022 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.

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). 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). 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)., Published as part of 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, pp. 187-223 in Phytotaxa 575 (3) on pages 210-212, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Hlubikova, D., Ector, L. & Hoffmann, L. (2011) Examination of the type material of some diatom species related to Achnanthidium minutissimum (Kutz.) Czarn. (Bacillariophyceae). Algological Studies 136 / 137: 19 - 43. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0019","Cantonati, M. & Lange-Bertalot, H. (2006) Achnanthidium dolomiticum sp. nov. (Bacillariophyta) from oligotrophic mountain springs and lakes fed by dolomite aquifers. Journal of Phycology 42 (6): 1184 - 1188. https: // doi. org / 10.1111 / j. 1529 - 8817.2006.00281. x","Bicudo, D. C., Fonseca, B. M., Bini, L. M., Crossetti, L. O., Bicudo, C. E. M. & Araujo-Jesus, T. (2007) Undesirable side-effects of water hyacinth control in a shallow tropical reservoir. Freshwater Biology 52 (6): 1120 - 1133. https: // doi. org / 10.1111 / j. 1365 - 2427.2007.01738. x","Bicudo, D. C., Zanon, J. E., Ferragut, C., Crossetti, L. O., Faustino, S. B. & Bicudo, C. E. M. (2020) Garcas Reservoir trophic state dynamics: a 20 - year synthesis. Hoehnea 47: e 722019. https: // doi. org / 10.1590 / 2236 - 8906 - 72 / 2019"]}

Achnanthidium peruvianum E.Morales & Ector in Morales et al. (2011: 121) (Figs 166–193) Synonym: Achnanthes kryophiloides M.H.Hohn in Patrick et al. (1966), p. 468, pl. 1, Figs 5, 6 Morphometric features:— 8.0–20.2 µm long; 2.3–4.3 µm wide; 28–34 striae in 10 µm; striae mainly composed of 4–5 areolae. Taxonomical remarks:— The species presents similar morphological features to another South American species called Achnanthidium sehuencoensis, which has almost the same valve outline. However, SEM analysis separated the two due to the deflected terminal raphe fissures, the non-expanded central raphe fissures, and lower stria density (20–24/10 µm, Morales et al. 2009) compared to A. peruvianum. Achnanthidium petersenii (Hustedt) C.E.Wetzel, Ector & Jüttner (in Jüttner et al. 2019: 66) has wider fascia on the raphe valve. However, on the rapheless valves, the axial area is moderately broad, and the central area broadly rhombic-lanceolate (Jüttner et al. 2019). In A. peruvianum, the axial area is narrow and linear, while the central area is undifferentiated with short striae (Morales et al. 2011). Distribution and ecological information:— Achnanthidium peruvianum was described from the Rondos River in Peru, South America (Hohn in Patrick et al. 1966). In our samples, the species was rare (abundance, Published as part of 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, pp. 187-223 in Phytotaxa 575 (3) on page 207, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Morales, E. A., Ector, L., Fernandez, E., Novais, M. H., Hlubikova, D., Hamilton, P. B., Blanco, S., Vis, M. L. & Kociolek, J. P. (2011) The genus Achnanthidium Kutz. (Achnanthales, Bacillariophyceae) in Bolivian streams: a report of taxa found in recent investigations. Algological Studies 136 - 137: 89 - 130. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0089","Morales, E. A., Fernandez, E. & Kociolek, J. P. (2009) Epilithic diatoms (Bacillariophyta) from cloud forest and alpine streams in Bolivia, South America 3: diatoms from Sehuencas, Carrasco National Park, Department of Cochabamba. Acta Botanica Croatica 68 (2): 263 - 283.","Juttner, I., Wetzel, C. E., Williams, D. M. & Ector, L. (2019) Investigations of the type materials of Achnanthes parallela J. R. Carter and Achnanthes petersenii Hustedt (Bacillariophyceae) with comments on the genus Rossithidium Round & Bukhtiyarova. Botany Letters 167 (1): 57 - 69. https: // doi. org / 10.1080 / 23818107.2019.1668297"]}

