European Journal of Taxonomy 79: 1-19

http://dx.doi.org/10.5852/ejt.2014.79

This work is licensed under a Creative Commons Attribution 3.0 License.

ISSN 2118-9773

www. europeanj ournaloftaxonomy. eu

2014 • Van de Vijver A. & Kopalova K.

Research article

Four Achnanthidium species (Bacillariophyta) formerly identified as

Achnanthidium minutissimum from the Antarctic Region

Bart VAN DE VIJVER 12 & Katerina KOPALOVA 3

botanic Garden Meise, Department of Bryophyta & Thallophyta,

Nieuwelaan 38, B-1860 Meise, Belgium

Email: [email protected] (corresponding author)

^University of Antwerp, Department of Biology, ECOBE,

Universiteitsplein 1, B-2610 Wilrijk, Antwerpen, Belgium

3 Charles University in Prague, Faculty of Science, Department of Ecology,

Vinicna 7, CZ-12844 Prague 2, Czech Republic

Abstract. Four taxa belonging to the complex of species around Achnanthidium minutissimum were

found during the ongoing taxonomic revision of the Antarctic freshwater and l imn o-terrestrial diatom

flora. Two taxa were previously described as Achnanthidium lailae and A. sieminskae. Two others were

formerly identified as A. minutissimum but detailed light and scanning electron microscopical observations

revealed sufficient morphological differences compared to the type of A. minutissimum , to justify their

separation and description as new taxa: Achnanthidium indistinctum and A. maritimo-antarcticum. The

morphology and ecology of all four taxa are discussed comparing the species with morphologically

similar taxa. The biogeographical consequences of the splitting of the former A. minutissimum complex

in the Antarctic Region are discussed.

Keywords. Achnanthidium , Bacillariophyta, Antarctic Region, new species, biogeography.

Van de Vijver B. & Kopalova K. 2014. Four Achnanthidium species (Bacillariophyta) formerly identified as

Achnanthidium minutissimum from the Antarctic Region. European Journal of Taxonomy 79: 1-19. http://dx.doi.

org/10.5852/eit.2014.79

Introduction

Based on the list of all published diatom records (40 entries) from the Antarctic Region in Kellogg &

Kellogg (2002), Achnanthes minutissima Kiitz. (Kiitzing 1833) is one of the most commonly observed

diatom taxa in the (sub-)Antarctic Region. Apart from the Antarctic Continent, the species has been

reported from all investigated localities on the sub-Antarctic Islands in the southern Indian Ocean

(Bourrelly & Manguin 1954; Le Cohu 2005; Van de Vijver et al. 2002) and the Maritime Antarctic

Region (e.g., South Shetland Islands, South Orkney Islands) (Jones et al. 1993; Oppenheim 1994).

The species often forms large populations on the sub-Antarctic islands, dominating the aquatic diatom

communities in lakes and pools (Van de Vijver et al. 2001, 2002,2004,2008). All findings contributed to

the generally accepted idea that A. minutissima was atypical cosmopolitan species showing a worldwide

distribution on all continents (Ector 2011).

European Journal of Taxonomy 79: 1-19 (2014)

The correct identity and biogeographical distribution of this taxon has for a long time been obscured

by force-fitting and taxonomic drift. As a correct interpretation of the type of A. minutissima was

virtually unknown, all identifications were based on the short description in Kiitzing (1833). In 1994,

Czamecki transferred the species to the genus Achnanthidium as Achnanthidium minutissimum (Kiitz.)

Czarnecki (Czamecki 1994). The type material of Achnanthes minutissima remained unstudied until

Potapova & Hamilton (2007) published for the first time several SEM images from the type material.

Following a taxonomic workshop in Luxemburg in 2009 (Ector 2011), it was clear that the old concept

of A. minutissimum was no longer acceptable and that a revision of all populations formerly identified as

A. minutissimum was necessary. A special volume of Algological Studies (published in 2011) describing

the workshop results, was entirely dedicated to the study of the Achnanthidium minutissimum- complex.

A large number of new taxa were described, split off from the former catch-all taxon A. minutissimum

and some of its varieties. Since 2009, more than 15 new Achnanthidium species have been described

such as A. caravelense Novais & Ector (Novais et al. 2011), A. acerosum Van de Vijver, Lange-Bert.

& Jarlman (Van de Vijver et al. 2011a) and A. acsiae Wojtal, E.Morales, Van de Vijver & Ector (Wojtal

et al. 2011). Simultaneously to the revision of the European and North American Achnanthidium

minutissimum complex, several Antarctic populations have been re-investigated which resulted in the

separation of two new species with a purely Antarctic distribution. In 2009, Zidarova et al. described

Achnanthidium lailae Van de Vijver (Zidarova et al. 2009) from James Ross Island located in the

northern Weddell Sea, close to the Antarctic Peninsula. Three years later, Achnanthidium sieminskae

Witkowski, Kulikovskiy & Riaux-Gobin (Witkowski et al. 2012) was described from the sub-Antarctic

lies Kerguelen in the southern Indian Ocean.

