C Nova Hedwigia, Beiheft 143, p. 381–390 Stuttgart, July 2014 © 2014 J. Cramer in der Gebr. Borntraeger Verlagsbuchhandlung, Stuttgart, Germany www.borntraeger-cramer.de DOI 10.1127/1438-9134/2014/019 1438-9134/2014/019 $ 2.50 Type analysis of Aulacoseira gessneri (Hustedt) Simonsen (Bacillariophyceae) from South America Carlos E. Wetzel 1*,2 , Luc Ector 1 and Denise C. Bicudo 2 1 Public Research Centre – Gabriel Lippmann, Department of Environment and Agrobiotech- nologies (EVA), 41 rue du Brill, 4422 Belvaux, Luxembourg 2 Instituto de Botânica de São Paulo, Núcleo de Ecologia, Av. Miguel Stéfano 3687, CEP 04301- 012, São Paulo, SP, Brazil * Corresponding author: wetzel@lippmann.lu With 21 figures Abstract: Melosira gessneri, a colonial planktonic diatom, was first described from tropical South America (Tapajós River, Brazilian Amazon) and was later transferred to the genus Aulacoseira without detailed analysis of the type material. The species is reported in recent literature, mainly from pristine areas of Brazil like the Pantanal region and the Amazon basin in high abundances. Besides its occurrence in the type locality, Aulacoseira gessneri was reported in one sample from the lower reach of the Amazon River (Gurupá River) in the 1960’s and more recently from the Pantanal region in central-western region of Brazil (La Plata basin). The objectives of this work are to analyse using light and scanning electron microscopy the type material described by Hustedt, as well as to verify the current distribution of the species. Confusion of this taxon with other common species such as A. muzzanensis and A. islandica are likely to be related to a poor knowledge of tropical South American planktonic diatom species. The presence of A. gessneri in large hydrographic basins such as distinct tributaries of the Amazon River and massive abundance records from the Pantanal region suggests that the species has been overlooked in other geographic areas and is probably more widely distributed in the South American continent. Key words: Aulacoseira, Amazon, Brazil, centric diatoms, Pantanal, plankton, Tapajós River, type material. Introduction Aulacoseira Thwaites is an ecologically important and ubiquitous diatom genus, with cosmopolitan and endemic species found in ecologically diverse lacustrine and lotic freshwaters, but it has been described as a character-poor and thus a “difficult” genus (Crawford & Likhoshway 2002). Although considered a relatively small genus, a review of the literature made by Edgar & Theriot (2004) reveals that the number of described species placed into the genus has risen to approximately 60, with 15 common varieties widely used until the 70’s. Fourtanier & Kociolek (2009, 2011) show that many more infraspecific taxa have been described and that the number goes up to 155 taxa when including all taxonomic categories below species. According to Algaebase there are currently 96 species (and infraspecific) names in the database, of which 44 have been flagged as currently accepted taxonomically (Guiry & Guiry 2013). However, as observed by Denys et al. (2003) even in well-studied areas such as European rivers and lakes, some important widely distributed planktonic diatom species are still poorly known, suggesting that species can often remain unidentified or mistaken for other taxa due to identification problems and their absence from standard reference works. Thus, publications concentrating on thorough study of type material aiming to clarify the identity of several diatom taxa, especially of those that are frequently reported in the ecological literature, are always helpful when taking taxonomic decisions. Some past and recent studies based on type material have clarified, better illustrated, and defined the taxonomic circumscription of several Aulacoseira species (e.g. Crawford 1981, Krammer 1991a, b, Crawford & Likhoshway 1999, Tuji 2002, Crawford et al. 2003, Houk 2003, 2007, Tuji & Houki 2004, Siver & Hamilton