The development of predictive models of the impacts of exotic species is one of the major goals of invasion biology. Understanding the mechanisms by which introduced species affect their host assemblages is an important prerequisite for the development of such models. Here we describe experiments that tested hypotheses about the effects and mechanisms of a recently introduced polychaete, Sabella spallanzanii, on other sessile invertebrates in sessile epifaunal assemblages in southern Australia. Substantial effects were considered likely because sabellids reach high densities in some areas and form a canopy of feeding fans that might affect recruitment to the substratum. Sabellids were cleared from pilings, and then recruitment to plates on cleared and uncleared pilings was compared. Different treatments were also applied to the settlement plates to test for effects at a smaller scale than that of entire pilings: plates either had ambient densities of fanworms attached, had fanworm mimics, or were left bare. Plates with mimics were used to test for the effects of physical structure on recruitment, and plates with live worms were compared to these to test for biological effects. S. spallanzanii affected the recruitment of several sessile taxa. On the larger (piling) scale, recruitment of barnacles, bryozoans, and one sponge was inhibited beneath fanworms, while recruitment of another sponge was enhanced. On the smaller scale, sponges, serpulid polychaetes, barnacles, and bryozoans were affected, but effects were mostly inhibitory at one site and positive at another. These effects appeared to be mostly due to physical structure (worm mimics produced the same results as real worms). Most taxa recruited to S. spallanzanii and mimic tubes, but this mechanism only caused significant enhancement of overall recruitment for a few taxa. Sampling of the water column around cleared and uncleared pilings showed that the abundance of planktonic organisms, including larvae, was lower beneath fanworm canopies. The results show that S. spallanzanii strongly influences recruitment of other sessile taxa, but the effects varied between spatial scales and sites and among taxa. This implies that development of predictive models of the effects of similar invaders will require detailed knowledge of the responses of individual species that comprise local assemblages.