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Analysis of the type material of Achnanthidium jackii Rabenhorst (Bacillariophyta, Achnanthidiaceae) Bart Van de Vijver, Botanic Garden Meise, Research Department, Nieuwelaan 38, 1860 Meise, Belgium & University of Antwerp, Department of Biology – ECOBE, Universiteitsplein 1, 2610 Wilrijk, Belgium (corresponding author: bart.vandevijver@plantentuinmeise.be) Carlos E. Wetzel, Luxembourg Institute of Science and Technology (LIST), Environmental Research & Innovation (ERIN) Department, 41 rue du Brill, L-4422 Belvaux, Luxembourg Luc Ector, Luxembourg Institute of Science and Technology (LIST), Environmental Research & Innovation (ERIN) Department, 41 rue du Brill, L-4422 Belvaux, Luxembourg Achnanthidium jackii Rabenhorst (1861: n° 1003) was described in 1861 by Rabenhorst from decaying plants and algae from “Quellwasser bei Salem” (Municipality of Baden-Württemberg State, southern Germany). This species was named for the collector, Ap[othekar] Jack. The species was transferred several years later by Grunow to Achnanthes linearis var. jackii (Rabenhorst) Grunow (in Van Heurck 1880: pl. 27: fig. 24). Lange-Bertalot & Ruppel (1980: 20) lectotypified the species, choosing the exsiccata material 1003 (sub n° ZU 2/37), present in the Hustedt Collection (Bremerhaven, BRM) as the lectotype. The exsiccata published by Rabenhorst under the title Algen Europa’s (1861-1882) was distributed to a large number of herbaria and research institutes worldwide, each having today a (relatively) complete set of this material. Material of Achnanthidium jackii from the Salem source was studied several times by various authors although never in full detail. Lange-Bertalot & Ruppel (1980) showed two light micrographs (LM) (Lange-Bertalot & Ruppel 1980: figs 120, 121) from the lectotype, together with several other figures representing A. jackii from different localities. More light micrographs of the type appeared in Lange-Bertalot & Krammer (1989, pl. 51: figs 22–26’) and Potapova & Hamilton (2007, figs 4 i, j). The latter authors also showed four scanning electron microscopy (SEM) pictures (Potapova & Hamilton 2007, figs 3a–d). Unfortunately, to date a detailed analysis of A. jackii had not been made. Novais et al. (2015) analysed several varieties of the Achnanthidium minutissimum (Kützing) Czarnecki (Czarnecki 1994: 157) including Achnanthes minutissima var. cryptocephala Grunow (in Van Heurck 1880: pl. 27, figs 41–44). The latter, based on type material from Belgium (Van Heurck Type du Synopsis des diatomées de Belgique n° 238), was characterized by the presence of a fairly large fascia in the central area, protracted, rostrate to subcapitate apices and weakly radiate striae on both valves composed of 3–4, rarely two areolae. This variety was recombined as Achnanthidium neocryptocephalum (Grunow) Novais & Van de Vijver (in Novais et al. 2015: 116). The type population examined showed a large similarity with (the few) illustrations of the type A. jackii making a detailed analysis of the latter type necessary to examine possible conspecificity. Here we detail, using LM and SEM observations, the original Rabenhorst material (Alg. Europa’s n° 1003) of A. jackii using the exsiccata material present in the Van Heurck collection at Botanic Garden Meise (BR), Belgium. A comparison between both taxa is made. Achnanthidium jackii Rabenhorst (Figs 1–22) Light microscopy (Figs 1–17): Frustules in girdle view rectangular, weakly arched with slightly recurved apices. Valves linear-lanceolate to weakly lanceolate with more or less convex margins and broadly rounded, protracted, rostrate apices. Valve dimensions (n=30): length 8–17 µm, width 3.0–3.9 µm. Raphe valve concave (Figs 3–10). Axial area narrow, linear, only very weakly widening towards the central area. Central area forming a clear, often asymmetrical, fascia. Shortened striae absent in the central area. Raphe filiform, straight with indistinct proximal raphe

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endings. Terminal raphe fissures indiscernible in LM. Transapical striae weakly radiate to almost parallel throughout the entire valve, becoming more radiate towards the apices, 25–30 in 10 µm, becoming denser near the apices (up to 35 in 10 µm). Rapheless valve convex (Figs 11–17). Axial area very narrow, linear-lanceolate, widening near the central area. Central area variable ranging from absent to forming a small fascia to occasionally asymmetrical due to absence of one central stria or the spacing of two central striae (Figs 11, 12). Transapical striae weakly radiate throughout, more radiate near the apices, 28–30 in 10 µm, up to 34 near the apices.

Figs 1–22. Achnanthidium jackii Rabenhorst. LM and SEM material from Rabenhorst 1861,

exsiccata n° 1003. Figs 1–17. LM showing frustules in girdle view (Figs 1, 2), raphe valves (Figs 3–10) and rapheless valves (Figs 11–17). Scale bar = 10 µm. Figs 18–22. SEM pictures showing raphe valves (Figs 18, 21) and rapheless valves (Figs 19, 20, 22). Scale bars = 10 µm.

