14 April 2005

Darwin’s snails

"No facts seem to me so difficult as those connected with the dispersal of Land Mollusca."
Charles Darwin, letter to J. D. Dana, 29 September 1856

Darwin appears to have spent considerable effort to understand the dispersal mechanisms of animals, especially land snails, that live on isolated islands. This subject comes up repeatedly in his letters. For example, in his letter of 23 May 1855 to W. D. Fox, he announced his plans to test the survival of land snails immersed in sea water1: "I am going to try land-snail shells & their eggs also. in [sic] sea-water." Two years later, in another letter to Fox dated 8 February 1857, he reported his results1: "I have just had a Helix pomatia withstand 14 days well in Salt-water; to my very great surprise." And then again to Charles Lyell three days later1: "I have just had Helix Pomatia quite alive & hearty after 20 days under sea-water; & this same individual about six-weeks ago had a bath of 7 days."

The subject was still on his mind when he wrote to Alfred Russel Wallace on 1 May 18571: "One of the subjects on which I have been experimentising & which cost me much trouble, is the means of distribution of all organic beings found on oceanic islands & any facts on this subject would be most gratefully received: Land-Molluscs are a great perplexity to me."

In On the Origin of Species2, he proposed two plausible mechanisms by which land snails could be dispersed: on birds and on driftwood. Of course, he didn't forget to mention the results of his experiments: "And I found that several species in this state [hibernation] withstand uninjured an immersion in sea-water during seven days: one of these shells was the Helix pomatia, and after it had again hybernated I put it in sea-water for twenty days, and it perfectly recovered. As this species has a thick calcareous operculum, I removed it, and when it had formed a new membranous one, I immersed it for fourteen days in sea-water, and it recovered and crawled away: but more experiments are wanted on this head."

Land snails do get around. In 1883, the island of Krakatau was devastated by a volcanic eruption. The five species of land snails that had been recorded on the island before the eruption has not recolonized it, but two other species were first recorded on the island in 19083. One of these was the tree snail Amphidromus porcellanus that had probably rafted to the island. Darwin would have been fascinated.

1. Burkhardt, F. (editor) 1996. Charles Darwin's Letters. A Selection 1825-1859. Cambridge University Press.
2. Darwin, C. 1859. On the Origin of Species by Means of Natural Selection. Full text
3. Thornton, I. 1996. Krakatau. Harvard University Press.

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