Vacant niches in a species-poor system: resource partitioning based on diet quality for a Mexican clam in the Baltic Sea

Invasive species are often generalist species that can take advantage of formerly unexploited resources. The existence of such vacant niches is more likely in species-poor systems like the Baltic Sea. The suspension-feeding wedge clam, Rangia cuneata , native to estuarine environments in the Gulf of Mexico, was sighted for the first time in the southeastern Baltic in 2010 and a few years later in the north along the Swedish coast. To explore possible competition for food resources between R. cuneata and the three native clams inhabiting Baltic shallow soft bottoms, stable isotope and fatty acid analyses were conducted. There was no overlap between R. cuneata and any of the native species in neither stable isotope nor fatty acid niches. This suggests efficient partitioning of resources; multivariate analyses indicate that separation was driven mainly by δ ¹³ C and by certain fatty acids reflecting different phytoplankton groups. R. cuneata reflected seasonal variation in phytoplankton more than other clams and contained more of fatty acid biomarkers for cyanobacteria reflecting higher trophic plasticity. In conclusion, the addition of R. cuneata to the Baltic shallow soft bottoms suggest the existence of a vacant niche, however long-term effects on other species and on nutrients cycling require further studies.