Achnanthidium peetersianum C.E.Wetzel, Jüttner & Ector in Wetzel et al. (2019: 345) (Figs 144–165) Morphometric features:— 7.6–12.3 µm long; 2.0–2.7 µm wide; 29–34 striae in 10 µm; striae mainly composed of 3–5 areolae. Taxonomical remarks:— Our population is in agreement with the type species, besides it has slightly narrower valves when compared to the French population (2.8–3.3 μm, Wetzel et al. 2019). This species has narrow valves with subcapitate to small capitate apices, similar to A. macrocephalum (Hustedt) Round & Bukhtiyarova (1996: 349), although the latter taxon presents broadly capitate apices. Despite being difficult to separate, a reevaluation of A. macrocephalum type material was recently published in the same study as A. peetersianum (Wetzel et al. 2019). Using SEM micrographs, the authors showed that A. peetersianum presented predominantly rounded areolae and had a higher density per stria, while A. macrocephalum striae are composed of 1–2 elongated areolae of different lengths. Moreover, the axial area of the rapheless valve of the latter taxon is expanded, widening towards the central area. Recently, Yu et al. (2019) described two small species that resemble ours: A. subtilissimum P.Yu, Q. -M.You & Q.-X.Wang (in Yu et al. 2019: 161) and A. limosua P.Yu, Q. -M.You & Q.-X.Wang (in Yu et al. 2019: 158). The first species is likely a synonym of A. macrocephalum, and the second presents visibly larger valves (3.2–4.0 µm) than A. peetersianum. Achnanthidium indistinctum also bears several similarities with A. peetersianum. They can be distinguished in girdle view by the apparently thickened striae in the first taxon, which is not visible in our specimens (see Figs 158, 159). Concerning the valve view, A. indistinctum presents rostrate apices and an almost nonexistent central area that never forms a fascia. On the other hand, A. peetersianum exhibits subcapitate to capitate apices and a small rounded central area that rarely forms a fascia. Notably, SEM analysis can also help separate both taxa due to the distal raphe ends straight in A. indistinctum and bent in our specimens. Despite the slightly narrower valves in the latter taxon (1.8–2.2 µm, Van de Vijver & Kopalová 2014), the metric features of these two species overlap. Achnanthidium reimeri (Camburn) Ponader & Potapova (2007: 235) has a similar valve outline with our population, however, it presents longer and wider valves and a rhomboid central area on the rapheless valve (Ponader & Potapova 2007). Distribution and ecological information:— Recently described from Le Sauvigny River in France, it was one of the most well-distributed species found in this study (40 % of occurrence). It is plausible that this taxon was previously identified in Brazilian publications as A. macrocephalum. In this study, A. peetersianum occurred in all of the sampled habitats. Its ecological preference was for slightly acidic (pH optimum of 6.6–6.7), electrolytic-poor (optimum of 28.2–49 μS∙ cm-1) and oligo- to mesotrophic waters (TP optimum of 14.9–27.1 μg∙L- 1, TN optimum of 374.4–519.4 μg∙L- 1). We also observed this species in enriched waters (TP> 50 μg∙L- 1)., Published as part of 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, pp. 187-223 in Phytotaxa 575 (3) on pages 204-205, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Wetzel, C. E., Juttner, I., Gurung, S. & Ector, L. (2019) Analysis of the type material of Achnanthes minutissima var. macrocephala (Bacillariophyta) and description of two new small capitate Achnanthidium species from Europe and the Himalaya. Plant Ecology and Evolution 152 (2): 340 - 350. https: // doi. org / 10.5091 / plecevo. 2019.1628","Bukhtiyarova, L. & Round, F. E. (1996) Revision of the genus Achnanthes sensu lato. Psammothidium, a new genus based on A. marginulatum. Diatom Research 11: 1 - 30. https: // doi. org / 10.1080 / 0269249 X. 1996.9705361","Yu, P., You, Q. - M., Pang, W. - T., Cao, Y. & Wang, Q. - X. (2019) Five new Achnanthidiaceae species (Bacillariophyta) from Jiuzhai Valley, Sichuan Province, Southwestern China. Phytotaxa 405 (3): 147 - 170. https: // doi. org / 10.11646 / phytotaxa. 405.3.5","Van de Vijver, B. & Kopalova, K. (2014) Four Achnanthidium species (Bacillariophyta) formerly identified as Achnanthidium minutissimum from the Antarctic Region. European Journal of Taxonomy 79: 1 - 19. https: // doi. org / 10.5852 / ejt. 2014.79","Ponader, K. C. & Potapova, M. G. (2007) Diatoms from the genus Achnanthidium in flowing waters of the Appalachian Mountains (North America): Ecology, distribution and taxonomic notes. Limnologica 37 (3): 227 - 241. https: // doi. org / 10.1016 / j. limno. 2007.01.004"]}

Achnanthidium lusitanicum Novais & M.Morais in Novais et al. (2015: 129) (Figs 45–99) Morphometric features:— 6.5–16.5 µm long; 2.6–3.4 µm wide; 30–34 striae in 10 µm; striae mainly composed of 4–6 areolae. Taxonomical remarks:— Achnanthidium lusitanicum shares morphological similarities with A. eutrophilum (Lange-Bertalot) Lange-Bertalot (1999: 271) but our species has slender valves narrower than the latter taxon, which has rhombic-lanceolate to rhombic-elliptical frustules in small specimens and valve apices bluntly rounded to cuneate. Also, A. lusitanicum have a higher stria density (A. eutrophilum: 3.2–5.8 µm wide, striae 25–30/10 µm, Hlúbiková et al. 2011). Achnanthidium caravelense Novais & Ector (in Novais et al. 2011: 142) is similar to A. lusitanicum when analyzed by LM and SEM. However, it differs from A. lusitanicum mainly by the elliptic raphe valve with almost linear margins. The SEM images show that A. caravelense presents a lower number of areolae per striae (3–4, Novais et al. 2011). The valve outline of Achnanthidium palmeti Gassiole, Le Cohu & M.Coste (2013: 22) is very similar to A. lusitanicum, and the cell metrics overlap. However, some A. palmeti specimens have slightly wider valves (2.7–4.6 μm, Gassiole et al. 2013). The main character differing both species is the frustules that are arched with slightly recurved apices in A. palmeti and its terminal raphe fissures of the raphid valve are deflected towards the same side. Indeed, our species has straight raphe fissures and an almost flat valve face. In São Paulo state, Achnanthidium saprophilum (H.Kobayasi & Mayama) Round & Bukhtiyarova (1996: 349) shows valves with no inflation in the central portion as A. lusitanicum, and it also presents lower stria density (26–30 in 10 μm). Besides that, A. saprophilum showed presence of areolae with different shapes, a feature not observed in A. lusitanicum. In addition, a similar Brazilian population was cited as A. jackii by Bartozek et al. (2018). However, the cited species had linear-lanceolate valves with broadly rounded and rostrate apices, a central area always forming a fascia and lower stria density (28–30/10 µm, Van de Vijver et al. 2018). Notably, the central area of A. lusitanicum rarely produces a unilateral fascia, being often absent or small rounded with some shortened striae. Distribution and ecological information:— The taxon was found in all three habitats (occurrence of 10 %), but it was more often found in surface sediments from reservoirs. In the sediment assemblages, the ecological preferences of the taxa were for slightly alkaline (pH optimum of 7.3), medium cond. (optimum of 100.3 μS∙ cm-1) and eutrophic waters (TP optimum of 56.7 μg∙L- 1 and TN optimum of 1016.5 μg∙L- 1). It is worth mentioning that it also occurred in oligo- and mesotrophic waters.

