For an awful lot of their evolutionary history, snails were exclusively marine. At some point, they made the break from the briny and moved into fresh water and onto land. (Well, yes, you can file that one under the bleedin' obvious.) Among the molluscs formerly known as prosobranchs, the transitions happened independently among several lineages. Different groups abandoned the ocean at different times and by following different routes. Sea to land; sea to land to freshwater; sea to freshwater; sea to freshwater to land; sea to freshwater to land to freshwa … You get the picture*. But for the pulmonates, the shift was more straightforward.
Oh, wait. Did I just say straightforward? Well, it's a comparative thing.
Let me set the scene. In the old days, gastropod classification was a piece of cake. Prosobranchia (gills), Opisthobranchia (gill-ish), Pulmonata (gill-less). That was just about all you needed to know. Unfortunately, it wasn't very helpful because Prosobranchia was paraphyletic. While malacologists unraveled the complexities of that huge assemblage, the Opisthobranchia + Pulmonata group remained stable.
But, once again, it's a comparative thing. It turns out that Opisthobranchia (nudibranchs and their allies) is not monophyletic. For example, the strange little acochlidiaceans, which live between grains of sand, are more closely related to pulmonates than they are to other sea slugs. As for the sacoglossans … well, they pop up all over the place. In a recent study by Annette Klussmann-Kolb and colleagues, solar-powered sacoglossans clustered with the air-breathing limpets, Siphonaria. Does that make sense? As much as anything does in the world of gastropod phylogeny.
So what does this all mean for the origin of land and freshwater pulmonates?
The analysis by Klussmann-Kolb and colleagues revealed that the freshwater Hygrophila and the terrestrial Stylommatophora were both monophyletic — but weren't sister groups. When the researchers mapped habitats onto their phylogenetic tree, they found that Hygrophila had not only moved directly from the sea to freshwater without a terrestrial stopover but the group had done it only once. Having adopted a freshwater life style, the snails stayed there.
Stylommatophora made its transition to land via mangrove, saltmarsh and other intertidal environments. That group — which contains all the well-known land snails and slugs — probably also changed habitat only once. Its closest relatives (ellobiids, onchidiids) mostly occupy the marine marginal zone but a handful of them have become truly terrestrial. So the shift may have happened several times among the pulmonates but it has always taken the same route.
Well, I did say that it was comparatively straightforward.
Klussmann-Kolb, A, Dinapoli, A, Kuhn, K, Streit, B & Albrecht, C. (2008). From sea to land and beyond — new insights into the evolution of euthyneuran Gastropoda (Mollusca). BMC Evolutionary Biology 8:57 doi:10.1186/1471-2148-8-57. OPEN ACCESS.
* Don't ask me to trace the paths. I could tell you but you know what I'd have to do then.