A new species, Ctenophora hunanensis Bing Liu & Rioual, sp. nov., found in the Yongle River, a tributary of the Xiang River (Hunan Province, southern China) is described on the basis of morphological observations made under light and scanning electron microscopes. Ctenophora hunanensis is distinguished from other Ctenophora taxa by a unique combination of characters that includes its lanceolate valve outline with rostrate apices, sternum gradually becoming wider from valve apices to center, and a greater valve width than the other members of the genus. Ctenophora hunanensis inhabits the epilithic community in the headwaters of a freshwater river. Many abnormal valves of C. hunanensis were observed in the samples investigated and the most frequent morphological abnormalities consisted in a lack of symmetry relative to the apical axis caused by a unilateral incising in the middle part of the valve.

abnormal valve, central area, cribrum, Ctenophora, sternum

In recent years, the diatom flora of Hunan Province in southern China (Fig. 1) has been investigated by Dr. Liu from Jishou University and his collaborators and their research led to the descriptions of several species new-to-science (e.g. Liu et al. 2016, 2017a, 2017b, 2017c, 2018a, 2018b, 2018c, 2019a, 2019b, 2019c, 2020a, 2021; Long et al. 2021, 2022a, 2022b; Liu 2023; Yuan et al. 2023; Xu et al. 2024). Many of these new species were described from Dongting Lake, the second largest freshwater lake in China that is located in the northeast of Hunan and drains the entire river system of this province with only a few exceptions. By contrast, the diatom flora of the rivers of Hunan, such as the Xiang River and its tributaries, has been relatively underexplored until now with only a few reports available in the literature (Long et al. 2022c, Liu 2023, Yuan et al. 2023, Zheng et al. 2023). Thus, as part of an ongoing investigation on the Xiang River and its tributaries, epilithon samples were collected from the Yongle River (Fig. 1) and analysed. The analyses revealed the presence of specimens of the genus Ctenophora (Grunow) D.M. Williams & Round that could not be attributed to any known species from this genus.

Figure 1. 

Map of Hunan Province, in southern China, showing the location of the Yongle River, a tributary of the Xiang River and Dongting Lake. The Yongle River is the type locality of Ctenophora hunanensis sp. nov.

Ctenophora was raised to the generic level by Williams and Round (1986). As a genus, it is distinguished from other related genera by the type of valve striation (with transverse, uniseriate striae composed of poroids closed externally by complex cribra), the cribrum structure and the robust plain area (i.e. without perforations in the silica wall) that usually occupies the entire width of the valve in its center. For more than three decades, Ctenophora was thought to be monotypic, only represented by the species Ctenophora pulchella (Kützing) Williams & Round, which was originally described from England but considered as cosmopolitan. This situation changed with the discovery of Ctenophora sinensis Bing Liu & D.M. Williams (Liu et al. 2020b) from samples collected in Lake Qinghai (China). More recently, Williams and Van de Vijver (2023a, 2023b) re-examined type material from European historical collections and proposed the re-combination and transfer to Ctenophora of three species. An exhaustive discussion on the structural characters useful for the description of Ctenophora taxa and on the phylogenetic relationships of this genus with other genera such as Tabularia (Kützing) Williams & Round and Catacombas Williams & Round was also proposed in Williams and Van de Vijver (2023b).

This paper describes Ctenophora hunanensis Bing Liu & Rioual sp. nov., as a new freshwater species from China and further contributes to the under-appreciated diversity of Ctenophora as well as to the investigation of the diatom flora of Hunan Province.

The diatom samples of this study were collected from the Yongle River which runs through Anren County in the south of Hunan Province (Fig. 1). The Yongle River is a headwater tributary of the Xiang River which is the largest river in Hunan. Benthic diatom samples were collected on March 24, 2024. 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. Immediately after being retrieved from the river bed, each stone was placed on a plastic plate and its surface was brushed using a toothbrush, with the brushed-off diatom samples being washed into 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, water temperature, pH, and conductivity were measured in situ with a portable multimeter (HQ40D, Hach Company, USA).

The laboratory methods are also the same as in Liu (2023) and consist as follows: “The collected diatom samples which were 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 acronyms follow Index Herbarium http://sweetgum.nybg.org/science/ih/). Samples were further examined using a field emission scanning electron microscope (SEM, Carl Zeiss Microscope, model Sigma HD) available at Huaihua University, China. For SEM analysis, several drops of the cleaned diatom material were air-dried onto glass coverslips. The coverslips were attached to aluminum stubs using double-sided conductive carbon strip and sputter-coated with platinum (Cressington Sputter Coater 108auto, Ted Pella, Inc.). The terminology used in the description and in the discussion mainly follows Round et al. (1990) and Liu et al. (2020b).

