Few transformations in the history of life have been as extreme as the embrace of the ocean by seagrass. Like whales and dolphins, modern seagrasses descend from land-dwelling ancestors. But marine mammals surface for air. Seagrasses often live entirely submerged. Why did they take the plunge, turning their back on the land to become sea creatures? The world’s continents are fringed by vast expanses of sand and mud. Until seagrasses came along, no plant or seaweed could grow for long in such shifting, unstable conditions. Seagrasses brought to these sea bottoms buried creeping stems, strong roots, and pliable leaves. Not only could seagrasses tolerate this oozy, muddy habitat, they built more of it.

Alongside their sinewy body form, the ancestors of seagrass carried with them from dry land two useful features that no sea creature possessed. The first is flowers, the second a molecule, lignin. To these useful inheritances from their land ancestors, seagrasses added a suite of innovations that helped them to thrive in the salty, wave-pounded margins of the oceans.

Seagrass flowers are perhaps the most visually underwhelming blooms in the world. When I first encountered them at Blackness [Castle, UK,] in the early summer, I didn’t even realize they were flowers. On a later visit, after looking at diagrams online, I bent a few of the thicker eelgrass blades and saw a row of lighter-green chevrons tucked within the blades’ folds. That was it. Unimpressive to human eyes, but amazing in seawater currents.

Under a microscope, the elegance of the seagrasses’ flowers is more apparent. Although different seagrass species take different forms, all dodge the destructive power of water’s drag by miniaturization and streamlining. Water slips right over them. Petals are gone or reduced to scalelike shields. Some tropical species, like paddle grass, have long, threadlike stigmas that finger the water for pollen. Others, like the ribbon weed that grows in the bays of Gathaagudu, Australia, poke their flowers from tiny spikes atop leafless stems. The more violent the shore, the more streamlined the flower. Eelgrass is perhaps the most extreme of all, flowering inside a stem with its sexual parts barely poking out. The lashings of springtime Atlantic storms can do no harm.

The female stigmas and the male pollen of seagrasses use the flow and viscosity of water to their advantage. Tiny eddies form around them, encouraging pollination. Pollen grains are, in most seagrasses, threadlike. When they come near a stigma, instead of drifting past, as would a spherical pollen grain, they turn in the water current and spin toward the stigma. If they miss, a swirl of current swoops them up to the next stigma. Water currents do most of the work of pollination, but in turtle grass, a tropical species, small animals ferry pollen, too.

One of the great advantages of flowers is the maternal care they provide. In seagrass, the fruit and seed are like a bathyscape, a well-provisioned protective capsule in which the embryo can drift and explore. Some seagrasses float for months over hundreds of kilometers, others settle within hours a few meters from their parent. In a few species in the Indian and Pacific Oceans, naked embryos cling to their mother flowers and are nourished through placenta-like connections until they are ready to break off.

Once they start growing, another legacy from land ancestors helps seagrasses. Their buried roots and stems are strong as wire. Their leaves, although flexible, are very hard to tear. This toughness is imparted by lignin. A molecule originally evolved on land to build tree trunks and support leaves, lignin is what gives plant material its strength. Wood is full of it, cotton wool has none. Paper is wood with the lignin digested away. By bringing to the seas this terrestrial invention, seagrasses grew in ways that no other ocean species could, in rugged mats of almost unbreakable fibers. Lignin also resists decay, helping seagrass to build up sediment as it grows.

Flowers and lignin were helpful bequests from seagrass land ancestors. The rest of seagrasses’ success in the ocean came from their own innovations in body structure and collaboration.

Air channels run through the interior of seagrass leaves, stems, and roots. These act as self-filling scuba tanks. Oxygen made by the leaves during photosynthesis travels through the channels to the rest of the plant, including roots buried in oxygen-starved mud. Air channels also buoy the leaves, holding them up to the light.

Water reflects and absorbs light, so from a plant’s perspective the underwater world is like growing in perpetual shade. Seagrasses cram the emerald chloroplasts that do the work of photosynthesis into the very top layer of the leaf. Seagrass light-gathering pigments are reconfigured, each seagrass species finding the pigment palette that best embraces the blue-green watery light of its home.

