LAUSR

The Asian clam (Corbicula fluminea) is one of the rapidly spreading, very successful aquatic invasive species, which has become established widely in many parts of the world. Its spread is assumed to be by both passive and active dispersal. However, the importance of active pedal movement in dispersal is hardly known. Since there was no direct evidence of this phenomenon, field observations were combined with laboratory experiments to find out if the clams move upstream actively, and how this is affected by the quality of the substrate, the density of the clams, and the water velocity. Field observations were conducted at a small watercourse with no waterborne transport. Experiments were done in an indoor artificial stream system, where the distances moved by adult clams were measured via digital image analysis. Substrate grain size, starting density of clams, and water velocity significantly affected clam movement. Fine grain sediment and slow flow velocity both facilitated spread, while there was no clear pattern of density-dependent dispersal. Also, we found no clear preference for either upstream or downstream movement. The maximum distance moved in the lab experiments predicts no more than 0.15 km/y active pedal movement in an upstream direction, while our field observations detected a much faster (0.5–11 km/y) upstream movement, which might be explained by passive dispersal, such as via human transport and ecto- or endozoochory. Overall, it seems that active movement of the species cannot read to long-distance migration.