A search for causes of polymorphism in Clithon oualaniensis (Lesson) (Gastropoda; Prosobranchia).

Western Clithon from India and Ceylon differs from eastern Clithon from the Malay Peninsula and Hong Kong as regards shell pattern and coloration (Grüneberg 1976, 1978). The western form lives in regions with high surface salinity whereas the eastern form lives in regions with low surface salinity. This situation may either reflect a causal relation, or it may be the result of a coincidence. The evidence presented in this paper speaks against a causal relation and suggests that other factors with a similar geographical distribution may be at work.