Corbicula fluminea invasion in Lake Maggiore: a three years ecological study.

The number of reports of non-indigenous species (NIS) of the aquatic environments are increasing rapidly worldwide. The rate of invasions of freshwater environments is rising in many countries, mainly caused by an increase in human activities. The growing interest in invasive species is due to their negative impact on biodiversity and ecosystem functioning. Some species are recognized among the most dangerous invasive species of freshwater ecosystems (eg C. fluminea, D. polymorpha and A. woodiana). This study aims to evaluate the C. fluminea invasion in Lake Maggiore, inspecting different chraracteristics of populations settled in southern littoral part. Field and semi-natural condition observations are carried out to evaluate growth and reproductive rate of Corbicula in the environment recently colonized. Lab experiments were performed to measure filtration rate. To evaluate possible competitive interactions between alien and native species, dietary sources and trophic niche overlap were determined through measurements of the stable isotope compositions of C (δ13C) and N (δ15N) respectively. Lake Maggiore offers a favorable environment for the establishment of the Asian clam Corbicula fluminea. The picture that emerges from the study of C. fluminea populations is quite complex, with differences among the five sampling sites. Larger size class organisms are well represented in Angera and Arona (southern sites), while are (still) almost absent in Feriolo and Monvalle (northern sites). The density and biomass of C. fluminea observed are rather high, with peaks reaching up to 3000 ind.m-2 and 2500 g m-2. The populations size structure provides clues to understand the process of invasion and its phases. The populations size structure in the different sites presents wide differences, indicative of different demographic dynamics. The timing of colonization of Lake Maggiore seems to follow this order: Monvalle <Feriolo <Brebbia <Angera <Arona. It is also possible to assume that the colonization of this environment began in 2007, for expansion of the population already established in its effluent, the river Ticino. The reproductive period of C. fluminea in Lake Maggiore showed a single reproductive peak, from late Spring to Autumn. The recruitment of new organisms is scarce at the site of Arona, while the opposite situation is observed in Feriolo. The number of larvae produced by each adult is highly variable, with values ranging from a few hundred to 16000 larvae per individual. Generally in the sites of Angera, Brebbia, Feriolo and Monvalle mortality is rather low, while at Arona is considerably higher. At Angera was observed a phenomenon of mass mortality in Autumn 2012. C. fluminea has a high growth rate and in this environment there are no factors able to limit the growth and spread of this species. The growth rates are affected by seasonal variations in temperature: in Summer and Autumn organisms show higher growth rate values than in the other seasons (almost 2 times higher). The growth rate is higher for small size organisms, as expected comparing young and adults organisms of the same species. In spite of similar functional roles, Unionids and C. fluminea are likely to differ in filtering efficiency and trophic niche. Our preliminary results suggest that Corbicula has a noticeably higher filtration capacity than the most abundant Unionid species (U. elongatulus) and is able to shift its diet along the year. The results of stable isotope analysis highlight a niche overlap between C. fluminea and native species of bivalves, indicating that C. fluminea has lower trophic fidelity than native Unionids. Feeding plasticity of C. fluminea is likely of major importance for competing successfully indigenous bivalves in Lake Maggiore, thus promoting the overdomination of the alien species in the area. Indeed, Corbicula seems to stand already as a keystone littoral species in Lake Maggiore, likely being a strong competitor for space and food to native mussels. However, besides the competition with bivalve invasive species the decline observed for the native species of Lake Maggiore (U. elongatulus abundance decreases about 90 % from 2004 and 2012) is likely due to several factors. Severe drought is responsible for periodical mass mortalities which affect both native Unionids and invasive clams, but these latter can recover and recolonize the environment more rapidly than native species. The specimens that moving from one environment to another can bring along their parasites, which can find the conditions necessary to establish a new population. In the course of this study two new species of parasites of Unionids (cercariae of Bucephalidae and Rhodeus amarus) have been found. The infection of the gonads due to cercariae leads to the castration of bivalves and can be fatal, while the presence of the larvae of R. amarus in the gills reduces the rate of respiration and filtration. The presence of D. polymorpha is recognized in the lake since the 1990’s and is often found anchored to the shell of other bivalve species. High infestation of Dreissena can cause the death by asphyxia. The combination of all these factors provides sufficient explanation for the observed decline of native species, however, the invasion of C. fluminea appears to be a worrying phenomenon. After 3-4 years from the entry into the lake this clam had already colonized a third of the shoreline. To date, this bivalve is found in all suitable habitats distributed along the entire perimeter of the lake. It is possible to conclude that eradication is an expensive solution and the methodologies used often have negative impacts on native species. Moreover, it is not possible to eradicate a species from the environment, in addiction the high growth rate and the plasticity of the alien species recorded in Lake Maggiore favour them respect native species. Mass mortality events can promote the replacement of the native by alien species.