Strong population genetic structuring for ocean island caecilian

Long distance dispersal of animals with low vagility has been treated as an enigma in biogeography. Truly oceanic islands (those which arose without ever having contact with the mainland) are excellent places for these investigations as all flora and fauna must have dispersed over an oceanographic barrier (see le Roux et al 2014; de Queiroz 2014). The island of São Tomé in the Gulf of Guinea turns out to be particularly interesting as it has a number of endemic amphibians, themselves intolerant of sea-water, and for whom a particular form of rafting was postulated by C·I·B core team member, John Measey. In their article, Measey et al (2007) suggested that salt intolerant amphibians could have reached São Tomé and the other Gulf of Guinea islands, via rafts that float on freshwater outflow from the Congo River.

The São Tomé caecilian, Schistometopum thomense

The São Tomé caecilian, Schistometopum thomense

In an article just published in the scientific journal PLoS-ONE, Measey together with colleagues Stoelting and Drewes investigated the population genetics of one such amphibian endemic to the island of São Tomé, the São Tomé caecilian, Schistometopum thomense. In the first population genetic study of any caecilian amphibian, Stoelting et al (2014) found that there was very high geographic structuring on the island, which itself is only 45 km long. “On the one hand, it’s what you might expect from a small animal that lives underground and burrows through the soil,” said Stoelting, “but on the other it’s a very small island and, given its age [˜13 Mya] and apparent absence of competitors, you might have expected that the animals would have had time to disperse throughout.” The team suggested that the deep genetic structuring they found in the mitochondrial DNA might be a result of volcanic eruptions on the island. “These may have produced isolated pockets of caecilians that were then able to recolonize areas as they became vegetated”: explained Measey.

The article offers key insights into the invasion process, and shows that even species which have dispersed thousands of kilometres may still have major challenges to dispersing just a few kilometres in their new range. Measey contrasted the findings in the two studies: “It’s a little like taking 10 hours to fly 10 000 km between Cape Town and London, and then taking five hours to walk the 20 km from the airport into central London.”

Read the paper:

Stoelting RE, Measey GJ, Drewes RC (2014) Population genetics of the São Tomé Caecilian (Gymnophiona: Dermophiidae: Schistometopum thomense) reveals strong geographic structuring. PLoS ONE 9(8): e104628. doi:10.1371/journal.pone.0104628

For more information contact John Measey at

Further reading

  • De Queiroz, A. (2014). The monkey's voyage: how improbable journeys shaped the history of life. Basic Books.
  • Measey, GJ, Vences, M, Drewes, RC, Chiari, Y, Melo, M, & Bourles, B (2007). Freshwater paths across the ocean: molecular phylogeny of the frog Ptychadena newtoni gives insights into amphibian colonization of oceanic islands. Journal of Biogeography, 34(1), 7-20.
  • Le Roux, J. J., Strasberg, D., Rouget, M., Morden, C. W., Koordom, M., & Richardson, D. M. (2014). Relatedness defies biogeography: the tale of two island endemics (Acacia heterophylla and A. koa). New Phytologist, 204: 230–242. doi: 10.1111/nph.12900.