Een pleistocene vondst van Pisidium conventus Clessin in Nederland en de huidige geografische verspreiding van deze soort in Europa

Mr. G. Spaink (1972a, b), Rijks Geologische Dienst, Haarlem, Netherlands, collected a large series of fossil (Weichselian) shells of P. conventus Clessin in a boring near the city of Eindhoven. This is the third Pleistocene record for this species in Europe (fig. 1, locality F3). All three are from periglacial deposits of the last glaciation. A description of the Recent distribution of the species with many new localities in central Europe is given. Outside the Fenno-scandinavian main area, so far 77 localities (lakes and tarns) are known in Europe. The 51 central European localities are situated in the drainages of the Rhine (fig. 1, nos. 1-4), the Danube (fig. 1, nos. 5-7), the Po (fig. 1, nr. 8) and the Rhône rivers (fig. 1, no. 9) respectively. The northernmost localities for P. conventus are lake Lomvatn (73°15’N), Novaya Zemlya, USSR (Odhner, 1923), and Zeta Lake (71°N 106°38’W), Victoria Island, Canada (Heard, 1963). The most southerly localities are Cayuga Lake (42°30’N), N.Y., USA (Heard 1963) and in Europe Lake Garda (45°30’N), Italy. The highest locality of the whole area lies in the Alps: Tauernmoossee (2003 m.s.m.) in the Hohe Tauern. The highest locality in the Fenno-scandinavian area is lake Gjuvvatn (1837 m alt.) in southern Norway (Ökland, 1971). The altimetric situation of the 51 central European localities is as follows: 36 localities below 800 m altitude, 10 localities between 800 and 1200 m altitude, no localities between 1200 and 1600 m altitude, 5 localities between 1600 and 2000 m altitude. Taking into consideration the fact that 80% of these localities are situated outside the alpine ranges, and moreover that only a small part of the species area lies in the arctic zone (fig. 1), the often used areographic definition “arcticalpine” should be replaced by the term “northern pre-alpine”. The general distribution of P. conventus is traditionaly called “circumpolar”, but as fig. 3 shows, the term “holarctic” is more appropriate. The deepest records are from the Lago Maggiore, to 350 m. (Lenz, 1954), and Lake Geneva and Lake of Constance, both to 300 m. (Zschokke, 1911: 157). P. conventus has often been considered a glacial relict in central Europe. However, the author cites a number of cases of post-glacial migration of this species in the alpine region. In his opinion, transportation by means of birds is an underestimated factor in the interpretation of the discontinuous distribution. There is still no evidence for the glacial relict hypothesis of P. conventus in central Europe.