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Shades of green indicate the number of species reported for a given area. Seagrasses are found across the world, from the tropics to the Arctic. Antarctica is the only continent without seagrasses. While most coastal regions are dominated by one or a few seagrass species, regions in the tropical waters of the Indian and western Pacific oceans have the highest seagrass diversity with as many as 14 species growing together. Many seagrass species live in depths of 3 to 9 feet (1 to 3 meters), but the deepest growing seagrass ( Halophila decipiens) has been found at depths of 190 feet (58 meters). Because they depend on light for photosynthesis, they are most commonly found in shallow depths where light levels are high. Seagrasses grow in salty and brackish (semi-salty) waters around the world, typically along gently sloping, protected coastlines. The entire genome of one seagrass, the eelgrass Zostera marina, was sequenced in 2016, helping us understand how these plants adapted to life in the sea, how they may respond to climate warming, and the evolution of salt tolerance in crop plants. Scientists are studying what genes were lost and which were regained as seagrasses evolved from algae in the sea to plants on land, and then transitioned back to the sea. They have no flowers or veins, and their holdfasts simply attach to the bottom and are generally not specialized to take in nutrients. In contrast, seaweeds (algae) are much simpler organisms. The roots and rhizomes (thicker horizontal stems) of seagrasses extend into the sediment of the seafloor and are used to store and absorb nutrients, as well as anchor the plants. Instead, they have a thin cuticle layer, which allows gasses and nutrients to diffuse directly into and out of the leaves from the water. Unlike flowering plants on land, however, they lack stomata-the tiny pores on leaves that open and close to control water and gas exchange. Like other flowering plants, their roots can absorb nutrients. Veins transport nutrients and water throughout the plant, and have little air pockets called lacunae that help keep the leaves buoyant and exchange oxygen and carbon dioxide throughout the plant. Chloroplasts in their tissues use the sun's energy to convert carbon dioxide and water into sugar and oxygen for growth through the process of photosynthesis. Like their relatives, seagrasses have leaves, roots and veins, and produce flowers and seeds. Seagrasses belong to a group of plants called monocotyledons that include grasses, lilies and palms. (Courtesy of the Integration and Application Network (), University of Maryland Center for Environmental Science )Įven though seagrasses and seaweeds look superficially similar, they are very different organisms.

Algae on the seafloor have a holdfast and transport nutrients through the body by diffusion, while seagrasses are flowering vascular plants with roots and an internal transport system. What Are Seagrasses? A Plant, Not a SeaweedĪlgae or "seaweeds" (left) differ from seagrasses (right) in several ways.

Work is ongoing around the world to restore these important ecosystems. Seagrasses provide many important services to people as well, but many seagrasses meadows have been lost because of human activities. Seagrasses provide shelter and food to an incredibly diverse community of animals, from tiny invertebrates to large fish, crabs, turtles, marine mammals and birds. Although they often receive little attention, they are one of the most productive ecosystems in the world. Seagrasses can form dense underwater meadows, some of which are large enough to be seen from space. They evolved around 100 million years ago, and today there are approximately 72 different seagrass species that belong to four major groups. Seagrasses have roots, stems and leaves, and produce flowers and seeds. They are often confused with seaweeds, but are actually more closely related to the flowering plants that you see on land. Seagrasses are so-named because most species have long green, grass-like leaves. Seagrasses are found in shallow salty and brackish waters in many parts of the world, from the tropics to the Arctic Circle.