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During an evaluation of the diatom diversity and water quality of a small brook near Morogoro (Tanzania), an unknown Planothidium species was observed that could not be identified using the currently available literature. The past 10 years, a large number of Planothidium species has been revised resulting in a better characterisation of some well-known taxa such as P. lanceolatum (Brébisson ex Kützing 1846: 247) Lange-Bertalot (1999: 287), P. frequentissimum (Lange-Bertalot 1993: 4) Lange-Bertalot (1999: 282) and P. delicatulum (Kützing 1844: 75) Round & Bukhtiyarova (1996: 353), but also in the description of a lot of new species (Compère & Van de Vijver 2009, Jahn et al. 2017, Kulaš et al. (2020), N’Guessan et al. 2014, Stancheva 2019, Stancheva et al. 2020, Van de Vijver et al. 2013, 2018, Wetzel et al. 2013, 2014, 2019). Several of these studies dealt with Planothidium species from the African Continent. Compère & Van de Vijver (2009) analysed and illustrated the type material of P. engelbrechtii (Cholnoky 1955: 16) Round & Bukhtiyarova (1996: 353), whereas N’Guessan et al. (2014) discussed the morphology and taxonomic history of two species formerly described by Carter & Denny in 1982: P. miotum (J.R.Carter & Denny 1982: 286) Lange-Bertalot (1999: 278) & P. piaficum (J.R.Carter & Denny 1982: 286) C.E.Wetzel & Ector (in N’Guessan et al. 2014: 460), completed with the description of P. comperei C.E.Wetzel et al. (in N’Guessan et al. 2014: 456), a new Planothidium species from Ivory Coast.

