Cryptic invasion and intraspecific hybridisation in the freshwater snail Theodoxus fluviatilis in the river Rhine

Intraspecific cryptic invasion involves the spread and establishment of non-native genetic lineages within the species’ native range. Individuals of non-native lineages can displace native individuals or hybridise with them. Hybridisation homogenises unique characteristics of geographically distinct populations and thus reduces the potential for future species diversity. In Europe, the freshwater snail Theodoxus fluviatilis has different haplotypes depending on the geographically separated water system. Since 2005, an upstream spread of T. fluviatilis with haplotype F31, which originates in the Ponto-Caspian region, has been recorded in the Danube and later in the river Rhine in Germany. Shortly after the invasion of snails with non-native haplotype, we collected 301 individuals of T. fluviatilis from 11 locations in the Upper and High Rhine. We used a mitochondrial marker (COI) for the determination of the snails’ haplotype: 19 individuals belonged to the native haplotype F28 (6.3 %), 159 (52.8 %) to the non-native haplotype F31 and 123 (40.9 %) to another non-native haplotype “euxinus” (also of Ponto-Caspian origin). Using eight microsatellites we examined whether individuals of the invading non-native haplotypes hybridise with individuals of the native haplotype F28. We found 12 hybrids (4.0 %) among the 301 snails examined. Hybrids occurred in 7 out of the 11 populations examined. We provide evidence that hybrids with the native haplotypes occur as early as the first 1–2 generations after the invasion of the non-native haplotypes. This indicates that the native haplotype of T. fluviatilis is unlikely to persist in the High Rhine in the long term.