During a survey of the freshwater diatom flora from the Maritime Antarctic Region, two other taxa

have been found that were previously identified as A. minutissimum. Analysis of their ultrastructure

using scanning electron microscopy revealed significant differences with the type of Achnanthidium

minutissimum and with all other recently split-off taxa justifying their description as new taxa. The

present paper gives a formal description for both new taxa, Achnanthidium maritimo-antarcticum

sp. nov. and A. indistinctum sp. nov., and discusses the morphology, ecology and distribution of the two

other Antarctic taxa, A. lailae and A. sieminskae.

Material and methods

Samples used in this study were collected during several field trips from different aquatic habitats on

the South Shetland Islands (Livingston Island), James Ross Island and the sub-Antarctic Islands in

the southern Indian (lies Crozet, lies Kerguelen, Prince Edward Islands) and Atlantic Ocean (South

Georgia) (Fig. 1). Details on sampling methods and physico-chemical analyses can be found in Van

de Vijver & Beyens (1996), Van de Vijver et al. (2001, 2002, 2004, 2008), Kopalova et al. (2013) and

Kopalova & Van de Vijver (2013).

Diatom samples for LM observation were prepared following the method described in Van der Werff

(1955). In total, ten sampling sites from various Antarctic localities were examined in this study, with

all samples listed in Table 1. Subsamples of the original material were oxidized using 37% H 0 Cf and

heating to 80°C for approximately lh. The reaction was further completed by the addition of KMn04.

Following digestion and centrifugation (three times 10 minutes at 3700x g), the material free of organic

matter was diluted with distilled water for sample mounting to avoid excessive concentrations of diatom

valves and frustules on the slides. A subsample from the organic-free material was mounted in Naphrax®

for diatom community studies. The slides were analysed using an Olympus BX53 microscope, equipped

with Differential Interference Contrast (Nomarski) and an Olympus UC30 digital camera. Samples

and slides are stored at the BR-collection, property of the Belgian federal government and given in

permanent loan to the Botanic Garden Meise (Belgium). For scanning electron microscopy (SEM),

parts of the oxidized suspensions were filtered through a 1 -pm Isopore™ polycarbonate membrane

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VAN DE VIJVER B. & KOPALOVA K., Achnanthidium minutissimum complex in the Antarctic

filter (Merck Millipore). The stubs were sputter-coated with a Gold-Palladium layer of 10 nm and

studied in a ZEISS ULTRA SEM microscope at 3 kV (Natural History Museum London, UK). Diatom

terminology follows Ross et al. (1979), Round et al. (1990) and Potapova & Hamilton (2007). For

comparison, the following publications were consulted: Krammer & Lange-Bertalot (1991), Kobayashi

(1997), Potapova & Ponader (2004), Ivanov & Ector (2006), Potapova (2006), Monnier et al. (2007),

Ponader & Potapova (2007), Potapova & Hamilton (2007), Zidarova et al. (2009), Hlubikova et al.

(2011), Morales et al. (2011), Van de Vijver et al. (2011a, 2011b) and Witkowski et al. (2012).

40’

2 0 1

SouJfKGeorgia'

Falklc

Island

litfh

tAmericl

80’

20 ’

Prince Edwarc

[arion Island

CrozetBlands

7 , Antarctic NZ

Continent

80’

Fig. i. The Antarctic region with the location of the sampled islands and archipelagos.

Tablel. List of samples used in this study.

Sample ID

Geographic locality

Source

C-BW184

Crozet

Van de Vijver et al. (2002)

KER-POCO008

Kerguelen

this study

KER-BW417

Kerguelen

Van de Vijver et al. (2001)

He-D273

Heard Island

Van de Vijver et al. (2004)

PEI-D50

Prince Edward Islands

Van de Vijver et al. (2008)

SG-W398

South Georgia

Van de Vijver & Beyens (1996)

LIV-BYM051

Livingston Island

this study

LIV-BY055

Livingston Island

Kopalova & Van de Vijver (2013)

JRI2009-CLW58

James Ross Island

this study

JRI2009-CLW60

James Ross Island

this study

European Journal of Taxonomy 79: 1-19 (2014)

Results

Class Bacillariophyceae Haeckel emend. Medlin & Kaczmarska 2004 (Medlin & Kaczmarska 2004)

Subclass Bacillariophycidae D.G.Mann in Round et al. 1990 (Round et al. 1990)

Order Achnanthales Silva 1962 (Silva 1962)

Family Achnanthidiaceae D.G.Mann in Round et al. 1990 (Round et al. 1990)

Genus Achnanthidium Kiitz. 1844 (Kiitzing 1844)

Achnanthidium indistinctum Van de Vijver & Kopalova sp. nov.

Figs 2-28

Morphological observations

Light microscopy (Figs 2-24)

Frustules short, rectangular in girdle view, bent around the transapical axis (Figs 2-4). Mantle striae

often apparently thickened in LM (Fig. 4). Valves narrowly lanceolate with slightly to clearly convex

margins and distinctly protracted, rostrate apices (Figs 2-24). Valve dimensions (n=30): length 8.5-13.0

pm, width 1.8-2.2 pm. Raphe valve (Figs 5-15) concave with a very narrow linear axial area, almost

invisible towards the valve apices, almost not widening towards the central part of the valve. Central

area very small, indistinct, bordered by several (usually two) more widely spaced striae. Raphe straight,

filiform, almost invisible, with simple proximal raphe endings. Distal raphe fissures not discernible

in LM. Striae moderately radiate throughout the entire valve, becoming indiscernible (due to higher

density) near the apices. Rapheless valve (Figs 16-24) slightly convex with narrow, linear axial area, not

or very slightly widening towards the valve centre. Central area almost non-existing, never forming a

fascia or subfascia. Central striae very weakly more distantly spaced. Striae wealdy radiate throughout,

only very slightly more densely spaced near the apices, 30-35 in 10 pm.

