This study describes a new species of Pinnularia, P. hupingensis sp. nov., on the basis of light and scanning electron microscope images. Pinnularia hupingensis sp. nov. is characterised by its linear valve outline, extremely divergent striae, and very large hexagonal central area occupying ca. 1/5–1/8 of the valve length. The primary and secondary sides of the valve and the internal proximal raphe fissures are discussed. The new species is compared to similar taxa of the genus Pinnularia.

Central area, divergent striae, Huping Mountain, Pinnularia hupingensis, valvocopula

The genus Pinnularia belongs to the artificial category of symmetric biraphid diatoms and is characterised by two girdle-appressed plate-like chloroplasts and alveolate striae (Round et al. 1990). According to the AlgaeBase website, there are 880 accepted species names, 501 accepted varieties and 127 accepted formae in the genus at present (Guiry and Guiry 2023). Hence Pinnularia is a very large and speciose genus, with numerous species described from Chinese material (e.g. Zhang et al. (2016); Liu et al. (2018a); Kociolek et al. (2020); Deng et al. (2021); Zhang et al. (2022)). The monograph of Krammer (2000), which provides the descriptions and illustrations of many Pinnularia taxa, still represents a very valuable source for taxon identification, although numerous other references are also needed since many new Pinnularia species have been described in the 21^st century. The different valve sizes, outlines, the arrangements and densities of the striae on the valve surface, the central areas, the shapes of the distal raphe fissures and the ornaments on the valve surface are the most useful characters to discriminate the taxa within the genus Pinnularia (e.g. Round et al. (1990); Krammer (2000)).

There are two main types of stria arrangements on the valve surfaces of Pinnularia taxa. In one type, the striae are radiate throughout the valve surface, such as in P. hustedtii F. Meister (Williams et al. 2022). In the other type, the striae are divergent, i.e. the striae are radiate only in the middle of the valve and abruptly become strongly convergent near the two apices, such as in P. superdivergentissima H. Germain & Chaumont (Chaumont and Germain 1976; Krammer 2000). If the central area in Pinnularia exists, it can be hyaline without any ornaments, such as in P. lacustrigibba Poulíčková, D.G. Mann & Kollár (Poulíčková et al. 2018) and P. qinghainensis Bing Liu & S. Blanco (Deng et al. 2021) or it may have lunate markings on each side of the central nodule, such as in P. brandelii Cleve (Krammer 2000, plate 97, figs 7–12) and P. stomatophora (Grunow) Cleve (Krammer 2000, plate 100, figs 1–8). Interestingly, in the closely-related genus Caloneis, the lunate markings are more common. Both Caloneis lewissi Patrick and C. schumanniana (Grunow) Cleve have lunate markings on each side of the central nodule (Stancheva et al. 2009). Levkov and Williams (2014) observed Caloneis species from the ancient lakes Ohrid and Prespa and described eighty new species, all of them with lunate markings. The lunate markings seen under the light microscope (LM) are actually lunate depressions as seen under the scanning electron microscope (SEM) (e.g. Krammer (2000), plate 98, figs 7, 8; Stancheva et al. (2009), fig. 39). The stria arrangement and the central area are two important characteristics for the new species described in this paper as they can be used for its distinction from similar species.

Hunan Province is situated in south-central China. Most of the streams/rivers in Hunan are tributaries of four major river systems: the Xiang, Zi, Yuan and Li Rivers. These four major rivers run from the south of Hunan to its north and terminate in Dongting Lake, the largest lake in Hunan that discharges into the Yangtze River (the longest river in China). The diatom flora of Hunan remains underexplored, although Liu and his collaborators have described many new diatom species from Hunan in recent years (e.g. Liu et al. (2018b, 2019, 2020, 2021, 2022); Liu (2023)). This paper further contributes to the investigation of the diatom flora of Hunan by providing the description of a new Pinnularia species collected from a river from this Province.

The diatom samples of this study were collected from the Xie River which originates in the Huping Mountain National Nature Reserve (29°50'–30°09'N, 110°29'–110°59'E, 230 m a.s.l.). The Huping Mountain National Nature Reserve is located in the northwest of Shimen County, Hunan Province and is bordered by the Wufeng and Hefeng Counties of Hubei Province. This reserve was approved by the Chinese government as a national nature reserve in 1994 and it has a total area of 66.6 km² and a core area of about 54.5 km² (Tian et al. 2019). The Xie River is a headwater tributary of the Li River which is one of the four large rivers in Hunan Province. Epilithic diatom samples were collected on 14 March 2021. The method of collecting the diatom samples is the same as in Yuan et al. (2023) and consists of sampling numerous submerged stones showing yellow-brown surfaces that indicate the presence of diatoms. Each stone was placed on a plastic 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 Company).

