Diatom Research (2008), Volume 23 (l), 233-242 THE UNMASKING OF A SUB-ANTARCTIC ENDEMIC: PSAMMOTHIDIUM ABUNDANS (MANGUIN) BUKHTIYAROVA ET ROUND IN EUROPEAN RIVERS Bart Van de Vijver* National Botanic Garden of Belgium, Department of Blyophytes & Thallophytes, Domein van Bouchout, B-1860 Meise, Belgium Martyn Kelly Bowburn Consultancy, 1 I Monteigne Drive, Durham DH6 5QB, United Kingdom Saul Blanco Area de Ecologia, Universidad de Ledn, E-24071 Ledn, Spain Amelie Jarlman Jarlman HB, Stora Tvargatan 33, S-223 52 Lund, Sweden Luc Ector Public Research Center - Gabriel Lippmann, Department of Environment and Agro-Biotechnologies, 41, rue du Brill, L-4422 Belvaux, Luxemburg Psammothidium abundans (Manguin) Bukhtiyarova & Round was described 50 years ago from the sub-Antarctic Kerguelen archipelago (southern Indian Ocean). Apart from single occurrences in South Africa, southern Australia and Tierra del Fuego, the reported distribution of this species is restricted to the (sub-)Antarctic Region. In the northern hemisphere, the species was only very sporadically reported from two rivers in Scotland. During surface water surveys in Sweden, United Kingdom and Ireland (as part of water quality investigations), several populations of an Achnanthoid species similar in appearance to P. abundans have been found in rivers and brooks. The morphological analysis of the European forms, together with a thorough examination of the type material (including lectotypification of P. abundans), could not reveal any morphological differences between the European and sub-Antarctic (type) populations. This paper is the result of the morphological study of this species and raises questions about how the species ended up so far from its original distributional range. After elimination of all possible dispersion abilities, it is most likely that the species has always been present in the European rivers but has been overlooked till now. Possible causes for this are the lack of appropriate identification literature during standard European water monitoring, species drift and force-fitting and the fact that many rivers have not been thoroughly studied. INTRODUCTION The sub-Antarctic islands have a unique freshwater diatom flora (e.g. Van de Vijver et al. 2001, 2002, 2004) comprised of a large number of species that are considered to be endemic since they have until now not been found elsewhere in the world. Diatom endemism estimates for this region range from 40 to more than 55% of the total number of observed * e-mail: vandevijver@br.fgov.be; tel. +32 2 260 09 41; fax. +32 2 260 09 45