Odontidium Kützing (1844: 44)

Jüttner, Ingrid, Williams, David M., Levkov, Zlatko, Falasco, Elisa, Battegazzore, Maurizio, Cantonati, Marco, Vijver, Bart Van De, Angele, Catalena & Ector, Luc, 2015, Reinvestigation of the type material for Odontidium hyemale (Roth) Kützing and related species, with description of four new species in the genus Odontidium (Fragilariaceae, Bacillariophyta), Phytotaxa 234 (1), pp. 1-36 : 6-7

Odontidium Kützing (1844: 44)

Type species: Odontidium hyemale (Roth) Kützing (1844: 44 , pl. 17, fig. 4).

Taxonomy:— Williams (1985: 74, 124) provided a detailed diagnosis of both subgenera Diatoma and Odontidium : Odontidium species have a wider and diffuse sternum; a complex girdle structure with differentiation of the cingulum into three components with a larger number of structurally complex copulae and a distinct pleural band; non-functional spines, which are simple projections that can, but need not, result in a ribbon colony shape; and they have extra-elliptical lobed chromatophores ( Geitler 1967, Ettl & Brezina 1975, 1978). In Diatoma the sternum is narrow, spines are either absent or fewer in number and non-functional, and their colonies are predominantly zig-zag or, less frequently, starshaped; there are also differences in the attachment of the valvocopula to the virgae. The scanning electron microscope allowed an understanding of the spine structure and hence the type of colony formation achieved by each species. When the data were analysed cladistically, both Diatoma and Odontidium could be distinguished as independent entities and were as sister taxa relative to Meridion C. Agardh (1824 : xiv), another genus with heavily silicified transapical ribs, hence the conclusion that their status as sub-genera of Diatoma was appropriate ( Williams 1985).

A subsequent morphological analysis, with the addition of more taxa, also supported the separation of two groups corresponding to Diatoma and Odontidium , and also placed them as sister taxa to each other ( Williams 1990a: 93, fig. 1). Of significance was the fact that the heavily silicified transapical ribs, at first thought to be a defining character of a group comprising Diatoma , Odontidium and Meridion , was not, and was determined to have evolved more than once. First for a group comprising Diatoma and Odontidium , second for a group comprising Distrionella D.M. Williams (1990b: 175) , Meridion C. Agardh (1824 : xiv; both the iso- and heteropolar species, as understood in Williams 1985), Tabellaria Ehrenberg ex Kützing (1844: 127) and Tetracyclus Ralfs (1843: 105) , the ribs having been lost or modified in Tabellaria ( Williams 1990a: 93, fig. 1). In spite of Williams (1990a: 95) concluding that “There remains no justification for their [ Diatoma and Odontidium ] separation at generic level”, continuing morphological investigations yielded even more inconclusive results concerning the relationships between Diatoma and Odontidium and, in some cases, they were revealed as paraphyletic with respect to each other and the remaining freshwater araphid diatoms ( Williams 1994a). Furthermore, inconclusive relationships have emerged from several recent molecular studies ( Medlin et al. 2008: 91, fig. 2, Sato et al. 2008a: 387, Fig. 55 , 2008b: 523, fig. 38, Lobban et al. 2015: fig. 1). Therefore, while the relationships amongst araphid freshwater genera are still largely unknown, the two groups of species in Diatoma and Odontidium , which are recognised as genera or subgenera, appear to be well characterised and are best considered as two separate monophyletic groups that need not be sister taxa.

