Introduction, spread and establishment of the invasive clam "Corbicula" spp. in Switzerland

Invasive species are of great concern in conservation biology, in economy and as well as in human health. River and lakes are vulnerable ecosystems and are prone to human-caused introductions of non-indigenous species. Research on the ecology of invasive species, their impact on native communities and the introduction pathways is needed to manage these invasive species and to prevent further introductions.
The present thesis concentrates on aquatic invasive invertebrate species and documents the spread of the Asian clam Corbicula spp. into Switzerland. In particular, we focused on Corbicula fluminea and examined its ecological preference, population structure and dispersal in the river Rhine. Furthermore, we analysed the clam’s and other non-indigenous molluscs’ possible impact on the native biodiversity in a sandy flat in Lake Neuchâtel.
A literature survey about the influence of invasive non-indigenous species on the biodiversity in the river Rhine documented great changes in river morphology, water quality and species composition during the last century. River engineering, pollution and the opening of canals connecting formerly separated river systems, allowed the invasion of many new species. Some of them reached extraordinary densities and hampered the development of native taxa. Many typically riverine species were replaced by large numbers of euryoecious and non-indigenous species. Competition for food and space, the hosting of parasites and intraguild predation were the main biological mechanisms underlying the alteration of the river community. Most studies on invasive species showed negative correlations between introduced invasive species and native biodiversity but experimental evidence is so far lacking.
The Asian clam C. fluminea has been introduced in the Lower Rhine with ballast water of cargo ships in the second half of the 1980ties and was first recorded in Basel, Switzerland, in 1995. There, the clam established a well-structured population. It occurred initially in the internationally navigable section of the river Rhine and in the Canal de Huningue, which obtains water containing clam larvae from the river Rhine. Our field survey showed that the clam preferred slowly flowing, shallow water sites with fine-grained sediment, mainly sand. A substratum-choice experiment in the river Rhine confirmed the empirical evidence.
In spring 2011, there is still no evidence that the clam has colonized any tributaries of the Rhine in the region of Basel. However, in 2003, C. fluminea passed a weir and was found 22 km upstream of Basel where cargo shipping is absent. In the same year, records were made in Lake Constance and in Lake Neuchâtel and subsequently in other Swiss lowland lakes. Today, their interconnecting rivers have not yet been colonized apart from some river sections near lake outlets and the river Aare between Aarau and its confluence with the river Rhine.
We present evidence for separate introductions of an identical haplotype of C. fluminea into five lakes in Switzerland based on mitochondrial DNA-sequences of the COI in specimens sampled. This indicates passive dispersal of the clam by human activities and/or waterfowl. All but one of the 72 genetically analysed individuals were assigned to a single haplotype of C. fluminea. A second haplotype belonging to the sister species Corbicula fluminalis was found only in the river Rhine near Basel. Recent records of specimens which were not genetically analysed all showed the shell morphology of C. fluminea, confirming the dominance of this clam in Swiss rivers and lakes. Passive dispersal by waterfowl, recreational boating and other human activities may facilitate the further spread of C. fluminea in Switzerland. However, the clam might be less successful in colonizing rivers with rapid current, which might be important refuges for native species.
We investigated a sandy flat in Lake Neuchâtel using SCUBA diving to assess the abundance of macroinvertebrate species, and to analyse the impact of non-indigenous mollusc species (C. fluminea, Dreissena polymorpha, Potamopyrgus antipodarum and Lithoglyphus naticoides) on the native macroinvertebrate community. The non-indigenous molluscs dominated the sand flat in numbers and influenced the composition and diversity of native macroinvertebrates, mostly by transforming the sandy substratum into a partly hard substratum habitat. Differences observed in community composition between shallow (< 3.5 m) and deep sites (≥ 5 m) resulted to a certain extent from the reduced abundance of D. polymorpha at shallow sites most probably due to depth-selective feeding of ducks. A shell decay study showed that the shells of C. fluminea and D. polymorpha persist for a longer period in the sediment than those of native molluscs. Therefore, shells of these non-indigenous molluscs have a long-lasting impact through modification of sandy habitat. Several native taxa benefit from such ecosystem engineering, but for other taxa with more specific habitat requirements it might be harmful.
In Lake Neuchâtel, high densities of C. fluminea negatively influenced the abundance of sand preferring native taxa. Negative impacts of C. fluminea in industrial facilities such as clogging of water pipes along the river Rhine are also known. Consequently, we recommend classifying C. fluminea as an invasive species in Switzerland and further monitoring and studying this clam.