Planothidium africanum Van de Vijver, Gogne, Hoogsteyns, Van de Velde, Vlaminck, Kabota, Teunen & C.E.Wetzel sp. nov. (Figs 1–28 LM, 29–35 SEM) Valves lanceolate (in the largest specimens) to elliptic-lanceolate with distinctly convex margins and protracted, rostrate but truncated apices. Valve dimensions (n=40): length 11–25 μm, width 5.5–6.5 μm. Rapheless valve (Figs 1–14, 29, 30, 33 & 35): Axial area usually narrow, straight, linear, occasionally weakly lanceolate and broadening near the central area. Irregularly shaped, shallow depressions present in both central and axial area (Figs 29, 33). Central area with a unilateral large horseshoe-shaped hyaline area. Cavum distinctly visible, internally passing over the neighboring striae (Figs 30, 35). Striae on the opposite site only weakly to almost shortened. Striae parallel to weakly radiate near the central area, becoming more radiate near the apices, 15–16 in 10 µm. Striae multiseriate, composed of three rows of small, rounded areolae at the valve margins, becoming biseriate towards the axial area (Fig. 29, 33). On the mantle, irregular groupings of small, rounded areolae present, continuing the valve face striae on the mantle (Fig. 33). Internally, striae located in deep grooves (Fig. 30). Raphe valve (Figs 15–28, 31, 32 & 34): Axial area very narrow, linear, not widening towards the central area. Central area asymmetrical, rectangular to slightly wedgeshaped, bordered on each side by 2–3 shortened striae, whereas on the opposite side, striae either lacking or extremely shortened. Raphe branches straight with expanded, drop-like central raphe pores (Fig. 31). Terminal raphe fissures unilaterally deflected (Fig. 31). Striae weakly radiate throughout the entire valve, 14–16 in 10 μm. Internally, central nodule clearly raised (Fig. 32). Internal central raphe endings weakly deflected into opposite directions, terminating onto the central nodule (Fig. 32). Internally, striae located in broad, deep grooves (Fig. 32). Cingulum composed of multiple open girdle bands (Fig. 34). Type:— TANZANIA. Morogoro, sample L10 stone, coll. date VII.2022, leg. Van de Velde (holotype slide BR-4778= Fig. 6, isotype slide 422, University of Antwerp, Belgium). PhycoBank registration: — www.PhycoBank.org/103609 Ecology & associated diatom flora:— Sample L10 Stone (S 06° 49’ 00.1” / E 037° 42’ 56.8”) was collected from some submerged stones in a small, shallow (Navicula species [N. rostellata Kützing (1844: 95), N. schroeteri Meister (1932: 38), N. symmetrica R.M. Patrick (1944: 5)], Achnanthidium exiguum (Grunow in Cleve & Grunow 1880: 21) Czarnecki (1994: 157), Cocconeis cf. euglypta, Planothidium africanum, P. rostratum (Østrup 1903: 35) Lange-Bertalot (1999: 279), several (non-identified) Gomphonema species, and Geissleria cf. decussis with lower numbers of Seminavis strigosa (Hustedt 1949: 44) Danielidis & Economou-Amilli (in Danieldis & Mann 2003: 30), Halamphora sp. and Pleurosigma salinarum (Grunow 1878: 116) Grunow (in Cleve & Grunow 1880: 54). Based on Lange-Bertalot et al. (2017) and Taylor et al. (2007), this diatom flora is typical for eutrophic, freshwater with higher electrolyte content to even brackish, and β- to α-mesosaprobic conditions, pointing to a high degree of pollution. Taxonomic comments:— Table 1 lists the morphometric features of several similar Planothidium species. Planothidium africanum has a distinct cavum, excluding several species bearing a comparable valve outline but with a sinus from being conspecific. Planothidium californicum Stancheva & N.Kristan (in Stancheva et al. 2020: 8) has a similar valve outline and similar valve dimensions, but lacks the typical cavum and hence can be relatively easily separated from the new species. Planothidium dubium (Grunow in Cleve & Grunow 1880: 23) Round & Bukhtiyarova (1996: 352) and P. reichardtii Lange-Bertalot & Werum (in Werum & Lange-Bertalot 2004: 172) have a similar elliptic-lanceolate valve outline with truncated, rostrate apices and an asymmetrical central area in the raphe-bearing valve. But both taxa also lack the distinct cavum structure excluding hence all conspecificity. The African species discussed in N’Guessan et al. (2014) sufficiently differ not to be confused with P. africanum. Planothidium comperei shows the highest resemblance based on its valve outline, although most valves have more convex valve margins. The stria structure of the rapheless valve also shows some similarity with the multiseriate nature of the striae at the valve margins becoming biseriate near the axial area. But the species can be separated from P. africanum in having an almost symmetrical central area in the raphe-bearing valve with a more radiate striation pattern, contrary to P. africanum having more parallel striae and a distinctly asymmetrical central area with on one side an almost complete lack of (or very shortened) striae. The most similar species is P. incuriatum C.E.Wetzel, Van de Vijver & Ector, split off from P. biporomum (M.H. Hohn & Hellerman 1963: 273) Lange-Bertalot (1999: 275) in 2013. Most likely, P. africanum, P. incuriatum and P. biporomum are close to each other, given a similar shape in cavum structure, a similar stria structure with a multiseriate base at the valve margin and a biseriate part at the axial area, and similar shallow markings in the axial and central area. However, Planothidium incuriatum has a more elongated, slender valve outline with more protracted apices, a symmetrical central area in the raphe-bearing valves (contrary to the distinctly asymmetrical central area in P. africanum) and slightly wider valves (6.5–7.0 µm versus 5.5–6.5 µm in P. africanum). The striae in the rapheless valve of P. africanum have a distinct grouping of 3 series of areolae at the valve margin, a feature not observed in P. incuriatum that have a very faint third series of areolae at the margin (see for instance Wetzel et al. 2003, fig. 51). This grouping is also visible in the mantle areolae (Fig. 33). Finally, other rostrate, cavum-bearing taxa such as P. rostratum (present in the same sample as P. africanum) and P. rostratoholarcticum Lange-Bertalot & Bąk (in Bąk & LangeBertalot 2014: 354) differ in having multiple slit-like grooves in their axial and central area, so far not observed in P. africanum. Contrary, P. africanum possesses an irregular pattern of small, shallow depressions in the axial area, not seen in P. rostratum and P. rostratoholarcticum. Wetzel et al. (2019) discuss several small-celled cavum-bearing Planothidium species such as P. frequentissimum but they can all be separated based on their valve dimensions (usually shorter and/or narrower), shape of the apices (typically broadly rounded, hardly protracted in P. frequentissimum) and valve outline, most of them being more strictly lanceolate instead of the elliptic-lanceolate valves observed in P. africanum., Published as part of Vijver, Bart Van De, Gogne, Britt, Hoogsteyns, Gabrielle, Velde, Levente Van De, Vlaminck, Luna, Kabota, Sija A., Teunen, Lies & Wetzel, Carlos E., 2023, Planothidium africanum sp. nov., a new freshwater diatom (Bacillariophyta) species from Tanzania, pp. 281-286 in Phytotaxa 585 (4) on pages 282-284, DOI: 10.11646/phytotaxa.585.4.4, http://zenodo.org/record/7703860, {"references":["Kutzing, F. T. (1844) Die Kieselschaligen Bacillarien oder Diatomeen. Nordhausen, pp. [i-vii], [1] - 152.","Meister, F. (1932) Kieselalgen aus Asien. Gebruder Borntraeger, Berlin, pp. [1] - 56.","Patrick, R. M. (1944) Estudo Limnologico e Biologico das Lagoas da regiao Litoranea Sul-Riograndense II. Some New Diatoms from the Lagoa dos Quadros. Boletim do Museu Naciona, nova serie, botanica 2: 1 - 6.","Cleve, P. T. & Grunow, A. (1880) Beitrage zur Kenntniss der arctischen Diatomeen. Kongliga Svenska Vetenskaps-Akademiens Handlingar 17 (2): 1 - 121.","Czarnecki, D. B. (1994) The freshwater diatoms culture collection at Loras College, Dubuque, Iowa. In: Proceedings of the 11 th International Diatom Symposium. Memoirs of the California Academy of Sciences 17: 155 - 174.","Ostrup, E. (1903) Freshwater diatoms. In Flora of Koh Chang. Part VII. Contributions to the knowledge of the Gulf of Siam. Preliminary Report on Botany. Results Danish Expedition to Siam (1899 - 1900). Botanisk Tidsskrift 25 (1): 28 - 41.","Lange-Bertalot, H. (1999) Neue Kombinationen von Taxa aus Achnanthes Bory (sensu lato). Iconographia Diatomologica 6: 270 - 283.","Hustedt, F. (1949) Diatomeen von Sinai-Halbinsel und aus dem Libanon-Gebiet. Hydrobiologia 2: 24 - 55.","Grunow, A. (1878) Algen und Diatomaceen aus dem Kaspischen Meere. In: Schneider, O. (Ed.) Naturwissenschaftliche Beitrage zur Kenntnis der Kaukasuslander, auf Grund seiner Sammelbeute. Dresden Burdach, Dresden, pp. 98 - 132.","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. Koeltz Botanical Books, Schmitten- Oberreifenberg, 942 pp.","Taylor, J. C., Harding, W. R. & Archibald, C. G. M. (2007) An Illustrated Guide to Some Common Diatom Species from South Africa An Illustrated Guide to Some Common Diatom Species from South Africa. WRC Report No TT 282 / 07. Water Research Commission, Pretoria, pp. 1 - 178 pls.","Stancheva, R., Kristan, N. V., Kristan, W. B, III & Sheath, R. G. (2020) Diatom genus Planothidium (Bacillariophyta) from streams and rivers in California, USA: diversity, distribution and autecology. Phytotaxa 470 (1): 1 - 30. https: // doi. org / 10.11646 / phytotaxa. 470.1.1","Round, F. E. & Bukhtiyarova, L. (1996) Four new genera based on Achnanthes (Achnanthidium) together with a re-definition of Achnanthidium. Diatom Research 11 (2): 345 - 361. https: // doi. org / 10.1080 / 0269249 X. 1996.9705389","Werum, M. & Lange-Bertalot, H. (2004) Diatoms in springs from Central Europe and elsewhere under the influence of hydrologeology and anthropogenic impacts. Iconographia Diatomologica 13: 3 - 417.","N'Guessan, K. R., Wetzel, C. E. Ector, L., Coste, M., C. Cocquyt, C., Van de Vijver, B., Yao, S. S., Ouattara, A., Kouamelan, E. P. & Tison- Rosebery, J. (2014) Planothidium comperei sp. nov. (Bacillariophyta), a new diatom species from Ivory Coast. Plant Ecology and Evolution 147 (3): 455 - 462. https: // doi. org / 10.5091 / plecevo. 2014.981","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.","Wetzel, C. E., Van de Vijver, B., Hoffmann, L. & Ector, L. (2013) Planothidium incuriatum sp. nov. a widely distributed diatom species (Bacillariophyta) and type analysis of Planothidium biporomum. Phytotaxa 138 (1): 43 - 57. https: // dx. doi. org / 10.11646 / phytotaxa. 138.1.6","Wetzel, C. E., Van de Vijver, B., Blanco, S. & Ector, L. (2019) On some common and new cavum-bearing Planothidium (Bacillariophyta) species from freshwater. Fottea 19 (1): 50 - 89. https: // doi. org / 10.5507 / fot. 2018.016","Bak, M. & Lange-Bertalot, H. (2014) Four small-celled Planothidium species from Central Europe proposed as new to science. Oceanological and Hydrobiological Studies 43 (4): 346 - 359. https: // doi. org / 10.2478 / s 13545 - 014 - 0152 - 9"]}

Platessa arborea sp. nov. new monoraphid diatom species is described from the tree moss habitat of India’s Sikkim state from the Eastern Himalayan biodiversity hotspot. This new species was described based on light and scanning electron micrographs and compared to morphologically similar taxa. Platessa arborea seems to have an affinity for the aerial habitat and has certain unique morphological features distinct from all the other Platessa species, such as its narrowly rounded apices, wide axial area, and the presence of predominantly uniseriate striae with unusually thick striae density. In India, there have been previous reports of some species from the genera Achnanthes and Cocconeis, which have been eventually transferred to the genus Platessa. However, this is the first formal documentation of the Platessa genus from India.