Scanning electron microscopy (Figs 25-28)

Striae in the rapheless valve composed of a series of 3-4 distinct, rounded to square areolae (Fig. 25).

Slit-like areolae apparently never present. Central striae less dense than in the rest of the valve (Fig. 25).

Near the apices, striae only composed of 1-3 rounded areolae (Fig. 25). Mantle areolae elongated, slit¬

like, often enlarged (Fig. 25). Striae on the raphe valve, ca. 36 in 10 pm, composed of 2-3 irregularly

shaped (mostly rounded or elongated rectangular) areolae (Fig. 26). Near the apices, stria density

higher (up to 38 in 10 pm), composed of fewer areolae (Fig. 26). Internal areolae openings on both

valves covered by hymenes (Figs 27, 28). External raphe branches straight terminating in simple, short

proximal raphe endings (Fig. 26). Distal raphe fissures straight, terminating just beyond the last apical

stria (Fig. 26). Internally, proximal raphe endings shortly bent into opposite directions (Fig. 28). Distal

raphe endings terminating in small helictoglossae (Fig. 28).

Etymology

The specific epithet was given because the species is rather small and can be easily overlooked.

Type material

Holotype

Slide BR-4361 (National Botanic Garden, Meise, Belgium).

Isotypes

Slides PLP-251 (UA, University of Antwerp, Belgium).

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VAN DE VIJVER B. & KOPALOVA K., Achnanthidium minutissimum complex in the Antarctic

Type locality

Byers Peninsula, Livingston Island, South Shetland Islands, sample BYM051 (62°38 , 20.1”S

61°06 , 44.2”W) (Leg. B. Van de Vijver; coll, date 15 Jan. 2009).

Ecology, distribution and associated diatom flora

So far, A. indistinctum has only been found on Livingston Island (South Shetland Islands). The type

population was observed living on submerged mosses collected in a large lake on the central plateau,

located between Midge Lake and Limnopolar lake. The lake had an alkaline pH (7.35) with a low

specific conductance value (97 pS/cm) and a high phosphate level (5.2 mg/1). The flora in the sample was

Figs 2-28. Achnanthidium indistinctum Van de Vijver & Kopalova sp. nov. Light and scanning electron

micrographs of the type population on Byers Peninsula (Livingston Island). 2-4. LM views of some

frustules in girdle view. 5-15. LM views of raphe valves. 16-24. LM views of rapheless valves. 25. SEM

external view of an entire rapheless valve. 26. SEM external view of an entire raphe valve. 27. SEM

internal view of an entire rapheless valve. 28. SEM internal view of an entire raphe valve. Scale bars

represent 10 pm.

European Journal of Taxonomy 79: 1-19 (2014)

dominated by several Psammothidium species [P. abundans (Manguin) Bukht. & Round (Bukhtiyarova

& Round 1996), P. papilio (D.E. Kellogg, M. Stuiver, T.B. Kellogg & G.H. Denton) Van de Vijver &

Kopalova (Kopalova et al. 2012), P. subatomoides (Hust.) Bukht. & Round (Bukhtiyarova & Round

1996)], Nitzschia cf. perminuta (Grunow) M.Peragallo (Peragallo 1903), Brachysira minor (Krasske)

Lange-Bert, in Lange-Bertalot & Moser (Lange-Bertalot & Moser 1994), Sellaphora seminulum

(Grunow) D.G.Mann (Mann 1989) and Amphora sp. It is possible that the species is also present on

Signy Island (South Orkney Islands) based on the illustrations in Oppenheim (1994). The distribution

in the Antarctic region and other geographical areas is unclear due to the insufficient discrimination

between species in the Achnanthidium minutissimum- complex. All published records should be verified

in order to know the exact biogeography of this and all other taxa.

Achnanthidium maritimo-antarcticum Van de Vijver & Kopalova sp. nov.

Pigs 29-53

Morphological observations

Light microscopy (Figs 29-49)

Frustules in girdle view rectangular, bent around the transapical axis (Figs 29,30). Valves linear-lanceolate

with parallel to slightly convex margins and clearly protracted, rostrate, occasionally subcapitate apices

(Figs 31-49). Valve dimensions (n=25): length 12-15 pm, width 2.3-2.7 pm. Raphe valve (Figs 31-40)

concave with a linear axial area, narrow to almost absent near the valve apices, hardly widening towards

the central part of the valve. Central area irregular, formed by several more widely spaced striae. Raphe

straight, filiform, invisible, with indistinct straight, simple proximal raphe endings. Distal raphe hardly

discernible in LM. Striae slightly radiate throughout the entire valve, becoming more radiate and more

densely spaced near the apices, 30- 33 in 10 pm, up to 40^12 in 10 pm near the valve apices. Rapheless

valve (Figs 41-49) slightly convex with very narrow, linear axial area, widening to form a wealdy

elliptically, never transapically elongated central area, never forming a fascia or subfascia. Central striae

slightly more distantly spaced. Striae slightly radiate throughout, only slightly more denser near the

apices, 30-32 in 10 pm.

