The grove snail, Cepaea nemoralis, is a common European land mollusk that has become a globally recognized organism for the study of evolutionary biology. The snail’s scientific significance stems from its highly visible and genetically determined shell variation, which serves as a textbook example of natural selection operating in real-time.
Identifying the Grove Snail
The grove snail is a medium-sized terrestrial species, typically reaching a shell width between 18 and 25 millimeters and a height of 12 to 22 millimeters. Its shell is globular and glossy, consisting of four and a half to five and a half whorls. The coloration is diverse, with the shell’s background color ranging from bright yellow to pink or various shades of brown.
A distinctive feature that helps in identifying mature C. nemoralis is the apertural lip, which is the thickened, slightly out-turned margin of the shell opening. In most regions, this lip is a dark purple-brown color, which differentiates it from its close relative, the white-lipped snail, Cepaea hortensis. The shell may also display a pattern of up to five dark, spiral bands, or it may be entirely unbanded.
Global Distribution and Preferred Habitat
The native range of Cepaea nemoralis spans Western and Central Europe, extending from southern Scandinavia down to the Iberian Peninsula and into parts of Central Europe. Its dispersal has been further expanded by human activity, with successful introductions establishing populations across North America, including parts of the United States and Canada, during the 19th century.
Typical habitats include dense vegetation such as hedgerows, damp meadows, and woodlands. They are also commonly found in gardens, cultivated fields, and coastal dunes. This ecological flexibility contributes to the snail’s success and the high population densities often observed in suitable microhabitats.
The Puzzle of Shell Polymorphism
The most studied trait of the grove snail is its shell polymorphism, which refers to the existence of multiple distinct shell forms within a single population. This variation is governed by two independent, highly variable genetic characteristics: the background color and the presence or absence of dark banding. The shell colors are generally categorized as brown, pink, or yellow, and the inheritance of these colors follows a simple dominance hierarchy.
The brown color allele is dominant over pink, and both brown and pink are dominant over the yellow allele. Separately, the dark banding pattern is controlled by a different set of alleles, allowing for shells to be unbanded or to possess up to five dark spiral stripes. The genes controlling these two major traits—color and banding—are situated very close together on the same chromosome, forming what scientists call a “supergene.”
This tight genetic linkage ensures that the color and banding traits are inherited together as a functional unit, greatly reducing the frequency of recombination and maintaining specific combinations of color and pattern.
Evolutionary Drivers: The Role of Predation
The maintenance of such extensive polymorphism is directly linked to the primary selective force acting on the snail: predation by visually hunting birds, specifically the Song Thrush (Turdus philomelos). Thrushes are specialist snail predators that use a characteristic behavior to access the soft body of the mollusk. They carry the snail to a hard object, often a stone or rock, known as an “anvil,” and repeatedly smash the shell against it until it breaks.
This predatory method creates a fossil record of selection, as the broken shells accumulate at the anvil sites, allowing researchers to determine which shell morphs are being preferentially consumed. The thrush’s visual hunting creates a strong selective pressure for crypsis, or camouflage, with the most conspicuous snails being removed from the population.
For instance, yellow shells tend to survive better in open, grassy habitats, particularly in the spring and summer when the background is green. Conversely, brown and pink shells, often unbanded, are more protected in darker environments like dense woodland or leaf litter, especially during the winter. The relative survival of a specific shell morph depends entirely on how well its color and pattern blend into the immediate habitat background.