Platessa arborea C. Radhakrishnan, S. Sherly & B. Karthick sp. nov. (Figs 1–67, Fig. 9 represents the holotype) LM Description (Figs 1–60):—Valve elliptical with narrowly rounded apices. Raphe-sternum valve (RSV): Length: 6.5–9.5 µm, width: 3.5–4.5 µm, ratio length/width: 1.6–2.2 (n = 60). Axial area narrow and hardly discernible, visible only near center of the valve; central area nearly rectangular shape, bordered by 2–3 shorter striae frequently more widely spaced. Striae radiate throughout the valve, 22–24 in 10 µm. Raphe slightly filiform expanded proximal raphe endings. Sternum valve (SV): wide axial area, Length: 6.0–9.0 µm, width: 3.5–4.5 µm. Striae densely arranged 22–24 in 10 µm, parallel at centre, radiate towards apices. SEM Description (Figs 61–67):—Raphe-sternum valve (RSV): Externally, raphe filiform, slight curvature found near ends (Figs 61 & 63). Proximal raphe ends positioned in a broadened shallow groove, distal raphe located in the shallow groove with cone-shaped endings, and both ends slightly bend to the same side (Fig. 61). Striae mostly uniseriate; however found biseriate at 2–3 striae near apices. (Fig. 61). Areolae, apically slit-like throughout the valve, only at the center of the valve it is round-shaped towards axial area (Fig. 61). Internally, proximal raphe ends are strongly hooked, a very remarkable feature which is infrequent in other Platessa species, and its distal ends are curved on opposite sides, terminating in well-developed helictoglossae (Figs 64, 65). Striae lowered between raised virgae. Width of the interstriae is wider than the striae. Striae internally rectangular to round in shape and mostly covered with hymen (Figs 64, 65). An apparent depression is observed on the central nodule (Figs 64, 65). Sternum valve (SV), externally, axial area covered with several irregular shape depressions distributed on the surface of the valve (Fig. 62). Striae uniseriate, however, found biseriate at more than 10 striae near the valve mantle (Fig. 62). Interstriae is raised, and the width of striae is unequal (Fig. 62). Internally, axial area is flat and features are not easily visible (Fig. 66). Biseriate striae found near the mantle with hymenated areolae. Interstriae raised its width slightly more than the striae (Fig. 67). Holotype (designated here):—Slide #58/55, Sample #2878; deposited at the Diatom Collection, Agharkar Research Institute Herbarium (AHMA), Pune, India. Type locality:— INDIA, Sikkim, composite tree moss sample collected on 22 November 2019 on the way to Khecheopalri Lake, West Sikkim district (27.34829 °N, 88.19187 °E; elevation 1794 m a.s.l.) by Radhakrishnan Cheran. Etymology:—Named after the habitat (tree) on which it was found. In Latin, the tree is called an Arbor., Published as part of Sherly, Sheena, Radhakrishnan, Cheran & Karthick, Balasubramanian, 2022, Platessa arborea sp. nov. (Bacillariophyceae): A new tree moss dwelling diatom from the Eastern Himalayas, India, pp. 151-158 in Phytotaxa 552 (2) on pages 152-153, DOI: 10.11646/phytotaxa.552.2.2, http://zenodo.org/record/6690982

The epiphytic diatoms attached to seaweeds and benthic diatom assemblages, have been collected along the Seogwipo coast in Jeju Island, South Korea. In the last three-year survey, 80 diatom taxa inhabiting warmer oceans have been found, including those in the tropical and subtropical regions such as the Pacific Ocean, the Caribbean Sea, the western Indian Ocean, and other warmer regions. These diatom flora shows a worldwide pattern that concentrated around the Tropic of Cancer. This survey of local areas reveals the existence of many rare or retrieved taxa that were not well known until recently. Furthermore, some of them, including Aulacodiscus affinis, Leudugeria janischii, Hyalosynedra lanceolata, Trachyneis velata var. ornata, Lyrella concilians have been reported for the first time since the first entry or haven been reported only once. Of the diatom flora described here, 48 taxa are new to South Korea and previously unrecorded in the local area.

