Typically, snails coil as they grow. The exact shape and characteristics of the coil are known to be a combination of genetic and environmental factors, depending on the snail. There is an interesting story involving snails and the young Jean Piaget. Piaget is famous for his work in psychology, but before that, when he was quite young, he worked with birds and mollusks. His work was published and otherwise disseminated and experts in these areas, unaware that he was a teenager, assumed he was an adult expert on natural history. Anyway, Piaget studied, among other things, a group of snail species in the Alps that had a diverse range of shapes from roundish to more elongated. It appeared that these snails were well adapted to their environment. Elongated snails, which are more delicate than round ones, lived in low energy environments such as lakes, while the round ones were found in quickly running water, where being round gave more mechanical protection from getting bashed about.
As part of his research he collected specimens of these snails from lakes and various streams, and raised them in aquariums. When the snails of the various species grew up in the aquariums, they all looked like each other (they looked like the still water lake snails); the morphology that separated these different species in the wild vanished in the lab. It turned out that the snails differentiated via some sort of reaction to their environment. What seems to have been an excellent example of Darwinian adaptation was actually an excellent example of norm of reaction (the range of variation a given genotype produces in various environments).
Piaget also studied a group of plants in the same region that exhibited the same phenomenon; viny, bushy, tree-ish, etc. forms were the same species growing in different habitats. Between the snails and the plants, Piaget is said to have more or less given up on "Darwinism." He was wrong, of course. Rejecting Darwinian concepts of evolution gave Piaget the chance to become a very famous psychologist but he lost the opportunity to explore and publish on a key feature in evolutionary biology, reaction norms.
But enough of reaction norms. Most of the morphological differences between snail species is probably more genetic than it is environmental, despite Piaget's bad luck in choice of model species. Some years ago I had the pleasure of working with fresh water snail expert Pete Williamson, and from him I learned that one way to differentiate among species of snails was to develop a polynomial formula that described the shape of the coiled shell. One could then measure a number of individuals and assign variance terms to each element of the polynomial, and in this way test hypotheses about differences between species or subspecies, or evolutionary change over time. Pete's work on African fresh water snails was one of the first examples of punctuated equilibrium to be worked out in a major taxon following Gould and Eldridge's definition of that concept. He also was able to pin down various "turnover" events when a number of species went extinct and new ones arose that associated in time with key events in human evolution in East Africa. (I talk about Pete in some of my Congo Memoirs, starting with this one. He's piloting the boat we were in that sank in Lake Edward.)
And all this leads us to a new paper, published in BioMed Central's open access journal EvoDevo. The press release I received this morning seems quite good so I'll just reproduce it below. The paper, being Open Access, is available here. (Let me know if that link does not work for you!)
Snail shell coiling programmed by protein patterning
Snail shells coil in response to an lopsided protein gradient across their shell mantles, finds research in BioMed Central's open access journal EvoDevo. In contrast the shell mantle of limpets, whose shells do not coil, have a symmetrical pattern of the protein Decapentaplegic (Dpp).
There are many hundreds of different kinds of gastropods (slugs snail and limpets) - second only in number of species to insects. They have adapted to live on land as well as in fresh water and marine environments, and have altered their physiology to survive in different habitats and to exploit different niches. The ancestral snail is thought to have had a coiled shell but during evolution some snails have lost their shells to become slugs, and some, limpets and false limpets, have independently lost the ability to coil their shells.
In order to find out why some gastropods have straight and some have coiled shells researchers from the University of Tokyo looked at the pattern of Dpp during shell growth. Dpp was first identified in fruit flies where it is necessary for the correct development of limbs, wings and other organs - decapentaplegic describes the 15 things missing in the absence of the gene dpp. Dpp is also found in the shell gland of gastropods, an early structure which begins to form a developing shell. However its presence in the mantle, which takes over shell production as the animal develops, was unknown.
In all four animals tested, limpets Patella vulgata and Nipponacmea fuscoviridis, and the right-hand coiled pond snail Lymnaea stagnalis along with a sinistral coiled lab-developed snail, dpp expression matched shell shape. There was also a Dpp protein gradient spreading away from this which was also symmetrical in limpets but had left/right asymmetry for the pond snails, matching the handedness of shell coiling.
Keisuke Shimizu, who led this study, commented, "This molecular mechanism driving for shell coiling persists from early developmental stages though adult life as the shell is replaced. It also provides an explanation for how shell coiling has been lost several times during the evolution of gastropods by the relatively easy loss of asymmetric Dpp."
Here's a picture:
Shimizu, K., Iijima, M., Setiamarga, D., Sarashina, I., Kudoh, T., Asami, T., Gittenberger, E., & Endo, K. (2013). Left-right asymmetric expression of dpp in the mantle of gastropods correlates with asymmetric shell coiling EvoDevo, 4 (1) DOI: 10.1186/2041-9139-4-15