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)., Published as part of 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, pp. 187-223 in Phytotaxa 575 (3) on page 196, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Novais, M. H., Juttner, I., Van de Vijver, B., Morais, M. M., Hoffmann, L. & Ector, L. (2015) Morphological variability within the Achnanthidium minutissimum species complex (Bacillariophyta): comparison between the type material of Achnanthes minutissima and related taxa, and new freshwater Achnanthidium species from Portugal. Phytotaxa 224 (2): 101 - 139. https: // doi. org / 10.11646 / phytotaxa. 224.2.1","Marquardt, G. C., Costa, L. F., Bicudo, D. C., Bicudo, C. E. M., Blanco, S., Wetzel, C. E. & Ector, L. (2017) Type analysis of Achnanthidium minutissimum and A. catenatum and description of A. tropicocatenatum sp. nov. (Bacillariophyta), a common species in Brazilian reservoirs. Plant Ecology and Evolution 150 (3): 313 - 330. https: // doi. org / 10.5091 / plecevo. 2017.1325","Kutzing, F. T. (1844). Die kieselschaligen Bacillarien oder Diatomeen. W. Kohne, Nordhausen. https: // doi. org / 10.5962 / bhl. title. 64360","Van Heurck, H. (1880) Synopsis des Diatomees de Belgique. Atlas. Anvers, Ducaju et Cie, pls 1 - 30.","Rabenhorst, L. (1861 - 1862) Die Algen Europa's, Fortsetzung der Algen Sachsens, resp. Mittel-Europas. Decades I-CIX, numbers 1 - 1600 (or 1001 - 2600). Dresden.","Van de Vijver, B., Wetzel, C. E. & Ector, L. (2018) Analysis of the type material of Achnanthidium jackii Rabenhorst (Bacillariophyta, Achnanthidiaceae). Notulae Algarum 55: 1 - 4.","Peres, F., Le Cohu, R. & Delmont, D. (2014) Achnanthidium barbei sp. nov. and Achnanthidium costei sp. nov., two new diatoms from French rivers. Diatom Research 29 (4): 387 - 397. https: // doi. org / 10.1080 / 0269249 X. 2014.890956"]}

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)., Published as part of 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, pp. 187-223 in Phytotaxa 575 (3) on pages 194-195, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Van de Vijver, B., Ector, L., Beltrami, M. E., de Haan, M., Falasco, E., Hlubikova, D., Jarlman, A., Kelly, M., Novais, M. H. & Wojtal, A. Z. (2011) A critical analysis of the type material of Achnanthidium lineare W. Sm. (Bacillariophyceae). Algological Studies 136 - 137: 167 - 191. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0167","Pinseel, E., Van de Vijver, B. & Kopalova, K. (2015) Achnanthidium petuniabuktianum sp. nov. (Achnanthidiaceae, Bacillariophyta), a new representative of the A. pyrenaicum group from Spitsbergen (Svalbard Archipelago, High Arctic). Phytotaxa 226 (1): 63 - 74. https: // doi. org / 10.11646 / phytotaxa. 226.1.6","Van de Vijver, B. & Kopalova, K. (2014) Four Achnanthidium species (Bacillariophyta) formerly identified as Achnanthidium minutissimum from the Antarctic Region. European Journal of Taxonomy 79: 1 - 19. https: // doi. org / 10.5852 / ejt. 2014.79"]}

Achnanthidium tropicocatenatum G.C.Marquardt, C.E.Wetzel & Ector (Figs 238–275) Morphometric features:— 6.7–18.1 µm long; 2.3–3.1 µm wide; 36–42 striae in 10 µm; striae mainly composed of 4–6 areolae. Taxonomical remarks:— Achnanthidium tropicocatenatum and A. catenatum are similar. In the protologue of the first species, many characters were taken into account to provide the differences between them (e.g. SEM, LM, geometric morphometric analysis, and ecological preferences). Nevertheless, it is possible to distinguish both taxa by the less inflated central portion, fewer capitate apices and higher stria density in A. tropicocatenatum (28–30 striae/10 µm in A. catenatum, Bílý & Marvan 1959). Also, in A. catenatum, the girdle view is distinct, with the valve being strongly curved with the apices sharply pointed. In contrast, in A. tropicocatenatum, the valve is less arched with apices slightly curved (Marquardt et al. 2017). Marquardt et al. (2017) commented about the confusion made with two different populations in Novais et al. (2015), probably joining two different species in one taxon (A. lusitanicum in Novais et al. 2015). The first population from Janeiro de Baixo, Zêzere River (Tejo basin, Portugal), is considered the type material of the species and differs from A. tropicatenatum by its rostrate apices, or broadly rounded ones in smaller specimens. Distribution and ecological information:— The taxon was recently described despite its presence in Brazilian literature being misidentified as A. catenatum. Indeed, in our samples, the species A. tropicocatenatum had the highest frequency of occurrence (61 %). Its ecological optimum was calculated for all habitats. The species showed an ecological preference for slightly acid to neutral (pH optimum of 6.7–6.9), low to medium cond. (optimum of 53.7–94.3 μS∙ cm-1) and mesotrophic waters (TP optimum of 19.8–37.3 μg∙L- 1 and TN optimum of 504.2–665.8 μg∙L- 1). Achnanthidium tropicocatenatum was previously described from oligotrophic conditions in São Paulo state (Marquardt et al. 2017); however, it presented greater tolerance in our study area and was identified from oligo- to eutrophic conditions., Published as part of 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, pp. 187-223 in Phytotaxa 575 (3) on page 210, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Bily, J. & Marvan, P. (1959) Achnanthes catenata sp. n. Preslia 31: 34 - 35.","Marquardt, G. C., Costa, L. F., Bicudo, D. C., Bicudo, C. E. M., Blanco, S., Wetzel, C. E. & Ector, L. (2017) Type analysis of Achnanthidium minutissimum and A. catenatum and description of A. tropicocatenatum sp. nov. (Bacillariophyta), a common species in Brazilian reservoirs. Plant Ecology and Evolution 150 (3): 313 - 330. https: // doi. org / 10.5091 / plecevo. 2017.1325","Novais, M. H., Juttner, I., Van de Vijver, B., Morais, M. M., Hoffmann, L. & Ector, L. (2015) Morphological variability within the Achnanthidium minutissimum species complex (Bacillariophyta): comparison between the type material of Achnanthes minutissima and related taxa, and new freshwater Achnanthidium species from Portugal. Phytotaxa 224 (2): 101 - 139. https: // doi. org / 10.11646 / phytotaxa. 224.2.1"]}

