Seagrass is often mistaken for marine algae, or seaweed, due to its fully submerged habitat in shallow coastal waters. However, seagrass is classified in the Kingdom Plantae. Seagrasses are the only known group of flowering plants (angiosperms) that have evolved to live and reproduce entirely within the marine environment.
What Makes Seagrass a True Plant
Seagrass possesses the physical characteristics of a true plant. Unlike algae, seagrasses feature true roots and horizontal, underground stems called rhizomes. The rhizomes anchor the plant into the soft seafloor sediment while actively absorbing nutrients. This thick network allows the plant to spread laterally and establish dense underwater meadows.
The presence of a vascular system, consisting of xylem and phloem, is used to transport water, nutrients, and sugars throughout the organism. Seagrasses also reproduce sexually by producing flowers, pollen, and seeds. This ability to flower and form seeds beneath the water is a significant biological difference from simpler marine life forms.
Distinguishing Seagrass from Seaweed
Although they share an aquatic environment, seagrass and seaweed are fundamentally distinct life forms. Seaweed is a type of macroalgae, classified in the Kingdom Protista, not the Kingdom Plantae. Algae lack the specialized tissues found in true plants, possessing a simple body structure known as a thallus.
Seaweed uses a structure called a holdfast for attachment to surfaces, but it does not absorb nutrients. Algae absorb all necessary nutrients directly from the surrounding water across their entire surface. Seagrass, conversely, relies on its true roots and vascular system to draw nourishment from the substrate and circulate it internally. Furthermore, seaweed reproduces using spores, a simpler method than the sexual reproduction involving flowers and seeds seen in seagrass.
Life in a Salty Environment
To survive in the harsh, saline conditions of the ocean, seagrasses have developed adaptations. They are halophytes, meaning they tolerate high salt concentrations by actively regulating osmotic pressure within their cells. This is achieved by mechanisms that allow the plants to selectively release excess salts like sodium and potassium.
Another adaptation is the presence of air canals (aerenchyma), which are spongy tissues that transport oxygen down to the submerged roots and rhizomes buried in the low-oxygen sediment. As flowering plants, seagrasses must pollinate underwater, which they manage through hydrophilous pollination where thread-like pollen is carried by water currents. Because they rely on photosynthesis, seagrasses are restricted to growing in shallow, clear coastal zones where enough sunlight can penetrate the water column.
Ecological Importance
Seagrass meadows provide substantial ecological benefits to the marine environment. These dense underwater beds serve as crucial nursery habitats, offering shelter and food for the juvenile stages of many commercially important fish, crustaceans, and invertebrates. The physical structure of the leaves and rhizomes also plays a significant role in improving water quality.
The root systems stabilize the seafloor sediment, preventing coastal erosion and reducing suspended particles in the water. This trapping of sediment clarifies the water, promoting light penetration. Seagrasses are highly efficient carbon sinks, capturing and storing carbon (known as “blue carbon”) in the sediment below the meadows. Seagrass meadows are capable of storing carbon at rates up to twice as fast as terrestrial forests.