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Abstract. This study was conducted to describe the shell shapes of Vivipara angularis collected from four different locations in Lake Dapao, one of the deepest lakes in Mindanao, Philippines. Geometric morphometric tools specifically relative warp (RW) and euclidean distance matrix analysis (EDMA) were used to describe the shapes. Results showed there was no population differentiation in the snail as the shell shapes in all four populations were concentrated towards the means shape revealed by relative warp analysis. A closer look at variations in shell shapes based on the results of EDMA revealed that individual variations were due to differences in selected interlandmark distances observed along the spire, body whorl and aperture of the shell. Observed variations in shell shapes can be argued to be due to ecological factors, developmental differences and/or maybe environmentally induced.
Key Words: EDMA, PCA, relative warp analysis, histogram.
Introduction. Lake Dapao is one of the deepest lakes in the Philippines (BIMP-EAGA 1997) and second largest lake in the Province of Lanao del Sur, as old as Lake Lanao (Figure 1). The area is located 7°48" North latitude and 124°03" East longitude. The pH value ranges from 6.5-6.7, and the dissolved oxygen level from 7.8 to 8.2 p.p.m. (Haribon Foundation 2012). This lake is an isolated and restricted body of water where Vivipara angularis, one of the most common and widely distributed freshwater gastropod can be found. Populations of the snail inhabit the lake's vicinity and are also used as food by some of the local residents in the area. At present however, an introduced species known worldwide as Pomacea canaliculata was found to be increasingly populating the lake. It is hypothesized that the population structures of freshwater snails around the lake might be affected by the introduction of the invasive P. canaliculata since they outcompete the native species populations for calcium resources, habitat, and spawning sites. To be able to test this hypothesis, selected populations were analyzed by describing morphological shapes of the shell using geometric morphometric methods, specifically landmark-based analysis to capture shape variation in shells (Torres et al 2011; Moneva et al 2012a). Similar study of the patterns of variation within populations of this species was analyzed from another lake using relative warps and it was observed to...