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Cocconeis crozetensis, a new monoraphid diatom fromsubantarctic freshwater and moss habitatsOscar E. Romero a & Bart Van de Vijver ba Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Universidad de Granada, Granada,Spainb Department of Bryophytes & Thallophytes, National Botanic Garden of Belgium, Meise,Belgium

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To cite this article: Oscar E. Romero & Bart Van de Vijver (2011): Cocconeis crozetensis, a new monoraphid diatom fromsubantarctic freshwater and moss habitats, Diatom Research, 26:1, 89-98

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Diatom ResearchVol. 26, No. 1, March 2011, 89–98

Cocconeis crozetensis, a new monoraphid diatom from subantarctic freshwater and moss habitats

OSCAR E. ROMERO1* & BART VAN DE VIJVER2

1Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Universidad de Granada, Granada, Spain2Department of Bryophytes & Thallophytes, National Botanic Garden of Belgium, Meise, Belgium

Ultrastructural observations of one previously misidentified freshwater species of a Cocconeis (Bacillariophyceae) from the CrozetArchipelago, Indian Sector of the Southern Ocean, are presented. Cocconeis crozetensis sp. nov. possesses small valves (apical axis:10–13 μm; transapical axis: 6.7–8.4 μm). The main frustule features that distinguish the proposed new species are: (1) the sternumvalve (SV) areolae with an external vola and the internal vela, (2) the submarginal area of the raphe-sternum valve, and (3) the mor-phology of valvocopula and fimbriae of the SV. Possible morphological relationships with several other small Cocconeis, such asC. neodiminuta, C. neothumensis, C. placentula var. pseudolineata and C. pseudothumensis, as well as its morphological similaritieswith placentuloid Cocconeis are discussed. Reliable identification of C. crozetensis sp. nov. under light microscopy can prove diffi-cult for less-experienced diatomologists and ecologists, and the use of electron microscopy proves to be unavoidable for its correctidentification.

Keywords: Cocconeis, freshwater, moss habitats, monoraphid, subantarctic

IntroductionDiatoms are one of the most abundant algal groups in fresh-water and (semi-)wet terrestrial subantarctic ecosystems(Jones 1996, Van de Vijver & Beyens 1999). Île de la Pos-session, the largest of the five islands of the subantarcticCrozet Archipelago in the Indian Sector of the SouthernOcean, has a highly diverse diatom community. In theirwide account of the freshwater diatom flora from Île de laPossession, Van de Vijver et al. (2002) identified over 200species found in more than 500 aquatic, moss and soil sam-ples. The richness and diversity of the subantarctic diatomflora are evidenced by a number of studies published inrecent years, many of them reporting several new entities forscience (Van de Vijver et al. 2002, Riaux-Gobin & Romero2003, Van de Vijver & Gremmen 2006, Riaux-Gobin et al.2007, 2009, Al-Handal et al. 2008).

As expected from ecological studies dealing with theentire diatom community, some species might have beenmisidentified due to force-fitting (Tyler 1996) and species-drift. This is the case for a small species of CocconeisEhrenberg, originally identified as C. neothumensis Kam-mer by Van de Vijver et al. (2002). After thorough obser-vations, some ultrastructural features of this small-sizedCocconeis did not correspond with the original descrip-tion of C. neothumensis (Krammer 1991). In this work,this formerly misidentified taxon is described as a newspecies: C. crozetensis sp. nov. Light and scanning elec-tron microscopy observations are presented. The ecology

∗Corresponding author. Email: oromero@ugr.es

(Received 7 July 2010; accepted 26 October 2010)

of C. crozetensis is addressed and its relationship withseveral morphologically related Cocconeis is thoroughlydiscussed.

The Crozet ArchipelagoThe Crozet Archipelago (45◦48′ to 46◦26′S, 50◦14′ to52◦15′E) is a group of five small volcanic islands locatedin the southern Indian Ocean (Fig. 1), 2400 km north ofthe Antarctic Continent and 2400 km southeast of SouthAfrica, just north of the Antarctic Convergence. The cli-mate, in general, is oceanic and cold with a mean airtemperature of 5 ◦C and a total annual precipitation exceed-ing 2200 mm per year. Strong winds coming from the westand northwest sweep the islands. The main island of thearchipelago is Île de la Possession (Fig. 1) with a surfacearea of 156 km2. The topography is dominated by severalpeaks as a result of past volcanic activity, culminating atthe Pic du Mascarin (934 m). Deep glacial valleys extendfrom north to east and steep cliffs border the western andsouthern coasts.

