In a previous post, I discussed the possibility of the transfer of land (and aquatic) snails by birds and listed some literature providing records of snails found on birds. The presence of snails on remote volcanic islands can only be due to transport by humans, birds (and perhaps other animals), floating debris and the wind (also discussed briefly in this post). One such group of islands is the Tristan da Cunha archipelago midway between South Africa and South America. Since these islands were never connected to a continent, all species endemic to them must have evolved from ancestors that arrived after the islands formed volcanically. Therefore, one puzzle in a case like this is to determine who those ancestors were and where they came from.
A group of land snails endemic to the Tristan da Cunha islands were until recently placed in a genus of their own, Tristania (family Clausiliidae). However, anatomical studies suggested that Tristania belonged to the genus Balea, which is represented in Europe and in the Azores by several species of land snails.
Balea perversa. Shells of adults are 8-10 mm long. (Picture from Kerney & Cameron, 1979. A Field Guide to the Land Snails of Britain and North-west Europe.
In a paper published in this week's Nature, Gittenberger et al.1 demonstrate, using mitochondrial DNA sequence comparisons, that the Balea species in Tristan da Cunha (and also those in the Azores) evolved from a single ancestral species that was probably transported from Europe over the ocean first to the Azores and then to the Tristan da Cunha islands.
However, the possible culprits that may have been responsible for the dispersal of these snails are yet to be identified. In the supplementary information to their paper, the authors discount 2 species of migratory birds, the greater shearwater (Puffinus gravis) and the Arctic tern (Sterna paradisaea), because these birds apparently do not come ashore in Europe. They suggest that nonspecific wading birds are more likely to have been responsible for the transport.
To clinch the case, all we need now is a bird caught with live Balea on its body.
1. Gittenberger E., Groenenberg D. S. J., Kokshoorn B.& Preece R. C. 2006. Nature, 439:409.