31. Achnanthes citronella (A. Mann) Hustedt (Figs 85, 86) Basionym: Cocconeis citronella A. Mann (reproduced by Riaux-Gobin et al. 2015) Synonym: Achnanthes mammalis var. reticulata Cleve. Type locality: Jolo Jolo, Sulu, Philippines. References: Lobban et al. 2012. p. 285. pl. 38, figs 5, 6; Riaux-Gobin et al. 2011, p. 13, pl. 1, fig. 6, pl. 8, figs 1–3; Riaux-Gobin et al. 2015, p. 104, figs 15–17, 25, 26, 33, 35–41. Morphometrics: Valves 38–53 (28–68) μm long, 21–29 (19–35) μm wide, transapical striae in sternum valves 9–10 (7–12) in10 μm, in raphe valves 20 (17–26) in 10 μm. Remarks: Achnanthes citronella has been reported in Tahiti (Ricard 1977) and the Penghu Islands of Taiwan (Li 1978), Cuba (Foged 1984), Fujian coasts of China (Jin et al. 1985), Palawan of Philippines (Podzorski & Håkansson 1987), Kowloon Bay of Hong Kong (Glenwright & Dickman 1997), Mascarene Islands in the western Indian Ocean (Riaux-Gobin et al. 2011), Guam, Society Islands and the Caribbean Sea (Lobban et al. 2012), the coast of the Baja California Peninsula of Mexico (López-Fuerte et al. 2015), Florida Bay (Frankovich & Wachnicka 2015), and the southern India (Rai & Rajashekhar 2015). It is a diatom taxon living in warmer coasts in view of its distribution. This taxon was found rarely as an epiphyte on seaweeds along the Seogwipo coast of Juju Island, and it is a new report to South Korea., Published as part of Joh, Gyeongje, 2021, Distribution and frequent occurrence of diatom taxa (Bacillariophyta) inhabiting warmer oceans in Seogwipo coast of Jeju Island, southernmost Korea, pp. 1-67 in Phytotaxa 517 (1) on page 24, DOI: 10.11646/phytotaxa.517.1.1, http://zenodo.org/record/8061058, {"references":["Riaux-Gobin, C., Compere, P., Hinz, F. & Ector, L. (2015) Achnanthes citronella, A. trachyderma comb. nov. (Bacillariophyta) and allied taxa pertaining to the same morphological group. Phytotaxa 227 (2): 101 - 119. https: // doi. org / 10.11646 / phytotaxa. 227.2.1","Lobban, C. S., Schefter, M., Jordan, R. W., Arai, Y., Sasaki, A., Theriot, E. C., Ashworth, M., Ruck, E. C. & Pennesi, C. (2012) Coral-reef diatoms (Bacillariophyta) from Guam: new records and preliminary checklist, with emphasis on epiphytic species from farmer-fish territories. Micronesica 43: 237 - 479.","Riaux-Gobin, C., Romero, O. O., Compere, P. & Al-Handel, A. Y. (2011) Small-sized Achnanthales (Bacillariophyta) from coral sands off Mascarenes (Western Indian Ocean). Bibliotheca Diatomologica Vol. 57, J. Cramer, Stuttgart, Germany, 234 pp.","Ricard, M. (1977) Ecology of diatoms from two Polynesian lagoons. Nova Hedwigia 54: 389 - 405.","Li, C. W. (1978) Notes on marine littoral diatoms of Taiwan I. Some diatoms of Pescadores. Nova Hedwigia 29: 787 - 812.","Foged, N. (1984) Freshwater and littoral diatoms from Cuba. Bibliotheca Diatomologica. Vol. 5. J. Cramer, Germany, 121 pp.","Jin, D. (Chin, T. G.), Cheng, Z., Lin, J. & Liu, S. (1985) The marine benthic diatoms in China. Vol. 1. China Ocean Press, Beijing, 313 pp.","Podzorski, A. C. & Hakansson, H. (1987) Freshwater and marine diatoms from Palawan (a Philippine island). Bibliotheca Diatomologica Vol. 13. J. Cramer, Berlin, 245 pp.","Glenwright, T. & Dickman, M. (1997) Diatom assemblages in surficial sediments along a transect between the Ninepin Island group and Kowloon Bay, Hong Kong. In: Morton, B. (Ed.) Proceedings of the 8 th International marine Biological Workshop: The marine fauna of Hong Kong and southern China IV. Hong Kong University Press, Hong Kong, pp. 339 - 359.","Lopez-Fuerte, F. O., Siqueiros-Beltrones, D. A. & Yabur, R. (2015) First record of benthic diatoms (Bacillariophyceae and Fragilariophyceae) from Isla Guadalupe, Baja California, Mexico. Revista Mexicana de Biodiversida 86: 281 - 292. https: // doi. org / 10.1016 / j. rmb. 2015.04.016","Frankovich, T. A. & Wachnicka, A. (2015) Epiphytic diatoms along phosphorus and salinity gradients in Florida Bay (Florida, USA), an illustrated guide and annotated checklist. In: Entry, J. A., Gottlieb, A. D., Jayachandran, K. and Ogram, A. (Eds.) Microbiology of the Everglades ecosystem. CRC Press, Boca Raton, Florida, pp. 241 - 288. https: // doi. org / 10.1201 / b 18253 - 15","Rai, S. V. & Rajashekhar, M. (2015) Ecological measurements of phytoplankton communities in the Arabian Sea water off Kerala and Karnataka coasts (South India). Algological Studies 149: 39 - 60. https: // doi. org / 10.1127 / algol _ stud / 2015 / 0205"]}

32. Achnanthes trachyderma (Meister) Riaux-Gobin, Compère, Hinz & Ector (Figs 75–77) Basionym: Cocconeis trachyderma Meister. Type locality: ‘ Shell of a marine snail from Nagasaki’, Nagasaki, Japan. Reference: Riaux-Gobin et al. 2015, p. 108, figs 19, 20, 27–29. Morphometrics: Valves 33–37 (24–32) μm long, 16–18 (12–18) μm wide, transapical striae 8–11 (10–12) in 10 μm of sternum valve, and 21 (23–26) in 10 μm of raphe valve. Local specimens are a little larger than the type specimens. Remarks: Achnanthes trachyderma was reported from a few regions, such as Tahiti and Moorea Islands of the southern Pacific Ocean (Riaux-Gobin et al. 2015), and Rodrigues Island of the western Indian Ocean (Al-Handal et al. 2016). It is a typical tropical taxon inhabiting warmer coasts. This taxon is rare epiphyte on seaweeds along the Seogwipo coast in Jeju Island, and is a new report to South Korea., Published as part of Joh, Gyeongje, 2021, Distribution and frequent occurrence of diatom taxa (Bacillariophyta) inhabiting warmer oceans in Seogwipo coast of Jeju Island, southernmost Korea, pp. 1-67 in Phytotaxa 517 (1) on pages 24-25, DOI: 10.11646/phytotaxa.517.1.1, http://zenodo.org/record/8061058, {"references":["Riaux-Gobin, C., Compere, P., Hinz, F. & Ector, L. (2015) Achnanthes citronella, A. trachyderma comb. nov. (Bacillariophyta) and allied taxa pertaining to the same morphological group. Phytotaxa 227 (2): 101 - 119. https: // doi. org / 10.11646 / phytotaxa. 227.2.1","Al-Handal, A. Y., Compere, P. & Riaux-Gobin, C. (2016) Marine benthic diatoms in the coral reefs of Reunion and Rodrigues Islands, West Indian Ocean. Micronesica 3: 1 - 78. [http: // micronesica. org / volumes / 2016]"]}

33. Planothidium quarnerensis f. rhombica (Giffen) Witkowski & Riaux-Gobin (Figs 89, 90) Basionym: Cocconeis quarnerensis f. rhombica Giffen. Type locality: Mahé Island, Seychelles. Reference: Riaux-Gobin et al. 2018, p. 117, figs 45–48. Morphomerics: Valves 13–24 (15–18) μm long, 7–11 (10–11) μm wide, transapical striae 10–11 (8–9) in 10 μm of sternum valve, 11–12 (9) in 10 μm of raphe valve. Remarks: The structure of vestigial striae is distinct in the axial area of sternum valve. Local specimens are slightly longer, but narrower in width, compared with the type ones. This taxon was reported only from the type locality, Mahé Island of the Seychelles, and probably tropical taxon in warmer coasts. It was rare epiphytes on seaweeds along the Seogwipo coast in Jeju Island, and this is a new report to South Korea., Published as part of Joh, Gyeongje, 2021, Distribution and frequent occurrence of diatom taxa (Bacillariophyta) inhabiting warmer oceans in Seogwipo coast of Jeju Island, southernmost Korea, pp. 1-67 in Phytotaxa 517 (1) on page 25, DOI: 10.11646/phytotaxa.517.1.1, http://zenodo.org/record/8061058, {"references":["Riaux-Gobin, C., Witkowski, A., Igersheim, A., Lobban, C. S., Al-Handal, A. Y. & Compere, P. (2018) Planothidium juandenovense sp. nov. (Bacillariophyta) from Juan de Nova (Scattered Islands, Mozambique Channel) and other tropical environments: a new addition to the Planothidium delicatulum complex. Fottea 18: 106 - 119. https: // doi. org / 10.5507 / fot. 2017.019"]}