The vegetation on Île de la Possession is mainlydominated by grasses and mosses. The native vascu-lar flora consists of only 22 species occurring up to analtitude between 200 and 300 m. The immediate areaaround the Alfred Faure base and other cabins on theisland is partly covered by introduced, mainly Europeanplants (Carcaillet 1993, Frénot et al. 2006). Additionalinformation on the geography, climatology and vegetation

ISSN 0269-249X print/ISSN 2159-8347 online© 2011 The International Society for Diatom ResearchDOI: 10.1080/0269249X.2011.575118http://www.informaworld.com

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Fig. 1. (Upper) The Crozet Archipelago in the Indian Sector of the Southern Ocean. (Lower) Location of the sampling sites on Île de laPossession (also see Table 1).

of Île de la Possession can be found in Frénot et al. (1989)and Bougère & Bougère (1998).

Materials and methodsAll samples used in this study were taken from poolsand moss vegetation in the Vallée des Géants, near Baiedu ‘la Pérouse’, on the southwestern side of Île de laPossession (Fig. 1). Table 1 lists the main physical and(measured) chemical characteristics of the analyzed sam-ples. The Vallée des Géants, one of the wettest places onÎle de la Possession, is under the continuous influence ofheavy winds and sea spray which greatly influence thecomposition of its diatom community.

Diatom samples for light microscopy (LM) observa-tion were prepared following the method described in Vander Werff (1955). Small parts of the samples were cleanedby adding 37% H2O2 and heating to 80 ◦C for ∼1 h. Thereaction was completed by addition of KMnO4. Follow-ing digestion and centrifugation (3 × 10 min at 3700 g),the resulting clean material was diluted with distilledwater to avoid excessive concentrations of diatom valveson the slides. Cleaned diatom material was mounted in

Naphrax�. The slides were analyzed using an Olympus�

BX51 microscope, equipped with Differential InterferenceContrast (Nomarski) and the Colorview I Soft ImagingSystem�. Samples and slides are curated at the NationalBotanic Garden in Meise, Belgium. For scanning elec-tron microscopy (SEM), part of the suspension was filteredthrough polycarbonate membrane filters with a pore diam-eter of 1 μm, pieces of which were fixed on aluminumstubs after air-drying. The stubs were sputter-coated with50 nm of Au and studied in a Zeiss DSM 950 SEM at30 kV (Centro de Instrumentación Científica, Universidadde Granada, Spain).

The terminology recommended in Anonymous (1975),Ross et al. (1979), Holmes et al. (1982) and Round et al.(1990) was used for the description of frustule features.The classification proposed by Round et al. (1990) forsuprageneric taxa has been adopted. The sternum valve andraphe-sternum valve are identified as SV and RSV, respec-tively (Romero 1996). The striae density was counted at thecentre of the valve face and also at the margin opposite thecentre of the valve if the striae were markedly radiate. Inthe species description, AA is used for the apical axis, TAfor the transapical axis.

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Cocconeis crozetensis Romero & Van de Vijver 91

Tab

le1.

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216

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itude

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ObservationsCocconeis crozetensis Romero & Van de Vijver sp. nov.(Figs 2–30)

References: non Cocconeis neothumensis Krammer (1991);non Cocconeis neothumensis sensu Van de Vijver et al.2002.

Holotype: Slide BR-4205 (National Botanic Garden, Bel-gium).

Isotypes: Slide PLP-153 (University of Antwerp), SlideBRM-ZUH7/49 (Hustedt Collection, Bremerhaven).

Type locality: Vallée des Géants, Île de la Possession, CrozetArchipelago, sample BM224. Collected by Bart Van deVijver, 28 December 1997.

Etymology: The species epithet refers to Crozet Archipelago,where the species was discovered.

Description. Valvae ellipticae. Longitudo 10–13 μm, lati-tudo 6.7–8.4 μm.

Sternumvalva. Valvae leviter convexae secus sternum lin-eare ad anguste lanceolatum. Striae transapicales radiatae,uniseriatae, 20–26 in 10 μm, constantes ex 1–6 areolis clarediscernendis in microscopio photonico. Areolae occlusaeexterne volis habitu morphologico irregulares internequesubquadrangulares. Valvocopula aperta.

