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

Ji-Yan Long', John Patrick Kociolek’, David M. Williams’,
Bing Liu', Wen-Hui Mo', Jin-Hua Chen!

| College of Biology and Environmental Sciences, Jishou University, Jishou 416000, China 2 Museum of
Natural History and Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colo-
rado-80309, USA 3 Department of Life Sciences, the Natural History Museum, Cromwell Road, London SW7
SBD He

Corresponding author: Jin-Hua Chen (cjh_jsu@jsu.edu.cn)

Academic editor: Yasen Mutafchiev | Received 21 December 2021 | Accepted 2 June 2022 | Published 20 June 2022

Citation: Long J-Y, Kociolek JP, Williams DM, Liu B, Mo W-H, Chen J-H (2022) Two new freshwater species
of Surirella (Bacillariophyta) from the Wuling Mountains, China. PhytoKeys 201: 35-49. https://doi.org/10.3897/
phytokeys.201.79626

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.
wufluminensis sp. nov. and S. suiningensis sp. nov. Surirella wufluminensis 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. Surirella suiningensis has nat-
rowly 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.

Keywords

Helictoglossa-like process, new species, Surirella, twisted frustule, ultrastructure

Copyright Ji-Yan Long et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

36 Ji-Yan Long et al. / PhytoKeys 201: 35-49 (2022)

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 Kiitzing 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 Kiitzing. Koci-
olek 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; 1930, 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 spe-
cies 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 cli-
mate. 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

Two new freshwater species of Surirella 37

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 photo-
graphed using a Leica DM3000 light microscope (LM) at x1000 magnification (objec-
tive NA 1.25) and a Leica MC190 HD digital camera. The holotype slides are depos-
ited 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 sputter-
coated with platinum for 20 seconds (Cressington Sputter Coater 108auto, Ted Pella,
Inc.). Samples were examined and imaged using a field emission scanning electron mi-
croscopy Sigma HD (Carl Zeiss Microscopy) available at Huaihua University, China.

Terminology related to valve morphology follows Ruck and Kociolek (2004) and
Van de Vijver et al. (2013), and costa-stria bundle (CSB), over-fibula costa (OFC) fol-
lows Liu et al. (2019).

Results

Class Bacillariophyceae Haeckel
Order Surirellales D.G. Mann
Family Surirellaceae Kiitzing
Genus Surirella Turpin

Surirella wufluminensis Bing Liu & Kociolek, sp. nov.

Figs 1-4

Holotype. Holotype BM! 81892, specimen circled on slide, illustrated as Fig. 1D;
isotype JIU! G202106, specimen circled on slide, illustrated as Fig. 1C.

Type locality. Cutna. Hunan province: the course of Wu River, a sampling point
at Changpu Town, Suining County, 26°34.59'N, 110°09.19'E, 300 ma.s.L., collected
by Bing Liu, March 22, 2021.

Description. LM (Fig. 1). Valves twisted, sometimes exhibiting linear-lanceolate
valve outline (Fig. 1D), with twisted and deflected apices (Fig. 1[A—C). Valve dimen-
sions (n = 27): 198-295 um long, 41-50 um wide. Valve face smooth without undu-
lations, valve midline sigmoid, fibulae very distinctive, 20—28 in 100 um. The degree
of rotation (twist) of the valves differs (Fig. 1A—D) as each position relative to the
observer is different. Two type valves are observed under LM: one is counterclockwise
twisted (Fig. 1A, C), the other is clockwise twisted (Fig. 1B, D), also seen in SEM
(Fig. 2A—P).

38 Ji-Yan Long et al. / PhytoKeys 201: 35-49 (2022)

Twisted apex

=>
%

1 Noh

Figure |. Surirella wufluminensis sp. nov., LM, x400 A, C two counter-clockwise twisted valves B, D two
clockwise twisted valves. Micrograph of the holotype specimen (Fig. A) and isotype specimen (Fig. D).
Scale bar: 50 pm.

SEM (Figs 2-4). Valve twisted, but visual discrepancies of degree of twist differs (Fig.
2A-F). Valve surface flat without undulations but mound-like siliceous outgrowths scat-
tered throughout (Figs 2A—C, 3A—D). Very shallow depressions close to mantle, corre-
sponding to beneath, internal fibulae (Fig. 3B, four arrows). Raised longitudinal ridges
produced on both sides of raphe (Fig. 3C). Externally, costae (thickened silica ribs) ab-
sent, external openings of areolae slit-like on both valve face and mantle (Fig. 3D, E).

