Many bottom-dwelling marine invertebrates produce free-swimming larvae that colonize new surfaces. To identify a suitable habitat, these motile larvae preferentially settle on areas that are covered by bacterial biofilms. However, the bacterial cues and animal sensory systems required to mediate this microbe-animal interaction are poorly understood. Our lab discovered that larvae of the marine tubeworm Hydroides elegans are induced to settle by an array of phage tail-like structures termed MACs (Metamorphosis Associated Contractile structures) produced by the marine bacterium Pseudoalteromonas luteoviolacea. MACs are genetically and evolutionarily related to the contractile tails of bacteriophage where each structure contains an inner tube surrounded by a contractile sheath, a tail spike at the end of the tube and a baseplate complex. Contraction of the sheath drives the inner tube and tail spike through the target cell membrane and often deliver effectors to host cells. In this study, we discovered a putative effector that we hypothesize is injected by MACs into Hydroides larvae causing them to settle and metamorphose. Using molecular cloning, metamorphosis assays, and mass spectrometry, we tested the ability of P. luteoviolacea mutants to induce Hydroides metamorphosis. We found a gene product that is necessary for the MAC’s ability to induce metamorphosis that was termed JF50_12615. Intriguingly, JF50_12615 mutants lack electron dense cargo within the contractile MAC structure when observed by Electron Cryo-Tomography. Our results show that cargo found in MAC tails is responsible for P. luteoviolacea’s ability to induce the metamorphosis of the marine tubeworm H. elegans. Our findings are the first to identify a bacterial effector directly mediating animal metamorphosis.