Scanning electron microscopy (Figs 50-53)

Striae in the raphe valve composed of a series of 3-4 distinct, rounded to square areolae terminating in

one transapically elongated, rectangular areola (Fig. 50). Central striae composed of 2 rounded areolae

and one transapically elongated areola near the valve margin (Fig. 50). Occasionally, short slit-like

areolae interposed between the central striae. Near the apices, striae only composed of 2-3 rounded

areolae (Fig. 50). Mantle areolae elongated, slit-like. Striae on the rapheless valve composed of 2-5

irregularly shaped (mostly rounded or elongated rectangular) areolae (Fig. 51). Near the apices, striae

showing lowest number of areolae (Fig. 51). Internal areolae openings on both valves covered by

hymenes (Figs 52, 53). External raphe branches straight to weakly curved terminating in simple, short

proximal and distal raphe endings (Fig. 50). Distal raphe endings terminating just beyond the last apical

stria (Fig. 50). Internally, proximal raphe endings short, slightly bent. Distal raphe endings terminating

in small helictoglossae (Fig. 52).

Etymology

The specific epithet refers to the geographical area where the new species was found: Maritime Antarctic

Region. The hyphen can and should be maintained in the species name based on art. 60.9 of the

International Code of Nomenclature for algae, fungi and plants (McNeill et al. 2012).

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VAN DE VIJVER B. & KOPALOVA K., Achnanthidium minutissimum complex in the Antarctic

Type material

Holotype

Slide BR-4362 (National Botanic Garden, Meise, Belgium).

Isotypes

Slides PLP-252 (UA, University of Antwerp, Belgium).

Type locality

Byers Peninsula, Livingston Island, South Shetland Islands, sample BY055 (62°37’41.7”S

61°06 , 30.4”W) (Leg. B. Van de Vijver; coll, date 15 Jan. 2009).

Figs 29-53. Achnanthidium maritimo-antarcticum Van de Vijver & Kopalova sp. nov. Light and scanning

electron micrographs of the type population on Byers Peninsula (Livingston Island). 29-30. LM views

of some frustules in girdle view. 31-40. LM views of raphe valves. 41-49. LM views of rapheless valves.

50. SEM external view of an entire raphe valve. 51. SEM external view of an entire rapheless valve.

52. SEM internal view of an entire raphe valve. 53. SEM internal view of an entire rapheless valve.

Scale bars represent 10 pm.

European Journal of Taxonomy 79: 1-19 (2014)

Ecology, distribution and associated diatom flora

The type population of A. maritimo-antarcticum was found in a large lake located on the central plateau

of Byers Peninsula. The lake is characterized by a weakly alkaline pH (7.5) and a low conductivity

(130 pS/cm). The ion composition in the lake is mainly dominated by Na + (20.0 mg/1) and Ca 2+

(12.1mg/l). Both nutrients (N-NO,+N-NO,) and phosphate were rather low (25 pg/1 and 10 jug/1 resp.).

The shoreline of the lake was completely covered with partly submerged mosses. Dominant taxa in the

sample included Fragilaria capucina s.l. Desm. (Desmazieres 1825), Staurosirella sp., Navicula cremeri

Van de Vijver & Zidarova in Van de Vijver et al. (Van de Vijver et al. 201 Id) and Nitzschia cf. perminuta.

Smaller populations were found in other lakes such as Limnopolar Lake but these populations never

exceed 1% of the total diatom composition. Due to force-fitting and taxonomic drift, the distribution of

the new taxon in the Antarctic Region is unclear.

Achnanthidium lailae Van de Vijver in Zidarova et al. (Zidarova et al. 2009)

Figs 54-77

Morphological observations

Light microscopy (Figs 54-74)

Frustules in girdle view narrow, rectangular, bent around the transapical axis, apices weakly recurved

(Figs 54-56).Valves linear to very slightly linear-lanceolate with almost parallel margins and non-

protracted, broadly rounded, never rostrate or capitate apices (Figs 57-74). Valve dimensions (n=30):

length 10-14 pm, valve width 1.8-2.5 pm. Raphe valve (Figs 57-64) concave with a rather narrow,

linear to linear-lanceolate axial area, widening towards the central area. Central area forming a typical

rectangular fascia. Shortened marginal striae occasionally present in the central area. Raphe straight

to weakly undulating with inconspicuous straight proximal raphe endings. Distal raphe fissures not

discernible in LM. Striae weakly but still distinctly radiate near the valve center, becoming more radiate

near the apices, 30-33 in 10 pm. Rapheless valve (Figs 65-74) slightly convex with moderately broad,

clearly lanceolate axial area, widening near the valve centre. Central area elongated, rhombic lanceolate,

never expanding into a fascia due to several longer marginal striae in the central area. Striae parallel to

weakly radiate near the valve centre, more radiate near the apices, 28-30 in 10 pm.