Achnanthidium paulistanum L.F.Costa & D.Bicudo sp. nov. (Figs 117–143) Description:— LM: Valves narrowly linear-lanceolate, with subrostrate to rostrate apices (Figs 117–134); 8.4–15.0 µm long, 2.1–2.6 µm wide. Raphe valve: axial area narrow and linear; central area small rounded, almost absent, with 1–2 shortened striae on both sides of the valve, and never forming a fascia (Figs 117–126). Raphe filiform straight. Transapical striae slightly radiate throughout the entire valve, becoming denser and more strongly radiate towards the apices, 30–31 in 10 µm (Figs 117–126). Rapheless valve: axial area narrow and linear, slightly widening towards the central area; central area very small rounded or absent, with 1–2 shortened striae on both sides of the valve (Figs 127–134). Transapical striae slightly radiate throughout the entire valve, becoming denser towards the apices, 30–32 in 10 µm (Figs 127–134). Girdle view rectangularly arched, with apices recurved to the rapheless valve (Figs 135–137). SEM: Raphe terminating right after the last stria (Figs 138–140). Central raphe fissures close, straight and slightly expanded (Figs 138–140). Terminal raphe fissures short, deflected (Figs 138–140). Striae mainly composed of 3–5 rounded to elongated areolae, sometimes slit-like near the valve margin (Figs 138–143). Mantle with one row of slitlike areolae (Figs 139, 140). Type:— BRAZIL. São Paulo state: São Pedro, SP –304, km 127, periphyton from a small lake, A.A.J.Castro & C.E.M.Bicudo, 20th March 1990 (holotype SP-255724! = Fig. 118, isotype: BR-4617). Etymology: —The specific epithet refers to people born in the state of São Paulo (in Portuguese, the “ paulistas ”). Taxonomical remarks:— Achnanthidium paulistanum sp. nov. is characterized by a linear or narrowly lanceolate valve shape, with subrostrate apices. In contrast, A. brasiliense sp. nov. has linear-lanceolate valves and rostrate apices. Notably, the slightly deflected terminal raphe fissures are always present in A. paulistanum sp. nov. valves, a feature that was not observed in A. brasiliense sp. nov. specimens. The central area never forms a fascia and is almost absent in the new species. In this way, the main characters differing in both species are the terminal raphe fissures and the central area shape of the raphid valve. The deflected terminal raphe fissures differentiate the species from others, including A. indistinctum specimens with valve outline very similar to A. paulistanum sp. nov. It also can be distinguished by the girdle view: the Brazilian species has apices recurved to the rapheless valve and mantle striae almost indiscernible in LM, whereas the Antarctic one presents girdle view with apices straight and mantle striae thickened in LM. In valve view, A. indistinctum has slightly higher stria density and slightly narrower valves (1.8–2.2 µm wide, 30–35 striae in 10 µm, Van de Vijver & Kopalová 2014). Achnanthidium maritimo-antarcticum, another Antarctic species, differs by having parallel to slightly convex margins, appearing sometimes undulated, and clearly protracted apices, whereas A. paulistanum sp. nov. has convex to slightly convex margins and apices not clearly differentiate from the main body. In ultrastructure, A. maritimo-antarcticum possesses terminal raphe fissures straight to weakly curved (Van de Vijver & Kopalová 2014). Due to its linear shape, Achnanthidium acutum sp. nov. is morphologically close to this taxon, but A. paulistanum sp. nov. can be narrowly lanceolate. These species also differ with respect to the apices, which are acutely rounded in the first and subrostrate in the latter. Nevertheless, the slightly deflected terminal raphe fissures and the higher stria density in A. paulistanum sp. nov. can be used to separate both taxa. Distribution and ecological information:— The taxon was rare and occurred only in the epiphytic type sample., Published as part of 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, pp. 187-223 in Phytotaxa 575 (3) on pages 203-204, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Van de Vijver, B. & Kopalova, K. (2014) Four Achnanthidium species (Bacillariophyta) formerly identified as Achnanthidium minutissimum from the Antarctic Region. European Journal of Taxonomy 79: 1 - 19. https: // doi. org / 10.5852 / ejt. 2014.79"]}

Achnanthidium saprophilum (H.Kobayashi & Mayama) Round & Bukhtiyarova (Figs 215–237) Morphometric features:— 6.0–15.3 µm long; 2.5–3.2 µm wide; 26–32 striae in 10 µm; striae mainly composed of 4–5 areolae. Taxonomical remarks:— The species can be confused with smaller specimens of A. tropicocatenatum, however, they differ by the subcapitate to capitate apices and higher stria density (36–45 in 10 µm) in A. tropicocatenatum. Achnanthidium peetersianum presents small capitate apices and has narrower valves (2.0–2.7 µm) than A. saprophilum. Also, externally terminal raphe fissures are slightly bent to the same side in A. peetersianum, at the same level as the striae on the valve face, whereas in our species external terminal raphe fissures are always straight and terminating after the last striae. Despite displaying different raphid valves with linear-lanceolate to rhombic-lanceolate in A. vandevijveri sp. nov., the rapheless valves present almost the same valve outline as A. saprophilum. However, the valves outline in A. vandevijveri sp. nov. are slightly wider and longer. Achnanthidium acsiae Wojtal, E.Morales, Van de Vijver & Ector (in Wojtal 2011: 226) has similar valve outline and metric features as A. saprophilum, however, the former has the central area of raphe valve often forming a bilateral fascia, a feature never observed in our population. Achnanthidium saprophilum rarely shows a unilateral fascia. Still, the stria density is higher than ours and they are composed of less areolae (28–36 striae in 10 µm, 3–4 areolae per stria, Wojtal et al. 2011). Achnanthidium pseudolineare differs from A. saprophilum by the rapheless valve, presenting linear valves with parallel margins and broadly rounded apices. The stria density is also different between the species, being higher in A. pseudolineare (32–40 in 10 µm, Van de Vijver et al. 2011). Achnanthidium costei Pérès & Le Cohu (in Pérès et al. 2014: 390) also has linear valves with parallel margins, but this outline is characteristic of the raphe valves (Pérès et al. 2014). Its rapheless valves shows another distinct feature: the axial area is wider and lanceolate. Achnanthidium saprophilum has both valves linear-lanceolate with convex margins, and the axial area straight on rapheless valves. Concerning Achnanthidium saprophilum, despite the number of areolae per striae in the isotype being higher (4–7, Hlúbiková et al. 2011) than ours (4–5), the areolae morphology of the valves from our population is variable, including apically elongated areolae, as described for A. saprophilum (Kobayasi & Mayama 1982). Distribution and ecological information:— The species occurred in planktonic, periphytic and surface sediment assemblages in low abundance (, Published as part of 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, pp. 187-223 in Phytotaxa 575 (3) on pages 208-210, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Wojtal, A. Z., Ector, L., Van de Vijver, B., Morales, E. A., Blanco, S., Piatek, J. & Smieja, A. (2011) The Achnanthidium minutissimum complex (Bacillariophyceae) in southern Poland. Algological Studies 136 - 137: 211 - 238. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0211","Van de Vijver, B., Ector, L., Beltrami, M. E., de Haan, M., Falasco, E., Hlubikova, D., Jarlman, A., Kelly, M., Novais, M. H. & Wojtal, A. Z. (2011) A critical analysis of the type material of Achnanthidium lineare W. Sm. (Bacillariophyceae). Algological Studies 136 - 137: 167 - 191. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0167","Peres, F., Le Cohu, R. & Delmont, D. (2014) Achnanthidium barbei sp. nov. and Achnanthidium costei sp. nov., two new diatoms from French rivers. Diatom Research 29 (4): 387 - 397. https: // doi. org / 10.1080 / 0269249 X. 2014.890956","Hlubikova, D., Ector, L. & Hoffmann, L. (2011) Examination of the type material of some diatom species related to Achnanthidium minutissimum (Kutz.) Czarn. (Bacillariophyceae). Algological Studies 136 / 137: 19 - 43. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0019","Kobayasi, H. & Mayama, S. (1982) Most pollution-tolerant diatoms of severely polluted rivers in the vicinity of Tokyo. Japanese Journal of Phycology 30: 188 - 196."]}

