﻿Two new freshwater species of Surirella (Bacillariophyta) from the Wuling Mountains, China

﻿Abstract Two sympatric Surirella species found at the same specific locality in the Wuling Mountains of China are documented with light and scanning electron microscope. Both species are new to science and named S.wufluminensissp. nov. and S.suiningensissp. nov.Surirellawufluminensis has large frustules that are either clockwise or counterclockwise twisted when viewed with the light microscope, and possesses distinctive fibulae, mound-like outgrowths on the valve surface throughout, raised longitudinal ridges on both sides of the raphe, and two helictoglossa-like processes at one apex internally. Surirellasuiningensis has narrowly ovate valve outline, distinctive fibulae, troughs alternating with crests from pole to pole, and two helictoglossa-like processes at one apex internally. These two species do not produce costae on the valve surface in contrast to many species in Surirella. This study provides a further two examples of the wide range of morphological diversity in the genus Surirella.


Introduction
The diatom genus Surirella Turpin (1828) includes numerous taxa commonly found in benthic habitats (Hustedt 1930;Krammer and Lange-Bertalot 1991; but see also Hustedt 1942 for a consideration of species from large lakes that may be planktonic). Formal morphological (Ruck and Kociolek 2004) and molecular (Ruck et al. 2016) analyses suggested that the former diagnosing features of Surirella and closely-related genera were not supported and that some species of Surirella, including its generitype S. striatula Turpin and the Pinnatae groups of Surirella, are more closely related to some species previously included in Campylodiscus C.G. Ehrenberg ex Kützing than they are to other species previously included in Surirella (the Fastuosoid group) (Ruck and Kociolek 2004;Ruck et al. 2016). Wang (2018) considered the surirelloid diatoms from inland habitats of China and recognized 47 different taxa (including only two new species) within Surirellaceae: nine taxa in Cymatopleura W. Smith, 29 taxa in Surirella, four taxa in Stenipterobia Brébisson ex Van Heurck, and five taxa in Campylodiscus Ehrenberg ex Kützing. Kociolek et al. (2020) listed 33 freshwater Surirella taxa described from China before 2000, including those described by Mereschkowsky (1906) and Skvortzov (1927;1929a, b, c;, 1938, 1976. Post-2000 until 2019 another 3 new taxa were described (Kulikovskiy et al. 2012;Liu et al. 2019). Among the above 36 taxa listed by Kociolek et al. (2020), only three species, Cymatopleura xinjiangiana Q-M You & J. P. Kociolek, C. aquastudia Q-M You & J. P. Kociolek, Surirella tientsinensis Skvortzov, were also mentioned in Wang (2018). And most recently Liu et al. (2021) described a new species from China and included the record of another species in the flora of the country.
There are a few taxa in Surirella sensu stricto possesseing 'twisted' frustules, such as S. aquastudia (Kociolek & Q. You) Kociolek, S. xinjiangiana (Q. You & Kociolek) Kociolek, and S. dongtingensis Bing Liu & Ector (You et al. 2017;Liu et al. 2021). They often appear to different visual discrepancies due to the degree of twist or their position relative to the observer. There are also a few taxa in Surirella sensu stricto without costa-stria bundles (sensu Liu et al. 2019), such as S. stalagma M.H. Hohn & Hellerman and S. atomus Hustedt (English 2011). In this study, we describe two new species belonging to Surirella sensu stricto characterized by twisted frustules and the valves lacking costae (thickened siliceous ribs).

