A new species, Ulnaria shun-biseriata sp. nov., was found in the Shun River of Hunan Province, southern China, and its morphology was described based on light and scannning electron microscope obervations. Ulnaria shun-biseriata is characterized by its lanceolate valve outline, apiculate valve apices, slightly undulate valve margins, mostly biseriate striae, variable central area, and closed valvocopula. Many abnormal valves of U. shun-biseriata were observed in the samples investigated and the most frequent morphological abnormalities consisted of a lack of symmetry relative to the apical axis caused by a unilateral expansion in the middle part of the valve.

abnormal valve, biseriate striae, pre-normal valve, Ulnaria, valvocopula

Hunan Province, is situated in southern China to the south of both the Yangtze River and Dongting Lake. It has an area of 210,500 km², is a major rice-producing region with a population that exceeded 66.6 million inhabitants in 2020. Dongting Lake, the second largest freshwater lake in China, locates in the northeast of Hunan, and drains the entire river system of Hunan with only a few exceptions. There are four major rivers in Hunan, i.e. the Xiang, Zi, Yuna and Li Rivers, all of which flow into Dongting Lake.

In recent years, the diatom flora of Hunan has been explored by Dr. Liu from Jishou University and his collaborators and their research led to the descriptions of more than 30 species new-to-science (e.g. Liu 2023; Liu et al. 2016, 2017a, 2017b, 2017c, 2018a, 2018b, 2018c, 2019a, 2019b, 2019c, 2020a, 2021; Long et al. 2021, 2022a, 2022b; Yuan et al. 2023; Xu et al. 2024). However, the diatom flora of the Xiang River has been underexplored until now. The Xiang River is one of the principal tributaries of the Yangtze River, China’s largest river, and is the largest and longest river in Hunan with a total length of 800 km. The Xiang River’s source is located in the mountains in the northern part of the Zhuang Autonomous Region of Guangxi and flows northeast into Hunan Province. During its course into Dongting Lake, the Xiang River is joined by many tributaries (e.g. the Xiao, Lei and Lu Rivers) to form a very large drainage area. There are very limited reports on the diatom flora of the Xiang River (Long et al. 2022c; Liu 2023; Yuan et al. 2023; Zheng et al. 2023).

The diatom genus Ulnaria (Kützing) Compère is a freshwater genus which has been intensively studied by Liu (2023). In this monograph, Liu (2023) provided many insights on the life history and living cells, as well as morphological observations on the ultrastructure of the basal siliceous layer, with details on the central area, ocellulimbus, rimoportula, valvocopula, and configuration of girdle bands for 63 Ulnaria taxa. Moreover, 15 new Ulnaria species have been found and described from Hunan Province (Liu et al. 2017b, 2019a, 2019c, Liu 2023), one of which (U. pandurata-uniseriata) was found in the Shun River – a small tributary of the Xiang River. This paper further contributes to the investigation of the diatom flora of the Xiang River by providing the description of a new Ulnaria species, U. shun-biseriata sp. nov., collected from the Shun River, one of its tributary.

The diatom samples of this study were collected from the Shun River, a headwater tributary of the Xiang River, which runs through Lanshan County in the south of Hunan. Epilithic diatom samples were collected on October 5, 2021. The method of collecting the diatom samples is the same as in Liu (2023) and consists of sampling numerous submerged stones showing yellow-brown surfaces that indicate the presence of diatoms. Each stone was placed on a stainless-steel plate and its surface was brushed using a toothbrush, with the brushed-off diatom samples being washed onto the plate. The diatom samples were transferred into two 100 ml sampling bottles. One bottle was fixed with 70% ethanol and the other was left unfixed. At the time of sample collection, temperature, pH, and conductivity were measured in situ with a portable multimeter (HQ40D, Hach, Colorado, USA).

The laboratory methods are also the same as in Liu (2023). To alleviate any plagiarism concerns, we acknowledge the repetition herein: “The collected diatom samples to which was added 70% alcohol were processed (cleaned) for microscopic examination with 10% HCl and 30% H₂O₂. Permanent slides were prepared using Naphrax mountant and examined using a Leica DM3000 light microscope (LM). Slides are deposited in the Herbarium of Jishou University, Hunan, People’s Republic of China (JIU) (Herbarium acronym according the Index Herbarium http://sweetgum.nybg.org/science/ih/). Samples were also examined using scanning electron microscope (SEM). Several drops of the cleaned diatom material were air-dried onto glass coverslips. The coverslips were attached to aluminium stubs using double-sided conductive carbon strip and sputter-coated with platinum (Cressington Sputter Coater 108auto, Ted Pella, Inc.). Samples were examined and imaged using a field emission scanning electron microscope (Carl Zeiss Microscope, model Sigma HD) available at Huaihua University, China”.

The terminology in the diatom descriptions and in the discussion mainly follows Liu (2023). In particular, the term viminule refers to the interconnecting tiny ribs between the two adjacent virgae which define areolae in biseriate or multiseriate striae.

Ulnaria shun-biseriata sp. nov. can be distinguished from all other species of Ulnaria by a unique combination of characters that includes its lanceolate valve outline, slightly undulate valve margins, mostly biseriate striae, and variable central area.

The most similar species to U. shun-biseriata is U. oxybiseriata D.M. Williams & Bing Liu because both species have overlapping ranges of valve dimensions and are characterized by apiculate apices and biseriate striae (Table 1). Ulnaria shun-biseriata however, has generally wider valves and has undulate valve margins which distinguish it from U. oxybiseriata whose valve margins are straight (not undulate). Moreover, U. oxybiseriata has linear-lanceolate valves, an outline not observed for U. shun-biseriata.

