Sea grasses provide habitat for fish, sea horses, crustaceans and others; food for sea turtles, waterfowl and marine mammals.Sea grasses provide habitat for fish, sea horses, crustaceans and others; food for sea turtles, waterfowl and marine mammals.
right sunlight filters down through the clear Mediterranean waters off the coast of Spain, illuminating a lush meadow just below the surface. Blades of strikingly green grass undulate in the currents. Painted comber fish dart among clumps of leaves, and technicolor nudibranchs crawl over mounds. Porcelain crabs scuttle by tiny starfish clinging to the blades. A four-foot-tall fan mussel has planted itself on a rock outcropping. A sea turtle glides by.
This rich underwater landscape has been shaped by its humble covering, Posidonia oceanica. Commonly known as Neptune grass, it is one of about 70 species of seagrasses that have spread, over millions of years, across the globe’s coastal shallows, embracing and buffering continental shelves from Greenland to New Guinea. Seagrasses provide habitat for fish, sea horses, crustaceans and others; food for sea turtles, waterfowl and marine mammals; and nurseries for an astounding 20 percent of the largest fisheries on the planet.
“Seagrasses are the forgotten ecosystem,” Ronald Jumeau, a United Nations representative from the Republic of Seychelles, writes in a 2020 U.N. report. “Swaying gently beneath the surface of the ocean, seagrasses are too often out of sight and out of mind, overshadowed by colorful coral reefs and mighty mangroves.” But, he says, they “are among the most productive natural habitats on land or sea.”
Emmett Duffy, director of the Smithsonian’s Tennenbaum Marine Observatories Network, shares that view of seagrasses as underappreciated but essential: “They’re like the Serengeti grasslands of Africa—but hardly anybody knows about them.”
Yet this invisible ecosystem, once you do see it, has a primal if uncanny draw, at once alien and familiar, a remembered dream of a submerged meadow. This may be because, unlike seaweeds (which are algae, not plants) and corals, seagrasses are terrestrial immigrants. When the largest dinosaurs were in their heyday, these grasses drifted from dry land into the sea.
They have changed little since then. Like land grasses, they grow leaves, roots, rhizomes, veins and flowers. Their modest adaptations to the marine environment include aquatic pollination, neutrally buoyant seeds that can drift with the current before settling, and leaves that manage saltwater. These adaptations have led seagrasses to cover some 116,000 square miles of the world’s ocean floors, along every continent except Antarctica. Typically preferring depths of less than ten feet, most seagrasses are modest in height, but some can reach 35 feet long, such as the showy, ribbonlike Zostera caulescens, which grows off the coast of Japan.
Seagrasses have survived, not just as species, but often as individual clones, for thousands of years. Scientists studying Posidonia oceanica meadows in the Mediterranean Sea estimate that the largest clone, which stretches more than nine miles, has been around, sending out slow-growing rhizomes, for tens of thousands of years, and possibly as many as 200,000 years. It could be the oldest-known organism on Earth.
Throughout these millennia seagrasses have not only greened undersea landscapes but have also actively shaped them—“ecological engineers,” as researchers say. Roots hold seafloor sediment in place. Leaves help to trap floating sediment, improving water clarity. Seagrasses slow currents and help protect shorelines from storms. And they efficiently filter out polluting chemicals even as they cycle nutrients, oxygenate the water and pull carbon dioxide into the seafloor. The new U.N. report estimates that seagrasses may perform up to 18 percent of the ocean’s carbon sequestration, even though they cover only about 0.1 percent of the ocean floor.
And they don’t do all this hard work silently. Carlos Duarte, a leading international seagrass expert at King Abdullah University of Science and Technology, on the banks of the Red Sea, in Saudi Arabia, describes a “scintillating sound when you lie in seagrass meadows,” which comes from the bursting of oxygen bubbles seagrasses produce and which sound, he says, “like little bells.” These faint peals may serve as clarion calls to some creatures that rely on seagrass meadows. For example, fish whose larvae, floating through the water column in search of a suitable place to land and mature, may depend on the sound for guidance.
Like many other ecosystems, seagrasses are also facing rapid decline. Approximately 7 percent of global seagrass coverage disappears each year, similar to the loss of coral reefs and tropical rainforests. This decline also threatens species that depend on seagrasses for food and habitat, including endangered manatees, green sea turtles, chinook salmons, and dugongs, and it serves as a warning of greater devastation to come.
Originally published at smithsonianmag