The chloroplast haplotypes, ecology, growth, and reproduction of the introduced Japanese green alga Codium fragile were evaluated in four salt marsh pannes from Brave Boat Harbor, Kittery, Maine. All chloroplast haplotypes were identical and they matched the subsp. fragile delineated by Provan et al. Frond stature was seasonally variable and showed a pronounced increase in length from summer to early fall. Residual (i.e., overwintering) spring fronds were highly variable in stature compared to previous fall values, with some fronds only being ~ 0.5–1.0 cm. Frond stature and frequency patterns in the different pannes were related to their proximity to tidal channels, plus patterns of storm damage. Thus, one panne, which was closest to a tidal channel and had the lowest tidal level, exhibited the largest population frequency and smallest mean frond stature. The highest percent occurrence of gametangia occurred during late summer to early fall when the largest fronds were present. The reproductive pattern in the panne closest to a tidal channel peaked sharply following extensive physical damage to the panne, including diminished depth, enhanced desiccation, and anoxic conditions.