PHYSA POPULATION AT UKALA, MADHYA PRADESH INDIA- AN EXAMPLE OF ECOPHENOTYPIC VARIATION

Variation in gastropod shell population may be ecophenotypic or, genetic. Conchological difference within freshwater snail occurs due to: 1) Fluvial condition and stream size, 2) River substrate, and 3) Presence or absence of predators. End Cretaceous is characterized by outpouring of basaltic lava essentially in Western and Central parts of Indian Peninsula. The entire volcanism occurred in a series of phases punctuated by terrestrial aquatic conditions which is represented by thin (around 1metre) volcanosedimentary sequence commonly known as intertrappean horizon. Very often these intertrappean horizons contain fresh water mollusca like Physa, Lymnea, Viviparus, and Unio. The Physa specimens show high morphological diversity (especially in shape and size- however number of whorls remains more or less same). The Physa populations of Hislop have been clustered into three varieties belonging to single species- Physa prinseppi normalis, Physa prinseppi elongata, and Physa prinseppi inflata. Intertrappean horizons are generally not very much extensive but present as discrete, detached outcrops. One of such outcrops is the present location Ukala village in Dhar district, Madhya Pradesh, India. The physid fossils here display a wide variety in shape and size. These fossils have been to some extent deformed by overburden compaction by huge Deccan Trap Basalt. They display different degrees of deformation probably due to initial different orientations with respect to horizontal surface. The bivariate (length vs. width) plot of the present physid fossils interestingly show rectilinear arrangement of data with no intermediate clustering. Slight scattering of data may be due to change in size and shape for predator avoidance and overburden compaction. Hence, Ukala population of Physa represents a single variable species Physa prinseppi. The existing varieties of Physa may be obliterated. Keywords: Physa, Cretaceous, Intertrappean, India, Ecophenotypic variation