Description: Bioinvasions are amongst the aspects of global change. Continual ship traffic, in particular increasing during the past 250 years, caused an exponential increase in the number of species introductions into new environments. However, not all transported species become successful invaders. Unfavorable transport conditions in particular kill high proportions of species in transit. Species comprising the ability to maintain fitness under adverse conditions may survive the transport more likely than species lacking this ability. Hence, stress robust species may become successful invaders more frequently. In fact, stress tolerance has been shown to be different between native and invasive species. However, the reason why invasive species are more robust than natives against stress has yet not been investigated. Possibly, the higher stress tolerance is caused by evolutionary changes which made certain species in general more tolerant to abiotic stress than species remaining in their native ranges. Also possible is that the conditions during the invasion process select for genotypes with a high robustness to abiotic stress. As a consequence, invasive populations comprise a higher mean stress tolerance to their physical environment than native populations. Therefore, I looked at tolerances to physical stress of an invasive population of Crepidula fornicata and a native population of Ciona intestinalis, in the Irish Sea. Comparable experiments were conducted with a native and a second invasive population of C. fornicata, originated at the east and west coasts of North America, and one invasive population of C. intestinalis, sampled in Japan. I applied two different stresses in an experiment with two sequential stress phases. An intermediate recovery phase should assure that individuals entering the second stress phase comprise only individuals with a high stress tolerance. This approach addresses a putative pre-selection for genotypes with a high robustness to stress. In small pilot studies individuals of both species were subjected to different levels of varying environmental stresses, among them hyposalinity, thermal stress during submersion, and oxygen depletion. Hyposalinity could be found to be the only significant stress for C. fornicata. Hence, I applied low salinity conditions in both sequential stress phases. Hypoxia and thermal stress could both be found to affect the survival of C. intestinalis significantly. Stress induced mortality was monitored as a measure for stress tolerance in all experiments. Results were analyzed either within or among populations of the same species. Although low salinity conditions in both stress phases influenced the survivorship of C. fornicata significantly no evident for an occurring pre-selection for a robust genotype could be found. Hypoxia affected the survival of C. intestinalis significantly during the first stress phase of the main study. No effect of heat stress and no evident for an occurring pre-selection could be found in the second stress phase. Nevertheless, a difference in the performances of native and invasive populations of the same species was detected. In both species, the invasive population was more tolerant to the applied abiotic stress than the native population. These results suggest that a higher stress tolerance is rather arisen during the invasion process than an effect of evolution. As no evidence for a preselection could be found the increase in stress tolerance of the invasive population may be caused either by multiple invasions introducing a higher genotypic diversity among individuals in the target area or by an increased phenotypic plasticity arisen due to the prevalent abiotic and biotic conditions in the new environment. Which of the two models explains the higher tolerance to abiotic stress better, needs still further investigations