Understanding the population dynamics of invasive species, such as the green mussel Perna viridis (Linnaeus, 1758), can aid in explaining the success of newly introduced populations and help predict the potential for spread. During a two-year field study of established populations in the invaded region of southwest Florida, year round gametogenesis and continuous spawning capabilities were observed through histological analysis of mussels collected monthly. This was supported by overall stable energetic reserves as measured through proximal biochemical composition (protein, glycogen and lipid content). However, egg outputs in the summer (6.4 × 10 6 ± 2.6 × 10 6 eggs / female) were significantly higher than egg outputs of winter-spawned mussels (7.7 × 10 4 ± 1.4 × 10 4 eggs / female). Stability in biochemical composition, suggests temperature and food availability were sufficient year round, allowing for the maintenance of reserves and active gametogenesis. Protein ranged from 409.0–628.0 mg g −1 , glycogen from 44.3–158.5 mg g −1 and total lipids from 7.4–13.5 mg g −1. Year-round reproductive capabilities supported by sufficient energy reserves may help explain the rapid colonization and high densities of green mussels along the southeastern United States and suggests the potential for competition with native species, particularly the eastern oyster Crassostrea virginica (Gmelin, 1791).