ACTA BOT. CROAT. 82 (2), 2023 87 Acta Bot. Croat. 82 (2), 87–95, 2023 CODEN: ABCRA 25 DOI: 10.37427/botcro-2023-009 ISSN 0365-0588 eISSN 1847-8476 Placoneis modaomensis sp. nov. (Bacillariophyta; Cymbellaceae), a new species from Guangdong Province, China Yu-Jie Li1, Ji-Shu Guo1, Hong-Ping Ni1, Ying-Yan Huang2*, John Patrick Kociolek3, Yan-Ling Li1* 1 Yunnan University, School of Ecology and Environmental Science, Institute for Ecological Research and Pollution Control of Plateau Lakes, Kunming 650500, China 2 Ministry of Ecology and Environment, Administration for Pearl River Basin-South China Sea Ecology and Environ- ment, Center for Ecology and Environment Monitoring and Research, Guangzhou, 510611, China 3 University of Colorado, Museum of Natural History and Department of Ecology and Evolutionary Biology, Boulder, Colorado-80309, USA Abstract – One new species, Placoneis modaomensis, found in a freshwater environment from a tributary of the Pearl River, which lies in Modaomen Channel, Zhuhai County, Guangdong Province, China, is described based on light and scanning electron microscope observations. P. modaomensis sp. nov. has the morphological features that are typical for the genus, including external terminal raphe fissures curved to the opposite sides and areolae with internal volate occlusions. The new species is similar to P. amphibola (Cleve) E.J. Cox, P. amphiboliformis (Metzeltin, Lange-Bertalot & Soninkhishig) Vishnyakov, P. parvapolonica Lange-Bertalot & Wojtal, P. clementispronina Lange-Bertalot & Wojtal and P. nanoclementis Lange-Bertalot & Wojtal in the shape of the valves and in having coarse striae but it can be easily distinguished by the two main morphological characteristics: external central raphe endings bent in the opposite di- rections, and areolae covered by volate occlusions externally. The latter feature appears to be new for taxa assigned to the genus Placoneis. Data on the associated diatom flora and its ecology are also given. These findings increase our understanding about the morphology of Placoneis in general and the distribution of the genus in China. Keywords: diatoms, morphology, new species, Placoneis, taxonomy Introduction The genus Placoneis Mereschkowsky was erected by Mereschkowsky in 1903 for a group of species showing a single chloroplast with a central bridge and lateral lobes (Mereschkowsky 1903). With the shift to an emphasis on using frustular features to diagnose taxa (e.g., Hustedt 1930), species assigned to Placoneis were considered part of the large genus Navicula Bory (1822). Due to its structure of the chloroplast, Cox (1987) resurrected the genus Placoneis with P. gastrum (Ehrenberg) Mereschkowsky (Basionym: Pinnularia gastrum Ehrenberg) as the type species. Phylo- genetic analysis of Placoneis based on morphological and molecular data showed that the genus was part of the Cym- bellales, a group with valves that are asymmetrical to the apical and/or transapical axes, despite having symmetrical valves. Support for this phylogenetic placement includes the straight and expanded central raphe endings and more or less hooked distal ones, striae composed of rounded areolae which are internally closed by volae (tectulum), and a single chloroplast with a central bridge and lateral lobes extending under the valves (Cox 1987, 2003, 2004, Mann and Stickle 1995, Bruder and Medlin 2007). To date, more than 136 spe- cies are recognized to be part of this genus (Guiry and Guiry 2022). The genus Placoneis has a relatively wide distribution range, including Europe (Cox 1987, Bruder and Medlin 2007, Levkov and Williams 2011, Kulikovskiy et al. 2016, Vishnyakov 2020), Asia (Mayama and Kawashima 1998, Metzeltin et al. 2009, Kulikovskiy et al. 2012, Pomazkina et al. 2019, Kezlya et al. 2020), North America (Johansen et al. 2004, Kociolek and Thomas 2010, Kociolek et al. 2014), South America (Metzeltin and Lange-Bertalot 1998, Metzeltin and Lange-Bertalot 2007, Straube et al. 2013, Maidana et al. * Corresponding author e-mail: yanlingli@ynu.edu.cn, yingyan.Huang@zjnhjg.mee.gov.cn mailto:yanlingli@ynu.edu.cn mailto: yingyan.Huang@zjnhjg.mee.gov.cn LI Y.-J., GUO J.-S., NI H.-P., HUANG Y.-Y., KOCIOLEK J.P., LI Y.-L. 88 ACTA BOT. CROAT. 