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Convergent evolution of shell colour patterns in two Caribbean terrestrial prosobranch snails

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Abstract

Two species of helicinid land snails, the Jamaican endemic Eutrochatella pulchella and the Cuban endemic Viana regina, display a colour pattern of white spots on a greenish background. Microstructural and optical investigations of the shells have revealed that the colour patterns are non-homologous. In E. pulchella the white spots are associated with areas of granular shell structure, in which the crystals have an irregular, peak-like form. In V. regina, there is no structural modification of the shell mineral associated with the spots. Modification of the organic matrix is responsible for the appearance of spotting. In V. regina, the shell pigment is yellow and located in the inner part of the shell. It appears greenish as seen from the outside through the translucent shell. The pigment fluoresces yellow under ultraviolet light. The greenish shell pigment of E. pulchella is non-fluorescent and is uniformly distributed through the shell (except in the white spots).

Since the colour patterns are non-homologous, their similarity must have arisen through convergent evolution. The adaptive value of the colour patterns is thus indicated. Both species live exposed on limestone rocks and their colour patterns (at least in the case of E. pulchella) cause them to be very cryptic. Predation by visually oriented predators (perhaps birds) is a probable cause of selection for shell colour pattern.

The association of polymorphism with wide habitat breadth and of monomorphism with narrow habitat breadth is proposed. This relationship is seen in the two helicinid species, both of which show little within-population variation as compared with polymorphic snails (e.g. helicids), which occupy a wide range of habitats. The generally monomorphic character of populations of Jamaican snails may be a result of habitat specialization associated with the unusually high species diversity on the island (some 450 species in 11,400 km2).

The need for studies on the relationship of shell colour and structure is pointed out.

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