Constraint of morphological variation in gastropod snails
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Existent forms of many living groups are unevenly distributed in the realm of geometrically possible morphology. In particular, this biased distribution is well known in gastropod shell morphology. As a cause of this, several factors such as defense, efficiency of shell construction and postural stability were considered. Previous studies have explained the biased distribution by a single factor, but there are no researches that considered multiple factors simultaneously in gastropod shell forms. Here, I talk a theoretical morphological study that explains the biased distribution in gastropod snails by two factors, postural stability and efficiency of shell construction. The result shows that postural stability is high when spire is higher and width of umbilicus is narrower. In contrast, the efficiency of shell construction is high when spire is lower and width of umbilicus is wider. This implicates that two functions are conflicting. Actual shell forms are distributed in the area where both functions are satisfied to some extent. It suggests that these two functions are in the relation of a functional trade-off in actual gastropod snails. In addition, I discuss the difference of distributions between terrestrial and marine shell forms. |
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