Is seaweed a plant?

Seaweed is an informal name for the many kinds of marine algae that grow along seashores. Although they often look like plants, seaweeds are not plants. They belong to three broad groups—green, red, and brown algae—that occupy different branches of the tree of life. Plants themselves evolved from ancient green algal ancestors, eventually spreading across nearly every terrestrial environment.

Like plants, seaweeds contain chlorophyll and other pigments that allow them to use sunlight to make food through photosynthesis. Some even have structures resembling roots, stems, and leaves, but the similarity is largely superficial: These structures are not true plant organs and lack the specialized tissues found in plants. A seaweed’s rootlike holdfast, for example, is primarily an anchor against ocean currents rather than a vascular organ for absorbing water and nutrients, as in plants.

Adding to the potential confusion, there are true plants that live in coastal marine environments, often inhabiting the same ecosystems as seaweeds. For example, eelgrass and other seagrasses are flowering plants that evolved from land-dwelling ancestors and later adapted to life underwater. They have true roots, stems, and leaves, produce flowers and seeds, and often form extensive underwater meadows.