Grove Snail - Coloration

Coloration

Apart from the band at the lip of the shell, grove snails are highly polymorphic in their shell colour and banding. The background colour of the shell can sometimes be so pale as to be almost white; it can also be yellow, pink, chestnut through to dark brown, and the shells can be with or without dark bandings. The bands vary in intensity of colour, in width and in total number, from zero up to a total of six.

The polymorphism has been intensely studied from 1940 onwards for its heredity, evolution and ecology. Researchers have variously asserted that the cause is random genetic drift, different natural selection pressures in different areas (the snail often has darker camouflage in woodland, lighter in rough grassland) with mixing by migration, and balanced polymorphism. Balanced polymorphism could arise when a predator like the Song Thrush has a given 'search image', so it tends to see and kill snails of a particular colour and pattern. Natural selection then favours a diversity of colours and patterns. However it appears that no one explanation is the whole answer: most probably, the polymorphism has several causes, including selection of paler, more reflective colours in hot environments to reduce water loss.

Different coloration and banding of the shells of Cepaea nemoralis:

• without dark bandings

• with one band

• with more bands