Scanning electron microscopy (Figs 75-77)

Striae on the raphe valve composed of 2-3 small areolae (Fig. 77). Areolae close to the axial area

and at the apices rounded. Marginal areolae sometimes narrow, transapically elongated and hence

slit-like, sometimes fused with the second areola (Fig. 77). Striae of the rapheless valve composed of

2-3 rounded to slit-lilce external areola openings (Fig. 75). Mantle areolae slit-like. Internal areolae

openings covered by hymenes (Figs 76, 77). When removed due to sample preparation, very narrow

struts visible separating the areolae (Fig. 76). Raphe slightly undulating becoming narrower towards the

apices (Fig. 77). Proximal raphe endings almost straight, inconspicuous. Distal raphe fissures wealdy

deflected, continuing slightly beyond the last striae, never onto the mantle (Fig. 77). Internally, proximal

raphe endings shortly bent into opposite directions, terminating in a thickened central pore (Fig. 77).

Distal raphe endings terminating on small helictoglossae.

Ecology, distribution and associated diatom flora

The type population was found in a large circumneutral lake (pH = 7.1) on Ulu Peninsula on James

Ross Island (Zidarova et al. 2009). Since then, several other large populations were discovered in lakes

on Clearwater Mesa, a volcanic tableland on James Ross Island next to Ulu Peninsula. All populations

were observed in alkaline lakes (pH 8.1-8.7) with relatively high conductivity (1000-2000 pS/cm). The

samples were dominated by Halamphora sp., Pinnularia australomicrostauron Zidarova et al. (Zidarova

et al. 2012), Nitzschia cf. commutata Grunow in Cleve & Grunow, and Achnanthes coarctata (Breb.)

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VAN DE VIJVER B. & KOPALOVA K., Achnanthidium minutissimum complex in the Antarctic

Grunow in Cleve & Grunow (Cleve & Grunow 1880) and Gomphonema sp. So far, no populations were

found on other islands in the Maritime Antarctic Region (Kopalova & Van de Vijver 2013, Van de Vijver

et al. unpubl. res.).

Achnanthidium sieminskae Witkowski, Kulikovskiy & Riaux-Gobin (Witkowski et al. 2012)

Figs 78-103

Morphological observations

Light microscopy (Figs 78-99)

Frustules in girdle view rectangular, clearly bent around the transapical axis and with clearly recurved

apices (Figs 78-80). Valves linear to narrowly linear-lanceolate with almost parallel to slightly convex

Figs 54-77. Achnanthidium lailae Van de Vijver. Fight and scanning electron micrographs of a

population on Clearwater Mesa (James Ross Island). 54-56. FM views of some frustules in girdle view.

57-64. FM views of raphe valves. 65-74. FM views of rapheless valves. 75. SEM external view of an

entire rapheless valve. 76. SEM internal view of an entire rapheless valve. 77. SEM external (right) and

internal (left) view of an entire raphe valve. Scale bars represent 10 pm.

European Journal of Taxonomy 79: 1-19 (2014)

margins and clearly protracted, rostrate to even capitate apices (Figs 81-99). Valve dimensions (n=25):

length 9.5-18.0 pm, width 1.9-3.1 pm. Raphe valve (Figs 81-91) concave with a very narrow linear

axial area, almost not discernible near the valve apices. Central area elliptical to rounded, very small,

almost indistinct, formed by 2-3 more widely spaced striae. Central striae often lacking extending the

central area up to the valve margin. Raphe straight, filiform with simple, straight proximal raphe endings.

Distal raphe hardly discernible in LM. Striae weakly radiate near the valve center, becoming parallel and

even convergent near the apices, well visible in LM, ca. 32 in 10 pm, more densely spaced near the valve

ends. Rapheless valve (Figs 92-99) slightly convex with narrow, linear axial area, widening to form a

weakly elliptically, never transapically elongated central area. Fascia never present. Central 2-3 striae

more distantly spaced. Striae weakly radiate throughout, becoming more parallel to even convergent

near the apices, ca. 35 in 10 pm.

Figs 78-103. Achnanthidium sieminskae Witlcowski, Kulikovskiy & Riaux-Gobin. Light and scanning

electron micrographs of a population on lies Kerguelen. 78-80. LM views of some frustules in girdle

view. 81-91. LM views of raphe valves. 92-99. LM views of rapheless valves. 100. SEM external view

of an entire raphe valve. 101. SEM internal view of an entire raphe valve. 102. SEM external view of an

entire rapheless valve. 103. SEM internal view of an entire rapheless valve. Scale bars represent 10 pm.

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VAN DE VIJVER B. & KOPALOVA K., Achnanthidium minutissimum complex in the Antarctic

Scanning electron microscopy (Figs 100-103)

Valve face of the raphe valve clearly concave. Valve face/mantle margin forming a weakly raised

hyaline border separating valve face striae from mantle areolae. Striae, ca. 32-34 in 10 pm, composed

of 2-3 small, rounded areolae (Fig. 100) followed near the valve center by a thin, slit-lilce, marginal

areola (Fig. 100). Near the apices, striae more dense, ca. 40 in 10 pm, composed of only 3 rounded

areolae. Mantle areolae slit-like (Figs 100, 102). Raphe almost straight with simple, straight proximal

raphe endings (Fig. 100). Distal raphe fissures weakly deflected, continuing slightly beyond the last

striae terminating on the valve face/mantle junction. Internally, proximal raphe endings shortly bent

into opposite directions (Fig. 101). Distal raphe endings terminating in small helictoglossae (Fig. 101).

