Abstract
The thick-shelled mussel Unio crassus Philipsson is a legally protected, endangered species. This paper reports on its previously unknown localities in Romincka Forest (NE. Poland). Out of the four largest rivers of the Forest, live individuals were found in the Bludzia and B3edzianka; only empty shells were found in the Jarka River, and neither live mussels nor shells were found in the ytkiejmska Struga. U. crassus avoids canalised river sections, while water quality does not seem to affect its distribution in the examined rivers.
Highlights
The thick-shelled mussel Unio crassus Philipsson, 1788 is a Eurasian species inhabiting fast-flowing rivers with sandy or sandy-gravelly bottom (PIECHOCKI & DYDUCH-FALNIOWSKA 1993, ZAJ¥C 2004)
In north-eastern Poland it was previously recorded from the Wigry National Park and the Suwa3ki Landscape Park (ZAJ¥C 2004), but not from Romincka Forest
The thick-shelled mussel was found in three rivers of Romincka Forest: live individuals in the Bludzia and B3êdzianka; only shells in the Jarka
Summary
The thick-shelled mussel Unio crassus Philipsson, 1788 is a Eurasian species inhabiting fast-flowing rivers with sandy or sandy-gravelly bottom (PIECHOCKI & DYDUCH-FALNIOWSKA 1993, ZAJ¥C 2004) It was common in rivers of entire Europe. The upper section of the B3êdzianka River (its Russian part is called Krasnaya) has a mountain stream character (numerous rapids, stony or gravelly bottom, deep valley) In the forest it changes into a typical, Magdalena Marzec slow, lowland river and this section is canalised, with predominantly sandy and muddy bottom. The Jarka (in its lower reaches called Go3dapa) is a right-bank tributary of the Wêgorapa, which joins the Pissa River on the Russian side It has a varied character, and within the forest it is natural. On the five-grade scale of water quality, all these rivers were classified as unsatisfactory quality i.e. class IV because of their colour, chemical oxygen demand, faecal coliform bacteria count and overall coliform bacteria count
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