Amphicocconeis ruatara Riaux-Gobin sp. nov. SEM Figs 49–61. Table 1 Description:— SEM. Valves oblong-elliptic to elongate, with slightly rostrate apices (Fig. 51). Axially bent in cingular view. Valve length 15 ± 2 µm, 14–17 µm. Valve width 6 ± 2 µm, 6–8 µm; L/ W 6.2 ± 0.8; n = 54 (SEM) (Table 1). SV convex with a concave narrow sternum (Figs 49–50). One row of large and transapically elongated areolae on each part of the sternum, one marginal row of dense and short areolae and, in between these two sectors, short areolae arranged more or less in quincunx. Often a low crest (or crista marginalis) delineating the marginal row of dense and short areolae, as a sort of short mantle (Figs 49–50, arrow). SV striae parallel in mid-valve, slightly radiate and denser on apices, uniseriate. SV striae 22 ± 3 in 10 µm. SVVC open, narrow (or sometimes larger), composed of short fused fimbriae (27 in 10 µm), separated from each other by an irregularly biseriate row of pores (Figs 52–54), festooned edge. Head pole of the SVVC with very short fimbriae. RV flat, dense striae composed of one narrow macro-areola, short supplementary marginal striae on mid-valve (Fig. 55). Raphe filiform. Proximal raphe endings externally simple and relatively close to each other, central nodule slightly elevated externally and unique for the genus (Figs 55–56, 59). Terminal raphe endings largely hooked on the mantle (Fig. 58). Raphe sternum somewhat elevated internally, with raphe slit slightly undulating. Proximal raphe endings simple and coaxial internally (Fig. 57). Central area small. RV striae 51 ± 6.5 in 10 µm; 42–60 in 10 µm. RVVC open, 30–50% valve width, 51 rows of uniseriate pores in 10 µm (Figs 60–61). Holotype:—Stub12 12/02/2020, BM001222887. Holotype specimen illustrated in Fig. 49. Syntypes:—Whole slide from the sample RAI 20, deposited at NHM (BM 81917 material). Etymology:—The specific epithet ruatara refers to the Ruatara rock (Raivavae Isl.) on the intertidal coastal environment where was first found the new taxon. Locality:—Ruatara rock (‘Rocher de l’Homme’), Raivavae (Austral Islands, South Pacific), RAI 20 and RAI 18 (intertidal red macroalgal turfs on fissures of the rock). Geo localization: S 23° 51.274’; W 147° 39.595’; T°C 23°.7, salinity Remarks:—Some similarities of A. ruatara sp. nov. with A.discrepans (A.W.F.Schmidt) Riaux-Gobin, Witkowski, Ector & Igersheim (Riaux-Gobin et al. 2018: figs 15–16), but with some differences: The axial rows of SV areolae on each part of the sternum are larger on A. ruatara than on A. discrepans, along with (in most cases) the presence of a crista marginalis delineating a narrow mantle on the new taxon. The SVVC in A. ruatara has short fused fimbriae with irregularly uni- to biseriate rows of pores, in contrast with a marginal row of one small pore in A. discrepans. The RVVC of A. ruatara shows dense uniseriate rows of pores (51 in 10 µm), while showing one marginal and elongate pore in A. discrepans (30–31 in 10 µm; Riaux-Gobin et al. 2018: figs 17–20)., Published as part of Riaux-Gobin, Catherine, Garcia, Marinês, Witkowski, Andrzej, Saenz-Agudelo, Pablo, Coste, Michel & Daniszewska-Kowalczyk, Genowefa, 2021, New Amphicocconeis (Bacillariophyta) from Raivavae and Tahiti Islands (South Pacific) and Porto Belo (Brazil), with re-examination of Psammococconeis, pp. 30-54 in Phytotaxa 513 (1) on pages 49-52, DOI: 10.11646/phytotaxa.513.1.2, http://zenodo.org/record/5306935, {"references":["De Stefano, M., Sacchi, U., Totti, C. & Romero, O. E. (2006) Cocconeis distans Gregory and Amphicocconeis debesi (Hustedt) De Stefano comb. nov. (Bacillariophyta), an intricate taxonomical history. Botanica Marina 49: 438 - 449. https: // doi. org / 10.1515 / BOT. 2006.055","Riaux-Gobin, C., Ector, L., Witkowski, A. & Igersheim, A. (2018) Achnanthales from historical Grunow collection in Porto Subzanski, Croatia. Botanica Marina 61 (6): 573 - 593. https: // doi. org / 10.1515 / bot- 2018 - 0045","De Stefano, M. & Marino, D. (2003) Morphology and taxonomy of Amphicocconeis gen. nov. (Achnanthales, Bacillariophyceae, Bacillariophyta) with considerations on its relationship to other monoraphid diatom genera. European Journal of Phycology 38: 361 - 370. https: // doi. org / 10.1080 / 09670260310001612646","Ehrenberg, C. G. (1838) Atlas von Vier und Sechzig Kupfertafeln ze Christian Gottfried Ehrenberg uber Infusionsthierchen. pp. pls I- LXIV. Leipzig: Verlag von Leopold Voss."]}

Amphicocconeis catharinensis Riaux-Gobin & Garcia. SEM, Figs 7–17. Table 1 Description:— SEM.Valves robust, oval-elliptic, with round apices. Valve length 18 ± 3 µm, 14–22 µm. Valve width 11 ± 2 µm, 9–13 µm; n = 10 (SEM) (Table 1). SV: surface convex, striae radiate and regularly spaced, uniseriate, composed of large rectangular areolae. On the valve face, each areola is positioned on the bottom of a depression with opening bordered by digit flaps (Figs 8–9). Areolae are internally closed by hymenes (Figs 10–12). On the mantle, each oblong areola is separated from the next by a granular and concave plain structure of triangular to rectangular shape (Figs 8–9, arrows). Short marginal striae not observed. 11 ± 1 SV stria in 10 µm. SVVC relatively narrow with big pores corresponding to each areola. RV: surface flat, striae radiate, composed of macro-areolae (large along the margin, short along the raphe, with short areolae dispersed in between, Fig. 13). Terminal-distal raphe endings strongly curved on same side (Figs 13, 15). Proximal raphe endings slightly enlarged and curved on same side (opposite to the terminal raphe endings, Fig. 16). 26 ± 2.5 RV stria in 10 µm. RVVC 1/4 of the valve width, with marginal transapical oblong pores corresponding to each macro-alreola (Fig. 17, twin arrowheads). Remarks:—The SV of Amphicocconeis catharinensis was illustrated as being the SV of Psammococconeis disculoides in Garcia (2001: figs 21, 23). The specific ornamentation of the SV mantle, with a concave structure in between each oblong areola (Figs 8–9) seems unique. The RV of A. catharinensis has some similarities with that of A. disculoides and A. debesi (large area with short radiate macro-areolae), but these short areolae in median-area are in very low density in A. catharinensis (Fig. 13), furthermore the SV of A. catharinensis is highly dissimilar to that of Amphicocconeis disculoides (Hustedt) De Stefano & Marino (figs 2–3, 6– 7 in De Stefano & Marino 2003; Figs 1). Type locality:—Zimbros, Peninsula do Porto Belo, Santa Catarina State, Brazil. Holotype:—Stub 4 19/06/2020, BM001222889. Holotype specimen illustrated in Fig. 7. Etymology:—The epithet refers to the location of the type habitat of this taxon in Brazil (locality Santa Catarina State).

