Skip to main content

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 and have roots, stems and leaves, seaweed are multi-cellular algae and have little or no vascular tissues. The two differ in reproduction, structure, and how they transport nutrients and dissolved gases. The table and diagram below illustrate some of these distinctions.

Illustration reproduced with permission from the Smithsonian Marine Station at Fort Pierce:

drawing comparing algae and seagrass
Feature 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/Clarification 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.