Seaweed
"Seaweed" is a colloquial term and lacks a formal definition. A seaweed may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae. As these three groups do not have a common multicellular ancestor, the seaweed are in a polyphyletic group. In addition, some tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered to be seaweed.
Caulerpa brachypus
In Florida, this seaweed has been dubbed the "killer algae". First recorded in the state in 1999, by 2003 it had shown explosive growth and spread widely, with some near shore reefs in Palm Beach County becoming so overwhelmed that fish and lobsters were no longer present. During hurricanes in 2004, it was scoured from the rocky reefs and disappeared, but over the next few years, it became re-established, once more forming dense stands. It is thought to become so abundant on Florida's coast because of the sewage outflows which result in extra nutrients being present in the water. Such extensive algal cover is likely to kill corals, sponges and other sessile invertebrates, and force more mobile organisms to move elsewhere.