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., Published as part of 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, pp. 187-223 in Phytotaxa 575 (3) on pages 207-208, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Novais, M. H., Juttner, I., Van de Vijver, B., Morais, M. M., Hoffmann, L. & Ector, L. (2015) Morphological variability within the Achnanthidium minutissimum species complex (Bacillariophyta): comparison between the type material of Achnanthes minutissima and related taxa, and new freshwater Achnanthidium species from Portugal. Phytotaxa 224 (2): 101 - 139. https: // doi. org / 10.11646 / phytotaxa. 224.2.1","Lange-Bertalot, H. (1999) Neue Kombinationen von Taxa aus Achnanthes Bory (sensu lato). In: Lange-Bertalot, H. (Ed.), Iconographia Diatomologica. Annotated Diatom Micrographs. Vol. 6. Phytogeography - Diversity - Taxonomy. Koeltz Scientific Books, Konigstein, Germany, pp. 276 - 289.","Lange-Bertalot, H. & Metzeltin, D. (1996) Oligotrophie-Indikatoren. 800 Taxa reprasentativ fur drei diverse Seen-Typen. Kalkreich - Oligodysroph - Schwach gepuffertes Weichwasser [Indicators of oligotrophy. 800 taxa representative of three ecologically distinct lake types. Carbonate buffered - Oligodystrophic - Weakly buffered soft water]. Iconographia Diatomologica 2: 1 - 390.","Hlubikova, D., Ector, L. & Hoffmann, L. (2011) Examination of the type material of some diatom species related to Achnanthidium minutissimum (Kutz.) Czarn. (Bacillariophyceae). Algological Studies 136 / 137: 19 - 43. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0019","Meister, F. (1935) Seltene und neue Kieselalgen. Bericht der Schweizerischen Botanischen Gesellschaft 44: 87 - 108.","Miao, M., Li, Z., Hwang, E. - A., Kim, H. - K., Lee, H. & Kim, B. - H. (2020) Two new benthic diatoms of the genus Achnanthidium (Bacillariophyceae) from the Hangang River, Korea. Diversity 12 (7): 285. https: // doi. org / 10.3390 / d 12070285","Potapova, M. G. & Ponader, K. C. (2004) Two common North American diatoms, Achnanthidium rivulare sp. nov. and A. deflexum (Reimer) Kingston: morphology, ecology and comparison with related species. Diatom Research 19 (1): 33 - 57. https: // doi. org / 10.1080 / 0269249 X. 2004.9705606"]}

Achnanthidium sp. (Figs 317–327) LM observations:— Valves linear-elliptic to linear-lanceolate, with broadly rounded apices to subrostrate (Figs 317– 327); 7.4–11.6 µm long, 3.5–4.1 µm wide. Raphe valve: axial area narrow and linear, central area small rounded, with 1–2 shortened striae on both sides of the valve (Figs 317–321). Raphe filiform straight. Transapical striae slightly radiate throughout the entire valve, becoming denser and strongly radiate towards the apices; 24–28 in 10 µm (Figs 317–321). Rapheless valve: axial area narrow and linear, widening towards the central area, central area absent or lanceolate (Figs 322–327). Transapical striae radiate throughout the entire valve, becoming denser and strongly radiate towards the apices; 23–27 in 10 µm (Figs 322–327). Taxonomical remarks:— Achnanthidium modestiformis (Lange-Bertalot) Van de Vijver (in Van de Vijver et al. 2002: 17) presents similar morphometric features (12–17 µm long, 3.5–4 µm wide, stria density 24–25/10 µm, as Achnanthes modesta Manguin (in Bourrelly & Manguin 1954: 22). However, the Brazilian population has broadly rounded to rostrate apices, whereas they are more capitate in A. modestiformis. Moreover, Achnanthidium sp. has shorter valves. Another similar specimen cited in Brazilian literature is Achnanthes acares M.H. Hohn & Hellerman (1963: 273) (Schneck et al. 2008). The illustrations presented by Hohn & Hellerman (1963) concerning this taxon show a different central area, elliptical in the raphe valve and coarse striae resembling Planothidium. Krammer & Lange-Bertalot (1991) illustrated the holotype of this species using LM, however, they only showed the raphe valves of the specimens, which probably do not correspond to a monoraphid taxon. Additionally, another Brazilian population resembled ours, but it was not identified at the species level (Santos et al. 2011, Figs 21–24, as “ Achnanthidium sp. ”). More studies are necessary concerning the identity of this taxon. Distribution and ecological information:— Achnanthidium sp. was rare (occurrence of, Published as part of 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, pp. 187-223 in Phytotaxa 575 (3) on pages 214-215, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Van de Vijver, B., Frenot, Y. & Beyens, L. (2002) Freshwater diatoms from Ile de la Possession (Crozet Archipelago, Subantarctica). Bibliotheca Diatomologica 46: 1 - 412.","Bourrelly, P. & Manguin, E. (1954) Contribution a la flore algale d'eau douce des Iles Kerguelen. Memoires de l'Institut Scientifique de Madagascar, series B 5: 5 - 58.","Hohn, M. H. & Hellerman, J. (1963) The taxonomy and structure of diatom populations from three Eastern North American rivers using three sampling methods. Transactions of the American Microscopical Society 82 (3): 250 - 329. https: // doi. org / 10.2307 / 3223932","Schneck, F., Torgan, L. C. & Schwarzbold, A. (2008) Diatomaceas epiliticas em riacho de altitude no sul do Brasil. Rodriguesia 59 (2): 325 - 338. https: // doi. org / 10.1590 / 2175 - 7860200859205","Krammer, K. & Lange-Bertalot, H. (1991) Bacillariophyceae 4. Teil: Achnanthaceae, Kritische Erganzungen zu Navicula (Lineolatae) und Gomphonema. Gesamtliteraturverzeichnis Teil 1 - 4. In: Ettl, H., Gartner, G., Gerloff, J., Heynig, H. & Mollenhauer, D. (Eds.) Susswasserflora von Mitteleuropa Vol. 2 / 4. Gustav Fischer Verlag, Stuttgart-Jena, pp. 1 - 437.","Santos, E. M., Tremarin, P. I. & Ludwig, T. A. V. (2011) Diatomaceas perifiticas em Potamogeton polygonus Cham. & Schltdl.: citacoes pioneiras para o estado do Parana [Periphytic diatoms on Potamogeton polygonus Cham. & Schltdl.: first records from Parana State]. Biota Neotropica 11 (3): 303 - 315. https: // doi. org / 10.1590 / S 1676 - 06032011000300025"]}