Nomenclature:— Kützing first described the genus Odontidium Kützing (1844: 44) , without identifying its type species, in his ‘Die Kieselschaligen Bacillarien oder Diatomeen’ as belonging to his “Familia III. Fragilarieae ”, alongside Denticula Kützing (1844: 43 , another genus described as new in this monograph), Fragilaria Lyngbye (1819: 182) and Diatoma Bory (1824: 461) , the latter being divided into two “parts”, “ Formae laevissimae ” and “ Formae striatae ” ( Kützing 1844: 47). Kützing’s description of Odontidium was short: “ Individua quadrangula, a latere secundario transversim striata, lanceolata, in fasciam biconvexam arcte conjuncta ” [Individuals quadrangular, with transverse striata (= transapical ribs) extending from one side to the other; lanceolate, colonies biconvex, closely linked]. He included eight species, Odontidium mesodon (Ehrenb.) Kützing (1844: 44) , O. turgidulum (Ehrenb.) Kützing (1844: 44) , O. hyemale , O. pinnatum (Ehrenb.) Kützing (1844: 44) , O. striolatum (Ehrenb.) Kützing (1844: 45) , O. syriacum (Ehrenb.) Kützing (1844: 45) (all six based on taxa previously described by Ehrenberg as belonging to the genus Fragilaria ), O. glaciale Kützing (1844: 44) , and O. glans (Ehrenb.) Kützing (1844: 45) . The latter was first described by Ehrenberg as a species of Navicula ( Bory 1822: 128) and subsequently as a species of Biblarium Ehrenberg (1843b: 47) and is currently known as Tetracyclus glans (Ehrenberg) F.W. Mills ; see Williams (1987). Odontidium glaciale Kützing was described as new by Kützing (1844: 44). In his Species Algarum, Kützing added a further 7 species, O. leptoceros (Ehrenb.) Kützing (1849: 13) , O. amphiceros (Ehrenb.) Kützing (1849: 13) , O. granulatum (Ehrenb.) Kützing (1849: 13) , O. pinnulatum (Ehrenb.) Kützing (1849: 13) , O. rotundatum (Ehrenb.) Kützing (1849: 13) , O. nodulosum Kützing (1849: 13) and O. polyedrum (Ehrenb.) Kützing (1849: 14) ( Kützing 1849: 13–14; Kützing added the following to the diagnosis of the genus: “ Incolae aqua dulcis ”).

Over the next five decades, a number of new species were added to the genus (e. g. Rabenhorst 1853, Smith 1856, Grunow 1862) and many nomenclatural changes were made relative to i) the perceived status of the taxon in question (species, variety, etc., e. g. Grunow 1862, Rabenhorst 1864); ii) the differences, or otherwise, between Odontidium and Diatoma (e. g. Pfitzer 1871, Schaarschmidt 1880, 1881); and iii) the priority issues concerning the two names Odontidium and Diatoma if considered synonyms ( Patrick 1939). Of the new species added, after revision (and with the aid of scanning electron microscopy), some have now been placed in other genera but many still require investigation. Much later, in a paper dealing primarily with the nomenclatural priorities of Diatoma versus Odontidium, Patrick assigned Odontidium hyemale (Lyngbye) Kützing as the type of Odontidium ( Patrick 1939: 2) for the following reasons: “As this is the only species of those which were described by Kützing under the name Odontidium when it was created, which stands today as described by him, I would like to designate it as the type species” ( Patrick 1939: 6). It is difficult to understand Patrick (1939) entirely as she neglected the combination Odontidium mesodon (Ehrenberg) Kützing (1844: 44) in the synonymy of Odontidium hyemale var. mesodon (Ehrenberg) Grunow (1862: 357 ; Patrick 1939: 6) and ignores any mention of Odontidium glaciale . The former taxon, however, is somewhat confusing as Kützing described in the same monograph the two different species Odontidium mesodon (Ehrenberg) Kützing (1844: 44) and Diatoma mesodon Kützing (1844: 47) , the former based on Fragilaria mesodon Ehrenberg (1840: 211) , the latter based on Kützing’s material; Odontidium glaciale has since been identified as Odontidium cf. hyemale ( BM 17898, [Monte Rosa glacier], the type slide for Odontidium glaciale ). Therefore, Odontidium hyemale remains the best choice for the type of the genus Odontidium , a fact recognised by Ross when compiling the Nomina Conservanda for the 1952 International Code of Botanical Nomenclature ( Ross 1952: 95; Lanjouw 1952: 70).