Raphovalva. Valvae leviter concavae, leviter convexaesecus raphe–sternum. Area axialis angusta, linearis. Raphefiliformis, recta, poris centralibus clare visibilibus, approx-imatis. Pori terminales leviter expansi, attingentes latusinternum areae hyalinae submarginalis. Striae transapi-cales uniseriatae, radiatae, 22–26 in 10 μm, constantesex poris rotundatis (34–38 in 10 μm). Area hyalinaangusta submarginalis formans cristam silicificatam inlatere interno. Helictoglossa parva leviter elevata adsuntattingentes cristas internas. Valvocopula aperta, fimbriislatis.

Morphological observations (Figs 2–30)Valves elliptical (Figs 2–14, 19–20, 23, 26–27);valve dimensions (n = 50): length 10–13 μm, width6.7–8.4 μm.

SV. Slightly convex along the linear to narrow lance-olate sternum (Figs 2–7, 14, 19–20). Transapical striaeradiate and uniseriate, 20–26 in 10 μm, with 1 to 6areolae on each hemivalve (Figs 14, 19–20), clearlyvisible in LM (Figs 2–7). Areolae externally occluded byvolae of great morphological variability (Figs 15–18, 22),sub-quadrangular on the internal side, occluded by vela(Figs 19–22). Valvocopula open, fimbriae absent (Fig. 20).

RSV. Slightly convex along the raphe-sternum, otherwiseslightly concave (Fig. 23). Axial area narrow, linear with astraight, filiform raphe (Figs 8–13, 23, 26). External central

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Figs 2–13. Holotype specimens of Cocconeis crozetensis sp. nov., LM. Figs 2–7. SV. Figs 8–13. RSV. Scale bar = 10 μm.

pores expanded, close to each other (Fig. 24); terminalraphe pores also expanded, reaching the internal side ofthe submarginal hyaline area (Fig. 25). Central raphe poresdeflected in opposite directions on the internal side (Fig. 28).Central area small and elliptical, slightly asymmetricalfollowing the apical axis (Figs 24, 28). Transapical striaeuniseriate and radiate (Figs 8–13, 23, 26–27), 22–26 in10 μm, consisting of rounded poroids (34–38 in 10 μm)(Figs 24–29), occluded by a vela (Figs 29–30). Narrowsubmarginal hyaline area (Fig. 23) corresponding to awell-silicified rim on the internal side (Fig. 26). Small helic-toglossa, slightly raised, reaching the internal rim (Fig. 29).Valvocopula open in one end with short, wide fimbriae(Fig. 27).

Ecology and distribution. Cocconeis crozetensis sp. nov.has been found in small populations (1–10% of the totaldiatom community) in the Vallée des Géants, near Baiedu ‘la Pérouse’, Île de la Possession, thriving in semi-wetmosses and in the epibenthos of some small pools witha circumneutral to clearly alkaline pH (7.5–9.4) and highconductivity values (701–2090 μS cm−1) (Table 1). Thediatom flora in these samples is dominated by Planothid-ium quadripunctatum (Oppenheim) Sabbe, Planothidiumlanceolatum (Brébisson) Lange-Bertalot, Eolimna minima(Grunow) Lange-Bertalot, Opephora naveana Le Cohu andNavicula gregaria Donkin (Van de Vijver et al. 2002).

DiscussionIn their account of the diatom flora of the CrozetArchipelago, Van de Vijver et al. (2002) listed

C. neothumensis as the only Cocconeis being present inthe freshwater and semi-wet terrestrial habitats on Île dela Possession. In general, the small Cocconeis identified asC. neothumensis by Van de Vijver et al. (2002) correspondswell with the description of C. neothumensis (Krammer1991, Krammer & Lange-Bertalot 1991; Table 2). Krammer(1991) characterized C. neothumensis as having relativelyflat and slightly silicified valves, elliptical to lanceolate inshape. The RSV of C. neothumensis possesses a narrowvalvocopula with fimbriae easily recognizable under LMas a marginal chain of lighter or darker dots (depending onthe focus). As in C. crozetensis, the raphe of C. neothu-mensis is also straight and filiform, and the RSV striae andareolae are dense (Table 2). The axial area of the SV ofC. neothumensis ranges from narrowly to broadly lance-olate, while its SV striae are almost parallel and slightlyradiate, becoming strongly radial toward the valve apices(Krammer 1991; Table 2).

