Discrete or indiscrete? Redefining the colour polymorphism of the land snail Cepaea nemoralis

Abstract

Biologists have long tried to describe and name the different phenotypes that make up the shell polymorphism of the land snail Cepaea nemoralis . Traditionally, the view is that the ground colour of the shell is one of a few major colour classes, either yellow, pink or brown, but in practise it is frequently difficult to distinguish the colours, and define different shades of the same colour. To understand whether colour variation is in reality continuous, and to investigate how the variation may be perceived by an avian predator, we applied psychophysical models of colour vision to shell reflectance measures. We found that both achromatic and chromatic variation are indiscrete in Cepaea nemoralis , being continuously distributed over many perceptual units. Nonetheless, clustering analysis based on the density of the distribution did reveal three groups, roughly corresponding to human-perceived yellow, pink and brown shells. We also found large-scale geographic variation in the frequency of these groups across Europe, and some covariance between shell colour and banding patterns. Although further studies are necessary, the observation of continuous variation in colour is intriguing because the traditional theory is that the underlying supergene that determines colour has evolved to prevent phenotypes from “dissolving” into continuous trait distributions. The findings thus have significance for understanding the Cepaea polymorphism, and the nature of the selection that acts upon it, as well as more generally highlighting the need to measure colour objectively in other systems.

Eumelanin and pheomelanin pigmentation in mollusc shells may be less common than expected: insights from mass spectrometry

“…Using our newly developed HPLC–UV–MS method [44] which in contrast to HPLC–UV provides high selectivity, we unequivocally demonstrate the presence of eumelanin in five mollusc species belonging to three major clades of Conchifera. In addition, we found the first conclusive evidence of pheomelanin in a terrestrial gastropod ( Cepaea nemoralis ) known for its colour and banding polymorphism [45]. However, we also demonstrate that previous reports of eumelanin in two species ( Crassostrea gigas and Mizuhopecten yessoensis ) were possibly technical artefacts (see below), and that for a total of eight of the 13 species we investigated, which have brown/black pigmented patterns on their shells, we could find no evidence of melanin in their shells.…”

Fine-scale species delimitation: speciation in process and periodic patterns in nudibranch diversity

“…Therefore, even in a majority of other cases where we do not have data on early development, we can reasonably infer that ontogenetic periodicity must influence adult morphology in the majority of metazoans, since all of them possess a similar homeobox system of early development (Holland 2013). For example, colour and pattern polymorphism of land snail shells of the genus Cepaea has been shown to be caused by a complex interaction between gene expression and local environment, with both random and regular colour patterns (e.g., Jones et al 1977;Cook 2017;Davison et al 2019). The underlying genetic basis for the appearance of any characters can thus be either very complex or simple and irregular, but when such variations are brought up to higher taxonomic and phylogenetic levels, the periodic/quasi-periodic patterns become more evident, although still with irregularities.…”

Boldness and exploration vary between shell morphs but not environmental contexts in the snailCepaea nemoralis

“…1A-B). Genetic variation in terms of shell background color (from pale yellow to brown, but usually divided in yellow, pink and brown; Davison, Jackson, Murphy, & Reader, 2019) and in the number or width of dark bands has been well described in Mendelian terms (Jones et al, 1977). Modern genomics studies now aim to pinpoint the actual molecular/physiological underpinnings of shell color (Kerkvliet, Boer, Schilthuizen, & Kraaijeveld, 2017; Richards et al, 2013).…”