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



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