Internal areolae openings covered by hymenes (Fig. 101). Striae of the rapheless valve almost equally

spaced throughout the entire valve, 34-36 in 10 pm, composed of 2-5 rounded areolae sometimes

terminating in one slit-like areola (Fig. 102). Mantle areolae slit-like. Internal areolae openings covered

by hymenes (Fig. 103).

Ecology, distribution and associated diatom flora

Achnanthidium sieminskae was originally described as a brackish-littoral taxon from a tidal flat near

the Kerguelen Islands (Witkowski et al. 2012) but the authors already expressed some doubts about this

observation. Most likely, their population was the result of a riverine deposition in the marine Bossiere

fjord on Kerguelen. As they stated themselves, the only way to find out would be the analysis of the

living material which was unfortunately no longer available at the time of their publication. We reported

this taxon (as A. minutissimum) from a large number of purely freshwater habitats ranging from small

pools to larger lakes and even streams from all sub-Antarctic islands in both the Indian and Atlantic

Ocean (Van de Vijver & Beyens 1996; Van de Vijver et al. 2001, 2002, 2004, 2008). In many cases,

very large populations could be observed, occasionally up to 60-80% of all counted valves. These data

indicate that this is typical limno-terrestrial taxon preferring wet moss vegetations in pools, bogponds

and lakes with a pH between 5.2 and 7.4 and a low conductivity (< 160 pS/cm). These data confirm

the final conclusion in Witkowski et al. (2012) that the observed valves in their study are the result of

the redeposition of a freshwater species in a marine environment. Given the large amount of available

data and material, it is a pity the original authors did not verily their rather unusual observation more

carefully.

Discussion

All taxa discussed here clearly belong to the complex of species around Achnanthidium minutissimum

based on the simple, straight distal raphe endings contrary to the species in the A. pyrenaicum group

that have clearly deflected to even hooked distal raphe fissures (Kobayashi 1997; Potapova & Hamilton

2007). In the (even recent) past, all taxa discussed were identified as Achnanthidium minutissimum (e.g.,

Oppenheim 1994; Van de Vijver et al. 2002; Fe Cohu 2005; Kopalova & Van de Vijver 2013). Table 2

summarizes all important morphological features of all four taxa formerly identified as A. minutissimum.

The application of a more fine-grained taxonomy based on a narrow species concept (Mann 1999),

the detailed analysis of type material of so-called cosmopolitan taxa and as a consequence no longer

force-fitting (Tyler 1996) and taxonomic drift, clearly influenced our understanding of the biogeography

of Antarctic diatoms as shown in the taxonomic revision of several genera such as Muelleria (Van de

Vijver et al. 2010), Luticola (Van de Vijver & Mataloni 2008, Van de Vijver et al. 2011c), Hantzschia

(Zidarova et al. 2010) and Navicula (Van de Vijver et al. 20lid). A similar conclusion can now be made

for the genus Achnanthidium confirming once more the unique nature of the Antarctic diatom flora and

contradicting the Ubiquity hypothesis (Finlay & Clarke 1999) stating the cosmopolitan nature of all

micro-organisms. As a consequence, the genus Achnanthidium shows some interesting biogeographical

aspects. At present, only six Achnanthidium taxa are known from the Antarctic Region. Apart from the

four taxa discussed in this paper, two other Achnanthidium taxa, Achnanthidium modestiforme (Fange-

European Journal of Taxonomy 79: 1-19 (2014)

Table 2. Overview of the main morphological features of all four taxa formerly identified as A.

minutissimum s.l. reported from the Antarctic Region.

A. indistinctum

A. maritimo-

antarcticum

A. lailae

A. sieminskae

Valve length (pm)

8.5-13

12-15

10-14

9.5-18

Valve width (pm)

1.8-2.2

2.3-27

1.8-2.5

1.9-3.1

valve outline

narrowly lanceolate

linear-lanceolate

linear to very

slightly linear-

lanceolate

linear to narrowly

linear-lanceolate

apices

distinctly

protracted, rostrate

clearly protracted,

rostrate,

occasionally

subcapitate

non-protracted,

broadly rounded,

never rostrate or

capitate

clearly protracted,

rostrate to even

capitate

Rapheless valve

# striae in 10 pm

30-35

30-32

28-30

ca. 35

# areolae per stria

3-4, rounded to

rectangular

2-5, irregularly

shaped

2-3, rounded to

slit-like

2-5, rounded +

sometimes 1 slit-like

striation pattern

slightly radiate

throughout

slightly radiate

throughout

parallel to weakly

radiate near the

valve centre, more

radiate near the

apices

Striae weakly radiate

throughout, becoming

more parallel to even

convergent near the

apices

Raphe valve

# striae in 10 pm

ca. 36

30-33, up to

40^12 near the

valve apices

30-33

ca. 32-34

# areolae per stria

2-3

2-3, rounded

2-3, small

2-3, small, rounded,+

near the valve center

1 slit-like

striation pattern

moderately radiate

throughoout

slightly radiate

throughout, more

radiate towards

the apices

weakly but still

distinctly radiate

near the valve

center, becoming

more radiate near

the apices

weakly radiate near

the valve center,

becoming parallel

and even convergent

near the apices

(sub-) Antarctic

distribution

South Shetland

Islands

South Shetland

Islands

James Ross Island

South Georgia, Prince

Edward Islands,

lies Crozet, lies

Kerguelen, Heard

Island

_ r

VAN DE VIJVER B. & KOPALOVA K., Achnanthidium minutissimum complex in the Antarctic