Amphicocconeis cf. mascarenica Riaux-Gobin & Compère, 2011 (Figs 62–67). Table 1 Remarks:—The measurements and features of the specimens from the Pacific islands (Raivavae, Tahiti and Moorea Isls, Table 1) are close to those from the Mascarenes (Riaux-Gobin et al. 2011: tabl. 2), i.e., with valvocopulae with regularly biseriate rows of simple pores. This taxon is dominant on the Society Archipelago (Tahiti and Moorea Isls) as well as in the Austral Archipelago (Raivavae Isl.). In Tuamotu and Marquesas Archipelagos, Amphicocconeis are less abundant, but with, here again, A. cf. mascarenica as dominant taxon (pers. obs.)., Published as part of Riaux-Gobin, Catherine, Garcia, Marinês, Witkowski, Andrzej, Saenz-Agudelo, Pablo, Coste, Michel & Daniszewska-Kowalczyk, Genowefa, 2021, New Amphicocconeis (Bacillariophyta) from Raivavae and Tahiti Islands (South Pacific) and Porto Belo (Brazil), with re-examination of Psammococconeis, pp. 30-54 in Phytotaxa 513 (1) on page 52, DOI: 10.11646/phytotaxa.513.1.2, http://zenodo.org/record/5306935, {"references":["Riaux-Gobin, C., Compere, P. & Al-Handal, A. Y. (2011) New Amphicocconeis species off Mascarenes (Western Indian Ocean) and related taxa. Diatom Research 26 (2): 175 - 188. https: // doi. org / 10.1080 / 0269249 X. 2011.597958"]}

International audience; Psammococconeis and Amphicocconeis are recently created diatom genera showing a high morphological plasticity-variability, making particularly difficult identification only based on morphology. The sternum valve (SV) of Amphicocconeis is very similar to that of Cocconeis, whereas its raphe valve (RV) is highly dissimilar between the two genera. The RV in Amphicocconeis has curved terminal raphe fissures and striae composed of macroareolae with more or less complex arrangement, unique to each taxon. The Amphicocconeis valvocopulae are most of the time extended, with pores arranged along rows, or clusters of pores, and are an important taxonomic criterion. Several Amphicocconeis were originally described as Cocconeis and thereafter renamed after accurate examination with scanning electron microscopy (SEM). The number of described Amphicocconeis is low, while numerous species probably remain to be described, named or renamed. Several tropical Amphicocconeis from two Polynesian archipelagos are here investigated, from which three are described as new. Psammococconeis is a genus established prior to Amphicocconeis, originally described with simple terminal raphe endings, while their SV has characteristics close to Amphicocconeis. Psammococconeis is here re-defined as an invalid genus, after re-examination of the Brazilian type material of P. brasiliensis M.Garcia (renamed as Amphicocconeis brasilensis comb. nov.), and a new Amphicocconeis is described from this Brazilian material.

Planothidium marganaiensis G.G.Lai, Ector & C.E.Wetzel sp. nov. (Figs 2���65) Light microscopy (Figs 2���57):��� Frustules in girdle view rectangular, mostly single or forming short chains (up to two cells), clearly bent in the middle (Figs 29 ��� 32). Valves elliptical to weakly lanceolate with convex margins and subrostrate or slightly protracted apices (never capitate). Valve dimensions (n=59): length 5.0���12.3 ��m, width 2.8���4.0 ��m. Rapheless valve (Figs 2���28): Axial area linear, narrow and straight, sometimes lanceolate and slightly expanded in the middle of the valve. Central area asymmetrical with a unilateral small horseshoe-shaped hyaline area presenting a cavum. On the opposite side, absence of shortened striae or presence of one to two slightly shortened striae. Striae weakly radiate throughout the entire valve, 11���14 in 10 ��m, measured at the central part of the valve opposite to the unilateral expansion. Raphe valve (Figs 33���57): Axial area linear and narrow, slightly widened near the middle of the valve. Central area transapically rectangular to rounded, bordered by one to three shortened striae on each side, usually more distant than the others. Raphe branches straight with expanded, drop-like proximal raphe endings. Terminal raphe fissures not discernible in LM. Striae radiate throughout the entire valve, 11���14 in 10 ��m. Scanning electron microscopy (Figs 58���65):��� Rapheless valve (SV) (Figs 58���62): Valve surface with shallow, rounded to irregularly shaped depressions along the apical axis, mainly concentrated in the central area (Figs 58���60). Externally, striae multiseriate, composed of three to five rows of small same-sized round areolae (Figs 58���60). Striae continuing shortly onto the valve mantle, without interruption (Figs 58���59). Internally, cavum covered by a hood with a narrow aperture towards the mantle (Figs 61, 62). Raphe valve (RV) (Figs 63���65): Striae composed of three to five rows of rounded areolae, not extended over the valve mantle (Fig. 63). Virgae broader than striae, except near the apices. Externally, proximal raphe endings straight, terminating in expanded drop���like pores. Distal raphe fissures bent, continuing shortly onto the valve mantle (Fig. 63). Internally, striae sunken between raised virgae and composed of areolae covered by individual hymenes (Figs 64, 65). Central nodule raised (Fig. 65). Proximal raphe endings slightly deflected into opposite sides, terminating inconspicuously (Figs 64, 65). Distal raphe endings terminating on poorly developed helictoglossae, continuing shortly onto the valve mantle (Fig. 64). Type:��� ITALY. Carbonia-Iglesias: San Giovanni cave at Domusnovas (Monte Acqua, Marganai massif), 190 m a.s.l., 39��20���13.4������N; 008��37���37.1������E, sampling dates 21 September 2016 and 31 January 2017, collected by Giuseppina Grazia Lai (designated here, holotype: slide no. BR- 4636 Meise Botanic Garden, Belgium. Specimen here depicted in Fig. 9). Etymology:��� The name refers to the Marganai massif where the karst spring San Giovanni originates. Marganai (Margan��i) (Iglesias)���It is an elongated mountain or a mountain of a small mountain chain: it is likely that its name is to be compared���not derived���with Lat. margo, -inis ���margin��� (Indo-European). Cfr. Marghine (Source: Pittau, 2013)., Published as part of Lai, Giuseppina G., Ector, Luc, Padedda, Bachisio M. & Wetzel, Carlos E., 2021, Planothidium marganaiensis sp. nov. (Bacillariophyta), a new cavum-bearing species from a karst spring in south-western Sardinia (Italy), pp. 140-154 in Phytotaxa 489 (2) on pages 143-146, DOI: 10.11646/phytotaxa.489.2.3, http://zenodo.org/record/5756790, {"references":["Pittau, M. (2013) Toponimi della Sardegna Meridionale. Significato e origine. [http: // www. pittau. it / Sardo / top _ sard _ meridionale. html]"]}