One final comment on Patrick is necessary: she cited as the basionym of Odontidium hyemale Lyngbye’s Fragilaria hyemale ( Lyngbye 1819: 185) adding that “In Lyngbye’s description of this [ Odontidium hyemale ] he refers to Conferva hyemale Roth. However , this description seems not to have referred to a single species” ( Patrick 1939: 6). We disagree and have detailed our conclusions below under our account of Odontidium hyemale ( Figs 2–7 : reproductions of published drawings, material, slides, Figs 8-60 : type material and other populations).

Figures

FIGURES 53–60. Odontidium hyemale, SEM. 53–55, 57–60. NMW.C.2015.001.Faroe.2014.5. Eysturoy, Faroe Islands, stream. 56. NMW.C.2012.019.2012.Austria.13.sto.bryo.filalg. Area of Schwarzmoos lakes near Kühtai, Stubaier Alps, Austria, spring. 53. View of whole valve and girdle bands. 54, 56. View of whole valve. 55, 57. View of whole frustules in girdle view. 58. Internal view of whole valve. 59. Details of valve mantle and girdle band. 60. Internal view of apical pore field and rimoportula. Scale bars = 2 μm (Fig. 57), 1 μm (Figs 53–56, 58–60).

FIGURES 2–5. Reproductions of published drawings and type material. 2. Drawings of “Fr[agilaria] hyemalis” from Lyngbye (1819: tab. 63 E. 1.Algae tuft. 2, 4–6. Enlarged filaments. 3. Dried enlarged filaments). 3. Packet and dried material from C (C-A-94656) labelled as Conferva hyemalis Roth, possibly original material and seen by Agardh. 4. Reproduction of Lyngbye’s description of “Fragilaria hyemalis” with reference to Conferva hyemalis Roth (Lyngbye 1819: 185). 5. Reproduction of drawings from Kützing (1844: pl. 17, IV, 1–4) of Odontidium hyemale, based on Lyngbye’s material (“Originalexemplare”, collection Senator Binder, from the Faröe Islands, Kützing 1844: 44).

FIGURES 6–7. Reproductions of published drawings and type material. 6. Reproduction of Ehrenberg’s drawings of Fragilaria turgidula Ehrenberg (1838: 204). 7. Type slides for Odontidium hyemale Kützing (Faröe, BM 17900), Odontidium turgidulum Kützing (Faröe, BM 17908), and Diatoma ‘hiemale/hyemale’ (Eulenstein, Diat. spec. typ. No. 49, BM 21049, neotype).

FIGURES 8–31. Odontidium hyemale, LM, valve views. 8–24. BM 21049, “Diatoma hiemale/hyemale, Eul. Diat. spec. typ. No. 49”, Faroe Islands, neotype. 25–31. BM 17900, “Odontidium hyemale Kütz., Lyngbye 1832. 49.”, Faroe Islands. Scale bar = 10 μm.

FIGURES 32–35. Odontidium hyemale, LM. 32. Girdle view and valve view. 33–35. Girdle views. 32–33. BM 21049, “Diatoma hiemale/ hyemale, Eul. Diat. spec. typ. No. 49”, Faroe Islands, neotype. 34–35. BM 17900, “Odontidium hyemale Kütz., Lyngbye 1832. 49”, Faroe Islands. Scale bar = 10 μm.

FIGURES 36–43. Odontidium hyemale, LM, valve views. NMW.C.2015.001.Faroe.2014.5. Eysturoy, Faroe Islands, stream. Scale bar = 10 μm.

FIGURES 44–52. Odontidium hyemale, LM, valve views. NMW.C.2012.021.2012.Italy.76. Gesso della Valletta, Regional Natural Park of the Maritime Alps, Italy, spring. Scale bar = 10 μm.