The laboratory methods are also the same as in Liu (2023) and consist as follows: “The collected diatom samples to which 70% alcohol was not added were used to observe the living cells. A total of 100 μl diatom samples were transferred into a round chamber (diameter 14 mm, depth 0.35 mm) located in the middle of a custom-made slide by using a pipette, then examined using a Leica DM3000 light microscope (LM), equipped with a Leica MC190 HD digital camera. The collected diatom samples to which 70% alcohol was added were processed (cleaned) for microscopic examination with 10% hydrochloric acid (HCl) and 30% hydrogen peroxide (H₂O₂). Permanent slides were prepared using Naphrax mountant and examined using the same light microscope as above. 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 also examined using scanning electron microscopy (SEM). Several drops of the cleaned diatom material were air-dried on to glass coverslips. The coverslips were attached to aluminium stubs using double-sided conductive carbon strips and sputter-coated with platinum (Cressington Sputter Coater 108auto, Ted Pella, Inc.). Samples were examined and visualised using a field emission scanning electron microscopy (FESEM) Sigma HD (Carl Zeiss Microscopy) available at Huaihua University, China”. The terminology in the diatom descriptions and in the discussion mainly follows Round et al. (1990) and Krammer (2000).

Pinnularia hupingensis sp. nov. is characterised by its linear valve outline, extremely divergent striae and very large, hexagonal central area occupying ca. 1/5–1/8 of the valve length. The characteristics of P. hupingensis are summarised in Table 1 and compared to those of similar taxa. At first glance, P. hupingensis is similar to P. brandelii, but the latter has a lunate marking on either side of the central nodule (see Krammer (2000), plate 97, figs 7–12), whereas P. hupingensis does not have such lunate markings. Interestingly, Kezlya et al. (2022) noted that morphological distinctions, such as the surface markings in the central area, are consistent with phylogenic clades defined by genetic markers. The absence of such markings in P. hupingensis, therefore, excludes the possibility of this taxon being conspecific with P. brandelii. Pinnularia brebissonii and P. krammeri are more or less similar to P. hupingensis, but the former two species have different valve outlines and central areas (Table 1). The relative size of the central area, in particular, has been reported to be a stable character for the recognition of Pinnularia species for both small and large specimens (Kollár et al. 2019). Pinnularia hupingensis differs from P. superdivergentissima by having narrower valves (6.3–8.4 vs. 8–10 μm in width) and much higher stria density (13–14 vs. 7–9 in 10 μm). Furthermore, the ratio between the length of the central area and the length of the valve is ca. 1/5–1/8 in P. hupingensis, but ca. 1/9–1/10 in P. superdivergentissima.

The most similar taxon to P. hupingensis is Pinnularia sp1, a taxon morphologically similar to P. krammeri illustrated in Zidarova et al. (2012, p.17, figs 44–47). Pinnularia hupingensis and P. sp1 both have linear valve outline, weakly undulate valve margins, divergent striae, large central area and overlapping valve dimensions and stria density (Table 1). Zidarova et al. (2012) did not provide a formal description and stated: “A few valves could not be identified but have been given a provisional identification as Pinnularia sp1 until further observations can be made”. Morphologically, P. hupingensis is very similar to P. sp1 of Zidarova et al. (2012) and it cannot be excluded that the two taxa may be conspecific.

During the ontogeny of raphid diatoms, the valve side formed from the initial branching of the raphe sternum is termed the primary side and its opposite side is termed the secondary side. The secondary side may include the Voigt faults (or discontinuities) that mark the point of fusion of the sternum during the ontogeny of the valve (Round et al. 1990). Most authors do not discuss the primary and secondary sides of the Pinnularia taxa they described (e.g. Krammer (2000)) including in recent publications (Liu et al. 2018b; Moreno et al. 2020; Deng et al. 2021; Kulikovskiy et al. 2023), although there are a few exceptions, such as Zidarova et al. (2012) and Poulíčková et al. (2018). In P. hupingensis, the Voigt faults could be seen on the majority of the valves we observed.

In Pinnularia, the internal proximal raphe fissures can be continuous or interrupted by the central nodule. The internal proximal raphe fissures are continuous in Pinnularia lacustrigibba (Poulíčková et al. 2018) and P. hustedtii F. Meister (Williams et al. 2022), but interrupted by the central nodule in P. hupingensis as in P. qinghainensis. Clearly recognising these morphological structures is crucial to support the establishment of subgroups within the genus Pinnularia as discussed by Kulikovskiy et al. (2023).

The authors are grateful to Li Yuan, Ji-Yan Long and Wen-Hui Mo for assistance in the field and the laboratory.