Summary 1. Understanding the impacts of invasive species on natural ecosystems is an important component of developing management strategies. Habitat-forming invasive plants and sessile invertebrates often support a high diversity and abundance of native fauna, suggesting some benefits of invasion. However, the fitness responses of these native fauna, and thus the net benefit from their association with habitat-forming invasive species, are not well understood. 2. We determined how fitness-related life-history traits, patterns of resource allocation among life-history traits, and survivorship of an abundant bivalve, Anadara trapezia , responded to invasion by the habitat-forming seaweed, Caulerpa taxifolia , by transplanting A. trapezia into invaded and uninvaded habitats over a 12-month period. 3. Although A. trapezia recruits into C. taxifolia in high numbers, adult growth, body condition, shell condition, female reproduction and survivorship were all significantly lower in C. taxifolia compared to unvegetated sediment. Notably, we observed high mortality in C. taxifolia after heavy rainfall events, highlighting a potential link between sublethal effects on condition, stochastic environmental perturbation and survivorship. 4. In addition to the overall reduction in fitness, there were changes in scaling relationships between reproduction and body size following invasion. Female A. trapezia in C. taxifolia habitat allocated proportionally more resources to reproduction (including reproductive tissue and number of eggs per follicle) than those in unvegetated sediment despite their poor condition. Maximizing reproduction following invasion may impose a further cost to already stressed A. trapezia and contribute to the higher mortality observed when living in C. taxifolia . 5. Synthesis and applications . The full impact of habitat-forming invasive species is complex and understanding it cannot be based solely on descriptions of native species diversity or abundance. Our study has identified how the presence of long-lived species within habitat-forming invasive species may simply indicate an extinction debt. A decline in the fitness of A. trapezia in C. taxifolia appears to increase its probability of mortality in the long-term. We recommend that management approaches for C. taxifolia and other habitat-forming invasive species combine an understanding of impacts on species diversity, abundance and the fitness of associated fauna to provide a more pluralistic understanding of their effects.