Morphology, typification and critical analysis of some ecologically important small naviculoid species (Bacillariophyta)

The identity and nomenclatural history of several small-celled naviculoid taxa are revisited. The species discussed here are important from the ecological point of view since they are often dominant in benthic freshwater communities. The original concepts of several species that have suffered major taxonomic drift due to their entangled nomenclatural history are discussed, and forgotten epithets are resurrected. We examined the original material of Navicula aggerica E. Reichardt, Navicula atomoides Grunow, N. crassulexigua E. Reichardt, N. minima Grunow, N. minima var. typica R. Ross, N. minutissima (Kutz.) Grunow, N. saugerresii Desm., N. seminulum Grunow, N. seminulum var. intermedia Hust., N. seminulum var. radiosa Hust., N. stroemii Hust., N. subbacillum Hust., N. subseminulum Hust., N. tantula Hust., N. vasta Hust., N. ventraloides Hust., Stauroneis fonticola Hust., and Synedra minutissima Kutz. Several of these names were regarded as synonyms in many floristic works and, as such, remained forgotten or ignored. Analyses using light and scanning electron microscopy indicate conspecificity of Navicula minima (= Sellaphora seminulum sensu auct. nonnull.) with Sellaphora saugerresii (Desm.) C.E. Wetzel et D.G. Mann comb. nov., which has priority against N. minima. Synedra minutissima is lectotypified and transferred to Halamphora minutissima (Kutz.) C.E. Wetzel et Compere comb. nov. Navicula minutissima (Kutz.) Grunow 1860, nom. illeg. and Navicula minima Grunow pro parte, typo excl. designate one and the same species (valid and legitimate), currently known as Sellaphora aggerica (E. Reichardt) Falasco et Ector. We consider Sellaphora atomoides (Grunow) C.E. Wetzel et Van de Vijver comb. nov. (= Eolimna tantula sensu auct. nonnull.) and Sellaphora nigri (De Not.) C.E. Wetzel et Ector comb. nov. (= Eolimna minima sensu auct. nonnull.) to be separate species, although morphologically very similar. Sellaphora crassulexigua (E. Reichardt) C.E. Wetzel et Ector comb. nov. and Sellaphora subseminulum (Hust.) C.E. Wetzel comb. nov. are rarely encountered, but usually found in calcareous springs and aerial habitats, respectively. All species are transferred to the genus Sellaphora on the basis of their valve morphology, pending molecular studies confirming the monophyly of the group once living material of each can be located and brought into clonal culture. Additionally, 64 established taxa from Navicula s.l., Eolimna or Naviculadicta are formally transferred to Sellaphora. Navicula subminuscula Manguin is formally transferred to the genus Craticula Grunow.