82 (2), 2023 2017), West Africa (Fofana et al. 2014) and Antarctica ( Zidarova et al. 2009). In China, research on Placoneis has focused on the discovery of newly recorded species for the country, such as P. prespanensis Levkov, Krstic & Nakov (Li et al. 2010), P. explanata (Hustedt) S. Mamaya (Liu et al. 2012), P. interglacialis (Hustedt) E.J. Cox (Liu et al. 2012), P. opportuna (Hustedt) Chudaev & Gololobova (Lin et al. 2018), P. anglophila var. signata (Hustedt) Lange-Bertalot (Liu et al. 2020). New Placoneis species have been proposed from China by Gong et al. (2013) but otherwise only few new species of this genus have been reported. In Guangdong Province the diversity of diatoms is relatively rich (Qiu et al. 2016, Lin et al. 2018), but previous studies have mainly focused on Navicula Bory, Nitzschia Hassall, Cymbella C. Agardh and Gomphonema Ehrenberg (Wang et al. 2018, 2021). Our understanding of Placoneis from Guangdong Province is very limited (Wang et al. 2021). The purposes of our study are: (i) to describe a Placoneis species new to science from the tributary of the Pearl River: Modaomen Channel, Zhuhai County, Guangdong Prov- ince, China based on detailed morphological observation using both light and scanning electron microscopy, (ii) to discuss its characteristics compared to related species, and (iii) to provide ecological information on this new species. Materials and methods Modaomen is located in Zhuhai County, Guangdong Province, China, one of the eight major gates at the mouth of the Pearl River. The length of the Modaomen Channel is about 45 km, and the water depth varies from 5 to 13 m. The Channel is relatively straight and about 2200 m wide (Chen et al. 2014, Tong et al. 2018). In 2021, samples containing Placoneis were collected from the Modaomen Channel (22°24'20''N, 113°36'25''E). Channel water pH and specific conductance were measured using a YSI 650 multi-parameter display system (650 MDS, YSI Incorporated 1700/1725 Brannum Lane, Yellow Springs, OH 45387 USA) with a 600XL probe. The diatom samples were further processed as described by Battarbee (1986). After several rinses in distilled water, the partially cleaned diatom material was air-dried onto cover slips and mount- ed onto slides using Naphrax. The sample and slides were deposited in the Herbarium of the Institute for Ecological Research and Pollution Control of Plateau Lakes, Yunnan University, Kunming, P.R. China (YUK). The isotype slides were stored in the Key Laboratory of Biodiversity of Aquat- ic Organisms, Harbin Normal University. Morphological observations of specimens were made under oil immersion at 1000× magnification with light mi- croscopy (LM) using an OLYMPUS BX51-DIC research mi- croscope and a C5060 Olympus digital camera. At least 500 valves were identified and counted in each surface sediment sample. Cleaned material for scanning electron microscope (SEM) analysis was air-dried onto cover glasses, mounted onto stubs, and coated with 20 nm of Au (EMSCOP SC 500 sputter coater). Resulting stubs were examined in the LEO 1530 scanning electron microscope (SEM). Description of the new species follows the terminology provided by Round et al. (1990), Cox (2003), Metzeltin et al. (2009) and Lange- Bertalot and Wojtal (2014). Results Taxonomy Division Bacillariophyta Haeckel 1878: 95 Class Bacillariophyceae Haeckel 1878 emend D.G. Mann in Round et al. 1990: 651 Subclass Bacillariophycidae D.G. Mann in Round et al. 1990: 125 Order Cymbellales D.G Mann in Round et al. 1990: 653 Family Cymbellaceae Greville 1833: 263, 409 Genus Placoneis Mereschkowsky 1903: 3 Placoneis modaomenensis Y.-L. Li sp. nov. Fig. 1A–H; Fig. 1C is the holotype LM (Fig. 1): Valves elliptical to broadly elliptical, nearly symmetrical about the apical axis, with rostrate or rostrate- rounded apices. Length 25.0–32.5 μm, width 14.0–16.5 μm, length/width ratio 1.62–2.16, median 1.89 (n = 30). Raphe filiform, almost straight with slightly expanded, but not clearly deflected to any side. External central raphe ends slightly straight, no significant expansion or bending. Ex- ternal terminal raphe fissures hooked to opposite sides. Ax- ial area narrow, linear. Central area transverse, irregular to bow-tie-shaped, rarely asymmetrical, occupying nearly 1/2 of the valve width. Isolated pore is absent from the central area. Striae radiate throughout, 12–14 in 10 μm. Areolae visible, 11–14 in 10 μm. Fig. 1. Placoneis modaomensis, Light Microscopy (LM), Dif- ferential Interference Contrast (DIC). A–H – valve views, show- ing size range and variability of the holotype population. Scale bar = 10 μm. NEW DIATOM SPECIES FROM CHINA ACTA BOT. CROAT. 