Scanning electron microscopy (Figs 18–22): Axial area of the rapheless valve clearly lanceolate, widened towards the central area (Figs 19, 20, 22), on the raphe valve, axial area very narrow near the apices, widening towards the central area (Figs 18, 21). Striae in both valves composed of 3–4 rounded areolae and one marginal slit-like areola (Figs 18, 19). Near the apices striae composed of only 2–4 rounded to square areolae lacking slit-like marginal areolae (Figs 18, 19). Proximal raphe endings straight, indistinct, internally proximal endings deflected in opposite directions (Fig. 19). Terminal raphe fissures short, very weakly expanded, terminating at the apices just beyond the last striae (Figs 18, 21).

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Achnanthidium neocryptocephalum, described from Belgium in 1880, differs little from A. jackii. Novais et al. (2015) indicate a possible separation based on valve width based on earlier results (Potapova & Hamilton 2007). Specimens of A. jackii measured in Potapova & Hamilton (2007) showed a valve width ranging from 2.2–3.4 µm. However, analysis of 30 valves (both raphe and rapheless) resulted in a valve width varying between 3.0 and 3.9 µm which is entirely within the valve width range of A. neocryptocephalum. The striae in A. jackii on the raphe valve have a higher number of areolae per stria (3–5) whereas in all valves of A. neocryptocephalum, only 3–4, rarely 2 areolae are found per stria. The striae on the rapheless valve show no differences. All other morphological features, including valve outline, valve length, stria number, shape and size of both the axial area and the central area and the raphe structure do not differ between both taxa. A similar opinion was already indicated by Potapova & Hamilton (2007). Their multivariate analysis and cluster analysis of type specimens showed that A. jackii and A. neocryptocephalum (as A. minutissima var. cryptocephala) almost did not differ from each other. The detailed analysis of the type material of both taxa (A. neocryptocephalum in Novais et al. 2015 and A. jackii in this study), confirmed this observation. Achnanthidium jackii was described in 1861 and hence has priority over A. neocryptocephalum, a new name for Achnanthes minutissima var. cryptocephala Grunow 1880; priority at the species level for Achnanthidium cryptocephalum Nägeli ex Kützing 1849. Achnanthidium neocryptoceph-alum is thus a taxonomic synonym of A. jackii as a name has no priority outside its own rank (McNeill et al. 2012, Art. 11.2, Melbourne Code). Achnanthes minutissima var. cryptocephala is thus also a taxonomic synonym of A. jackii. We are grateful to Michael Guiry and an anonymous referee for providing corrections and careful revision of the manuscript. Czarnecki, D.B. (1994). The freshwater diatoms culture collection at Loras College, Dubuque,

Iowa. In: Proceedings of the 11th International Diatom Symposium (Kociolek, J.P., ed.). Memoirs of the California Academy of Sciences 17: 155-174.

Kützing, F.T. (1834 '1833'). Synopsis diatomearum oder Versuch einer systematischen Zusammenstellung der Diatomeen. Linnaea 8: 529-620, pls XIII-XIX [79 figs].

Kützing, F.T. (1849). Species algarum. pp. [i]-vi, [1]-922. Lipsiae [Leipzig]: F.A. Brockhaus. Lange-Bertalot, H. & Krammer, K. (1989). Achnanthes, eine Monographie der Gattung mit

Definition der Gattung Cocconeis und Nachtragen zu den Naviculaceae. Bibliotheca Diatomologica 18: 1-393, 100 pls.

Lange-Bertalot, H. & Ruppel, M. (1980). Zur Revision taxonomisch problematischer, ökologisch jedoch wichtiger Sippen der Gattung Achnanthes Bory. Archiv für Hydrobiologie, Supplement 60, Algological Studies 26: 1-31.

McNeill, J., Barrie, F.R., Buck, W.R., Demoulin, V., Greuter, W., Hawksworth, D.L., Herendeen, P.S., Knapp, S., Prado, J., Prud'homme van Reine, W.F., Smith, G.F., Wiersema, J.H. & Turland, N.J. (2012). International Code of Nomenclature for algae, fungi and plants (Melbourne Code) adopted by the Eighteenth International Botanical Congress Melbourne, Australia, July 2011 Regnum Vegetabile, Vol. 154. pp. [i]-xxx, 1-208. Königstein: Koeltz Scientific Books.

Novais, M.H., Jüttner, I., Hoffmann, L. & L. Ector (2015). Morphological variability within the Achnanthidium minutissimum species complex (Bacillariophyta): comparison between the type material of Achnanthes minutissima and related taxa, and new freshwater Achnanthidium species from Portugal. Phytotaxa 224(2): 101-139, 477 figs, 6 tables.

Potapova, M. & Hamilton, P.B. (2007). Morphological and ecological variation within the Achnanthidium minutissimum (Bacillariophyceae) species complex. Journal of Phycology 43(3): 561-575, 10 figs, 3 tables.

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Rabenhorst, L. (1861-1882). Die Algen Europa’s, Fortsetzung der Algen Sachsens, resp. Mittel-Europas. Decades I-CIX, numbers 1-1600 (or 1001-2600). Dresden.

Van Heurck, H. (1880). Synopsis des Diatomées de Belgique Atlas. pp. pls I-XXX. Anvers: Ducaju et Cie.