Shell Strength of Colour Morphs of the Mangrove Snail Littoraria Pallescens

The load required to break shells of the snail Littor-aria pallescens (Philippi) (Gastropoda: Littorinidae) has been established. Mass increases more than proportionally with size. Size for size, relatively heavy shells are thicker than relatively light ones, and the thicker shells are stronger. Three samples from different localities differ significantly in strength. Littoraria pallescens is polymorphic for shell colour. It is shown that in two of the three samples, both from Papua New Guinea, individuals of the dark morph are significantly heavier and stronger than those of the yellow morph, while there is no difference between morphs in the third sample, from Thailand. In the sample with the most robust shells the dark individuals are stronger, mass for mass, than yellows. Thus, colour morph is sometimes, but not always, associated with differences in shell characteristics which are likely to affect fitness. Where there is a difference, the dark morph is estimated to be on average about 22% stronger than the yellow morph. The difference between morphs is small compared with the large variation in shell characters associated with size.