82 (2), 2023 89 In SEM (Fig. 2 and Fig. 3): External raphe narrow, central raphe endings hooked opposite to each other (Fig. 2A, B, G, H, I). The terminal fissures curved, deflected in the opposite directions, extend onto the valve margin (Fig. 2C, D, E, F). Striae uniseriate, composed of round or elliptical areolae, extending to valve margin (Fig. 2A–F). Areolae covered by Fig. 2. Placoneis modaomensis, Scanning Electron Microscope (SEM), external views. A, B – external view of an entire valve. C, D, E, F – valve apices, striae with elliptic/rounded areolae and hook-shaped terminal raphe fissures bent onto the valve margin. G, H, I – external view of valve center, the central raphe endings hooked in the opposite directions from each other, note volate occlusions. Scale bar = 2 μm Fig. 3. Placoneis modaomensis, Scanning Electron Microscope (SEM), internal views. A, B, C – internal view of entire valve. D, E, F – internal view of valve apex with terminal raphe ends offset and bent slightly towards the margin. G, H – internal detail of central area showing raised central part and areolae covered by volate occlusions. Scale bar = 2 μm LI Y.-J., GUO J.-S., NI H.-P., HUANG Y.-Y., KOCIOLEK J.P., LI Y.-L. 90 ACTA BOT. CROAT. 82 (2), 2023 volate occlusions (Fig. 2G, H, I). Each areola has almost 1–4 siliceous protrusions internally, forming a variety of differ- ent shapes (Fig. 2A–I). Internal striae uniseriate, formed by elliptical areolae, separated by robust silica ribs (Fig. 3A–H). The raphe sternum clearly raised above the valve plane and expanded in the central area. Internal raphe straight, dis- continuous with intermissio, lying in a prominent and raised raphe sternum. Central raphe endings straight, not expanded. Terminal raphe ends terminate as helictoglossae, offset from the raphe branch, bent slightly towards the valve margin (Fig. 3D, E, F). Striae are uniseriate, distinctly radi- ate, formed by round to elliptical shaped areolae (11–14 in 10 µm) and covered by dentate occlusions (Fig. 3G, H). Type: – CHINA. Guangdong Province: Zhuhai County, Modaomen Channel, GD1, 22°24'20'' N, 113°36'25'' E, eleva- tion 0 m a.s.l., samples collected by Dr. Hong-Qu Tang, 26th July 2021. Holotype MDM202172601 in Coll. Yan-Ling Li, Yunnan University, Kunming, China. Fig. 1C is of the ho- lotype; Isotype YUNGL20220218, Harbin Normal Univer- sity, Harbin, China) Etymology: – modaomensis, referring to the type locality from which the new species was obtained. Associated diatom flora: Placoneis modaomensis is known from the Channel, situated at 0 m a.s.l. This species was as- sociated with Amphora linearis F. Meister (1935: 97), Seminavis strigosa (Hustedt) Danielidis & Economou-Amilli (2003: 30), Aulacoseira granulata (Ehrenberg) Simonsen (1979: 58), Gomphonema parvulum (Kützing) Kützing (1849: 65), Navicula schroeteri F. Meister (1932: 38), Navicula viridula var. rostellata (Kützing) Cleve (1895: 15), Nitzschia clausii Hantzsch (1860: 40) and Nitzschia frustulum (Kützing) Grunow (1880: 98). Ecology: The Modaomen Channel showed slightly alkaline conditions (pH 7.81), 29.8 °C of water temperature, 262 µS cm–1 of conductivity and 7.46 mg L-1 of dissolved oxygen (DO). Discussion Because Placoneis modaomenensis has symmetrical valve and radiate striae, it could easily be placed in one of four other genera: Paraplaconeis Kulikovskiy, Lange-Bertalot & Metzeltin (2012), Geissleria Lange-Bertalot & Metzeltin (1996), Rexlowea Kociolek & E.W.Thomas (2010) or Navicula Bory (1822). First, our new species resembles Paraplaconeis by symmetry of valve, but differs by the specific morphology of internal and external areolae patterns (Cox 1987, Cox 2003, Lange-Bertalot and Wojtal 2014). Second, P. modaomenensis is similar to the genus Geissleria on the basis of features ob- served with LM. These two genera are similar in terms of valve outline, but differ by the presence of the subpolar elon- gated areolae in the latter (Novais et al. 2013, Kulikovskiy et al. 2014). Third, P. modaomenensis is morphologically close to Rexlowea due to the valve outline, but these two genera are very distinct from one another by the arrange- ment, radiation and density of the striae (Kociolek and Thomas 2010). Fourth, P. modaomenensis can be confused with Navicula based on the valve symmetry. However, the former has exterior areolae covered by volate occlusions and internal areolae covered by dentate occlusions; while the lat- ter areolae are all individually covered on the inside by a weakly convex hymen (Lange-Bertalot 2001, Li and Qi 2018). The characters found in Placoneis modaomensis are compared with those of morphologically most similar genera in Tab. 1. While Placoneis seems easily placed as a member of Cymbellales, by virtue of its cytoplasmic features, its posi- tion in this lineage has been quite variable. Placoneis has been reported as outside the group of asymmetrical genera (Kociolek and Stoermer 1988, Nakov et al. 2014), deep with- in the cymbelloid lineage with some other genera naviculoid symmetry (Thomas et al. 2016, Kezlya et al. 2021) or in both positions depending upon the gene(s) used in the analysis (Bruder and Medlin 2007). Groupings within Placoneis are also enigmatic. Cox (1987) identified two groups within Placoneis, one with isolated