<p><strong>Four new species of monoraphid diatoms from Western Sichuan Plateau in China</strong></p>

Abstract

Four new monoraphid diatoms are examined and described from a high-altitude (&gt; 3000 m) region of western Sichuan, China. The descriptions are based on light and scanning electron microscopy observations, and the four new species are compared with similar taxa worldwide. Achnanthidium sichuanense sp. nov. has small, lanceolate-elliptical valves, with rounded apices. The outline and central area differentiate it from similar species. The unique characteristic of Eucocconeis undulatum sp. nov. is that it has valves that are undulate along the apical axis, forming an “H”-shaped area on both raphe and rapheless valves as seen in light microscope (LM). Platessa mugecuonesis sp. nov. and Platessa lanceolata sp. nov. are two small species, whose valve outlines and central areas make them different from other species in this genus. A. sichuanense sp. nov. and E. undulatum sp. nov. are discussed in terms of the relationship between “Achnanthidium &amp; Psammothidium” and “Eucocconeis &amp; Psammothidium”, as these genera are difficult to separate according to morphological characteristics.

Two new freshwater species of the genus Achnanthidium (Bacillariophyta, Achnanthidiaceae) from Qingxi River, China

“…The two new species possess characters that support their assignment to the A. pyrenaicum complex, deflected external distal raphe fissures ( Yu et al 2018 , 2019 b, You et al 2019 , 2021 ).…”

“…The species of the A. pyrenaicum complex have external distal raphe ends that are deflected or hooked to one side of the valve and slit-like areolar openings. The members of the A. exiguum complex have external distal raphe ends curved in opposite directions (Compère and Van de Vijver 2011;Karthick et al 2017;Yu et al 2018Yu et al , 2019aMiao et al 2020;Tseplik et al 2021;You et al 2021). A. exiguum and its relatives have been segregated into a new genus of Gogorevia Kulikovskiy, Glushchenko, Maltsev and Kociolek (Kulikovskiy et al 2020).…”

“…The members of the A. exiguum complex have external distal raphe ends curved in opposite directions ( Compère and Van de Vijver 2011 ; Karthick et al 2017 ; Yu et al 2018 , 2019a ; Miao et al 2020 ; Tseplik et al 2021 ; You et al 2021 ). A.…”

“…Presently, the number of species in the genus Achnanthidium is greater than 200 ( Marquardt et al 2017 ; Kociolek et al 2018 ; You et al 2021 ). Before the year of 2000, 11 new Achnanthes species had been described from China ( Hustedt 1922 ; Jao 1964 ; Jao et al 1974 ; Qi and Xie 1984 ; Zhu and Chen 1994 , 1996 ; Liu et al 2021 ).…”

“…It is, therefore, necessary to collect samples from the type locality, and re-evaluate their taxonomic position ( Liu et al 2021 ). From 2001 to 2021, 14 new Achnanthidium species have been described from China ( Liu et al 2016 ; Yu et al 2018 , 2019a , b , You et al 2019 , 2021 ; Liu et al 2021 ). During an investigation of the freshwater diatoms from the Qingxi River, two unknown Achnanthidium species were discovered.…”