Two new freshwater species of Surirella 39

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Figure 2. Surirella wufluminensis sp. nov., SEM A=C three valves in external view; note the visual discrep-
ancies of twist degrees between them as the position of each valve relative to the observer differs D-F three

valves in internal view; note the visual discrepancies of twist degrees between them. Scale bars: 20 um.

External distal raphe endings interrupted at both apices (Figs 3E G, 4D). Internally, distal
raphe endings interrupted only at one apex where two helictoglossa-like processes present
(Fig. 4E), raphe continuous at another apex (Fig. 4F). Mantle with same striation patterns

40 Ji-Yan Long et al. / PhytoKeys 201: 35-49 (2022)

Figure 3. Surirella wufluminensis sp. nov., external view, SEM A a whole valve B middle detail from A

showing the shallow depressions on the valve face correspond to the ones on the mantle (four arrows)
C-E details from Fig. B showing mound-like siliceous outgrowths on the valve surface (C, D, three wavy
arrows respectively), raised longitudinal ridges on both sides of the raphe (C, two arrows), slit-like external
openings of areolae on both valve face and mantle (D-F) F, G apical details from A, note interrupted
terminal raphe endings (see also Fig. 4D). Scale bars: 20 um (A); 3 um (B, C, F, G); 1 um (D, E).

as valve face and its base margin thickened (Figs 3E, F, 4C). Fibulae distinctive, extending
from mantle, spanning 1/3 to 1/2 of half valve width except near two apices where fibulae
meeting at midline (Figs 2D—F, 4A). Uniseriate striae 40-42 in 10 um (measured in SEM
images, n = 3). Internal openings of areolae rounded, rimmed (Fig. 4C, F). One portula
and ca. 15—23 uniseriate striae located between two adjacent fibulae (Fig. 4A, B).

Two new freshwater species of Surirella 4]

Figure 4. Surirella wufluminensis sp. nov., internal view, SEM A-C middle details from Fig. 2D, note
that fibulae extend 1/3 to 1/2 of half valve width (A), one portula between two adjacent fibulae (A, B),
uniseriate striae, and valve mantle with thickened base margin (C, white arrow) D, E apical details from
Fig. 2D, note two interrupted external terminal raphe endings (D, two arrows), two helictoglossa-like

processes (E, two arrows) F another apex showing continuous raphe at one apex (white arrow). Scale bars:

5 um (A); 2 um (B-F).

Etymology. Named after Wu River, where the species was found.
Ecology and distribution. Epilithic in a mountain river with oligotrophic
waters. The following environmental parameters were measured in the field. Con-

ductivity was 99.7 + 0.3 pS-cm™, pH was 7.9 = 0.1 and water temperature was
3 2i se Oe2F oe,

42 Ji-Yan Long et al. / PhytoKeys 201: 35-49 (2022)

Surirella suiningensis Bing Liu & D.M. Williams, sp. nov.
Figs 5—8

Holotype. Holotype BM! 81893, specimen circled on slide, illustrated as Fig. 5B; iso-
type JIU! G202107, specimen circled on slide, illustrated as Fig. 5A.

Type locality. Catna. Hunan Province: the course of Wu River, a sampling point
at Changpu Town, Suining County, 26°34.59'N, 110°09.19'E, 300 ma.s.L., collected
by Bing Liu, March 22, 2021.

Description. LM (Fig. 5). Valves narrowly ovate, heteropolar, with downward de-
flecting head and foot poles; headpole broadly rounded whereas footpole narrowly
rounded. Valve dimensions (n = 8): 148-173 um long, 49-55 um wide (measured at

Figure 5. Surirella suiningensis, sp. nov., LM. x400 A=C three valves showing narrow-ovate outline,
slightly heteropolar valves, straight valve median line, downward deflecting head and foot poles, distinc-
tive fibulae spanning 70-80% of half valve width except near each apex where a few fibulae meet at me-

dian line. Micrograph of holotype (B). Scale bar: 50 um.

Two new freshwater species of Surirella 43

Crest Trough

BG

Trough

™
Figure 6. Surirella suiningensis sp. nov., external view, SEM A one valve showing distinct continuous
siliceous median ridge, downward deflecting head and foot poles (two arrows), and pattern of troughs
alternating with crests B detail from A showing two troughs and one crest C, D apical details from A,
note two curved terminal raphe fissures (C, two arrows) E marginal detail of A, note the recessed valve
mantle with thickened base margin, no fenestrae existed, troughs on valve surface corresponding to those
on the mantle (two black arrows). Scale bars: 20 um (A); 2 um (B-E).

widest valve part). Valve face appearing smooth, without undulations and costae. Valve
midline straight, downward deflecting at each apex. Fibulae very distinctive, parallel
in valve middle part, radiate at two apices, 20-24 in 100 um. Fibulae extending from
mantle towards midline, spanning 70-80% of half valve width except near each apex
where few fibulae nearly meet at median line.