pores, external terminal raphe fissures deflected in directions opposite one another and straight internal central raphe endings, while the other group has features of isolated pores absent, external terminal raphe fissures deflected in the same direction and recurved internal central raphe endings. Kociolek and Thomas (2010) noted some species from Colorado, USA, that did not conform to this organization of the genus. For example, P. fourtanierii Kociolek & Thomas (2010: 204) has external terminal raphe fissures that are deflected in opposite direc- tions, but lack isolated pores and deflected internal raphe ends. P. coloradoensis Kociolek & Thomas (2010: 205) has external terminal raphe fissures that are deflected towards the same side, internal central raphe endings that are straight, but this species has isolated pores. In the case of P. modaomenensis, it has external terminal raphe fissures that curve in opposite sides, but has internal central raphe endings that are straight and no isolated pores. In addition, P. modaomenensis has areolae with volate occlusions posi- tioned on the valve exterior which appears to be unique within Placoneis. Presence of central raphe endings turned opposite to one another in P. modaomenensis is shared with P. uruguayensis Metzeltin et al., a species without many features in common with other Placoneis species. The statement of Levkov et al. that “the full range of variation of morphology with the genus has been underestimated” (2007, p. 116) seems even more true now than when it was offered more than 15 years ago. In terms of general valve shape and striae, P. modaomenensis shows some resemblance to P. amphibola (Cleve) E.J. Cox, P. amphiboliformis (Metzeltin, Lange-Bertalot and Soninkh- ishig) Vishnyakov, P. parvapolonica Lange-Bertalot & Wojtal, P. clementispronina Lange-Bertalot & Wojtal and P.  nanoclementis Lange-Bertalot & Wojtal. However, P. modaomenensis differs strikingly from these species that are morphologically similar with respect to two features by the following: 1) external central raphe endings are bent in the NEW DIATOM SPECIES FROM CHINA ACTA BOT. CROAT. 82 (2), 2023 91 Ta b. 1 . M or ph ol og ic al co m pa ris on o f P la co ne is to th e m os t s im ila r g en er a. Pl ac on ei s m od ao m en sis sp . n ov . Pl ac on ei s Pa ra pl ac on ei s G ei ss le ri a Re xl ow ea N av ic ul a Re fe re nc e Th is st ud y C ox 1 98 7, C ox 2 00 3, La ng e- Be rt al ot a nd W oj ta l 20 14 Ku lik ov sk iy et a l. 20 12 , La ng e- Be rt al ot a nd W oj ta l 20 14 N ov ai s e t a l. 20 13 , Ku lik ov sk iy et a l. 20 14 Ko ci ol ek a nd T ho m as 2 01 0 La ng e- Be rt al ot 2 00 1, L i a nd Q i 2 01 8 A re ol ae Be in g co ve re d by v ol at e oc cl us io ns ex te rn al ly, b ei ng co ve re d by d en ta te oc cl us io ns in te rn al ly Be in g cl os ed in te rn al ly b y vo la e O pe n in te rn al ly a nd oc cl ud ed ex te rn al ly St ra ig ht sl it- lik e p or es op en in gs w ith ou t h ym en s, pr es en ce o f t he su bp ol ar el on ga te d ar eo la e La rg e a nd m or e c oa rs el y ar ra ng ed p un ct a on th e m ar gi na l v al ve Be in g al l i nd iv id ua lly co ve re d on th e i ns id e b y a w ea kl y co nv ex h ym en St ri ae U ni se ria te , r ad ia te th ro ug h- ou t, ex te nd in g to v al ve m ar gi n U su al ly u ni se ria te b ut so m et im es b ise ria te , r ad ia te ne ar th e c en tr e o f t he v al ve , m or e p ar al le l o r c on ve rg en t, at th e a pi ce s U ni se ria te o nl y on th e v al ve m an tle , c on sis te nt ly bi se ria te o n th e v al ve fa ce Ra di at e t hr ou gh ou t, th e pr ox im al th re e o ne s w id er sp ac ed th an th e o th er s Ra di at e, w ith th e 1 st a nd o r 2n d st ria e a t t he ap ic es pa ra lle l o r, ra re ly, co nv er - ge nt U ni se ria te , r ar el y bi se ria te , pa ra lle l, ra di at e o r c on ve r- ge nt C hl or op la st — Si ng le , a sy m m et ric al ch lo ro pl as t, ce nt ra l p or tio n m or e o r l es s a lo ng th e a pi ca l ax is of th e c el l, fr om w hi ch lo be s e xt en d un de r t he va lv es — Si ng le , l ar ge p la st id is di vi de d in to tw o pl at es ly in g on e a ga in st ea ch v al ve , co nn ec te d by a b ro ad co lu m n — Tw o pl as tid s, lo ca te d on ea ch si de o f t he ap ic al a xi s, an d ea ch p la st id co nt ai ns a n el on ga te d, sl en de r p ro te in nu cl eu s R ap he sy st em Th e e xt er na l c en tr al ra ph e en ds sl ig ht ly st ra ig ht , t he te rm in al fi ss ur es d efl ec te d in th e o pp os ite d ire ct io ns ; t he in te rn al ce nt ra l r ap he en di ng s s tr ai gh t, te rm in al ra ph e e nd s t er m in at e a s he lic to gl os sa e Th e e xt er na l c en tr al ra ph e en di ng s a