Snail found 2,200 km from home hints at a hidden Pleistocene route across Asia

A small pond snail has turned up in the mountains of Kazakhstan, roughly 2,200 kilometers beyond the western edge of its known range, and its genes are telling a story that surprised the scientists who sequenced them. The species, Radix plicatula, was until now known only from East and Southeast Asia, from southern China and Vietnam to the Russian Far East. Finding it in the northern Tien Shan, within the Lake Balkhash drainage basin, places a distinctly Asian freshwater animal in a region whose mollusk fauna is otherwise dominated by widespread European and Siberian species. The discovery, published in Frontiers of Biogeography, raises a deceptively simple question: did this snail arrive recently with human help, or has it been hiding in Central Asia since the Ice Age?

The answer matters because the freshwater faunas on either side of the Tibetan Plateau are strikingly different. Central Asia hosts few endemic freshwater snails, and most of its species carry the genetic signatures of post-glacial recolonization from northern and western refugia. South and east of the Tibetan Plateau, by contrast, the faunas are highly endemic and deeply structured, the products of long isolation in a geologically restless landscape. Shared species between the two regions are rare, and when they do appear, the explanations range from modern accidental introductions, increasingly common as aquatic ecosystems are transformed, to relicts of ancient dispersal events during warmer, wetter epochs when Eurasia’s rivers and lakes were far more connected than today.

The new populations were found at two sites in southeastern Kazakhstan about 200 kilometers apart. The first was a small pasture pond on the shore of the Kapchagay Reservoir near the city of Konaev, sampled in November 2024 when air temperatures were below freezing and a thin film of ice covered part of the water. Seventeen specimens were collected from silt, stones, and aquatic vegetation. The second site, near the settlement of Chundzha, was a hydrothermal stream fed by an artificially drilled borehole with a constant outlet temperature of 16 to 21 degrees Celsius, where the snails were abundant both inside the borehole and in the outflow. Sixty-nine specimens were taken there. At both sites, the only other gastropod present was Physella acuta, a globally invasive North American species, and no other snails were recorded.

Morphological examination pointed firmly to Radix plicatula. The shells were small, with the largest reaching 8.8 millimeters in height, and the anatomy of the reproductive system, including a penis sheath about equal in length to the praeputium and a distinctive elongated trapezoidal pyriform body, matched recent descriptions of the species. Genetic analysis sealed the identification. The researchers amplified a 469-nucleotide fragment of the mitochondrial cytochrome c oxidase subunit I gene, the standard DNA barcoding marker, and built phylogenetic trees using maximum likelihood and Bayesian inference on a dataset of 65 sequences with Galba truncatula as an outgroup. The Tien Shan specimens fell squarely within the main Radix plicatula lineage, alongside sequences from eastern China near the species’ type locality.

What came next was the real surprise. Across the species’ full range, 235 analyzed sequences revealed 118 haplotypes, and the nine Tien Shan sequences alone contained six distinct haplotypes, yielding a haplotype diversity of 0.89 and a nucleotide diversity of 0.65 percent for a single geographically restricted population. The Tien Shan haplotypes sat just two to three mutational steps from their closest relatives in China, Japan, and Korea, with one haplotype differing by five steps, and they formed a star-like pattern typical of populations descended from a common ancestor that then expanded. Demographic tests reinforced that impression: Fu’s Fs was strongly negative and highly significant, and the mismatch distribution fit a model of sudden demographic expansion.

That level of diversity is hard to reconcile with a recent introduction. Freshwater snails carried by humans, whether through fisheries stocking, aquarium releases, or ballast, typically show pronounced founder effects, with one or a few haplotypes dominating and genetic diversity drastically reduced. Introduced South American populations of Galba truncatula, for example, retain only four COI haplotypes despite at least two independent colonization events, and most invasive populations of Pseudosuccinea columella worldwide belong to a single mitochondrial haplotype. Even the naturally colonizing Icelandic lineage of Ampullaceana balthica, roughly 5,000 years old, carries clear founder signatures with reduced diversity. The Tien Shan snails show a hint of the same star-like structure, but six haplotypes among nine individuals, several of them relatively divergent, looks far more like a population that has persisted and diversified in place than one founded recently from a handful of stowaways.

