NOTA

 

The nomenclatural history of Didymosphenia geminata (Lyngbye) M. Schmidt in Schmidt et al. (Bacillariophyta) and related taxa

 

La historia nomenclatural de Didymosphenia geminata (Lyngbye) M. Schmidt et al. (Bacillariophyta) y taxones relacionados

 

 

Saúl Blanco¹ & Luc Ector²

¹Department of Biodiversity and Environmental Management, University of León. E-24071 León, Spain. Current address: The Institute of the Environment. La Serna, 58, E-24007 León, Spain
²Department of Environment and Agro-biotechnologies (EVA), Public Research Centre - Gabriel Lippmann. Rue du Brill 41, L-4422 Belvaux, Luxembourg

e-mail: sblal@unileon.es

Recibido: 23 de septiembre 2013; Aceptado: 8 de octubre 2013

 

 

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The diatom genus Didymosphenia was erected by M. Schmidt in Schmidt et al. [36], although the question whether Didymosphenia was proposed as a genus or as a subgenus of Gomphonema remained unclear [13][40] and was only settled by the conservation of the genus name Didymosphenia M. Schmidt [22]. This genus originally comprised three species: (1) Didymosphenia sibirica (Grunow) M. Schmidt in Schmidt et al. ≡ Gomphonema geminatum var. sibiricum (“sibericum”) Grunow ≡ Gomphonema sibiricum (“siberica”) (Grunow) Mills non Gomphonema sibiricum (“sibirica”) Skvortzow & K.I. Meyer, (2) Didymosphenia curvirostra (“curvirostrum”) (Tempère & Brun) M. Schmidt ≡ Gomphonema geminatum var. curvirostra (“curvirostrum”) (Tempère & Brun) Cleve ≡ Gomphonema curvirostra (“curvirostrum”) Tempère & Brun, and (3) Didymosphenia geminata (Lyngb.) M. Schmidt ≡ Echinella geminata Lyngb. ≡ Gomphonema geminatum (Lyngb.) C. Agardh. However, no generitype was designated until the Special Committee for Bacillariophyta [39] designated Didymosphenia geminata (Lyngb.) M. Schmidt as type of the genus. This type is conserved since Didymosphenia is conserved against its nomenclatural (homotypic) synonym Dendrella Bory and its taxonomic synonym Diomphala Ehrenb. [22]. Gomphonema geminatum is also the type species designated and conserved by the former ICBN [23] for the genus Gomphonema C. Agardh (rejected versus Gomphonema Ehrenb., nom. cons.) with Gomphonema acuminatum Ehrenb. as its conserved type [22][23].

Currently, Fourtanier & Kociolek [14] list a total of 10 species validly described in or transferred to Didymosphenia : (1) Didymosphenia clavaherculis (Ehrenb.) Metzeltin & Lange-Bert. ≡ Diomphala clavaherculis Ehrenb., (2) Didymosphenia curvata (Skvortzow & Meyer) Metzeltin & Lange-Bert. ≡ Didymosphenia geminata var. baicalensis f. curvata Skvortzow & K.I. Meyer, (3) Didymosphenia curvirostra (Tempère & Brun) M. Schmidt (“curvirostrum”) (see above); (4) Didymosphenia dentata (Dorogostaisky) Skvortzow & K.I. Meyer ≡ Gomphonema dentatum Dorogostaisky, (5) Didymosphenia fossilis Horikawa & Okuno, (6) Didymosphenia geminata (Lyngb.) M. Schmidt, (7) Didymosphenia lineata Skabichevskii, (8) Didymosphenia pumila Metzeltin & Lange-Bert., (9) Didymosphenia sibirica (Grunow) M. Schmidt, and (10) Didymosphenia sublinearis Shirshov (actually absent in Shirshov [37], possibly a nomen nudum). Munda [28] also mentions “Didymosphenia borealis”, but this name does not seem to appear anywhere else, it is a nomen nudum or a mistake. Finally, a new species, Didymosphenia tatrensis Mrozińska, Czerwik-Marcinkowska & Gradziński has been described from the Western Carpathians [27]. With respect to Diomphala clavaherculis, Metzeltin & Lange-Bertalot [25] and Jahn & Kusber [20] treated this taxon as a clearly defined species not conspecific with Echinella geminata.