re st ra ig ht a nd sli gh tly ex pa nd ed ; t he in te rn al en di ng s u su al ly de fle ct ed to w ar ds th e se co nd ar y sid e, oc ca sio na lly ho ok ed in o pp os ite di re ct io ns Th e e xt er na l r ap he sl it to ge th er w ith te rm in al fis su re s o fte n cu rv in g pr im ar ily to o pp os ite si de s bu t fi na lly to th e s am e s id e Th e e xt er na l r ap he is a lm os t st ra ig ht ; t he in te rn al ra ph e i s no t d efl ec te d be tw ee n th e ce nt ra l n od ul e a nd he lic to gl os sa , i nt er na l r ib ha s n ev er b ee n ob se rv ed Th e e xt er na l p ro xi m al en ds sw ol le n, ex te rn al d ist al en ds ho ok ed Th e e xt er na l t er m in al fis su re s a re d efl ec te d to th e se co nd ar y sid e; th e i nt er na l ce nt ra l r ap he en ds a re n ot de fle ct ed LI Y.-J., GUO J.-S., NI H.-P., HUANG Y.-Y., KOCIOLEK J.P., LI Y.-L. 92 ACTA BOT. CROAT. 82 (2), 2023 Ta b. 2 . M or ph ol og ic al co m pa ris on o f t he cu rr en tly d es cr ib ed sp ec ie s i n Pl ac on ei s. P. m od ao m en sis sp . n ov . P. p ar va po lo ni ca P. a m ph ib ol a P. a m ph ib ol ifo rm is P. cl em en tis pr on in a P. n an oc le m en tis Re fe re nc e Th is st ud y La ng e- Be rt al ot a nd W oj ta l 20 14 C ox 2 00 3 M et ze lti n et a l. 20 09 La ng e- Be rt al ot a nd W oj ta l 20 14 La ng e- Be rt al ot a nd W oj ta l 20 14 Va lv e s ha pe El lip tic al to b ro ad ly el lip tic al a nd th e v al ve m ar - gi n m or e c on ve x Br oa dl y el lip tic al Li ne ar -e lli pt ic to li ne ar - la nc eo la te El lip tic al El lip tic al -la nc eo la te El lip tic al -la nc eo la te to el lip tic al C en tr al a re a Bo w -ti e, tr an sv er se Ex te nd ed tr an sa pi ca lly Bo w -ti e, tr an sv er se N ot g iv en Va ria bl e i n siz e Ex te nd ed tr an sa pi ca lly , va ria bl e i n sh ap e Is ol at ed p or es A bs en t Tw o N ot g iv en N ot g iv en Tw o Tw o Va lv e l en gt h (μ m ) 25 .0 –3 2. 5 16 –2 0 37 –7 5 34 –6 3 20 –4 0 14 –2 3 Va lv e w id th (μ m ) 14 .0 –1 6. 5 8. 0– 9. 5 22 –2 7 18 –2 4 10 –1 4 8. 0– 8. 6 N um be r o f st ri ae in 1 0 μm 12 –1 4 14 –1 5 7– 8 7– 8 11 –1 4 12 –1 3 N um be r o f a re ol ae in 10 μ m 11 –1 4 40 –4 4 N ot g iv en 10 –1 2 38 –4 1 40 –4 4 Ex te rn al te rm in al ra ph e fi ss ur es U nd ul at e, de fle ct ed in th e op po sit e d ire ct io ns , e xt en d to v al ve m ar gi n Tu rn in g fir st to o pp os ite sid es , fi na lly tu rn in g to th e sa m e s id e a s a t t he o pp os ite po le D efl ec te d to w ar ds o ne si de N ot g iv en W ea kl y sin uo us su ba pi ca lly an d de fle ct ed to o pp os ite sid es , a t o ne o f t he p ol es cu rv in g ba ck w ea kl y to th e sa m e s id e Sh or tly cu rv ed to ap pa r- en tly o pp os ite si de s Ex te rn al ce nt ra l ra ph e e nd -in gs H oo ke d in th e o pp os ite di re ct io ns fr om ea ch o th er D ist in ct ly ex pa nd ed ce nt ra l po re s, no t c le ar ly d efl ec te d to a ny si de Ex pa nd ed N ot g iv en Ex pa nd ed b ut h ar dl y de fle ct ed ce nt ra l p or es D ist in ct ly ex pa nd ed ce nt ra l po re s, no t c le ar ly d efl ec te d to a ny si de A re ol ae U ni se ria te , e xt er io r a re ol ae co ve re d by v ol at e o cc lu - sio ns a nd fo rm ed a v ar ie ty of d iff er en t s ha pe s, in te rn al ar eo la e a re m os tly ro un d to el lip tic al co ve re d by de nt at e o cc lu sio ns U ni se ria te , c irc ul ar to sli gh tly el on ga te ap ic al ly N ot g iv en Re gu la rly el on ga te d tr an sa pi ca lly Sm al l, ci rc ul ar to w ea kl y el on ga te d ap ic al ly Sl ig ht ly el on ga te d ap ic al ly NEW DIATOM SPECIES FROM CHINA ACTA BOT. CROAT. 82 (2), 2023 93 opposite directions, and 2) both internal and external are- olae are covered by volate occlusions. The characters found in P. modaomenensis are compared with other closely-relat- ed species of the genus in Tab. 2. Based on valve outline, the new species is similar to P.  amphibola, P. amphiboliformis, P. parvapolonica, P. clementis pronina and P. nanoclementis, and P. modaomenensis sp. nov. most resembles P. parvapolonica based on its more convex valve margin and the structure of raphe and areola. However, they can be easily separated from one another. For example, in terms of the central raphe endings, the central raphe endings of the new species hooked in the opposite di- rections from each other while they do not clearly deflect to any side in P. parvapolonica. Comparing the shape of the areolae, the exterior areolae come in a variety of shapes and internal areolae are mostly round to elliptical in the new species, but P. parvapolonica differs in having areolae whose exterior and internal openings are nearly circular to slight- ly elongated in shape. Taking into consideration