Extraordinary snail migration

Meet the rather brave snails who went on quite a journey...
03 November 2020

Interview with 

Richard Preece, Cambridge University

SNAIL BALEA PERVERSA

photo of balea perversa snail

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Now not all migratory species have the capacity to cover great distances unaided. Back in 2006, Cambridge University mollusc expert Richard Preece published research of snails making a rather phenomenal journey, and he told Katie Haylor about his work...

Richard - Some have been shown to travel enormous distances as passengers on the back of birds or other organisms. The ones I'm about to tell you about are about a centimetre long, two or three millimetres in width; long, slender things called Balea. I was fortunate enough to go on an expedition to the Tristan da Cunha islands in 1982. I spent most of the time on an island called Inaccessible Island, which as the name suggests is rather a difficult place to get to! These are volcanic islands right in the middle of the South Atlantic, about midway between South Africa and South America. With my interest, I was obviously keen to look at the species of snail that occur on those islands and nowhere else. Very few people have had the opportunity to do any field work there. I was also keen to see what introduced snails and other things occur on those islands, given their extreme remote position.

Katie - And it turns out that Richard isn't the first to go looking for snails on this island.

Richard - In fact, the first person to have collected snails on Tristan was a chap called Captain Carmichael, sent out to Tristan with the initial garrison; because basically the founding human population there were sent there to try and deny the French a base from which to rescue Napoleon; so we're talking in the immediate aftermath of Waterloo, a British garrison was set up there. And he was obviously bored out of his mind, and he actually resorted to collecting snails amongst other things; and he found some of these snails that no one else had ever seen before.

Katie - The question of how land snails ended up on remote islands was one that Charles Darwin himself pondered.

Richard - He comes into the story in a general way, because one of the things that fascinated him was trying to address the huge problem - how on earth do land snails - with such proverbially poor powers of dispersal - how on earth can they get to such remote islands? And these are islands that are oceanic, so they've never had any connection with mainland at all; so they must have gained access to those islands through some sort of aerial means. And since they don't actually physically fly, some other mechanism must be involved. And this was a subject that fascinated him for years, and indeed his very last paper was written about dispersal in molluscs.

Katie - Richard explained that there are two groups of snails on Tristan - the recent imports, several European species that have got there under human agency, likely from South Africa where they've also been introduced; and then there are two families, Balea and Succinea, which belong to a much more ancient colonisation. And they don't swim; they don't fly; but somehow at least one brave specimen made its way all the way over from Europe. Could they have been stowaways on a boat visiting the island? Well, Chris Smith asked Richard this previously on the Naked Scientists, and Richard explained that since the islands were only discovered in 1506, the amount of variation that now exists couldn't have occurred in that timeframe. Instead, it seems they hitched a ride. So how does a little snail manage to hitch a ride on migratory bird, cling on rather than falling into the depths of the ocean, and not only survive, but thrive, on an island so far from home?

Richard - Well this is a very interesting question, and a lot of people have tried to explore this. There is a little book written in the 1890s trying to document all the cases of passive dispersal in land snails, land and freshwater snails. Bivalves are known to clip themselves on to the legs of intersects or newts; birds have been shot and snails have been found amongst the plumage; but others have been shown to travel through the gut of a duck or something like that, a wader, and have actually survived the passage in the live state. So that is obviously another mechanism.

Katie - Rather handily, these snails are hermaphrodites, allowing a lonesome snail to start a colony potentially. And Richard's done some DNA detective work to check these really are the descendants of European snails.

Richard - I collected material that we dissected, and we showed from the dissections that, in fact, they were anatomically indistinguishable from Balea, European Balea. And we followed that up by looking at some DNA from the snails from Tristan, and indeed, everything points to the fact that they are indeed Balea. That family does not occur in Africa or anywhere in between, so that there's no other source population from which they could have arisen.

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