Seawater is a dense microbial suspension with >10 6 prokaryotic and >10 4 eukaryotic propagules per milliliter. Hence, submerged surfaces get immediately covered by biofilm-forming colonizers upon contact with seawater. Since biofilms may reduce individual fitness through decreasing motility and attractiveness or increasing shearing stress by water currents and infection risk by pathogens, marine organisms have evolved countermeasures to regulate the number of surface-colonizers; alternatively they tolerate settlement and biofilm-formation. Antimicrobial defense mechanisms co-evolved with potentially colonizing microbes. By contrast, non-native animals (neozoa) are confronted with novel microbial colonizers upon colonizing a new habitat, and are expected to be less well protected against surface-colonization. Here we present results of a thorough screening of the epithelial surface of the ctenophore Mnemiopsis leidyi, being non-native in European marine environments, for epithelial bacteria and archaea. Neither light-and electronmicroscopic inspection nor PCR-screening for bacterial and archaeal DNA of 134 adult specimens from different collection sites in the Western Baltic revealed any presence of prokaryotes on the surface epithelium of comb jellies in a recently invaded environment. A limited number of bacterial associates became evident from whole-body extracts of both juvenile and adult comb jellies. Their taxonomic diversity, however, was significantly lower in adult than in juvenile specimens, suggesting a maturation of anti-microbial defense upon ontogenetic development. The mechanisms underlying the effective defense of Mnemiopsis against microbial colonization, however, remain unknown. Based on our findings, we propose 1) to make use of invasion events as natural space-for-time experiments on how symbiotic interactions change upon environmental change; and 2) to study basal metazoan animals, such as ctenophores, to understand the evolutionary ba-* Corresponding author. S. Hammann et al. 238sics of symbiont-host interactions.