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Abstract
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.