I investigated whether Neogobius melanostomus, an invader of biodiversity “hot-spots” in Ontario facilitates or inhibits unionid mussel recruitment by serving as a host or as a sink for their parasitic larvae (glochidia). Infestation and metamorphosis rates of four mussel Species at Risk (Epioblasma torulosa rangiana, Epioblasma triquetra, Lampsilis fasciola, and Villosa iris) and one common species (Actinonaias ligamentina) on N. melanostomus were compared to rates on known hosts in the laboratory. All species successfully infested N. melanostomus, but only E. triquetra, V. iris, and A. ligamentina successfully metamorphosed, albeit at low rates. Neogobius melanostomus collected from areas of unionid occurrence in the Grand and Sydenham rivers exhibited body burdens of 39.4% and 5.1%, respectively. Analyses indicate that N. melanostomus serves more as a sink for glochidia than as a host for unionids, thereby limiting recruitment, which is a novel way by which N. melanostomus is affecting native mussel species.