"Rediscovery" of Encyonema ratpanati sp. nov. (Bacillariophyta, Cymbellaceae) in Victoria Falls, Namibia
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Bol. R. Soc. Esp. Hist. Nat., 115, 2020: pediente de paginación
“Rediscovery” of Encyonema ratpanati sp. nov.
(Bacillariophyta, Cymbellaceae) in Victoria Falls, Namibia
“Redescubrimiento” de Encyonema ratpanati sp. nov. (Bacillariophyta,
Cymbellaceae) en las Cataratas Victoria, Namibia
Adrián Llamazares, Eloy Bécares & Saúl Blanco
Laboratorio de diatomología. La Serna 58,
24007, León, España.
sblal@unileon.es
Recibido: 17 marzo de 2021, Aceptado: 27 de abril de 2021.
Publicado electrónicamente: 1 de mayo de 2021.
Keywords: Diatom, New species,Taxonomy, Namibia, Africa.
Palabras clave: Diatomea, Nueva especie,Taxonomía, Namibia, África.
Abstract
An Encyonema population collected in the Zambeze River near the Victoria Falls, Namibia,
is here described and illustrated in detail with the aid of LM and SEM microscopy.This taxon had
already been presented in the literature under the name E. volkii, the reasons leading to treat
this species as a new, independent taxon are here discussed. Encyonema ratpanati sp. nov. can be
easily distinguished from E. volkii by its more linear outline (length-to-width ratio up to 4.3 vs.
up to 3.5 in E. volkii), its lower degree of dorsiventrality, lack of rostrate apices throughout the
whole diminution series, wider axial area and denser areolation (>26 lineolae per 10 µm and
not 20–22 as in E. volkii).
Resumen
Se describe una población de Encyonema recogida en el río Zambeze, cerca de las cataratas
Victoria, Namibia, y se ilustra en detalle con la ayuda de microscopía óptica y electrónica. Este
taxón ya había sido presentado en la literatura bajo el nombre de E. volkii, se discuten aquí
las razones que llevan a tratar esta especie como un nuevo taxón independiente. Encyonema
ratpanati sp. nov. Puede ser fácilmente diferenciado de E. volkii por su contorno más lineal
(cociente largo/ancho de hasta 4.3 vs. hasta 3.5 en E. volkii), su menor grado de dorsiventralidad,
la ausencia de ápices rostrados a lo largo de toda la serie decreciente, el área central más ancha
una areolación más densa (>26 lineolas cada 10 µm y no 20–22 como en E. volkii).
1. Introduction
The genus Encyonema Kützing, 1834 is a relatively diverse group of freshwater
diatoms, with about 300 species currently accepted in the scientific literature (Guiry,
2021), most of them transferred from the genus Cymbella C.Agardh, 1830 or described
as new species in the monograph by Krammer (1997). This group of biraphid diatoms
is mainly characterized by a more or less developed degree of dorsiventrality, external
terminal raphe endings bent towards the ventral side, and—generally—lack of stigmata
or apical pore fields. Despite having been already proposed as a distinct genus in the
XIX century (Kützing, 1833), most subsequent authors treated Encyonema taxa within
Cymbella, merely distinguished by their their ability to form colonies in mucilage tubes.
However, this feature is actually present in few Encyonema species, most of them living in
mucous coatings on plants, wood or stones. It was not until 1982 when Encyonema was
resurrected first as a subgenus (Krammer, 1982) and then recognized again as a genus,
with an emended description (Round et al., 1990).The generitype E. paradoxum Kützing,
1834 resulted to be a later taxonomic synonym of E. leibleinii (C.Agardh) W.J.Silva et al.,
2013, which retains nomenclatural priority (Silva et al., 2013).
Despite constituting a strongly supported monophyletic group (Kermarrec et al.,
2011), there is a large heterogeneity among Encyonema members in terms of morphology
as well as growth habit (Liu et al., 2021), so that taxonomy has undergone many changes
doi: 10.29077/bol.115.ce03.llamazares ISSN: 2659-2703 -5-A. Llamazares, E. Bécares & S. Blanco
(Silva & Souza, 2015). Despite molecular data suggested a closer connection to the
gomphonemoid clade (Bruder & Medlin, 2007), the emplacement of Encyonema in the
Cymbellaceae is now considered clear (Nakov et al., 2014), in any case phylogenetically
more primitive than other cymbelloid forms (Pappas, 2005).
Prof. Ditmar Metzeltin (in Lange-Bertalot, 1993: pl. 134, figs 4-10) illustrated an
Encyonema population collected in Kunene River (Namibia/Angola), assigned in that
reference to the taxon Cymbella volkii Rumrich et al., 1993. In his reappraisal of the
cymbelloid diatoms, Krammer (1997) reconsiders this population under the name
“Encyonema sp.”, separated or, at least, doubtfully conspecific with (see the question
mark in pp. 80) E. volkii (Rumrich et al.) Krammer, 1997. Here we present evidences
supporting this segregation, describing and illustrating Encyonema ratpanati as a new
species from samples collected near the Victoria Falls, Namibia.