The general morphology of Ctenophora hunanensis Bing Liu & Rioual sp. nov. has all the characteristic features of the genus Ctenophora, including transverse, uniseriate striae composed of poroids closed externally by complex cribra, and the robust plain area that usually occupies the entire width of the valve in its center. Currently in AlgaeBase (Guiry in Guiry and Guiry 2023) for the genus Ctenophora there are five names taxonomically accepted as species. The five species include C. pulchella, the type species of the genus, C. saxonica (Kützing) D.M. Williams & Van de Vijver, C. sinensis B. Liu & D.M. Williams, C. subula (Sande Lacoste & Suringar) D.M. Williams & Van de Vijver, and C. vertebra (W. Gregory) D.M. Williams & Van de Vijver. Compared to the five other species of Ctenophora listed above, C. hunanensis is distinguished by its valve outline (lanceolate with rostrate apices), a sternum gradually becoming wider from the valve poles to center, and a much greater valve width than the other Ctenophora taxa (Table 1). Ctenophora sinensis and C. vertebra have very different valve outlines and can therefore be easily distinguished from C. hunanensis. Ctenophora hunanensis could be possibly confused with the smaller valves of C. pulchella and C. saxonica (and possibly C. subula?), but the difference in valve width remains (i.e., the ranges in width do not overlap). In addition, the central area of C. hunanensis is not buttressed while those of C. pulchella and C. subula are strongly buttressed.

Regarding the ultrastructure of the cribra, C. hunanensis possesses sieve-like closing plates composed of pegged struts that appear similar to those of C. sinensis, the other species described from China. In the three species described from Europe for which the structure has been illustrated, i.e. C. pulchella, C. saxonica, C. subula, the cribra appear as mesh-like structures. This difference in cribra structure may however be an artifact of the valve preservation, as the specimens illustrated by Williams and Van de Vijver (2023a, 2023b) were taken from historical collection at least 160 years old and may have been eroded to some extent.

Until recently as the genus was thought to be monotypic, Ctenophora was generally considered as an indicator of high electrolyte content in brackish waters, in marine coasts and river estuaries. When found in inland waters, Ctenophora was encountered in saline springs and salinized running waters. Its presence in freshwater was possible, but only in low numbers. It was also considered tolerant of polluted water, up to the α-mesosaprobic level (Lange-Bertalot et al. 2017). The discovery (or re-discovery) of other species of Ctenophora already suggested that populations of this genus could live in freshwater habitats. For example, Williams and Van de Vijver (2023a) reported that Ctenophora subula was found in the Netherlands as epiphytic on the filamentous green alga “Cladophora Sandii” (syn.: Cladophora rivularis (L.) Kuntze), which is a freshwater species. The salinity preference of Ctenophora vertebra is difficult to assess precisely although the type location “lacustrine sands, Glenshira, near Inveraray” in Scotland, also suggests it comes from a freshwater habitat (Williams and Van de Vijver, 2023b). The discovery of C. hunanensis in the headwaters of the Yongle River therefore confirms that the ecological niche of the genus Ctenophora is very broad, not just restricted to brackish conditions but also includes low-conductivity, unpolluted freshwater habitat.

Interestingly, the type population of C. hunanensis includes many abnormal valves (e.g., Figs 2C, E, H; 3B, C; 4A) which in the samples we investigated represented 23% of the specimens observed (7 abnormal valves out of 31). Most abnormal valves of C. hunanensis exhibit abnormal valve outlines lacking symmetry relative to the apical axis due to the incising of valve margin only occurring at one side (Figs 2C, E, H, 3B, C, 4A), which is a type of abnormality commonly reported in araphid diatoms (Falasco et al. 2009, 2021).

These observations further illustrate the morphological plasticity reported for some araphid genera such as Hannaea, Ulnaria and Fragilaria. For Ulnaria in particular, Zheng et al. (2024) also reported a large percentage of abnormal valves, with an asymmetrical incision on one margin of the valve, from the type population of Ulnaria shun-biseriata Bing Liu & Rioual, which was also described as a new species from a river in Hunan province. Further research is required to explain why diatom populations with large proportions of abnormal valves appear to be common in rivers of Hunan.

We are grateful to the anonymous referee and the editor for their helpful comments.