the occlu- sion of the areolae, the exterior and internal areolae are cov- ered by volate and dentate occlusions respectively in the P. modaomenensis sp. nov., while there is an absence of oc- clusions in exterior and internal areolae in P. parvapolonica. Besides, the size range of P. modaomensis is larger than P. parvapolonica (16–20 × 8.0–9.5 μm). Finally, the stria and areola density are much lower compared to P. parvapolonica (stria: 14–15 μm, areola: 40–44 in 10 μm). In summary, these differences are sufficient to justify the description of P. modaomensis as an independent species. Placoneis species are prevalent in freshwater bodies, in- cluding alkaline waters, mesotrophic and oligotrophic con- ditions (Cox 1987, Fujita and Ohtsuka 2005, Bruder and Medlin 2007, Kezlya et al. 2021). Of the five species similar to our new species, P. amphibola has a nordic-alpine distri- bution in Europe, and also occurs in freshwater fossil de- posits (Cox 2003). Except for P. amphibola, the relevant in- formation of nutrition, pH, and conductivity are not recorded, but other species have certain similarities or dif- ferences with our species. In terms of pH, P. parvapolonica, P. clementispronina and P. nanoclementis were found in al- kaline waters, but P. amphiboliformis was known from acid- ic rivers (Metzeltin et al. 2009, Lange-Bertalot and Wojtal 2014). From the nutritional level, P. clementispronina and P. nanoclementis were distributed in eutrophic waters, and P. parvapolonica was found in mesotrophic waters (Metzeltin et al. 2009, Lange-Bertalot and Wojtal 2014). As for the con- ductivity, P. parvapolonica and P. nanoclementis are found predominantly in waters with moderate to high conductiv- ity, whereas P. amphiboliformis was discovered rivers with low conductivity (Metzeltin et al. 2009, Lange-Bertalot and Wojtal 2014). Here, the new species of Placoneis was col- lected from alkaline waters with moderately high conduc- tivity, which is the most similar to P. parvapolonica and P. nanoclementis. In short, the discovery of this new species promotes the understanding of morphological features and ecological distribution about the genus Placoneis, and contributes to our understanding of diatom diversity, especially in Guang- dong Province. Acknowledgments This work was supported by the project of National Sci- ences and Foundation of China (Grant No. 42172206) and Yunnan Fundamental Research Project Q6 (202301AS070056). We thank Ms. Yulan Luo for her help with scanning electron microscopy observations. References Battarbee, R. W., 1986: Diatom analysis. In: Berglund, B. E. (ed.), Handbook of holocene palaeoecology & palaeohydrology, 527–570. John Wiley and Sons Ltd., Chichester, UK. Bory de Saint-Vincent, J. B .G. M., 1822: Bacillariées. Diction- naire Classique d’Histoire naturelle 2, 127–129. Bruder, K., Medlin, L. K., 2007: Molecular assessment of phylo- genetic relationships in selected species/genera in the navic- uloid diatoms (Bacillariophyta). I. The genus Placoneis. Nova Hedwigia 85(3–4), 331–352. https://doi.org/10.1127/0029– 5035/2007/0085–0331 Chen, W. H., Zou H. Z., Dong Y. J., 2014: Hydrodynamic of salt- water intrusion in the Modaomen waterway. Advances in Water Science 25(5), 713–723. [In Chinese with English ab- stract] https://doi.org/10.14042/j.cnki.32.1309.2014.05.003 Cox, E. J., 1987: Placoneis Mereschkowsky: the re-evaluation of a diatom genus originally characterized by its chloroplast type. Diatom Research 2(2), 145–157. https://doi.org/10.108 0/0269249X.1987.9704994 Cox, E. J., 2003: Placoneis Mereschkowsky (Bacillariophyta) re- visited: resolution of several typification and nomenclatural problems, including the generitype. Botanical Journal of the L i n nea n Soc ie t y 141(1), 53 – 83. ht t ps : //doi . org/10.1046/j.1095-8339.2003.00115.x Cox, E. J., 2004: Pore occlusions in raphid diatoms - a reassess- ment of their structure and terminology, with particular ref- erence to members of the Cymbellales. Diatom: the Japanese journal of diatomology, 20, 33–46. https://doi.org/10.11464/ diatom1985.20.0_33 Fofana, C. A. K., Sow, E. H., Taylor, J., Ector, L., Van de Vijver, B., 2014: Placoneis cocquytiae a new raphid diatom (Bacil- lariophyceae) from the Senegal River (Senegal, West Africa). Phytotaxa 161(2), 139–147. https://doi.org/10.11646/phyto- taxa.161.2.5 Fujita, Y., Ohtsuka, T., 2005: Diatoms from paddy fields in north- ern Laos. Diatom 21, 71–89. https://doi.org/10.11464/dia- tom1985.21.0_71 Gong, Z. J., Li, Y. L., Metzeltin, D., Lange-Bertalot, H., 2013: New species of Cymbella and Placoneis (Bacillariophyta) from late Pleistocene fossil, China. Phytotaxa 150(1), 29–40. https:// doi.org/10.11646/phytotaxa.150.1.2 