44 Ji-Yan Long et al. / PhytoKeys 201: 35-49 (2022)

= Se ae

“a7
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Figure 7. Surirella suiningensis, sp. nov., internal view, SEM A one complete valve showing fibulae ex-
tending close to median line and fibulae corresponding to the sinking of mantle (i.e. troughs, six arrows)
B detail from A showing only one portula between two adjacent fibulae and uniseriate striae C sinking of
mantle against the fibulae (two arrows) D, E apical details from B, note the raphe continuous at one apex
(D, arrow) whereas interrupted at the other apex (E, one arrow pointing at a helictoglossa-like process).

Scale bars: 20 um (A); 2 um (B, C, E); 1 um (D).

SEM (Figs 6-8). Valve smooth, canal raphe system located around valve margins,
slightly raised (Fig. 6A). Median line slightly raised, continuous from pole to pole,
downward deflecting at both apices (Fig. GA). Valve surface producing shallow troughs
and higher crests (Fig. GA, B), and troughs alternating with crests from pole to pole.

Two new freshwater species of Surirella 45

Figure 8. A-D Surirella suiningensis, sp. nov., internal view, SEM A one valve with valvocopula, note the

open nature of valvocopula (arrow) B apical detail from A, note two helictoglossa-like processes (two ar-

rows) C detail of open part of valvocopula D detail of valvocopula showing the rimmed areolae internally.

Scale bars: 20 um (A); 4 um (B); 1 pm (C, D).

Costae (thickened siliceous ribs) absent (Fig. 6B). Each trough corresponding to its
internal fibula, each crest composed of c. 16-27 uniseriate striae (Fig. 6B, E). Distal
raphe endings curved (Fig. 6C, D). Mantle with same pattern as valve surface, troughs
align with each other (Fig. GE). Internally, fibulae evident, extending from mantle to
median line, spanning 70-80% of half valve width. Striae uniseriate, 47-51 in 10 um.
External openings of areolae slit-like (Fig. 6B) while internal openings rounded and
rimmed (Fig. 7D). One portula and 16—27 uniseriate striae located between two ad-
jacent fibulae (Fig. 7B). Sinking of mantle against fibulae present (Fig. 7C). Raphe
continuous at one apex whereas interrupted at other apex (Figs 7E, 8B). Valvocopula
open (Fig. 8A, C) and rimmed areolae internally (Fig. 8D).

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.

46 Ji-Yan Long et al. / PhytoKeys 201: 35-49 (2022)

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 rec-
ognized 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), sug-
gesting 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) Kitzing,
S. rumrichorum Metzeltin & Lange-Bertalot (1998, figs 216: 2, 3; 219: 7), and S. hin-
ziae Cvetkoska, Levkov & P.B. Hamilton (in Cvetkoska et al. 2015, p. 187, fig. 32).

Surirella aquastudia, S. xinjiangiana, and S. dongtingensis all have twisted frus-
tules 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 Kiitzing (Wang 2018) — this species is now recognized as a member
of Iconella, as I. spiralis (Kiitzing) Ruck & T. Nakov. The only taxa similar to S. wuf-
luminensis, 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 Met-
zeltin 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).

Surirella wufluminensis and S. suiningensis occur together in the same freshwater
habitat. Their associated species comprise Pinnularia hustedtii Meister (see Williams et
al. 2022, p. 294), Diatoma vulgaris Bory de Saint-Vincent (1824, p. 461), Zabularia

Two new freshwater species of Surirella 47

fonticola (Hustedt) C.E. Wetzel & D.M. Williams (in Vigneshwaran et al. 2020,
p. 179), and some species of Fragilaria Lyngbye (1819, p. 182), Gyrosigma Hassall
(1845, p. 435), Navicula Bory de Saint-Vincent (1822, p. 128), Niteschia Hassall
(1845, p. 435), Sellaphora Mereschkowsky (1902, p. 186) and among others.

Acknowledgements

This research was supported by the Natural Science Foundation of Hunan [grant num-
ber 2022JJ30473], the Scientific Research Project of the Department of Education of
Hunan Province [grant number 21C0364] and the Graduate Scientific Research In-
novation Project of Jishou University [grant number JGY202167].

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