Thorough observations of the valves originally iden-tified as C. neothumensis by Van de Vijver et al.(2002) allowed the recognition of important ultra-structural features that basically differ from those ofC. neothumensis. Although on the internal valve face,the occlusion of the SV areolae of C. crozetensis aresimilar to those of C. neothumensis var. marina DeStefano, Marino & Mazzella (De Stefano et al. 2000) orC. neodiminuta Krammer (Krammer & Lange-Bertalot1991), the external vola is a unique and particular fea-ture. The external volae of the SV of C. crozetensismostly grow out from only one side of the foramen(Figs 16–17). Some volae possess two or three ear-lobes (Fig. 18). In general, the smaller volae appear in

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Table 2. Summary of morphometric data, morphological features, ecology and distribution of Cocconeis crozetensis sp. nov. and some morphologically related Cocconeis species.

C. placentula var.C. crozetiana C. neothumensis C. neodiminuta pseudolineata C. pseudothumensisThis study Krammer (1991) Krammer (1991) Krammer (1991) Krammer & Lange-Bertalot (1991)

AA (μm) 10–13 6.5–13.0 8–18 7.5–38.0 11.5–13.5TA (μm) 6.7–8.4 4.0–8.3 6–9 n.d. 8.2–9.5Valve outline Broadly elliptical Elliptical-lanceolate Broadly elliptical Broadly elliptical Broadly ellipticalSternum valve

Striae/10 μm 20–26 16–25 11–14 20–23 10–20Transapicalareolae in eachhemivalve

1–6 transapically elongated 3–4 transapically elongated 2–4 transapicallyelongated

3–6 transapicallyelongated

Mostly 3 transapically elongated

Areolae External vola, internal vela n.d. n.d. n.d. n.d.Axial area Narrow, slightly depressed Narrow to broadly

lanceolateLinear to lanceolate Linear to lanceolate Elliptic to lanceolate

Central area Slightly lanceolate Not differentiated Not differentiated Not differentiated LinearValvocopula Open on one end Absent n.d. n.d. n.d.Fimbriae Absent n.d. n.d. n.d. n.d.

Raphe-sternum valveStriae/10 μm 22–26 28–36 24–32 16–20(22) 35–40Areolae/10 μm 34–38 34–37 25–32 n.d. 10–20Raphe Straight, filiform Straight, filiform Straight, filiform Straight, filiform Delicate, straightforward and filiformCentral rapheendings

Closely located to each other Delicate, closely located toeach other

Small, closely located toeach other

Closely located to eachother

Closely located to each other

Terminal rapheendings

Well-separated from themarginal row of areolae

Small Difficult to observe withLM

Well-separated from themarginal row of areolae

n.d.

Central area Lanceolate-elliptical Elliptical Elliptical Elliptical EllipticalValvocopula Open in one end Narrow Narrow n.d. n.d.Fimbriae Wide, short Short n.d. n.d. n.d.

Ecology Subantarctic shallow freshwaterbodies

Freshwater, lakes Epyphytic, tycho-planktonic,lacustrine

Epyphytic Wet mosses, also fossil

Distribution Île de la Possession, CrozetArchipelago, Indian Sector,Southern Ocean

Possibly cosmopolitan Germany, Switzerland,Ireland

Not well established France, Germany, Switzerland

Notes: AA: apical axis; TA: transapical axis; n.d.: no data.

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Figs 14–18. Sternum valves of Cocconeis crozetensis sp. nov., SEM. Fig. 14. Complete SV, external view. Fig. 15. Detail of the areolationon the external valve surface. Fig. 16. Detail of areolae with broken vela (arrows). Figs 17–18. Detail of areolae showing the externalvolae. The white stars on Fig. 18 highlight volae with two/three earlobes. Scale bars = 1 μm (Fig. 14); 0.5 μm (Figs 15–18).

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Figs 19–22. Sternum valves of Cocconeis crozetensis sp. nov., SEM. Fig. 19. Complete SV, internal view. Fig. 20. Internal view of thevalve with valvocopula. Fig. 21. Detail of vela on the internal valve surface. Fig. 22. Broken valve, external view. Note the volae (blackarrows) and the vela (white arrows). Scale bars = 1 μm (Figs 19–20); 0.5 μm (Figs 21–22).