Could the genetic invasion paradox explain the pattern, with multiple introductions from different sources admixing to restore diversity? The authors consider this unlikely. In documented cases the introduced populations retain recognizable genetic contributions from geographically distant native lineages, which is not what the data show. The Tien Shan haplotypes form a compact group closely related to East Asian lineages and separated from them by only a few substitutions, consistent with expansion within a single evolutionary lineage. Moreover, Radix plicatula is a comparatively localized species, not a global invader repeatedly shuttled between continents, and the usual introduction routes into Central Asia appear closed to it: it has never been recorded in the large water bodies where fishery-associated introductions occur, such as Lake Balkhash, and it is not commonly kept in aquaria.

Rough age estimates from mitochondrial divergence point in the same direction. Depending on whether one assumes COI divergence rates of 1.0, 1.5, or 2.0 percent per million years, the mean within-lineage divergence of the Tien Shan population corresponds to roughly 0.32 to 0.64 million years, and the maximum observed divergence to 0.65 to 1.27 million years. These are coarse approximations rather than formal molecular dates, but they sit comfortably within the Middle to Late Pleistocene and match divergence times reported for other freshwater mollusks whose East Asian and Central Asian populations split during the same era. A recent introduction measured in decades or even centuries cannot plausibly generate this much mitochondrial diversity.

Species distribution modeling added an ecological twist. The team built 19 model variants using four algorithms, generalized linear models, boosted models, random forests, and maximum entropy, combined into weighted ensembles, and trained on occurrence records from GBIF and earlier studies alongside the new Kazakh finds. A model with pseudo-absences drawn from the entire study area performed best on paper, with an AUC of 0.99, but its projection confined the species’ potential niche to East and Southeast Asia, echoing an earlier study. The authors argue this approach is biologically misleading, because it mainly teaches the model to distinguish the region of current occurrence from everywhere else. A model using a restricted 50-kilometer buffer around occurrence records, plus land-cover variables, predicted dramatically broader suitability, sweeping across northern Central Asia and much of Europe. The current range, in other words, is not simply a map of the climate the snail can tolerate; dispersal limits and possibly competition with close relatives such as Radix euphratica appear to hold it back.

The most evocative result came from projecting the niche models onto paleoclimates spanning the Pliocene warm periods, the Last Interglacial, and the Pleistocene glacial cycles. Nearly every projection showed an almost continuous belt of suitable habitat north of the Tibetan Plateau and the great Asian mountain systems, a corridor absent under present-day conditions. Taking the modeling caveats seriously, paleoclimatic reconstructions carry uncertainty, and the Pleistocene projections used six variables instead of eight, the authors interpret this belt as a plausible, previously overlooked dispersal pathway linking East and Central Asia, likely traversed not as continuous rivers but through lake chains and bird-mediated passive transport of the snails themselves. A Tibetan route is considered less likely, since decades of intensive sampling have never found Radix plicatula on the plateau. If the northern corridor hypothesis holds, Central Asia’s freshwater fauna may owe more to Pleistocene contingency than to present-day climate, and the humble snail of the Tien Shan springs may be a living relict of a world far better connected than our maps suggest.

Subject of Research: Phylogeography and species distribution modeling of the pond snail Radix plicatula in Central Asia

Article Title: Recent introduction or evidence for a northern dispersal corridor between East and Central Asia? The case of Radix plicatula

Article References: Recent introduction or evidence for a northern dispersal corridor between East and Central Asia? The case of Radix plicatula. (n.d.). https://doi.org/10.21425/fob.19.194218

Image Credits: AI Generated

DOI: 10.21425/fob.19.194218

Keywords: Radix plicatula, freshwater snails, biogeography, Kazakhstan, Tien Shan, Pleistocene, dispersal corridor, phylogeography, species distribution modeling, mitochondrial DNA, paleoclimate, Tibetan Plateau