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.
Algae
Algae are prominent in bodies of water, common in terrestrial environments, and are found in unusual environments, such as on snow and ice. Seaweeds grow mostly in shallow marine waters, under deep; however, some such as Navicula pennata have been recorded to a depth of .