2. Materials and Methods
A sample of epilithic algae was collected on littoral rocks in the Zambeze River,
Zambia/Zimbabwe/Namibia border (17.92° S, 25.85° E, Figure 1), 50 m away from
Victoria Falls. The sample fixed with weak formalin (about a 0.5% final concentration
of formaldehyde) to stop biological
activity in the sample. A suspension
of clean diatom frustules was obtained
from that sample in the laboratory
by oxidation with hot (70–90 °C)
hydrogen peroxide (30% v/v). Some
drops of hydrochloric acid (3 M)
were added to remove calcium
carbonate inclusions. Light microscopy
(LM) slides were mounted using
Naphrax®. The identification of the
diatom species were carried out at
1000× magnification using a DIC light
(LM) Olympus BX60 microscope
equipped with an OPTIKA camera. A
subsample was analyzed by scanning
electron microscopy (SEM) at the
Electron Microscopy Unit (University
of Jaen, Spain), by placing a drop of
the cleaned sample on a conductive
metal structure and allowing it to dry
at room temperature. The samples
were subsequently coated with a 10
nm thick gold layer using a modular
high vacuum metallization system Figure 1. Type locality of E. ratpanati sp. nov. in the Zambeze River,
Namibia.
(QUORUM Q150T ES). SEM images
were obtained using a MERLIN (Carl
Zeiss) microscope operating at 20 kV. Images were processed with GIMP software
(Chastain & Pfaffman, 2006).
3. Results
Classification:
Phylum Bacillariophyta
Class Bacillariophyceae
Order Cymbellales
Family Cymbellaceae
Encyonema ratpanati sp. nov. (Figures 2, 3).
= Encyonema volkii pro parte (excl. typus) sensu Lange-Bertalot (1993, as “Cymbell
volkii”, figs 139: 4–10), sensu Krammer (1997, as “Encyonema sp.”, fig. 69: 15), sensu Krammer
(1997b, as “Encyonema volkii”, figs 191: 1, 2?, 3–5).
-6- Comunicaciones especializadas“Rediscovery” of Encyonema ratpanati sp.nov. (Bacillariophyta, Gomphonemataceae)
Diagnosis: valves lanceolate to
subrhombic, somewhat dorsiventral,
with a ventral margin less arched
than the dorsal and slightly protruded
(Figure 2). Apices acutely rounded,
not protracted. Valve size (median
[quartiles]): length 37.3 [37.1–37.8] µm,
width 10.7 [10.5–10.9] µm. The axial
area is widely lanceolate, broadening
at midvalve but with no distinct central
area (Figure 2). Stigmae or stigmoids
absent. Raphe branches filiform near
valve apices, broader in median portion
(Figure 3A). Terminal raphe fissures
Figure 2. Encyonema ratpanati sp. nov. LM. Individuals from the type externally curved towards the ventral
population in valve view. Scale bar: 10 µm. margin, sometimes ending in valve
mantle (Figure 3B). Central pores
slightly bent dorsally, with a T-shaped
intermissio (Figure 3C). Transapical
striae subradiate, 5.8 [5.3–6.0] in 10
µm, equally spaced throughout the
valve, formed by elongated lineolae
(Figure 3), 27 [26–29] in 10 µm.
Type: NAMIBIA: Zambeze River,
near Victoria Falls, on rocks. Coll. E.
Bécares, 10-VIII-2018. Holotype: LEB!
DIATOMEA-29.
Etymology: named after Ratpanat
Expeditions SL, who organized the
trip.
Accompanying taxa: Encyonema
ratpanati occurred along with Cavinula
Figure 3. Encyonema ratpanati sp. nov. SEM. Individuals from the type scutelloides (W.Smith) Lange-Bertalot,
population. A. Complete frustule in external oblique view. The
striae continue throughout the mantle. The valve/mantle transi- 1996, Mastogloia danseyi (Thwaites)
tion is soft. The raphe slit is wider in midvalve. B. Detail of the Thwaites, 1856, Placoneis apicalicostata
apex and the valvocopulae. The distal raphe fissure is curved Metzeltin & Lange-Bertalot, 2002,
towards the ventral side and ends externally in the mantle. C. and Sellaphora americana (Ehrenberg)
Internal valve view of a theca. Note the T-shaped intermissio. D.
Habitus. Note the perforated copula in the ventral side (frustule D.G.Mann, 1990.
on the left).
