Seagrasses are found in shallow salty and brackish waters in many parts of the world, from the tropics to the Arctic Circle. Seagrasses are so-named because most species have long green, grass-like leaves. They are often confused with seaweeds, but are actually more closely related to the flowering plants that you see on land. Seagrasses have roots, stems and leaves, and produce flowers and seeds. They evolved around 100 million years ago, and today there are approximately 72 different seagrass species that belong to four major groups. Seagrasses can form dense underwater meadows, some of which are large enough to be seen from space. Although they often receive little attention, they are one of the most productive ecosystems in the world. Seagrasses provide shelter and food to an incredibly diverse community of animals, from tiny invertebrates to large fish, crabs, turtles, marine mammals and birds. Seagrasses provide many important services to people as well, but many seagrasses meadows have been lost because of human activities. Work is ongoing around the world to restore these important ecosystems.
A Plant, Not a Seaweed
Even though seagrasses and seaweeds look superficially similar, they are very different organisms. Seagrasses belong to a group of plants called monocotyledons that include grasses, lilies and palms. Like their relatives, seagrasses have leaves, roots and veins, and produce flowers and seeds. 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. 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. Like other flowering plants, their roots can absorb nutrients. Unlike flowering plants on land, however, they lack stomata—the tiny pores on leaves that open and close to control water and gas exchange. Instead, they have a thin cuticle layer, which allows gasses and nutrients to diffuse directly into and out of the leaves from the water. 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. In contrast, seaweeds (algae) are much simpler organisms. They have no flowers or veins, and their holdfasts simply attach to the bottom and are generally not specialized to take in nutrients. 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. 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.
Where are Seagrasses found?
Seagrasses grow in salty and brackish (semi-salty) waters around the world, typically along gently sloping, protected coastlines. Because they depend on light for photosynthesis, they are most commonly found in shallow depths where light levels are high. 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). 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. Antarctica is the only continent without seagrasses.
Seagrasses are found across the world, from the tropics to the arctic. Shades of green indicate the number of species reported for a given area. The darker shades of green indicate more species are present. From Short, F. et al. 2007.
Seagrasses grow both vertically and horizontally—their blades reach upwards and their roots down and sideways—to capture sunlight and nutrients from the water and sediment. They spread by two methods: asexual clonal growth and sexual reproduction.
Asexual Clonal Growth: Similar to grasses on land, seagrass shoots are connected underground by a network of large root-like structures called rhizomes. The rhizomes can spread under the sediment and send up new shoots. When this happens, many stems within the same meadow can actually be part of the same plant and will have the same genetic code—which is why it is called clonal growth. In fact, the oldest known plant is a clone of the Mediterranean seagrass Posidonia oceanica, which may be up to 200,000 years old, dating back to the ice ages of the late Pleistocene. In some seagrass species, a meadow can develop from a single plant in less than a year, while in slow-growing species like Posidonia it can take hundreds of years.
Sexual Reproduction: Seagrasses reproduce sexually like terrestrial grasses, but pollination for seagrasses is completed with the help of water. Male seagrass flowers release pollen from structures called stamens into the water. Seagrasses produce the longest pollen grains on the planet (up to 5mm long compared to under 0.1mm for land plants typically), and this pollen often collects into stringy clumps. The clumps are moved by currents until they land on the pistil of a female flower and fertilization takes place. There is also evidence that small invertebrates, such as amphipods (tiny shrimp-like crustaceans) and polychaetes (marine worms), feed on the pollen of one seagrass (Thalassia testudinum), which could help to fertilize the flowers in a way similar to how insects pollinate flowers on land.
Self-pollination happens in some grass species, which can reduce genetic variation. Individual seagrass plants avoid this by producing only male or female flowers, or by producing the male and female flowers at different times. Just like land grasses, fertilized seagrass flowers develop seeds. Seagrass seeds are neutrally buoyant and can float many miles before they settle onto the soft seafloor and germinate to form a new plant. A few seagrass species such as the surfgrass Phylospadix can settle and live on rocky shores. Animals that eat seagrass seeds—including fish and turtles—may incidentally aid with their dispersal and germination if the seeds pass through their digestive tracks and remain viable.
The 72 species of seagrasses are commonly divided into four main groups: Zosteraceae, Hydrocharitaceae, Posidoniaceae and Cymodoceaceae. Their common names, like eelgrass, turtle grass, tape grass, shoal grass, and spoon grass, reflect their many shapes and sizes and roles in marine ecosystems. Seagrasses range from species with long flat blades that look like ribbons to fern or paddle-shaped leaves, cylindrical or spaghetti blades, or branching shoots. The tallest seagrass species—Zostera caulescens—was found growing to 35 feet (7 meters) in Japan. Some seagrass species are quick growing while others grow much more slowly. These distinct structures and growth forms affect how seagrasses influence their environment and what species live in the habitats they create.