Lai, Giuseppina G., Ector, Luc, Padedda, Bachisio M., Wetzel, Carlos E. (2021): Planothidium marganaiensis sp. nov. (Bacillariophyta), a new cavum-bearing species from a karst spring in south-western Sardinia (Italy). Phytotaxa 489 (2): 140-154, DOI: 10.11646/phytotaxa.489.2.3, URL: http://dx.doi.org/10.11646/phytotaxa.489.2.3

The type material of Achnanthes hauckiana Grunow has been investigated using light and scanning electron microscopy. The species, transferred to the genus Planothidium, was frequently reported from brackish and even marine environments. The original Grunow material was collected from a karstic spring near Trieste. Analysis of the material and of samples collected from the Croatian Krka River, resulted in a better understanding of the morphology of Planothidium hauckianum and a better characterization of the diatom flora of which it is a dominant component. The paper compares the species with similar Planothidium species from the P. delicatulum complex and revises published images of P. hauckianum in literature, making it clear that all reported and illustrated brackish populations of P. hauckianum belong to different, often unknown species. The presence of Planothidium hauckianum is limited to karstic spring areas, most likely showing a very restricted biogeographical distribution.

Achnanthes kohleriana Kopalov��, Zidarova & Van de Vijver sp. nov. Figs 1���24 Etymology The species is named after our friend and colleague Dr. Tyler Kohler (Charles University in Prague and University of Boulder, Colorado, USA) in recognition of his diatom ecology work in the Dry Valleys of the Antarctic Continent. Type Deception Island, South Shetland Islands, Antarctica, sample D13 (62��58���24.5��� S 60��43���03.2���W) (leg. R. Zidarova), coll. date: 21 Jan. 2013 (holo-: slide no. BR���4436; iso-: slide PLP���292, University of Antwerp, Belgium). Description Light microscopy (Figs 1���13) Frustules in girdle view bent, with concave raphe valve and convex rapheless valve (Fig. 13). Valves linear to linear-elliptic, with almost straight to weakly concave in the middle valve margins. Valve apices broadly rounded, not protracted. Valve dimensions (n = 24): length 38���45 ��m, width 9.0��� 10.5 ��m. Raphe valve (Figs 1���6): raphe distinctly lateral, curved. Proximal raphe endings deflected to one side terminating in drop-like expanded pores. Distal raphe fissures elongated and hooked. Axial area 1/4���1/5 of the valve width, almost linear, following the curvature of the raphe, not or only slightly widening toward the central area. Central area forming a rectangular to bow-tie-shaped fascia, lacking any striae near the margins. Transapical striae weakly to moderately radiate in the middle, becoming more radiate toward the apices, 11���12 in 10 ��m. Areolae well discernible in LM, rounded, c. 14���18 in 10 ��m. Rapheless valve (Figs 7���12): rapheless sternum narrow, located close to the valve margin. Striae parallel at the valve middle, becoming radiate toward the apices, 10���11 in 10 ��m. Areolae well discernible in LM, rounded, c. 14���18 in 10 ��m. Scanning electron microscopy (Figs 14���24) Raphe valve (Figs 14���19): valve face weakly concave in the middle. Apices presenting a rather larger hyaline zone (Figs 14, 16). Raphe slightly curved with tear-drop-shaped, weakly deflected proximal raphe endings, not extending in the central area (Figs 14, 17). Distal raphe fissures running a deep groove, hooked, elongated, continuing onto the mantle, terminating beyond the last striae (Figs 14, 16). Striae uniseriate, composed of almost rounded areolae with slightly recessed foramina, occluded by cribra with rounded perforations (Fig. 15). Small pseudosepta present near the valve apices (Fig. 19). Internally, proximal raphe endings strongly hooked backwards toward the poles (Fig. 18). Distal raphe endings straight, finishing onto small helictoglossae (Fig. 19). Striae separated by strongly thickened virgae (Fig. 19). Areolae with rounded foramina and recessed cribra (Fig. 19). Rapheless valve (Figs 20���24): valve face weakly convex (Fig. 20).A strongly thickened hyaline marginal ridge present at the valve face/ mantle junction (Figs 20, 21). Spines absent. Rapheless sternum narrow, located near the valve margin (Fig. 21), internally well discernible as a narrow hyaline line close to the valve margin (Fig. 24, arrows). Terminal orbiculi present on the valve mantle near the apices, occluded by a single, structureless silica flap (Fig. 23). Striae uniseriate on the valve face, becoming bi-seriate on the valve mantle (Fig. 23). Each stria composed of recessed rounded areolae, occluded by cribra with rounded or irregular perforations (Fig. 22). Internally, striae separated by strongly thickened virgae (Fig. 24). Areolae with rounded openings with noticeably recessed cribra (Fig. 24). Girdle composed of several open copulae, bearing a single row of rounded areolae, occluded by cribra (Fig. 20). Ecology and distribution So far Achnanthes kohleriana sp. nov. has been observed with certainty on several islands of the South Shetland archipelago (Livingston Island, Deception Island and King George Island). The largest population was found on Deception Island among wet mosses growing on a rock, located inland and far from the influence of sea sprays and with no nutrient input from sea birds or seals suggesting that the species is typically aerophilic. Other taxa present in the sample include Humidophila keiliorum Kopalov�� in Kopalov�� et al. (2015), H. deceptionensis Kopalov�� in Kopalov�� et al. (2015), Stauroneis pseudomuriella Van de Vijver & Lange-Bert. in Van de Vijver et al. (2004), S. pseudoschimanskii Van de Vijver & Lange-Bert. in Van de Vijver et al. (2004) and several Luticola species. Genus Planothidium Round & Bukht. (Round & Bukhtiyarova 1996), Published as part of Kopalov��, Kate��ina, Zidarova, Ralitsa & Vijver, Bart Van De, 2016, Four new monoraphid diatom species (Bacillariophyta, Achnanthaceae) from the Maritime Antarctic Region, pp. 1-19 in European Journal of Taxonomy 217 on pages 3-7, DOI: 10.5852/ejt.2016.217, http://zenodo.org/record/3840173, {"references":["Kopalova K., Kociolek J. P., Lowe R. L., Zidarova R. & Van de Vijver B. 2015. Five new species of the genus Humidophila (Bacillariophyta) from the Maritime Antarctic Region. Diatom Research 30: 117 - 131. http: // dx. doi. org / 10.1080 / 0269249 X. 2014.998714","Van de Vijver B., Beyens L. & Lange-Bertalot H. 2004. The genus Stauroneis in the Arctic and (sub-) Antarctic regions. Bibliotheca Diatomologica 51, J. Cramer, Berlin & Stuttgart.","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. http: // dx. doi. org / 10.1080 / 026924 9 X. 1996.9705361"]}