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)., Published as part of 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, pp. 187-223 in Phytotaxa 575 (3) on pages 201-202, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Witkowski, A., Kulikovskiy, M. & Riaux-Gobin, C. (2012) Achnanthidium sieminskae, a new diatom species from the Kerguelen Archipelago (Austral Islands). In: Wolowski, K., Kaczmarska, I., Ehrman, J. M. & Wojtal, A. Z. (Eds.) Current Advances in Algal Taxonomy and its Applications. W. Szafer Institute of Botany, Polish Academy of Sciences, Krakow, pp. 61 - 68.","Van de Vijver, B. & Kopalova, K. (2014) Four Achnanthidium species (Bacillariophyta) formerly identified as Achnanthidium minutissimum from the Antarctic Region. European Journal of Taxonomy 79: 1 - 19. https: // doi. org / 10.5852 / ejt. 2014.79","Lange-Bertalot, H. (1999) Neue Kombinationen von Taxa aus Achnanthes Bory (sensu lato). In: Lange-Bertalot, H. (Ed.), Iconographia Diatomologica. Annotated Diatom Micrographs. Vol. 6. Phytogeography - Diversity - Taxonomy. Koeltz Scientific Books, Konigstein, Germany, pp. 276 - 289.","Lange-Bertalot, H. & Krammer, K. (1989) Achnanthes eine Monographie der Gattung mit Definition der Gattung Cocconeis und Nachtragen zu den Naviculaceae. Bibliotheca Diatomologica 18: 1 - 393.","Krammer, K. & Lange-Bertalot, H. (2004) Bacillariophyceae 4. Teil: Achnanthaceae, Kritische Erganzungen zu Achnanthes s. l., Navicula s. str., Gomphonema. Gesamtliteraturverzeichnis Teil 1 - 4. In: Ettl, H., Gartner, G., Gerloff, J., Heynig, H. & Mollenhauer, D. (Eds.) Susswasserflora von Mitteleuropa Vol. 2 / 4. Spektrum Akademischer Verlag, Heidelberg-Berlin, pp. 1 - 468.","Lange-Bertalot, H., Hofmann, G., Werum, M. & Cantonati, M. (2017) Freshwater benthic diatoms of Central Europe: over 800 common species used in ecological assessment. English edition with updated taxonomy and added species. In: Cantonati, M., Kelly, M. G. & Lange-Bertalot, H. (Eds.) Koeltz Botanical Books, Schmitten-Oberreifenberg, pp. 1 - 942.","Silva-Lehmkuhl, A. M., Tremarin, P. I., Vercellino, I. S. & Ludwig, T. A. V. (2019) Periphytic diatoms from an oligotrophic lentic system, Piraquara I reservoir, Parana state, Brazil. Biota Neotropica 19 (2): e 20180568. https: // doi. org / 10.1590 / 1676 - 0611 - bn- 2018 - 0568","Yu, P., Kociolek, J. P., You, Q. & Wang, Q. (2018) Achnanthidium longissimum sp. nov. (Bacillariophyta), a new diatom species from Jiuzhai Valley, Southwestern China. Diatom Research 33 (3): 339 - 348. https: // doi. org / 10.1080 / 0269249 X. 2018.1545704","Pinseel, E., Vanormelingen, P., Hamilton, P. B., Vyverman, W., Van de Vijver, B. & Kopalova, K. (2017) Molecular and morphological characterization of the Achnanthidium minutissimum complex (Bacillariophyta) in Petuniabukta (Spitsbergen, High Arctic) including the description of A. digitatum sp. nov. European Journal of Phycology 52 (3): 264 - 280. https: // doi. org / 10.1080 / 09670262.2017.1283540"]}

Six species of the genus Achnanthidium Kutz. with straight terminal raphe fissures including Achnanthidium caledonicum (Lange-Bert.) Lange-Bert., A. sieminskae Witkowski, Kulikowskiy & Riaux-Gob., Achnanthidium neomicrocephalum Lange-Bert. & F.Staab and Achnanthidium minutissimum (Kutz.) Czarn. were studied using light and scanning electron microscopy, and shape analysis. The type of Achnanthes microcephala f. scotica J.R.Carter (synonym: A. caledonica) and a population of A. neomicrocephalum from its type locality were investigated. Two new species, Achnanthidium tirolense sp. nov. and Achnanthidium lacuslustense sp. nov., found in two oligotrophic lakes in Germany and Austria, were described. Achnanthidium tirolense sp. nov. is distinguished by its rhombic-lanceolate valves with a slightly inflated valve centre and an acute-angled fascia on the raphe valve, and Achnanthidium lacuslustense sp. nov. by its large capitate poles, broad fascia on the raphe valve and strongly curved frustules. The distribution of A. caledonicum, A. sieminskae, A. neomicrocephalum and A. minutissimum in relation to land use and their species associations were investigated in 52 rivers, streams, and lakes of Scotland. Achnanthidium sieminskae, A. caledonicum, and A. neomicrocephalum were found in areas with low human impact where seminatural vegetation was dominant. The latter two species are typical in mountainous areas. In contrast, A. minutissimum sensu stricto was found in locations where human impact was greater. Although further data are required to determine which environmental variables underlie these distributions, our results suggest that the treatment of these species should be revised in ecological status assessments, and taxa aggregated under A. minutissimum sensu lato should be distinguished in ecological and biogeographical studies.

