Dispersal influences several ecological and evolutionary processes, such as intraspecific competition, genetic drift and inbreeding. It can lead to phenotypic mismatch with the habitat when a locally adapted individual winds up in an environment with a divergent selection regime compared to the source habitat. The aim of this project was to compare dispersive traits in the freshwater isopod Asellus aquaticus from a cave habitat, with surface dwelling isopods collected upstream and downstream from the cave system. The subterranean stream (cave) represents a rare, geographically limited habitat which has a divergent selective pressure compared to the surrounding habitats. Experiments on dispersal were performed in the laboratory, in darkness with IR-equipment for visualization. Displacement was measured using one-dimensional test arenas. Compared to the surface phenotype, the cave phenotype was expected to have reduced fitness outside of the cave and unlikely to successfully disperse to new areas of similar suitable conditions. The results did not follow my main hypothesis that isopods from the cave would be less dispersive than individuals from the surface. The inconclusive results might derive from large variation in the data and divergent adaptations which yield similar expression of dispersal.

Knowledge about dispersion is of utmost importance for understanding populations’ reaction to changes in the environment. Expansion of a population range brings with it both spatial sorting and over time, spatial selection. This means that dispersion rates increases over time at the expanding edge. Most studies have so far been performed on continuously expanding populations. This study aims to bring more knowledge about dispersal biology in dynamic systems. I studied dispersal traits in two permanent and two seasonal vegetation habitats of an isopod (Asellus aquaticus), for which differentiation between habitat types has previously been shown. I quantified differences in displacement (dispersal rate) and three morphological traits, head angle (body streamline) and leg of the third and seventh pair of legs. Isopods from the seasonal vegetation had higher displacement rates than animals from permanent vegetation. This inclines that mechanisms driving spatial selection in expanding population ranges also exist in dynamic systems. The more streamlined isopods found in seasonal sites further points towards spatial sorting by dispersion capability. Because no effect of permanence was found on leg length and there was no correlation between streamlining and displacement, the higher dispersion among animals from seasonal habitats most likely derives from behavioral differences.