I came across this attractive land snail while doing some gardening. I was curious, as usual. For me, it doesn’t seem right not to know and understand the creatures sharing my property. And while I don’t want to become too mollusk-oriented here, considering the slug post, this one turned out to be too fascinating to not write about.
The snail is Cepaea nemoralis. It goes by many common names, Banded Wood Snail and Grovesnail seem to be the most frequently used. I soon learned (and you’ll anticipate my next statement) that they are not native to North America. C. nemoralis and its sister species C. hortensis are from western Europe and have been widely introduced, often via plant materials but occasionally intentionally. They have been on this side of The Pond for about 150 years.
It turns out C. nemoralis is a remarkably well-studied organism because it is, I was intrigued to discover, one of the most polymorphic species of any known animal, possessing extreme intraspecific variation of mitochondrial DNA.
The polymorphism is readily apparent if you run across many of these snails. The base color of the shells may be various shades of yellow, pink, or brown, and the shells may be unbanded, or have up to five bands. Bands may be complete, fused, or not fully pigmented (some handsome photos here). To keep things straight, a shorthand has been developed to categorize shell color and pattern, beginning with a letter signifying shell color, and five digits to record the number of bands. My snail would have the notation Y12345. Had any bands been fused, they would be noted parenthetically, and split bands would be shown with subscripts. I don’t know how bands that change color along their length are recorded, nor the variations in the color of the shell’s lip, or the snails body — all of which are further examples of polymorphism in this gastropod.
What makes a snail — this snail — so variable? It turns out, a whole lot of things. In any given population of C. nemoralis, several factors may act together, and their relative importance will vary from site to site. I found the classic paper on the subject, Polymorphism in Cepaea: A problem with too many solutions? both aptly named and a fine overview of the forces that influence polymorphism in Banded Tree Snails. Just a few examples:
Habitat: More structurally diverse habitats tend to have banded snail populations, while snails in more uniform habitats (grasslands, dunes) have fewer or no bands, thought to help in camouflage against predators. Color of the base of the shell is also influenced by habitat, again facilitating camouflage. All the ones I’ve found in my garden are yellow with bands. I found this one in a shady wet woods. Not only is the shell brown, so is the body.
Climate: Climate may influence shell color in two ways. First, shell color helps the snails thermoregulate. Darker shells absorb more heat than light shells; therefore, yellow shells tend to predominate in hotter climates. Rainfall also plays a role. Although most C. nemoralis have dark-lipped shells, populations with high percentages of white lips tend to be found in wetter areas. In the same places, dark-lipped C. hortensis (the uncommon morph) are found in higher proportions. Rainfall affects both these species in the same way (increasing the number of the less-common morph), but the mechanism is not known.
Conspicuousness — C. nemoralis is the most active of a suite of similiar species, making it more conspicuous to predators. This may further influence the high degree of polymorphism in this species.
All this selective pressure is complicated by elaborate genetics, which I don’t pretend to fully understand. I do know that since snails don’t move very far, they tend to mate with close, often related, neighbors, and that will cause the frequency of gene alleles to change over time. Looks like the sequencing of their DNA was completed fairly recently, so I’m sure more there will be even more interesting revelations on the evolutionary ecology of what Thomaz et al. called “a classic organism in ecological genetics.”
And I just thought I found a pretty snail, and wondered if maybe I could find something interesting about it.
Jones, J.S., B.H. Leith, and P. Rawlings. 1977. Polymorphism in Cepaea: A problem with too many solutions? Annual Review of Ecology and Systematics 8:109-143, with 201 references.
Terrett, J. A., S. Miles, and R. H. Thomas. 1996. Complete DNA sequence of the mitochrondrial genome of Cepaea nemoralis (Gastropoda: Pulmonata). Jrl. Molecular Evol. 42:160-168.
Thomaz, D., A. Guiller, and B. Clarke. 1996. Extreme divergence of mitochondrial DNA within species of pulmonate land snails. Proceedings of the Royal Society Ser. B: 263: 363-368.