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Seaweeds consist of several groups of multicellular algae: the red algae, green algae, and brown algae. As these three groups are not thought to have a common multicellular ancestor, the seaweeds are a paraphyletic group. In addition, tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered as seaweeds.
Seaweeds are popularly described as plants, but only red and green algae belong to the kingdom Plantae. They should not be confused with aquatic plants such as seagrasses (which are vascular plants).
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