Hitch-hiking snails

Geneticist Richard Preece, from the University of Cambridge, explains to Chris Smith how he has discovered evidence that snails hitch-hiked aboard migrating birds to travel from Europe to remote islands in the South Atlantic...

Richard - We wanted to understand how the snails on the Tristan da Cuhna Islands got there. We had suspicions that they might be related to a species that occurs in Europe, but what we've shown here is that these suspicions are very well founded. They are indeed exactly the same genus as the European one.

Chris - So the outstanding question is how does a single snail get to the islands. It doesn't swim and it doesn't have wings.

Richard - The most plausible way of getting there is to hitch hike on a migratory bird, maybe a wader. Waders are often found as stray vagrants on mid-Atlantic islands, and that would seem to be most likely vector.

Chris - How big are these snails then?

Richard - The adult snails are 7 millimetres, or maybe a little bit more when adult. These snails presumably weren't transported necessarily as adults. They might well have been transported as eggs or as juveniles. One of the things about these particular group of snails is that they have a very sticky slime. It's very noticeable if you tap them that the slime is tenacious and will stick to your fingers very readily.

Chris - So do you think that on eof these young snail would potentially have stuck itself onto a bird somewhere in Europe and then made its way ultimately 9000 kilometres away to the Tristan da Cuhna Islands?

Richard - Well it may not have done this in one leap. We know that these snails are hermaphrodite, so you only need a single individual to found a colony. That's quite important. It's quite interesting that this genus of snail also occurs in North Atlantic Islands, particularly on Madeira and on the Azores.

Chris - So it might have been some stepping stone down there then.

Richard - It could have been a stepping stone there, yes.

Chris - Just to play devil's advocate for a minute, how do you know that this wasn't a boat?

Richard - We know that the Tristan da Cuhna Islands were only discovered in 1506, and there are eight species of this snail on the islands. It is unprecedented for eight species of snail to have arisen in the 500 years that has elapsed since the discovery of the island.

Chris - When would that put the time at which the snails first took up residence?

Richard - We haven't got an accurate molecular clock to be able to pinpoint that, but perhaps in the future we might be able to get a better handle on that.

Chris - And given that these snails have travelled so far, do this make them the world's longest hitch hikers?

Richard - Well I think for a snail I don't know of a better example. It was quite interesting that way back in 1921 there was an article in Nature reporting for the first time the occurrence of one of these snails on the top of the mountain on Madeira. The article was claiming that that was an example of transportation by birds to that remote mountain peak. What we've shown here is not just that they were able to get to Madeira, but that they were able to get a good deal further, across the equator and right down into the South Atlantic.