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Figs 23–30. Raphe-sternum valves of Cocconeis crozetensis sp. nov., SEM. Fig. 23. Complete RSV, external view. Fig. 24. Detail ofproximal raphe ends, external view. Fig. 25. Detail of terminal raphe ends, external view. Fig. 26. Complete RSV, internal view. Fig. 27.Broken RSV, showing part of the internal valve surface of the SV. Fig. 28. Detail of proximal raphe ends, internal view. Fig. 29. Detail ofterminal raphe ends and helictoglossa (white star), internal view. Fig. 30. Broken RSV showing vela (arrows). Scale bars = 1 μm (Figs 23,26–27); 0.5 μm (Figs 24–25, 28–29); 0.2 μm (Fig. 30).

areolae located close to the valve margin (Figs 16, 22).The internal vela of the SV areolae of C. crozetensis(sub-quadrangular) differs from the one present inC. neothumensis var. marina (hymenes with radialperforations of different length, De Stefano et al.

2000), and C. neodiminuta and C. placentula var.pseudolineata (hymenes with parallel linear perforations,Krammer, 1991).

The general morphology of the RSV of C. crozeten-sis resembles that of the placentuloid Cocconeis. This

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pattern includes the following common valve features: theexternally convex SV possesses uniseriate striae and openvalvocopula with short fimbriae. The RSV, externally con-cave, has a straight, linear raphe-sternum, uniseriate striae,a submarginal hyaline area which separates the valve facefrom the mantle, and an open valvocopula (O.E. Romeroand M. DeStefano, unpub. data). The ongoing revision ofthe placentuloid Cocconeis will help to accurately definethe morphological relationships between C. crozetensis andthe placentuloid Cocconeis.

Although the SV of C. crozetensis is morphologi-cally similar to the SV of C. neodiminuta and C. pla-centula var. pseudolineata, C. crozetensis can be distin-guished by the more delicate structure of its SV. Valvesof C. neothumensis possess a higher areola and striadensity (Table 2) and are less silicified than those ofC. crozetensis. In addition to the above-mentioned smallCocconeis and other small species of the C. placentulagroup (Krammer & Lange-Bertalot 1991), the RSV ofC. crozetensis shows similarities with small Cocconeisfrom the southern hemisphere, such as C. californicaGrunow from Kerguelen Islands (Riaux-Gobin & Com-père 1996), C. margaritata Riaux-Gobin & Al-Handalfrom the Mascarenes Islands (Riaux-Gobin et al. 2010),and the Mediterranean C. neothumensis var. marina (DeStefano et al. 2000), as well as the episammic C. hau-niensis Witkowski, abundant in Baltic Sea coastal waters(Witkowski 1993).

The AA of C. crozetensis valves never exceeds 13 μm.Because of the overlapping in their main features and mor-phometric data compared with morphologically closelyrelated species (Table 2), the recognition of the RSV ofC. crozetensis can prove difficult. The higher number ofstriae in C. neothumensis and C. pseudothumensis (Table 2)make identification easy. However, the overlap in the rangeof striae in C. neodiminuta and some valves of C. placentulavar. pseudolineata (Table 2) can prove troublesome inidentifying C. crozetensis.

Small Cocconeis taxa are difficult to identify with lightmicroscopy (Riaux-Gobin et al. 2010). As demonstratedfor C. crozetensis, LM does not necessarily reveal sig-nificant features for taxon discrimination, such as thefine structure of striae and fimbriae. Therefore, the useof electron microscopy proves unavoidable as long asobservation of the valve and valvocopula ultrastructureis needed for the correct identification of small Coc-coneis species. Observations summarized in Table 2 sup-port this argument: main valve features of C. crozetensismostly overlap those of C. neodiminuta and C. neothu-mensis, and their reliable identification under LM canprove difficult for less-experienced diatomologists andecologists. In this regard, caution is advised sincethe ecology and the geographical distribution of thesesmall Cocconeis are very different and misidentifica-tion can lead to wrong ecological or palaeo-ecologicalinterpretations.

AcknowledgementsThis survey was made possible with the logistic and finan-cial support of the Institut Polaire Français – Paul-Emile Victor(IPEV) in the frame of the Terrestrial Ecology program 136 (YvesFrenot & Marc Lebouvier). O.E.R. was supported by the SpanishResearch Council (CSIC) and the European Union Program Syn-thesis of Systematics Resources (synthesys). Additional fundingfor B.V.V. was provided by the Science Foundation (Flanders,Belgium). Pierre Compère corrected the Latin diagnosis. Mr.Sven Bellanger prepared Fig. 1. C. Riaux-Gobin and one anony-mous reviewer are greatly acknowledged for their contributionsin correcting and improving this work.

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