Abstract
Background and aims – Two unknown benthic diatom species belonging to the genus Achnanthidium Kütz., and found in French and Nepalese freshwater habitats, were investigated. Both species are here described as new and compared with the original material of Achnanthes minutissima var. macrocephala Hust. [≡ Achnanthidium macrocephalum (Hust.) Round & Bukht.] from Indonesia.
 Methods – The morphology of three small and capitate Achnanthidium species was investigated using light microscopy (LM) and scanning electron microscopy (SEM).
 Key results – Achnanthidium coxianum sp. nov. (from the Himalaya) belongs to the group of species with hooked terminal raphe endings, while Achnanthidium peetersianum sp. nov. (from France) has variable terminal raphe endings which are usually slightly bent. Achnanthidium macrocephalum is a much rarer species and illustrations concerning its identity in the literature do not conform to the type studied here.
 Conclusions – The three species are similar in LM but clearly distinct in SEM. The shape of the areolae and terminal raphe endings separate the species. Achnanthidium macrocephalum has often been misidentified in studies from many areas of the world. It is similar to the new species in valve outline and in its small dimensions.
Highlights
Achnanthidium Kütz. is one of the most abundant and common diatom genera in freshwater ecosystems worldwide (e.g. Lange-Bertalot & Krammer 1989, Krammer & Lange-Bertalot 1991, Ponader & Potapova 2007, Potapova & Hamilton 2007)
The group of species that are similar to Achnanthidium minutissimum (Kütz.) Czarn. is one of the most complex of non-marine diatoms whose taxonomy and nomenclatural history has been the subject of intensive debate and revisions over several decades (Lange-Bertalot & Ruppel 1980, Lange-Bertalot & Krammer 1989, Krammer & LangeBertalot 1991, 2004, Round & Bukhtiyarova 1996, Potapova & Hamilton 2007, Ector 2011, Hlúbiková et al 2011, Van de Vijver et al 2011a, 2018, Novais et al 2015, Marquardt et al 2017, Krahn et al 2018)
Based on the terminal raphe endings of the raphe valve, the genus has been historically sub-divided into two main groups of species: the complex around Achnanthidium minutissimum with straight terminal raphe fissures and a second one which comprises species around Achnanthidium pyrenaicum (Hust.) H.Kobayasi with clearly deflected or hooked distal terminal raphe fissures (Czarnecki 1994, Round & Bukhtiyarova 1996, Kobayasi 1997, Van de Vijver et al 2011a, 2011b, Yu et al 2018, 2019)
Summary
Achnanthidium Kütz. is one of the most abundant and common diatom genera in freshwater ecosystems worldwide (e.g. Lange-Bertalot & Krammer 1989, Krammer & Lange-Bertalot 1991, Ponader & Potapova 2007, Potapova & Hamilton 2007). Based on the terminal raphe endings of the raphe valve, the genus has been historically sub-divided into two main groups of species: the complex around Achnanthidium minutissimum with straight terminal raphe fissures and a second one which comprises species around Achnanthidium pyrenaicum (Hust.) H.Kobayasi with clearly deflected or hooked distal terminal raphe fissures (Czarnecki 1994, Round & Bukhtiyarova 1996, Kobayasi 1997, Van de Vijver et al 2011a, 2011b, Yu et al 2018, 2019). The Achnanthidium minutissimum species complex is characterized by a usually slender, linear to linear-lanceolate valve outline, and by relatively short cells with a very fine and dense striation pattern (Van de Vijver et al 2011a). On the valve mantle a single row of (slightly) elongated areolae is present (Round & Bukhtiyarova 1996)
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