Among the other Ulnaria species from China whose ranges in valve length partly overlap with those of U. shun-biseriata, we should mention U. gaowangjiensis Bing Liu & D.M. Williams, U. sangzhi-biseriata Bing Liu and U. chengduoensis Bing Liu. Besides having generally narrower valves these three species have different valve outlines and different central areas. In addition, U. chengduoensis has uniseriate striae (Table 1).

We also checked the “Synedra” that the Russian taxonomist Boris Skvortzov described from Chinese material collected in the first part of the 20^th century. The list compiled by Kociolek et al. (2020) and again reported in Liu (2023) indicates that Skvortzov described 11 new “Synedra” taxa in three different papers published in 1928, 1935 and 1938. From the hand-drawings available in these publications and from the valve dimensions reported by Skvortzov, only Synedra ulna var. tenuirostris (Skvortzov 1938) appears comparable to Ulnaria shun-biseriata. However, Synedra ulna var. tenuirostris possesses linear-lanceolate valves with abruptly rostrate apices and a rectangular central area. Therefore, this taxon looks much closer to Ulnaria gaowangjiensis and/or Ulnaria sangzhi-biseriata than to U. shun-biseriata.

Like Ulnaria shun-biseriata, U. acus (Kützing) Aboal, U. dongtingensis Bing Liu, U. menyuanensis Bing Liu and U. undulata (Rabenhorst) Williams, also bear lanceolate valve with undulate margins. However, U. shun-biseriata can be differentiated from the latter four species by having generally much shorter and wider valves and by the stria type: the former bears mostly biseriate striae whereas the latter four species possess uniseriate striae (Table 1).

Interestingly, the type population of U. shun-biseriata includes many abnormal valves (Fig. 1J–Q) with a proportion of 45% (91 specimens observed in total, including 50 normal and 41 abnormal). Falasco et al. (2009, 2021) reviewed diatom teratological forms and summarized the most frequent types of abnormality: 1) abnormal valve outline (lack of symmetry, bent, incised, swollen, or notched profile); 2) unusual raphe system (fragmented, displaced, and bifurcated); 3) abnormal striation pattern (irregular, altered, fragmented, and branched); and 4) unusual raphe channel system (distorted, curved, and occasionally doubled back). Most abnormal valves of U. shun-biseriata exhibit abnormal valve outlines lacking symmetry relative to the apical axis due to the valve middle part only expanding on one side (Fig. 1J–Q), and the other types of abnormality were rarely observed.

These abnormal valves should not be confused with pre-normal frustules/valves as defined in Liu and Williams (2020b) and Liu (2023). During the life history of the araphid genera Hannaea and Ulnaria, there is a pre-normal vegetative period which is “the time between immediately after the initial cell’s first division and the presence of the first new normal vegetative cells. The cell, frustule, and valve occurring during this period can be termed ‘pre-normal vegetative cell, frustule, and valve” (Liu and Williams 2020b). The pre-normal valves in Hannaea and Ulnaria are irregularly shaped but they are produced only in the early period of the life history so that the pre-normal valves are larger than the normal valves. In U. shun-biseriata small valves also present abnormalities (e.g. Fig. 1P, Q) and therefore cannot correspond to pre-normal valves.

These observations further illustrate the morphological plasticity reported for some araphid genera such as Hannaea (Liu and Williams 2020b), Ulnaria (Liu 2023) and Fragilaria. In the genus Fragilaria in particular, occurrence of morphologically abnormal populations has long been reported. For example, Feldt et al. (1973) illustrated a variant population from Lake Superior (USA/Canada) of what they then tentatively identified as Synedra radians (Kützing) Grunow. These valves were characterized by being longitudinally asymmetric with a strongly incised central portion. Recently, a similar population from Lake Superior was re-investigated by Alexson et al. (2022), who thought that these valves were teratological forms of (possibly) Fragilaria limnetica Alexson & Reavie. Similarly, Cunningham and Whitson (1978) described from a lake in Iowa (USA) apparently stable populations of abnormal valves of F. cyclopum (Brutschy) Lange-Bertalot as Synedra cyclopum var. incisa Cunningham while Hoff et al. (2011) illustrated “notched” and “non-notched” morphotypes of F. flexura, a species resembling F. cyclopum that they described from a mountain lake in Kamchatka (Russian Far East). Type populations of several needle-shaped Fragilaria species were also illustrated with numerous, apparently deformed, valves such as F. billingsii Wengrat, C.E. Wetzel & E. Morales (Wengrat et al. 2016), F. neotropica P.D. Almeida, E. Morales & C.E. Wetzel (Almeida et al. 2016), F. huebeneri A. Schwartz, K.J. Krahn & C.E. Wetzel (Krahn et al. 2021), F. campyla (Hilse) Van de Vijver, Kusher & D.M. Williams, F. pseudofamiliaris Van de Vijver, T.M. Schuster, Kusber & D.M. Williams and F. metcalfeana Van de Vijver, D.M. Williams, Kusber & T.M. Schuster (the latter three species being illustrated in Van de Vijver et al. 2022). Krammer & Lange-Bertalot (1991) also suspected that F. montana (Krasske) Lange-Bertalot was not an independent species but teratological valves of F. crotonensis Kitton. Besides these long, needle-shaped species, a few small-sized Fragilaria species were also diagnosed as including asymmetrical valves such as F. deformis (W. Sm.) Van de Vijver & Ector (Van de Vijver et al. 2020) and F. irregularis Chudaev, Jüttner & Van de Vijver (Chudaev et al. 2021).

In summary, natural diatom populations may include initial valves, pre-normal valves, normal valves, and abnormal valves, and this morphological variability should be considered when establishing a new species in araphid diatoms.

Patrick Rioual acknowledges his current support from the National Natural Science Foundation of China (grant number 41888101).