Fragilaria undulata

Vijver, E. Cramer Bart Van de, 2022, Typification and transfer of Fragilaria undulata C. E. Cramer to the genus Pseudostaurosira (Staurosiraceae, Bacillariophyta) with some notes on the diatom species described by Carl E. Cramer, Notulae Algarum 225, pp. 1-5 : 2

Fragilaria undulata was later only very occasionally mentioned by other authors, most likely because De Toni (1892: 689) considered F. undulata C.E.Cramer to be a synonym of Fragilaria construens var. binodis (Ehrenberg) Grunow.

In 1951, Pierre Fusey (1921–1999) described a small-celled araphid taxon as Fragilaria construens [ var. binodis ] f. robusta Fusey ( Fusey 1951: 34, pl. 1: fig. 2) from a small pond (Étang du Pellinec) near the commune of Corlay, Côtes-d'Armor Départment, Brittany, France. The taxon was first raised to species level by Manguin as Fragilaria robusta (Fusey) Manguin ( Manguin 1954: 14, pl. 1: fig. 6) and later transferred to the genus Pseudostaurosira as P. robusta (Fusey) D.M. Williams & Round (1987: 278) . Although the original Fusey material was not examined for the current study, from a comparison of the drawing in Fusey (1951: pl. 1: fig. 2) with the F. undulata valves in Rabenhorst sample 1441, it is clear that both taxa are conspecific, making the former a junior heterotypic synonym of F. undulata C.E.Cramer.

However, although validly published, Cramer’s F. undulata is an illegitimate name as in 1835, another species bearing the name Fragilaria undulata was described by August K.J. Corda (1809– 1849), a Czech mycologist and botanist working as curator of the Zoology division at the Czech National Museum. Fragilaria undulata Corda ( Corda 1835: 207, pl. IV: figs, 39, 40) described a colony-forming diatom species from a sample collected in the thermal waters of Carlsbad (Karlovy Vary, Czech Republic). Attempts to find the Corda material were unsuccessful (O. Šída, Herbarium, National Museum Prague, pers. comm.). Judging from the drawing in Corda (1835) it is clear that both taxa are not even congeneric. Also, Ehrenberg (1854: 108) included the name F. undulata but this is considered a nomen nudum since only a name was mentioned lacking a description or an illustration (Art. 38.1).

Despite this illegitimate name, it is possible to validly and legitimately transfer F. undulata C.E.Cramer to the genus Pseudostaurosira as a replacement name (Art. 6.11) as P. undulata Van de Vijver & Kusber , nom. nov. The slide made from Rabenhorst sample 1441, kept in BR (Meise Botanic Garden, Belgium) is designated as lectotype for P. undulata .