Differential diagnosis: best
distinguished from Encyonema volkii by
its more linear outline (length-to-width ratio up to 4.3 vs. up to 3.5 in E. volkii), its lower
degree of dorsiventrality, lack of rostrate apices throughout the whole diminution series,
wider axial area and denser areolation (>26 lineolae per 10 µm and not 20–22 as in E.
volkii).The central protrusion is more evident also in E. ratpanati. Other Encyonema taxa
with overlapping morphometric data include E. alpinum (Grunow) D.G.Mann, 1990, with
a more elliptic outline (broader apices) and E. lacustre (C.Agardh) Pantocsek, 1901 (less
dorsiventral, with radiate striae). Of interest is also comparison with similar Encyonopsis
Krammer 1997 species such as Encyonopsis apiculata (Hustedt) Da Silva & Menezes,
2016 (with protracted apices), E. mendosa (Van Landingham) Da Silva & Menezes, 2016
(15 µm wide), E. lanceoelliptica Krammer, 1997 (less dorsiventral, wider axial area) or E.
dubitata (Cholnoky) Krammer, 1997 (larger length-to-width ratio).
Bol. R. Soc. Esp. Hist. Nat., 115, 2021 -7-A. Llamazares, E. Bécares & S. Blanco
4. Discussion
Lange-Bertalot et al. (1993) described Cymbella volkii from a sample
collected in 1988 by Dr. O.H. Volk in algal crusts on mica rocks at Gross
Barmen, Namibia. The type (‘Praep. Af-Fensteralgen 113 in Coll. Lange-
Bertalot, Botan. Institut Universitat Frankfurt a.M.’) is illustrated in the same
work (figs 134: 1–3), showing three individuals of an encyonemoid diatom
that fits well with the description provided. In the same plate, another
population collected at Kunene (Namibia/Angola border, figs 134: 4–10)
is presented which, although identified also as C. volkii, represents in our
opinion a different taxon, here described as E. ratpanati. Krammer (1997a)
intends to transfer C. volkii to the more suitable genus Encyonema, but fails
to make a full and direct reference to the basionym, the new combination is
effectively published in Krammer (1997b). In the first part of this monograph
(Krammer, 1997a) the type of C. volkii is again presented (as figs 69: 11–14,
note that the individual shown in Lange-Bertalot et al., 1993: fig. 134: 1 is the
same as Krammer’s fig. 69: 11), presenting a population that contrasts even
more markedly with E. ratpanati in terms of valve outline. Krammer (1997a)
includes two more specimens, one collected at Ngorongoro crater, Kenia
(figs 69: 9, 10), and other coming from ‘Kumene, Namibia’ [sic] (fig. 69: 15
= Lange-Bertalot et al., (1993): fig. 134: 4) which is identified in the plate as
“Encyonema sp.” (or as ‘E. volkii’ with a question mark in the corresponding
text, see pp. 80) and likely corresponds to E. ratpanati. This valve presents a
more linear shape, a wider axial area and a finer areolation that corresponds
rather to the features exhibited by E. ratpanati. Finally, Krammer (1997b)
illustrates another ‘E. volkii’ population (fig. 191: 1–5) from the Zambezi
River (Zambia) that also corresponds (with the possible exception of his Figure 4. Cymbella grossestriata
fig. 191: 2) with the species described in the present paper. The dubious var. curta. Iconotype,
conspecificity between the Gross Barmen type and the other populations, after Rich (1937). Scale
as already suggested by Krammer (1997a) can be definitely discarded after a bar: 10 µm.
closer examination of this new taxon (see above the differential diagnosis).
The diatom flora of Victoria falls had been already investigated (e.g.
Cholnoky, 1954; Hancock, 1979; Reichardt, 2007) but no records of E. volkii or
related species can be found. In his account of the diatom flora of the region, Rich
(1937) describes Cymbella grossestriata var. curta Rich, 1937 (actually an Encyonema,
Figure 4) with a close morphological resemblance with E. volkii (although smaller in
valve dimensions). Noteworthy, the images of E. volkii shown in Shinohara et al., (2014,
appendix S6, figs f, g) from Lake Malawi are compatible with the type material of this
species as illustrated in Krammer (1997a) and not with E. ratpanati. Apart from this and
the presence detected in the Buffelspoort Dam wall (Taylor et al., 2009), not illustrated,
no more occurrences of E. volkii exist to date.Thus, when the illustrations of E. ratpanati
published in the literature are excluded, the material available for E. volkii s. str. consists
only of few LM images, with no SEM data (the same also applies for the Ngorongoro
population), this points to the need of re-examining the type material of this taxon.
Acknowledgements
SEM images were kindly provided by Dr. A. Olenici. Technical and human support
provided by CICT of Universidad de Jaén (UJA, MINECO, Junta de Andalucía, FEDER)
is gratefully acknowledged. Two anonymous referees and the Editor are gratefully ack-
nowledged.
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