Seagrass Versus Seaweed

There are important distinctions between seagrasses and seaweed.

Seagrass can easily be confused with marine macroalgae, or seaweed, but there are many important differences between the two. While seagrasses are considered vascular plants, seaweed are protists (a taxonomic group that includes protozoans, prokaryotes, fungi, sponges, and algae). The two also differ in reproduction, structure, and the methods by which they transport nutrients and dissolved gases. The table and diagram below illustrate some of these distinctions.

Illustration showing some of these distinctions between seagrass and seaweed
Illustration reproduced with permission from the Smithsonian Marine Station at Fort Pierce http://www.sms.si.edu/irlspec/ComparAlgae_Seagr.htm.

--	Seagrass	Macroalgae (Seaweed)
Number of Species Worldwide	55	5,000-6,000
Reproduction	Have separate sexes Produce flowers, fruits, and seeds	Produce spores
Structure	Evolved from terrestrial plants and have tissues that are specialized for certain tasks Possess roots, leaves, and underground stems called rhizomes that hold plants in place	Relatively simple and unspecialized Holdfast anchors plant to a hard surface; does not possess roots extending below the surface
Transport/ Classification	Use roots and rhizomes to extract nutrients from the sediment; use leaves for extracting nutrients from the water Are categorized as vascular, with a network of xylem and phloem that transport nutrients and dissolved gases throughout the plant	Use diffusion to extract nutrients from the water Not plants or animals, but protists