Subclass: Alismatidae

Status: Not Listed

Classification: Plant


Seagrasses are underwater plants that evolved from land plants. They are like terrestrial plants in that they have leaves, flowers, seeds, roots, and connective tissues, and they make their food through photosynthesis. Unlike terrestrial plants, however, they do not have strong stems to hold themselves up—instead they’re supported by the buoyancy of the water that surrounds them. Seagrasses are a very important food source and habitat for wildlife, supporting a diverse community of organisms including fish, octopuses, sea turtles, shrimp, blue crabs, oysters, sponges, sea urchins, anemones, clams, and squid. Seagrasses have been called “the lungs of the sea” because they release oxygen into the water through the process of photosynthesis.


There are 26 species of seagrasses in North American coastal waters. They prefer to grow in shallow, sheltered, soft-bottomed coastal waters—both tropical and temperate.

Life History

Seagrasses can reproduce sexually or asexually. They are flowering plants that produce seeds. Pollen is carried through the water to fertilize female flowers. Seagrasses can also send out rhizome roots that can sprout new growth, so a single plant is capable of producing an entire underwater meadow.

The grasses help lessen the effects of strong currents, and also provide concealment and a place for eggs and larvae to attach. These factors make seagrasses a good nursery area for many fish and invertebrates, including commercially important fish species. Their leaves and stems also provide food for herbivores like sea turtles and manatees. Plankton, algae, and bacteria grow on seagrass stems, providing food for additional organisms. Dead seagrasses provide food for decomposers like worms, sea cucumbers, crabs, and filter feeders. Seagrasses improve water quality by trapping sediments, absorbing nutrients, and stabilizing sediment with their roots.


Seagrasses are very sensitive to water quality and are an indicator of the overall health of coastal ecosystems. Since they produce energy through photosynthesis they do best where the water is clear enough to allow sunlight to penetrate. Pollution, sedimentation, excessive nutrients, storms, disease, and overgrazing by herbivores all pose threats to seagrasses.

Fun Fact

Seagrasses are not true grasses. They are more closely related to terrestrial lilies and gingers than grasses.


Florida Fish and Wildlife Conservation Commission

Florida Keys National Marine Sanctuary

Oceanus Marine Construction and Technology

Ocean Portal, Smithsonian National Museum of Natural History

Seagrass Watch

Smithsonian Marine Station at Fort Pierce

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