Materials and methods
The study site is at the course of Wu River running through Suining County, located in the Wuling Mountains of China under a sub-tropical to warm temperate type climate. At the sampling site, epilithic algae were collected from numerous submerged stones showing yellow-brown surfaces indicating 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 into the plate. The diatom samples were transferred into a 100 mL sampling bottles and fixed with 70% ethanol. Two samples were collected from each site. Together with the sample collection, temperature, pH, and conductivity were measured in situ with a portable multimeter (HQ40D, HACH Company)-details are presented below in the 'Distribution and ecology' section of the species description.
Specimens for permanent slides were air-dried onto coverslips then mounted onto microscope slides using Naphrax. The slides were examined and specimens photographed using a Leica DM3000 light microscope (LM) at ×1000 magnification (objective NA 1.25) and a Leica MC190 HD digital camera. The holotype slides are deposited in the Natural History Museum, London, United Kingdom (BM) and isotypes slides are kept in the Herbarium of Jishou University, Hunan, People's Republic of China (JIU). For scanning electron microscopy (SEM) observation several drops of the selected cleaned diatom material were air-dried onto glass coverslips that were then attached to aluminium stubs using double-sided conductive carbon strips and sputtercoated with platinum for 20 seconds (Cressington Sputter Coater 108auto, Ted Pella, Inc.). Samples were examined and imaged using a field emission scanning electron microscopy Sigma HD (Carl Zeiss Microscopy) available at Huaihua University, China.
Etymology. Named after Suining County of Hunan Province, where the species was found.
Ecology and distribution. Epilithic in a mountain river with oligotrophic waters. Surirella suiningensis was found in the same sampling site with S. wufluminensis, for the environmental parameters, see above.

Discussion
The structure of the valves in the two new species argues for their placement in the genus Surirella, in the sense of Ruck et al. (2016). Both have direct communication between the raphe opening and the valve interior via simple portulae, a feature recognized by Ruck et al. (2016) as a synapomorphy for the genus. The structure of the raphe, which is discontinuous in both taxa, is akin to species in the Pinnatae group (= sensu stricto group) of Surirella (Ruck and Kociolek 2004;Ruck et al. 2016), suggesting they might be more closely related to species in that group than species in Cymatoleura W. Smith. The distinct presence of an internal helictoglossa-like process at the raphe terminations in the two species can be considered as a differentiating character from other "typical" Surirella species, which has, so far, been reported only in a few species such as Surirella robusta Ehrenberg, S. splendida (Ehrenberg) Kützing, S. rumrichorum Metzeltin & Lange-Bertalot (1998, figs 216: 2, 3; 219: 7), and S. hinziae Cvetkoska, Levkov & P.B. Hamilton (in Cvetkoska et al. 2015, p. 187, fig. 32).
Surirella aquastudia, S. xinjiangiana, and S. dongtingensis all have twisted frustules and all produce undulations on the valve surface from pole to pole. Previously, based on the presence of valve undulations, they would have been placed in the genus Cymatopleura. Surirella wufluminensis on the other hand, lacks undulations on its valve surface. Other similar species demonstrating valves with torsion include the former Surirella spiralis Kützing (Wang 2018) -this species is now recognized as a member of Iconella, as I. spiralis (Kützing) Ruck & T. Nakov. The only taxa similar to S. wufluminensis, in the sense of having twisted valves and lacking central valve undulations, are S. subcontorta Hustedt (in Schmidt 1942, plate 356, figs 1, 2), described from the African Rift Lake of Tanganyika, and S. uninodes Skvortzov (1937, p. 360) from Lake Baikal. Surirella subcontorta, as illustrated and described by (Hustedt 1942) is much wider at the 'headpole' than S. wufluminensis. Surirella uninodes is twisted near the middle of the valve, quite unlike S. wufluminensis (Hustedt 1942).
With regards to S. suiningensis, the ovate valve outline is reminiscent of Surirella davidsonii A.W.F. Schmidt, S. elegans Ehrenberg, S. slesvicensis Grunow (in Schmidt 1875, plate 21, fig. 19) as well as Iconella guatimalensis (Ehrenberg) Ruck & Nakov. Surirella suiningensis has narrow-ovate valve outline (i.e., its headpole is only slightly wider than its footpole) while S. davidsonii has an ovate valve outline (i.e. its headpole is wider than its footpole). SEM images of S. davidsonii have been published by Metzeltin and Lange-Bertalot (1998). Surirella elegans produces a lanceolate central region which S. suiningensis lacks. Surirella slesvicensis, described from a swamp in Europe, is similar to S. suiningensis in shape and overall proportions. De Toni (1892) suggested S. slesvicensis may be conspecific with S. elegans and S. subalpina Donkin (1869, p. 292) described from the U.K. There are no records of S. slesvicensis or S. subalpina being studied with electron microscopy (Gaul et al. 1993;Henderson and Reimer 2003).