Japweed Sargassum muticum
Japweed can reproduce all year and forms dense stands in quiet waters. Native to Japan (hence its common name), it was accidentally introduced to western North America and Europe, and is steadily expanding its range in these areas. Japweed out competes other seaweeds and in these regions is regarded as an invasive species. This long, bushy seaweed has numerous side-branches, which have many leaflike fronds up to 4 in (10 cm) long. Japweed's fronds bear small, gas-filled bladders, either singly or in clusters.