Guiry, M. D., Guiry, G. M., 2022: AlgaeBase. World-wide elec- tronic publication, National University of Ireland, Galway. Retrieved on May 5, 2022 from https://www.algaebase.org. Hantzsch, C. A., 1860: Neue Bacillarien: Nitzschia vivax var. elongata, Cymatopleura nobilis. Hedwigia 2, 1–40. Hustedt, F., 1930: Bacillariophyta (Diatomeae). In: Pascher, A., (ed.), Die Süsswasser-Flora Mitteleuropas, 10 (2 Auflage). Gustav Fischer, Jena. Johansen J. R., Lowe, R., Gómez, S. R., Kociolek, J. P., Makosky, S. A., 2004: New algal species records for the Great Smoky Mountains National Park, U.S.A., with an annotated check- list of all reported algal species for the park. Algological Studies 111(1): 17–44. https://doi.org/10.1127/1864– 1318/2004/0111–0017 Kezlya, E., Glushchenko, A., Maltsev, Y., Gusev, S., Kuznetsov, A., Kociolek, J. P., Kulikovskiy, M., 2020: Placoneis cattiensis LI Y.-J., GUO J.-S., NI H.-P., HUANG Y.-Y., KOCIOLEK J.P., LI Y.-L. 94 ACTA BOT. CROAT. 82 (2), 2023 sp. nov. — a new, diatom (Bacillariophyceae: Cymbellales) soil species from Cát Tiên National Park (Vietnam). Phyto- taxa 460(4), 237–248. https://doi.org/10.11646/phyto- taxa.460.4.1 Kezlya, E. M., Glushchenko, A. M., Maltsev, Y. I., Gusev, E. S., Genkal, S., Kociolek, J. P., Kulikovskiy, M. S., 2021: Three new species of Placoneis Mereschkowsky (Bacillariophyceae: Cymbellales) with comments on cryptic diversity in the P. elginensis-Group. Water 13(22): 3276. https://doi.org/10.3390/ w13223276 Kociolek, J. P., Stoermer, E. F., 1988: A preliminary investigation of the phylogenetic relationships of the freshwater, apical pore field-bearing cymbelloid and gomphonemoid diatoms (Bacillariophyceae). Journal of Phycology 24(3), 377–385. https://doi.org/10.1111/j.1529-8817.1988.tb04480.x Kociolek, J. P., Thomas, E. W., 2010: Taxonomy and ultrastruc- ture of five naviculoid diatoms (class Bacillariophyceae) from the Rocky Mountains of Colorado (USA), with the descrip- tion of a new genus and four new species. Nova Hedwigia 90(1–2), 195–214. ht tps://doi .org/10.1127/0029– 5035/2010/0090–0195 Kociolek, J. P., Laslandes, B., Bennett, D., Thomas, E., Brady, M., Graeff, C., 2014: Diatoms of the United States, 1. Taxonomy, ultrastructure and descriptions of new species and other rarely reported taxa from lake sediments in the western U.S.A. Cambridge University Press, Cambridge. Kulikovskiy, M. S., Lange-Bertalot, H., Metzeltin, D., Witkowski, A., 2012: Lake Baikal: Hotspot of endemic diatoms I. Icono- graphia Diatomologica 23, 1–607. Kulikovskiy, M., Gusev, E., Andreeva, S., Annenkova, N., 2014: Phylogenetic position of the diatom genus Geissleria Lange- Bertalot & Metzeltin and description of two new species from Siberian mountain lakes. Phytotaxa 177(5), 249–260. https://doi.org/10.11646/phytotaxa.177.5.1 Kulikovskiy, M., Glushchenko, A., Genkal, S. I., Kuznetsova, I., 2016: Identification book of diatoms from Russia. Filigran, Yaroslavl [in Russian]. Kützing, F. T., 1849: Species algarum. Friedrich Arnold Brock- haus, Leipzig. Lange-Bertalot, H., 2001: Navicula sensu stricto, 10 genera sep- arated from Navicula sensu lato, Frustulia. Diatoms of Eu- rope 2, 1–526. Lange-Bertalot, H., Wojtal, A. Z., 2014: Diversity in species com- plexes of Placoneis clementis (Grunow) Cox and Paraplaconeis placentula (Ehrenberg) Kulikovskiy, Lange-Bertalot & Metzeltin. Beihefte zur Nova Hedwigia 143, 403–420. https:// doi.org/10.1127/1438–9134/2014/021 Levkov, Z., Williams, D. M., 2011: Fifteen new diatom (Bacil- lariophyta) species from Lake Ohrid, Macedonia. Phytotaxa 30, 1–41. https://doi.org/10.11646/phytotaxa.30.1.1 Li, J. Y., Qi, Y. Z., 2018: Bacillariophyta, Naviaulaceae (III). In: Flora Algarum Sinicarum Aquae Dulcis. Science Press, To- mus, XXIII. [In Chinese with English abstract] Li, Y. L., Gong, Z. J., Wang, C.C., Shen, J., 2010: New species and new records of diatoms from Lake Fuxian, China. Journal of Systematics and Evolution 48(1), 65–72. https://doi. org/10.1111/j.1759–6831.2009.00059.x Lin, X. R., Rioual, P., Bai, Z. J., Peng, W., Sun, M. J., Huang, X. Z., 2018: The recent diatom flora in Lake Kanas, Xinjiang: unusual species and new records in China. Acta Hydrobio- logica Sinica 42(3), 641–654. [In Chinese with English ab- stract] https://doi.org/10.7541/2018.080 Liu, Y., Fan, Y. W., Wang, Q. X., 2012: Newly recorded species in Cymbellaceae and Gomphonemataceae from Great Xing’an Mountains, China. Acta Hydrobiologica Sinica 36, 496–508. [In Chinese with English abstract] Liu, Z. X., Liu, B., Quan, S. J., Long, J. Y., Mo, W. H., 2020: Two biraphid diatom species reported from China for the first time-Prestauroneis tumida and Placoneis anglophila var. signata. Acta Botanica Boreali-Occidentalia Sinica 40, 1784– 