Adding to Gomphonema geminatum and Echinella geminata, Dendrella geminata (Lyngb.) Bory and Lyngbyea pulvinata var. geminata (Lyngb.) Sommerfelt have been identified as nomenclatural synonyms of Didymosphenia geminata. Kützing [21] provided the following list of taxonomic or nomenclatural synonyms: Gomphonema herculeanum Ehrenb., Gomphonema pyriferum Suhr, Gomphonema intricatum Suhr, Gomphonema radicula Suhr, Gomphonema ampullaceum Grev. ≡ Echinella ampullacea Carmichael (nomen in Greville [16]) (also synonymized by Ralfs [34], Smith [38], Rabenhorst [33], Brun [37] and VanLandingham [43]), Gomphonema geminatum Grev., Crystalia (“Crystallia”) pulvinata Sommerfelt in Agardh (also synonymized by Rabenhorst [32], but Prollius [31] did not accept the synonymy), Dendrella lyngbyei Bory (also synonymized by Bory [6][7] and Drapiez [12]), Dendrella geminata (Lyngb.) Bory, and Vorticella pyraria O. Müll. ≡ Gomphonema pyrarium (O. Müll.) Ehrenb. (also synonymized by Agardh [1][2], Greville [15] and Dawson [10]). Rabenhorst [32] additionally considered Dendrella styllarioides Bory and Gomphonella olivacea Rabenh. as synonyms of Gomphonema geminatum (see the discussion about genus Gomphonella Rabenh. in Fourtanier & Kociolek [13]). Gomphonema herculeanum was later rejected as a synonym of Gomphonema geminatum and combined with Gomphoneis Cleve by Cleve in 1894 [13]. Concerning Suhr’s species, Kützing [21] seems to be the only one to have seen these diatoms, all later authors always citing Suhr “teste Kützing”; since these names are always cited as synonyms, they are not validly published and should be ignored. Gomphonema ampullaceum is regarded as an independent taxon by Harvey [18]. Vorticella pyraria is considered as synonym of Dendrella geminella Bory (and possibly of Dendrella styllarioides), but not of Echinella geminata in Bory [6][7] and Drapiez [12], and as a former synonym of Gomphonema truncatum Ehrenb. by Pritchard [30], opinion followed by many other authors. Guiry & Guiry [17] regard Gomphonema vulgare Bréb. in Brébisson & Godey as a synonym of Didymosphenia geminata. Lawson [24] mentions “Gomphonema coquedense Lang” (nomen nudum) as a possible synonym of Gomphonema geminatum. Finally, Antoine & Benson-Evans [3] presented an emended diagnosis of the generitype.

The tracking of diatom synonyms back to the XIX century is obscured by the lack of the precise descriptions and/or accurate illustrations available through modern microscopic techniques; e.g., Vorticella O. Müll. was treated as an animal in several early monographs (e.g., Pritchard [29][30], Deshayes & Edwards [11]), and is currently recognized as a genus of ciliates. Echinella Lyngb. (non O. Müll.) was also established for many forms that were not diatoms [43]. Moreover, in several cases the type material of these taxa has been lost or remains unavailable, which leaves their assignation as taxonomic synonyms in suspenso. In many cases, they could be considered only as synonyms pro parte.

Additionally, up to 26 infraspecific taxa have been validly described within Didymosphenia geminata [5], mainly published in the works of Skvortzow and Meyer about the lake Baikal. Metzeltin & Lange-Bertalot [25] rearrange all these into five species, accommodating many of the previously described varieties and forms into three different “morphotypes” of D. geminata (“geminata”, “capitata” and “subcapitata”). However, this classification, followed by few subsequent authors, is based on recent samples and original descriptions (and not on type materials), and thus might be not definitive [26]. The following taxa, originally described as varieties of Gomphonema geminatum, are not considered in detail in this monograph and deserve further comments:

Gomphonema geminatum var. hybridum (“hybrida”) Grunow in Van Heurck (Fig. 1A): this taxon lacks description, and only a single drawing is provided by Grunow in Van Heurck’s Atlas [42]. Dawson [10] suggested a connection with D. sibirica, but Metzeltin & Lange-Bertalot [25] did not cite G. geminatum var. hybridum in their list of synonyms, but they did it in that of D. curvata, with the addition of a question mark. This variety seems to represent a transitional form between D. sibirica and D. curvata according to the figures provided in Metzeltin & Lange-Bertalot [25]. Until the morphological variability of this taxon can be resolved with new findings, the combination of this taxon under Didymosphenia curvata is not recommended at this stage.

Gomphonema geminatum var. bipunctatum (“bipunctata”) Rattray (Fig. 1B): Dawson [10] hypothesized the conspecifity of this variety and “Didymosphenia fossilis”, a taxon presumably described in Barnard & Welch [4], although this short paper actually does not deal with gomphonemoid species. In any case, the micrograph of the lectotype of Didymosphenia fossilis Horikawa & Okuno [41] clearly shows a different taxon. Rattray [35] presents a detailed description of G. geminatum var. bipunctatum, evidencing a number of differences with respect to the nominal variety as illustrated in Smith [38]. The low stria density reported for this variety (6 in 10 µm) cannot be found in any of the taxa analyzed in Metzeltin & Lange-Bertalot [25], supporting therefore its status as an independent taxon. No further references were found in the literature of this taxon, considered “rare” in Rattray’s type material (diatomite from Loch Osabath, Isle of Lewis, Outer Hebrides).

Gomphonema geminatum var. norvegicum (“norvegica”) Holmboe [25] (Fig. 1C): the iconotype of this variety fits well with characteristic features of Didymosphenia, although the shape clearly differs from any other taxon described within this genus. In particular, the broad foot pole, giving the entire valve an isopolar outline, seems to be typical for this variety. According to the author, G. geminatum var. norvegicum exhibits a consistent morphology throughout all populations examined (five samples from Mjøsa and Grudbrandsdalen, Norway), so that it cannot be regarded as a “pseudo-taxon” sensu Metzeltin & Lange-Bertalot [25]. However, no more information was found in the bibliography advising against an eventual transfer to genus Didymosphenia.

 

Acknowledgements

P. Compère (National Botanic Garden of Belgium) is thanked for his insightful comments on a previous draft of this paper. Two anonymous reviewers kindly revised the manuscript.

 

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