On Snails and Shells

When you are walking besides a coast line you might notice some twisted shells clinging on to the rocks for dear life. Chances are you are witnessing just another day in the life of the Rough Periwinkle (Littorina saxatilis). A good observer might even notice that are some striking differences between these mollusks in particular areas of the coast line.

Indeed, specialized ecotypes occur in specific habitats that are dominated by crabs or waves. The crab or S-ecotype (S = sheltered) lives in areas where crabs are abound and therefor they have evolved large, thick shells with a small opening to withstand crab attacks. The wave or E-ecotype (E = exposed, which sounds like we are dealing with a naked snail) is constantly being bombarded by waves crashing onto the rocks and has subsequently evolved thin shells with large openings.


Shelly Genetics

These ecotypes cannot survive in each others habitat, as shown by transplant experiments. This is similar to plucking a businessman from Wall Street and dropping him in the middle of the African savanna. I don’t think he will be able to negotiate with a pack of lions (but see cartoon…). To get back to the snails, the two ecotypes do interbreed in narrow hybrid zones. And these areas provide excellent opportunities to study the genetics of speciation.

Johan Hollander and his colleagues quantified two hybrid zones from a morphological and a genetic perspective. They found that about 5% of the loci (i.e. a specific location in a gene) were more divergent between the ecotypes than expected. In one hybrid zone (located on the island Ramsholmen in Sweden), three loci were also associated with certain shell characteristics.

A businessman on the African savanna...
A businessman on the African savanna…

Portuguese Missing Links

This might not sound like exciting stuff for a layman. However, the evolution of shell shapes in snails is one of the most interesting processes to document. During my Master in Antwerp (Belgium), I studied the evolution of a group of snails on the Azores (Portugal) that go by the name Plutonia. These slimy critters still have a shell, but this is so small that they are unable to hide in it. Even more intriguing, one species (Plutonia atlantica) carries the shell inside its body. These snails are considered as the link between snails (with a shell) and slugs (without a shell). So, you can add these guys to the list of uncovered missing links, such as Archeopteryx and Tiktaalik. 

Plutonia atlantica
Plutonia atlantica


Hollander J, Galindo J, & Butlin RK (2015). Selection on outlier loci and their association with adaptive phenotypes in Littorina saxatilis contact zones. Journal of evolutionary biology, 28 (2), 328-37 PMID: 25439395

MORDAN, P. (2001). A SYSTEMATIC REVISION OF THE VITRINID SEMISLUGS OF THE AZORES (GASTROPODA:PULMONATA) Journal Molluscan Studies, 67 (3), 343-368 DOI: 10.1093/mollus/67.3.343


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