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.
Handling Editor: Mario BraunsAlthough the influence of invasive species on the distribution of native species is well-studied, our knowledge on the processes driving the co-occurrence of native gammarid species is limited. We studied the co-occurrence of two common European gammarids, Gammarus fossarum and Gammarus roeselii, along a stream continuum in Mecsek Mountains, SW Hungary.We hypothesized that (1) the large scale distribution is influenced by microhabitat diversity;(2) the distribution of each species is equal among the available microhabitats in the absence of a competitor; (3) microhabitat preference is evident in the case of coexistence; and (4) the mesohabitat structure influences the microhabitat preference. A longitudinal gradient was observed along the stream: The dominance of G. fossarum at upstream sections continuously transitioned into the dominance of G. roeselii through an intermediate section, where both species coexisted. This distribution cannot be completely explained by the microhabitat diversity. In cases of single species occurrences, no microhabitat preference was found in G. fossarum, whereas G. roeselii preferred lithal and biotic microhabitats. The presence of a competitor significantly modified the microdistribution: G. roeselii retreated from lithal microhabitats and G. fossarum almost disappeared from gravel. Moreover, the location of microhabitats within mesohabitats significantly affected the distribution of gammarids. Both species occurred in meso-and microhabitat combination resembling their habitat usually occupied along the stream continuum: G. fossarum was an indicator species for the fast-flowing shallow riffles with the dominance of lithal microhabitat, whereas G. roeselii was abundant in the slow-flowing deeper pools usually characterized by vegetation or organic matter. Our results suggest that, beyond the large scale longitudinal distributional pattern and species-specific lifecycles, the meso-and microhabitat characteristics of a stream can drive stable and long-term coexistence among G. fossarum and G. roeselii. K E Y W O R D S co-occurrence, Gammarus fossarum, Gammarus roeselii, habitat segregation, microhabitat preference
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