Bert.) Van de Vijver in Van de Vijver et al. (Van de Vijver et al. 2002) and A. australexiguum Van de

Vijver in Taylor et al. (Taylor et al. 2014), a taxon recently described in the complex of species around

A. exiguum (Grunow) Czarnecki (Czamecki 1994), were confirmed from the Region. All six taxa are so

far exclusively found in the Antarctic Region despite records of A. modestiforme from Argentina, Bolivia,

Brazil and Chile (Maidana 1996; Diaz Villanueva & Maidana 1999; Rumrich et al. 2000; Tremarin et

al. 2009; Morales et al. 2011) that are clear examples of force-fitting and most likely represent other, yet

undescribed taxa. Two of these six taxa have a distribution that is at present only restricted to the sub-

Antarctic Region (including all islands and archipelagos in the southern Indian Ocean and South Georgia

in the southern Atlantic Ocean): A. sieminskae and A. modestiforme , lacking at present any confirmed

records from other parts of the Antarctic Region. On the other hand, the Marit im e Antarctic Region

counts four Achnanthidium taxa (A. lailae, A. maritimo-antarcticum sp. nov., A. indistinctum sp. nov.

and A. australexiguum ), all of which so far never observed outside this Region. It is however unclear

whether this biogeography represents the complete distribution of these taxa or is just the consequence of

undersampling and underreporting. Kellogg & Kellogg (2002) list almost 40 records of Achnanthidium

minutissimum (usually as Achnanthes minutissima ) from mostly the Maritime Antarctic Region. All

records should be verified to complete our biogeographical knowledge of the species discussed in this

paper. Most literature records lack however (good) illustrations and even the retrieval of the original

material is not always very straightforward making new sampling in these areas a more obvious choice

to clarify the biogeography of these taxa.

Although molecular data would be interesting to confirm the separation of the taxa of the A. minutissimum

complex, it is clear that all taxa discussed here are morphologically distinct from each other andare not

conspecific with the former catch-all taxon A. minutissimum. Potapova & Hamilton (2007) already

presented some SEM images of the type material of A. minutissimum s.s. Based on these published

images and additional (but so far unpublished) observations on the type material of A. minutissimum

(Novais et al. submitted), it is clear that none of the observed populations in the Antarctic Region

represent A. minutissimum s.s.

Achnanthidium indistinctum can be separated from A. minutissimum based on several features

(Potapova & Hamilton 2007; Hlubilcova et al. 2011). Morphometrically, A. indistinctum has clearly

narrower valves (1.8-2.2 pm vs. 2.5-3.1 pm in A. minutissimum). Additionally, A. minutissimum has a

higher number of areolae per stria, usually 4-5 (Novais et al. submitted), a high number never observed

in A. indistinctum where 2—3(—4) is the rule. The mantle areolae in A. indistinctum are often enlarged

making them easily visible in LM on frustules in girdle view, similarly to for instance A. hoffmannii

Van de Vijver et al. (Van de Vijver et al. 2011a). Finally, the valve outline of A. minutissimum shows a

combination of more convex valve margins together with clearly rostrate to subcapitate apices, compared

to the weakly compared convex margins and weakly rostrate margins in A. indistinctum.

The second new taxon, A. maritimo-antarcticum can be separated from all other Achnanthidium taxa

by its more linear valve outline in combination with the protracted, broadly rounded apices. The striae

are solely composed of rounded to transapically elongated areolae, never presenting slit-like areolae as

often found in several Achnanthidium taxa such as A. hoffmannii , A. minutissimum and A. lailae.

Zidarova et al. (2009) already discussed the differences between A. lailae , A. minutissimum and several

other narrow, linear Achnanthidium taxa such as A. strictum Reichardt (Reichardt 2004) and A. lineare

W.Sm. (Smith 1855). The analysis of additional populations of A. lailae on Clearwater Mesa, James

Ross Island, confirm entirely the original separation of this taxon from the above mentioned taxa

A. minutissimum , A. strictum and A. lineare , based on the weakly deflected distal raphe fissures and

the higher number of areolae per stria in A. lailae. The only feature that need to be adjusted compared

European Journal of Taxonomy 79: 1-19 (2014)

to the original description, is the presence of slit-like areolae near the valve margins. In the original

description, the absence of these slit-like areolae was mentioned (Zidarova et al. 2009).