Achnanthidium minutissimum (Kütz.) Czarn. (Fig. 14) Memoirs of the California Academy of Sciences 17: 157 (Czarnecki 1994). — Basionym: Achnanthes minutissima Kütz., Linnaea 8 (5): 578, pl. 16, fig. 54 (Kützing 1833); Kützing, 50, pl. 4, fig. 54 (Kützing 1834); Kützing, Dec. VIII, no. 75 (Kützing 1834), in Kützing, Algarum Aquae Dulcis Germanicum, Decades I-XVI, numbers 1-160. Halis Saxonum, 1833-1836. DESCRIPTION LM (Fig. 14 A-AW) Frustules slightly arched, with convex RL, concave RV. Valves linear-lanceolate. Margins tapering gradually towards short protracted rounded, rostrate poles. Valve dimensions (n = 46): length 8.5-15 µm, width 2.5-3.5 µm. Central area on raphe valve varies from small rounded to a narrow rectangular fascia on one or both sides. Striae more widely spaced in valve centre. Raphe filiform, straight, central pores slightly widened, terminating close to margin of central area. Central area on rapheless valve indistinct or narrow elliptical. Axial area narrow linear on both valves, slightly widening towards valve centre. Striae radiate throughout on both valves, 30-35 in 10 µm (RV and RL), becoming denser at poles., Published as part of Jüttner, Ingrid, Hamilton, Paul B., Wetzel, Carlos E., De, Bart Van, Vijver, King, Lydia, Kelly, Martyn G., Williams, David M. & Ector, Luc, 2022, A study of the morphology and distribution of four Achnanthidium Kütz. species (Bacillariophyta), implications for ecological status assessment, and description of two new European species, pp. 147-176 in Cryptogamie, Algologie 20 (10) on page 155, DOI: 10.5252/cryptogamie-algologie2022v43a10, http://zenodo.org/record/7819378, {"references":["CZARNECKI D. B. 1994. - The freshwater diatom culture collection at Loras College, Dubuque, Iowa, in KOCIOLEK J. P. (ed.), Proceedings of the 11 th International Diatom Symposium, San Francisco, 12 - 17 August 1990. Memoirs of the California Academy of Sciences 17: 155 - 174.","KUTZING F. T. 1833. - Synopsis Diatomacearum oder Versuch einer systematischen Zusammenstellung der Diatomeen. Linnaea 8 (5): 529 - 620."]}

Achnanthidium neomicrocephalum Lange-Bert. & F.Staab (Figs 9-13) Süsswasserflora von Mitteleuropa, Bacillariophyceae. 4. Teil Achnanthaceae: 431, pl. 89, figs 1-13 (Krammer & Lange-Bertalot 2004). DESCRIPTION LM (Fig. 9 A-DZ) Frustules slightly arched, with convex RL, concave RV.Valves linear-lanceolate. Margins slightly tapering towards small protracted, capitate poles. Valve margins slightly inflated in valve centre, sometimes more strongly on one side. Valve dimensions (n = 31): length 12-30.5 µm, width at valve centre 2-2.5 µm. On RV, central area small rounded, bordered by 1-2 slightly shortened, more widely spaced striae on each side, or small transverse fascia. Raphe filiform, straight, central pores terminating close to margin of central area. Central area on rapheless valve indistinct. Axial area narrow, linear on both valves, slightly widening towards valve centre. Striae radiate throughout on both valves, 30-32 in 10 µm (RV and RL). SEM (Figs 10 A-H; 11A-F; 12A-H; 13A-D) Raphe located on slightly raised sternum, terminal raphe fissures straight terminating at valve/mantle junction. Internally, central raphe endings shortly deflected to opposite sides. Striae composed of 2-3 areolae (RV and RL). Areolae round or oval adjacent to axial area, sometimes slit-like adjacent to valve margin. Row of transapically elongated areolae on mantle separated from areolae on valve face by hyaline area.

Achnanthidium lacuslustense sp. nov. (Figs 17; 18) HOLOTYPE. — Germany. Lustsee, 14.IV.2013, I. Jüttner (holo-, NMW [NMW.C.2013.010.2013]; iso-, BM, BR, CANA). TYPE LOCALITY. — Germany. Upper Bavaria, Osterseen area, Lustsee, 47°48’33.509”N, 11°17’36.959”E. HABITAT. — Epiphytic and epipsammic in lake littoral. ETYMOLOGY. — The species was named after the location where it was found, the Lustsee in Upper Bavaria, Germany. REGISTRATION. — http://phycobank.org/103380 DESCRIPTION LM (Fig. 17 A-M) Frustules with strongly concave RV and convex RL with recurved poles.Valves lanceolate,margins tapering gradually towards large capitate poles.Width of poles slightly less or as wide as the maximum width at valve centre. Valve dimensions (n = 14): length 15.5-29.0 µm, width at valve centre 3-4 µm.Central area on RV a moderately broad fascia.Raphe filiform, straight.Central pores slightly expanded, terminating at margin of central area.Central area on RL rectangular, bordered by 2 short striae on each side. Axial area narrow linear on both valves. Striae radiate throughout on both valves, 28-30 in 10 µm (RV), 30 in 10 µm (RL). SEM (Fig. 18 A-H) Raphe located on raised sternum, central raphe endings slightly expanded, tear drop-shaped, terminal raphe fissures straight with tear drop-shaped ends, terminating at valve/ mantle junction. Internally, central raphe endings very slightly deflected to opposite sides, terminating on side of central nod - ule, terminal raphe endings form small helictoglossae. Striae straight on RV, composed of 3-4, rarely 2, areolae. Areolae round adjacent to axial area and at poles, others often slit-like. On RL striae straight, slightly curved at poles, composed of 3-4, rarely 2 or 5, areolae. Areolae mostly round adjacent to axial area, elsewhere often transapically elongate, internally occluded by individual hymenes. A row of transapically elongated areolae on mantle separated from areolae on valve face by a hyaline area.