Four monoraphid taxa belonging to the genera Achnanthes, Psammothidium and Planothidium were found during the ongoing taxonomic revision of the freshwater and limno-terrestrial diatoms of the Maritime Antarctic region. The present paper describes these four taxa as new based on detailed light and scanning electron microscopy observations: Achnanthes kohleriana Kopalová, Zidarova & Van de Vijver sp. nov., Planothidium wetzelectorianum Kopalová, Zidarova & Van de Vijver sp. nov., Psammothidium confusoneglectum Kopalová, Zidarova & Van de Vijver sp. nov. and Psammothidium superpapilio Kopalová, Zidarova & Van de Vijver sp. nov. The morphology and ecology of all four taxa are discussed and the species are compared with morphologically similar taxa.

International audience; Several diatoms in the family Achnanthaceae (Bacillariophyta), mainly from marine environments, have species with strongly apiculate, lemon-shaped valves. Some of them originally described under the genus Cocconeis (i.e., C. trachyderma or C. citronella), while others were first described as Stauroneis species [i.e. Stauroneis apiculata or S.(?) obesa]. Afterwards, Cocconeis citronella has been recombined within Achnanthes by Hustedt. The type material of C. citronella from Albert Mann’s collection, housed in the Smithsonian Institution (US), has been examined with light microscope; some ambiguities are pointed out and new details added to the original description. The intricate history of the latter taxon is redrawn and comparison with allied taxa are tentatively addressed. Cocconeis trachyderma is lectotypified and recombined as Achnanthes trachyderma comb. nov. Stauroneis apiculata and S.(?) obesa are recombined as Achnanthes apiculata comb. nov. and Achnanthes obesa comb. nov. respectively. The examination with light and scanning electron microscope of several marine samples from the Society Archipelago details the unique morphology of Achnanthes trachyderma which, until recently, has been often misidentified as Achnanthes citronella due to certain similarities between both taxa.

Wetzel, Carlos E., Hoffmann, Bart Van De Vijver Lucien, Ector, Luc (2013): Planothidium incuriatum sp. nov. a widely distributed diatom species (Bacillariophyta) and type analysis of Planothidium biporomum. Phytotaxa 138 (1): 43-57, DOI: 10.11646/phytotaxa.138.1.6, URL: http://dx.doi.org/10.11646/phytotaxa.138.1.6

Two new taxa similar to Achnanthidium minutissimum were common and abundant in samples collected during a survey of benthic diatoms in watercourses from mainland Portugal. They are described here as A . duriense , sp . nov . and A . lusitanicum , sp . nov . In addition, the recently described A . pseudolineare is reported from various localities and its ecology is described in detail. The new Achnanthidium species from Portugal were compared with the type material of morphologically similar taxa: Achnanthidium microcephalum , Achnanthes minutissima , A . minutissima var. cryptocephala , A . minutissima var. inconspicua and A . nana . The analysis of this type material confirmed that Achnanthes minutissima var. cryptocephala should be regarded as a different taxon, and a new combination is proposed, Achnanthidium neocryptocephalum , stat., comb. et nom . nov . Achnanthes minutissima var. inconspicua is now considered as a younger synonym of Achnanthidium lineare . Based on a detailed morphological study of specimens of Achnanthes nana from Scotland, Nepal and Portugal, this taxon is transferred to the genus Achnanthidium as Achnanthidium nanum , comb . nov .

Planothidium incuriatum sp. nov. is described from freshwater samples from Europe and South America. It is compared with the type material of Achnanthes biporoma (= Planothidium biporomum ), described from North America, to differentiate it from the new, related species. Species morphologies were compared using both light and scanning electron microscopy. Planothidium incuriatum can be differentiated from P. biporomum by its valve outline with more rostrate apices (instead of capitate) and by the areolation pattern of the striae in the raphe less valve (higher number of rows of areolae per stria). Distribution of P. incuriatum was determined from worldwide iconographic literature records where it was often identified as Planothidium biporomum among other names here detailed. While P. incuriatum appears to be widely distributed around the world, P. biporomum is, according to verified records, far more restricted.

A review of previously published hypotheses concerning phylogenetic relationships in the diatom family Achnanthaceae is presented. Data for characters such as valve ultrastructure, chloroplast number and morphology, and sexual reproduction for the genera Cocconeis, Achnanthes and Rhoicosphenia and their purported biraphid allies Mastogloia and Gomphonema are reviewed. Phenetic and cladistic analyses of the character state data suggest that the major genera of the Achnanthaceae may form distinct evolutionary lineages, which have each converged on the reduced raphe condition. Cocconeis appears to be more similar to Mastogloia and to represent one lineage, while Rhoicosphenia appears to be most similar to Gomphonema and to represent a second lineage. The genus Achnanthes may represent at least one additional independent lineage. Taxonomic implications of the present report are discussed.

A new combination in the genus Planothidium is proposed. The identity of Planothidium lagerheimii (Cleve) C.E.Wetzel & Ector comb. nov. (≡ Cocconeis lagerheimii Cleve), from South America is revisited. This rather common species often reported from freshwater and aerial environments was identified in previous studies either as Cocconeis rudis Frenguelli or Planothidium salvadorianum (Hust.) Lange-Bert. (≡ Achnanthes salvadoriana Hust.). After having measured the original images and based on photographs gathered in the literature we concluded that the three species, Cocconeis lagerheimii, Cocconeis rudis and Achnanthes salvadoriana, belong to the same taxon. Following the International Code of Nomencalture the correct name is the earliest legitimate one with the same rank. Thus the species having priority should be Cleve’s species described in 1894. Freshwater material from southern Brazil was investigated to illustrate ultrastructural details.