Our observations on the often very large freshwater populations of Achnanthidium sieminskae from lies

Kerguelen refine our knowledge on the morphology of this species. According to the original description

(Witlcowski et al. 2012), the number of areolae per stria on the rapheless valve was set to 3, sometimes 2

areolae although in the accompanying table, 3-5 areolae are mentioned. Based on our results, and a more

careful reanalysis of the illustrations in Witkowski et al. (2012), it is clear that the striae are composed

of 2-5 rounded, sometimes slit-lilce areolae. Moreover, the populations observed on lies Kerguelen and

the other sub-Antarctic islands in the southern Indian Ocean, fine-tuned the morphometric data for this

species. The observed length and width range in the freshwater and limno-terrestrial populations is larger

than in the original description. Some raphe valves seem to lack one or two striae in the central area. The

typical presence of 1-2 striae in the central area of A. sieminskae was used by Witkowski et al. (2012) as

a discriminating feature to separate the species from A. minutissimum. It is clear that this feature should

no longer be used. The separation of both taxa can also no longer be based on the denser striae near the

apices as analysis of the Kiitzing type material of A. minutissimum (Novais et al. submitted) showed the

presence of dense striae near the apices in the latter. Conspecificity of both taxa is however to be excluded

but the separation of both taxa is therefore only possible based on the valve outline with more linear

to linear-lanceolate valves (contrary to the typical lanceolate valves in A. minutissimum ), the presence

of clearly rostrate to capitate apices and differences in the number of areolae on the rapheless valve. In

their analysis of the type of A. minutissimum , Novais et al. (submitted) show a rapheless valve having

at least 5-6 rounded to weakly elongated areolae per stria whereas in the A. sieminskae valves shown in

this paper, only 2-3 (M) rounded and one slit-lilce areola are shown. It is also clear that the raphe in A.

sieminskae is usually more distinctly visible in TM than in A. minutissimum. Achnanthidium sieminskae

shows some similarity to a few other (usually recently described) Achnanthidium taxa. Witkowski et al.

(2012) compared their new species with A. jackii Rabenh. (Rabenhorst 1861) and A. lailae , although

any confusion with the latter is entirely to be excluded based on valve outline, dimensions and stria

structure. Achnanthidium ertzii Van de Vijver & Tange-Bert. in Van de Vijver et al. (Van de Vijver et al.

2011a), described from Madeira, shows a similar elongated valve outline but has narrower valves with a

lower number of areolae per stria (2-3 vs. 3-5 in A. sieminskae ) making conspecificity highly unlikely.

Achnanthidium maritimo-antarcticum , described in this paper, has comparable valve dimensions but

a more linear-lanceolate valve outline (compared to the typical lanceolate outline in A. sieminskae ), a

lower number of areolae per stria (max. 3) and less protracted, rostrate to subcapitate apices. The correct

taxonomic and morphological identity of other Achnanthidium taxa such as A. caledonicum (Lange-

Bert.) J.E.Slate & R.J.Stevenson (Slate & Stevenson 2007) is at present unclear making a comparison

between A. sieminskae and these taxa rather difficult. Achnanthidium caledonicum as it is currently

known, is much longer and has clearly capitate apices.

Conclusions

The present paper highlights the need for thorough morphological revisions of the entire Antarctic

freshwater and limno-terrestrial diatom flora applying a narrower species concept. The description of

two new Achnanthidium taxa split off from Achnanthidium minutissimum s.l. improves our knowledge

about the biogeography of species in this complex and allows their use in more precise ecological and

biogeographical studies.

Acknowledgements

The authors wish to thank Niek J.M. Gremmen and Touis Beyens for the collection of samples on Heard

Island, Prince Edward Islands and South Georgia. Sampling on Crozet and Kerguelen has been made

possible thanks to the logistic and financial support of the French Polar Institute-Paul-Emile Victor

_ r

VAN DE VIJVER B. & KOPALOVA K., Achnanthidium minutissimum complex in the Antarctic

in the framework of the terrestrial program 136 (Marc Lebouvier & Yves Frenot). Samples on Byers

Peninsula were taken in the framework of the IPY-Limnopolar Project POL2006-06635 (Ministerio de

Ciencia y Tecnologia, Spain). The authors would also like to thank the members of expeditions to the

Czech J.G. Mendel Antarctic Station for field support and assistance. K. Kopalova benefited from the

GA UK grant nr. 394211. Part of the research was funded within the Belspo CCAMBIO project and an

EU Synthesys grant to BVDV to visit the National History Museum in London, UK. Alex Ball, the staff

of the EMMA laboratory and Eileen J. Cox at the Natural History Museum are thanked for their help

with the scanning electron microscopy.

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Manuscript received: 4 November 2013

Manuscript accepted: 26 February 2014

Published on: 7 April 2014

Topic editor: Thomas Janssen

Desk editor: Natacha Beau

Printed versions of all papers are also deposited in the libraries of the institutes that are members of the

EJT consortium: Museum National d’Histoire Naturelle, Paris, France; Botanic Garden Meise, Belgium;

Royal Museum for Central Africa, Tervuren, Belgium; Natural History Museum, London, United

Kingdom; Royal Belgian Institute of Natural Sciences, Brussels, Belgium; Natural history Museum of

Denmark, Copenhagen, De nm ark