A tale about aquatic snails in the middle of the desert

Nowadays, central Australia is mostly a huge desert. It was not always like that: only five million years ago, this area had a completely different look, with humid forests, rivers, streams and lakes, a strong contrast to the current dry landscape. A significant portion of the Australian desert is, actually, full of water. Water that is not on the surface, but underground, forming one of the largest aquifer in the World: the Great Artesian Basin (GAB). Ocassionally, water of the GAB can reach the surface at very particular spots, generating artesian springs. They are about one million years old, and often constitute the only water source in many kilometers around, hosting a thriving community of small plants and animals that otherwise would not survive in the desert. The organisms living in such unique habitats have attracted the attention of scientists for a long time: fragile and isolated, their presence is difficult to explain. Take, for instance, the aquatic snails of the genus Trochidrobia. They are minute, delicate mollusks, endemic to a restricted area of central Australia, many of them at the edge of extinction; their lives depend entirely on these isolated groundwater springs, since they are not able to go elsewhere. A lot of questions will arise if we think about these striking snails. Did they arrive there by themselves? Were they “brought” to the springs? If so, how? If not, how and for how long were they present in that inhospitable area? In short: which is their story?

Organisms are not just present everywhere, we all know. It is not only that we assume them to be adapted to distinct environmental conditions, to a particular ecological niche; there is something else. Each taxon shows a distinct geographic distribution pattern that sometimes can be intriguing. Why can we find a species of shrub in these mountains, but it is absent elsewhere in the World? Why is it possible to find the same lizard in two particular islands but not in a third one of the same archipelago? Perceptive people have been wondering about these patterns since the early beginnings of natural history. Indeed, biogeography (the discipline that studies these issues) played a capital role in the development of the current evolutionary paradigm, and both, Darwin and Wallace, wondered about species distributions on islands. While the implication of the evolutionary process to answer these questions became evident more than a century ago, the actual reconstruction of the “story” that underlies each pattern was something that we could mainly hypothesize. We are living exciting times, since now we have the tools to reconstruct many of these stories, like the Trochidrobia snails one.

The puzzle was addressed ¹ by researchers from La Trobe University, The University of Adelaide and the South Australian Museum, who used some of the methods that are currently making possible to understand many biogeographical patterns. They sampled snails of the species Trochidrobia punicea, T. smithi and T. minuta along different complexes of GAB springs. They extracted their DNA, amplified specific regions of their genomes and compared all the obtained sequences. These DNA sequences are rather similar among all the sampled snails because they are homologous, this is, they derive from a common ancestral sequence (as well as all these snails evolved from a common ancestor). The divergence of the sequences along time from that ancestral sequence is caused by the accumulation of mutations on the DNA along time. Take two of these sequences: the more similar they are, the more recent was the last common ancestor that the two sampled snails share. Diverse statistical analyses allowed the authors to compare the similarity of all the studied sequences, resulting in a phylogenetic tree: a reconstruction of the evolution of this genus of snails. Assuming a rate of mutation (often called “molecular clock”) it is even possible to estimate when two lineages or species split. These are all usual ingredients of modern biogeography storytelling.

The phylogenetic tree pictures the relations between the three studied species of Trochidrobia, each of them sustained by a unique “stem” with high statistical support. Each species “stem” tracks back quite deep in the phylogeny of the group, and the splitting of the modern snail species (speciation events) are estimated at about 3.4 and 5.1 million years before present. This means that these snails were probably present in the area before the springs were formed, but after the beginning of the aridification trend. The scarce branching at the base of the species stems suggest that the current populations represents just a small fraction of the past genetic diversity, when there were still rivers and lakes in the area which progressively dried out. Trochidrobia snails are, all of them, survivors.

Within the species Trochidrobia punicea and T. smithi, some intraspecific lineages are also found (named with letters). They have strong statistical support, and cluster together snails living in the same geographical spring complex. Although some genetic flow within the same spring group cannot be rejected, there is no sign of genetic exchange among snail populations of different spring complexes, suggesting that each lineage has remained isolated throughout most its existence. Interestingly, the molecular clock estimates the origin of all these intraspecific lineages within an interval fully compatible with the origin of the springs, about one million years ago.

We have now the pieces to start the reconstruction of the story of these snails: we knew that before the Pliocene, central Australia was a much moister place, with rainforests, rivers and swamps. We have learnt that the ancestors of the Trochidrobia snails lived there, together with a rich biota, (including flamingoes or crocodiles) now vanished. During the past 5 million years, the desert progressively became the dominant habitat. The modern species of Trochidrobia arose and survived in the shrinking freshwater sources, becoming scarcer with time. It is reasonable to think that the greatest part of this diversity went extinct and that other sister species disappeared during this period. Finally, when the GAB springs were formed, they were simultaneously colonized by the surviving snails already present in the area becoming truly isolated, “trapped in the desert”.

Trochidrobia snails are relictual organisms, a treasure of Australian biodiversity. Their story reminds us the limitations of human time scale to understand what we see today. Five million years is a really short time for geological standards, but even that lapse has been enough to raise an enigmatic biogeographical pattern.