Achnanthidium tirolense sp. nov. (Figs 15; 16) HOLOTYPE. — Austria. Plansee, 18.IV.2019, I. Jüttner (holo-, NMW [NMW.C.2021.06.2019]; iso-, BM, BR, CANA). TYPE LOCALITY. — Austria. Tyrol, Kleiner Plansee, Plansee, 47°28’34.756”N, 10°46’50.861”E. HABITAT. — Epiphytic and epilithic in lake littoral. ETYMOLOGY. — The species was named after the region where it was found, Tyrol, Austria. REGISTRATION. — http://phycobank.org/103379 DESCRIPTION LM (Fig. 15 A-AG) Valves rhombic-lanceolate with slightly inflated valve centre. Margins tapering gradually towards long protracted, broadly rounded, rostrate poles. Valve dimensions (n = 25): length 14-23.5 µm, width at valve centre 3-4 µm. Central area on RV an acute-angled fascia. Raphe filiform, straight. Central pores slightly expanded, distantly placed at margin of central area. Central area on RL narrow lanceolate, sometimes a narrow acute-angled fascia on one or both sides formed by more widely spaced central striae. Axial area narrow linear on both valves. Striae radiate throughout on both valves, 28-30 in 10 µm (RV and RL). SEM (Fig. 16 A-F) Raphe located on slightly raised sternum, central raphe endings small, round, terminal raphe fissures straight with tear drop-shaped ends, terminating at valve/mantle junction. Internally, central raphe endings slightly deflected to opposite sides, terminal raphe endings form small helictoglossae. Striae straight or slightly curved, on RV composed of 3-5 areolae, on RL composed of 2-5 areolae. Areolae round, those adjacent to valve margin round or slit-like. One row of transapically elongated areolae on mantle separated from areolae on valve face by a hyaline area.

Jüttner, Ingrid, Hamilton, Paul B., Wetzel, Carlos E., De, Bart Van, Vijver, King, Lydia, Kelly, Martyn G., Williams, David M., Ector, Luc (2022): A study of the morphology and distribution of four Achnanthidium Kütz. species (Bacillariophyta), implications for ecological status assessment, and description of two new European species. Cryptogamie, Algologie 20 (10): 147-176, DOI: 10.5252/cryptogamie-algologie2022v43a10

Achnanthidium caledonicum (Lange-Bert.) Lange-Bert. (Figs 2-4) Iconographia Diatomologica 6: 270-283 (Lange-Bertalot 1999). — Achnanthes microcephala f. scotica J.R.Carter in Carter & Bailey- Watts, Nova Hedwigia 33: 534, pl. 1, fig. 31 (Carter & Bailey-Watts 1981). — Achnanthes minutissima var. scotica (J.R.Carter) Lange- Bert.in Lange-Bertalot & Krammer, Bibliotheca Diatomologica 18: 106, pl. 55, figs 4-12, pl. 57, fig. 11, pl. 67, figs 35-56 (Lange-Bertalot & Krammer 1989). — Achnanthes scotica (J.R.Carter) Lange-Bert., Bibliotheca Diatomologica 27: 8 (Lange-Bertalot 1993), nom. illeg., non Achnanthes scotica Flower & V.J.Jones, Diatom Research 4: 228, figs 1-7, 42-53 (Flower & Jones 1989). — Achnanthidium minutissimum var. scoticum (J.R.Carter) H.Cremer in Cremer & Wagner, Polar Biology 26: 108 (Cremer & Wagner 2003). — Basionym: Achnanthes caledonica Lange-Bert. in Lange-Bertalot & Moser, Bibliotheca Diatomologica 29: 95 (Lange-Bertalot & Moser 1994). DESCRIPTION LM (Fig. 2 A-AN) Frustules moderately arched, with convex rapheless valves, concave raphe valves, and recurved poles. Valves linear with large, subcapitate to capitate poles, the latter sometimes wider than the maximum valve width at valve centre. Valve margins tapering slightly towards a small constriction before poles. At valve centre, margins slightly inflated, sometimes unilaterally, or straight. Valve dimensions (n = 20): length 17.5-37.5 µm, width at valve centre: 3.0-3.5 µm. On raphe valve (RV), shape of central area variable, either a small transverse fascia, or oval to round, bordered by one or several slightly shortened striae. Raphe filiform, straight, central pores hardly widened, distantly placed at margin of central area. Central area on rapheless valve (RL) narrow lanceolate. Axial area narrow linear on both valves. Striae radiate throughout on both valves, more distantly placed in valve centre, 30-32 in 10 µm. SEM (Figs 3 A-F; 4A-D) Raphe located on slightly raised sternum, terminal fissures straight terminating at valve/mantle junction. Internally, central raphe endings shortly deflected to opposite sides, terminal raphe endings form small helictoglossae. Striae composed of 2-4 (RV) and 2-5 (RL) areolae. Areolae round or oval, areolae adjacent to valve margin slit-like. Row of transapically elongated areolae on mantle separated from areolae on valve face by hyaline area. On the type slide (lecto-, BM[BM 94890], ex J.R. Carter 3665), the species is common, but many specimens are positioned in girdle view. The central area is often a fascia, as mentioned by Carter (Carter & Bailey-Watts 1981), but sometimes less conspicuous, irregular, and formed by more widely spaced central striae., Published as part of Jüttner, Ingrid, Hamilton, Paul B., Wetzel, Carlos E., De, Bart Van, Vijver, King, Lydia, Kelly, Martyn G., Williams, David M. & Ector, Luc, 2022, A study of the morphology and distribution of four Achnanthidium Kütz. species (Bacillariophyta), implications for ecological status assessment, and description of two new European species, pp. 147-176 in Cryptogamie, Algologie 20 (10) on pages 152-153, DOI: 10.5252/cryptogamie-algologie2022v43a10, http://zenodo.org/record/7819378, {"references":["FLOWER R. J. & JONES V. J. 1989. - Taxonomic descriptions and occurrences of new Achnanthes taxa in acid lakes in the U. K. Diatom Research 4 (2): 227 - 239 https: // doi. org / 10.1080 / 0269 249 X. 1989.9705072","CREMER H. & WAGNER B. 2003. - The diatom flora in the ultraoligotrophic Lake El'gygytgyn, Chukotka. Polar Biology 26: 105 - 114. https: // doi. org / 10.1007 / s 00300 - 002 - 0445 - 0"]}