Migrating birds may have transported the Balea land snail across vast distances to remote islands.
Abstract
Darwin was fascinated by the transportation of land snails across great swathes of open ocean by birds — he even immersed snails in sea water to see how long they would survive1. Here we follow a molecular phylogenetic trail that reveals the incredible transequatorial dispersal of the land snail Balea from Europe to the Azores and the Tristan da Cunha islands, and back again. This long-distance dispersal is unexpected for what are proverbially considered the most pedestrian of creatures.
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An allegorical picture2 from the sixteenth century shows a bird carrying a snail (Fig. 1), but it was not until 1921 that the possibility of land-snail dispersal by birds was discussed for Balea3. It was suggested that the exceptionally tenacious slime and arboreal habit of Balea perversa, an ovoviviparous hermaphrodite, could aid passive dispersal by birds3. As land snails occur on even the most remote islands, this must be a frequent event, despite the limited number of well documented cases4.
In 1824, John Gray assigned two new species of land snail from the Tristan da Cunha islands to Balea, a genus of Clausiliidae thought to be restricted to the Palaearctic region, which includes northern Africa and Eurasia to the north of the Himalayas5. The Tristan archipelago consists of three main islands situated midway between South Africa and South America at about 37° S, with Gough Island lying about 350 km to the south-southeast (Fig. 2a). It lies 9,000 km and 8,500 km from the Azores and continental Europe, respectively.
On the basis of their extreme biogeographical disjunction, these Tristan land-snail species were subsequently transferred to a new genus, Tristania6. Additional species of Tristania were later described from other islands in the Tristan/Gough group7,8. To reach Tristan from either Europe or the Azores, where two Balea species occur9, a hostile gap of nearly 9,000 km has to be bridged, which hardly seems possible for a vulnerable pulmonate.
Despite renewed claims for the generic distinctness of Tristania, including the erection of the subfamily Tristaniinae10, anatomical evidence indicates that Tristania and Balea belong to the same genus8. We have analysed mitochondrial DNA sequence data for subunit I of cytochrome oxidase for different Balea species and find that they confirm this view: our neighbour-joining phylogenetic tree (Fig. 2b, and see supplementary information) indicates that Azorean and Tristan groups of Balea arose from a single ancestral species.
As the Baleinae originally radiated in the western Palaearctic region11,12, the most parsimonious scenario is that first the ancestral species reached the Azores, then, after additional long-distance dispersal, radiations resulted in two species in the Azores and at least eight in the Tristan/Gough archipelago. One of the Azorean species, B. heydeni, reached Madeira and then ‘returned’ to western Europe, where it exists as a distinct species only recently distinguished from B. perversa13 (the European B. perversa reached Iceland relatively late and remains indistinguishable from conspecific European populations).
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Gittenberger, E., Groenenberg, D., Kokshoorn, B. et al. Molecular trails from hitch-hiking snails. Nature 439, 409 (2006). https://doi.org/10.1038/439409a
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DOI: https://doi.org/10.1038/439409a
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