Pisidium moitessierianum Paladilhe (Lamellibranchiata) i porównanie jego populacji z plejstoceńską

Autor opisuje gatunek słodkowodnego małża Pisidium moitessierianum Paladilhe (nie znanego dotychczas z utworów plejstoceńskich Polski) z dwóch stanowisk interglacjalnych . Przy pomocy metody biometrycznej ustalono korelacje cech skorupki, przedstawiając wyniki graficznie. Taką samą analizę wykonano dla form współczesnych, żyjących w dwóch różnych środowiskach : w jeziorach i w rzekach. Uzyskane tą drogą dane umożliwiły wyróżnienie dwóch ekotypów współcześnie żyjącego małża , co pozwala przez porównanie określić przynależność ekologiczną form kopalnych i tym samym ustalić środowisko, w jakim żyły osobniki interglacjalne. Krzywa korelacyjna cech skorupki wskazuje poza tym na pewne tendencje rozwojowe tego gatunku i na zmianę proporcji od interglacjału eemskiego do dziś.

Results of the writer's investigations of the morphology of the shell in Recent and Pleistocene representatives of Pisidium moitessierianum Paladilhe are presented . The studied material has been collected from two Pleistocene localities of the Eemian Interglacial: Szeląg in Poznań and Bażantarnia in Elbląg ("Cardiumbank", Woldstedt, 1955); also from four recent sites : the Warta river near Konin, the Krutynia creek near Mikołajki, lake Dargin and lake Dobskie in the Mazury Lake District I(fig. 1). By using the biometrie method the variability of both the Recent and the Pleistocene populations have been studied. Two ecotypes have been differentiated in the recent forms, the lacustrine and the fluviatile. Curve d (fig. 4, 5) represents the length and height variation range of the complete recent material; it is one topped. However, upon separating that material into fluviatile (fig. 4b, 5b) and lacustrine (fig. 4c, 5c) individuals, two curves will be obtained, with two different tops expressing the variation range of two ecotypes. An analysis of the correlations of the two measured dimensions gives also different curves for the two ecological forms of recent representatives of t his species (fig . 2 b- c, 3 b-c). A correlation of the height and length ratio (w/dl) during growth of the shell (fig. 3b) shows that in fluviatile forms the length of the yongest shells exceeds the height (l: 1.17 ratio), while in mature shells the length very nearly equals the height (l : 1.02 ratio). In the lacustrine forms, however, the length of shell is greater than the height throughout all the growth stages (the ratio being 1:1.14 in the youngest, and 1:1.11 in the oldest specimens). A biometrie study of the Pleistocene specimens from Elbląg provides us with curves (fig. 2-5) showing the length and height variation range and the correlations of these characters. A comparison of the curves obtained for these fossil shells with recent ones of two ecotypes indicates strong similarities between curves for the fossil form and the now living fluviatile ecotype (fig . 2-5, lines a, b). In the oldest fossil individuals the length/height ratio during growth of shell is 1:1.05, that in the living forms being only 1:1.02 (fig .3 a- b) . On these values it may be inferred that in the lapse of time between the Eemian and today this ratio has diminished by 0.03, indicating a gradual change of proportions in the shell of the considered species. The same ratio in fossil and recent forms reveals a tendency during ontogeny for the height to be increased and made equal with the length. Well preserved fossil shells from Elbląg suggest that the sediment yielding them corresponds probably to original biotope of Pisidium moitessierianum Paladilhe, hence that the beds in which they occur are fluviatile deposits. Specimens from Szeląg are similar to recent lacustrine forms; however, owing to a small number of specimens, corresponding results are not shown graphically. The use of the biometrie method when comparing fossil and living molluscs in order to determine the nature of fresh-water deposits, is of particular significance in the absence from faunal assemblages of other ecologically characteristic forms.