1791. [In Chinese with English abstract] Maidana, N. I., Aponte, G. A., Fey, M., Schäbitz, F., Morales, E. A., 2017: Cyclostepanos salsae and Placoneis patagonica, two new diatoms (Bacillariophyta) from Laguna Cháltel in southern Patagonia, Argentina. Nova Hedwigia Beiheft 146, 89–102. https://doi.org/10.1127/1438–9134/2017/089 Mann, D. G., Stickle, A. J., 1995: Sexual reproduction and sys- tematics of Placoneis (Bacillariophyta). Phycologia 34(1), 74– 86. https://doi.org/10.2216/i0031-8884-34-1-74.1 Mayama, S., Kawashima, A., 1998: New combinations for some taxa of Navicula and Stauroneis, and an avowed substitute for a taxon of Eunotia. Diatom 14, 69–71. https://doi. org/10.11464/diatom1985.14.0_69 Meister, F., 1932: Kieselalgen aus Asien. Gebrüder Borntraeger Verlag, Berlin. Meister, F., 1935: Seltene und neue Kieselalgen I. Bericht der Sch- weizerischen Botanischen Gesellschaft 44, 88–108. Mereschkowsky, C., 1903: Über Placoneis, ein neues Diatomeen- Genus. Beihefte zum Botanischen Centralblatt 15, 1–30. Metzeltin, D., Lange-Bertalot, H., 1998: Tropical diatoms of South America I: About 700 predominantly rarely known or new taxa representative of the neotropical flora. Iconograph- ia Diatomologica 5, 3–695. Metzeltin, D., Lange-Bertalot, H., 2007: Tropical Diatoms of South America II. Special remarks on biogeography disjunc- tion. Iconographia Diatomologica 18, 1–877. Metzeltin D., Lange-Bertalot, H., Soninkhishig, N., 2009: Dia- toms in Mongolia. Iconographia Diatomologica 20, 3–686. Nakov, T., Ruck, E. C., Galachyants, Y., Spaulding, S. A., Theriot, E. C., 2014: Molecular phylogeny of the Cymbellales ( Bacillariophyceae, Heterokontophyta) with a comparison of models for accommodating rate variation across sites. Phycologia 53(4), 359–373. https://doi.org/10.2216/14-002.1 Novais, M. H., Wetzel, C. E., Van de Vijver, B., Morais, M. M., Hoffmann, L., Ector, L., 2013: New species and combinations in the genus Geissleria (Bacillariophyceae). Cryptogamie Al- gologie 34(2), 117–148. https://doi.org/10.7872/crya.v34. iss2.2013.117 Pomazkina, G. V., Rodionova, E. V., Sherbakova, T. A., 2019: Validation of 123 names of new diatom taxa from Lake Bai- kal. Limnology and Freshwater Biology 1: 181–198. https:// doi.org/10.31951/2658–3518–2019–A–1–181 Qiu, L. C., Wei, G. F., Li, X. J., Shi, L. S., Lin, S. Z., Han, B. P., 2016: Species diversity and temporal-spatial distribution of benthic diatoms in Jianjiang River, Guangdong Province. Journal of Tropical and Subtropical Botany 24: 197–207. [In Chinese with English abstract] https://doi.org/10.11926/j. issn.1005-3395.2016.02.011 Round, F. E., Crawford, R. M., Mann, D. G., 1990: The diatoms. biology and morphology of the genera. Cambridge Univer- sity Press, Cambridge. Simonsen, R., 1979: The diatom system: ideas on phylogeny. Bacillaria 2, 9–71. Straube, A., Tremarin, P. I., de Castro-Pires, E. C., Marquardt, G. C., Veiga Ludwig, T. A., 2013: Morphology, ultrastructure and distribution of Placoneis itamoemae sp. nov. (Cymbel- NEW DIATOM SPECIES FROM CHINA ACTA BOT. CROAT. 82 (2), 2023 95 laceae) from Brazil. Phytotaxa 76(3): 55–62. https://doi. org/10.11646/phytotaxa.76.3.13 Thomas, E. W., Stepanek, J. G., Kociolek, J. P., 2016: Historical and current perspectives on the systematics of the ‘enigmatic’diatom genus Rhoicosphenia (Bacillariophyta), with single and multi-molecular marker and morphological analyses and discussion on the monophyly of ‘monoraphid’diatoms. PloS One 11 (4), e0152797. https://doi. org/10.1371/journal.pone.0152797 Tong, C. F., Li, L., Meng, Y. Q., Wang, B., 2018: Analysis of strat- ification-mixing mechanism during spring tide of dry sea- son in the Modaomen waterway. Hydro-Science and Engi- neering, 48–57. [In Chinese with English abstract] https:// doi.org/10.16198/j.cnki.1009-640X.2018.01.008 Vishnyakov, V. S., 2020: Description of Placoneis mologaensis, a new diatom from the Rybinsk reservoir on the Volga river, European Russia. Phytotaxa 464(3): 217–226. https://doi. org/10.11646/phytotaxa.464.3.3 Wang, H. P., Li, D. D., Sun, S. J., Wang, H. J., 2018: The investi- gation of diatom species composition and database of diatom scanning electron microscope in the Pearl River of Guang- dong province. Chinese Journal of Forensic Medicine 33: 154–161. [In Chinese with English abstract] https://doi. org/10.13618/j.issn.1001-5728.2018.02.010 Wang, X. T., Huang, Y. Y., Huang, S. F., Li, S. J., Lei, Y. D., 2021: Atlas of common diatoms and benthic animals in the Pearl River Basin. China Water & Power Press, Beijing. Zidarova, R., Van De Vijver, B., Mataloni, G., Kopalova, K., Nedbalova, L., 2009: Four new freshwater diatom species (Bacillariophyceae) from Antarctica. Cryptogamie, Algologie 30(4): 295–310.