Seagrasses have transformed, lost, or duplicated dozens of genes. As is true of many revolutions in flowering plants, some seagrass lineages doubled then edited entire genomes. Genes for pumping sodium multiplied, to purge excess salt. Growth hormones and genes that respond to stress were reworked, some lost, others boosted. This is especially true for genes that battle herbivores. In the sea, old enemies are gone, but new grazers ply the meadows. Genes to counteract iron deficiency increased because seawater has just 1 percent the iron of fresh water or soil. Genes for making cell walls reverted to a formula found in algae. When Julia and I confused seagrass for green algae at Blackness, we unwittingly glimpsed a genetic truth. Seagrass has succeeded by becoming alga-like in some of its molecules and cells, all while keeping hold of some bonus land-plant traits.

Like orchids, seagrasses owe much of their success to partnerships. Clonal growth is the first community-building exercise. Instead of going it alone like a tree trunk, seagrasses self-multiply. The lush meadows that result make excellent hiding places for small invertebrate animals. These crustaceans, worms, fish, and others keep their home clean by grazing on the algae that encrusts seagrass leaves. This incessant nibbling stops algae from smothering the seagrass.fd

A three-way mutualism among seagrass, hatchet clams, and bacteria detoxifies some meadows. The clams shunt sulfur- and oxygen-rich water to bacteria that live inside the clams’ gills. The bacteria alchemize these chemicals into complex food molecules, some of which they send to their hosts, the clams. This chummy relationship lowers sulfur concentrations in the mud, allowing seagrass to thrive, which in turn provides a protective meadow for the clams. Corals, too, have a mutualistic relationship with some seagrasses. In the Caribbean, seagrass meadows give small corals a safe and food-rich home. The corals cluster around the base of seagrass shoots where they deter excessive grazing by sea turtles. Without their coral sheaths, the seagrass are often eaten down to the sediment. Without the seagrass, the corals would be swept away by currents. Wherever it grows, seagrass builds mutualisms.

Seagrasses embody adaptability and resilience. This is hope, in biological form.

Seagrasses are also hubs of microbial activity. The roots of all flowering plants on land export a little sugar to feed bacteria. Seagrasses turn the trickle into a gush. The sediment close to their roots can be as sweet as fruit nectars or tree sap, although to human tongues the mud tastes overwhelmingly of salt and decay. This sugary generosity comes with restrictions. Along with the sugars, seagrass roots suffuse their surroundings with aromatic chemicals, many of which are familiar to us as the aromas of coffee, red wine, and tree bark. These chemicals lock up the sugar. Most bacteria can’t benefit from the sweetness around them. But a few have a key to the lock. Among these are bacteria that supply nitrogen to seagrass roots, detoxify sulfur, and use hydrogen as a food. It seems that seagrasses are feeding helpful bacteria, while starving others. The plants may also be dumping excess sugars on sunny days, lacing this sweetness with aromatic chemicals to prevent harmful bacterial overgrowth.

Seagrasses feed the bacteria on their leaves and roots. In return, bacteria protect the plants. Like hatchet clams, root bacteria can purge sulfur. In one Mediterranean species, Posidonia oceanica or Neptune grass, bacteria inside roots snare dissolved nitrogen gas and turn it into ammonia and amino acids. The bacteria then shunt these nitrogen-containing molecules to the seagrass, a built-in supply of fertilizer. The seagrass reciprocates with sugars. This symbiosis was only recently uncovered and it is likely that other seagrasses, many of which are known to host nitrogen-supplying bacteria around their roots, have similarly hospitable internal arrangements.

To human senses, fibrous seagrass roots running through sediment can be boring. Just threads and mud. But at the scale of microbes and molecules, the rhizosphere, “root world,” roils with life.

No other plant has achieved as complete a transformation of its body, genes, and life cycle as seagrasses. Their success depends on a careful combination of atavism and innovation. They reversed the course of land plant evolution, going back to the seas four hundred million years after their ancestors crept onto land. In doing so, they became alga-like, especially in the structure of their leaves, but invented new flower shapes, air conduits, biochemistry, and ecological partnerships to accommodate the oceans. Remarkably, seagrasses took the plunge many times. All seagrasses descend from a group of flowering plants that includes the arums and water plantains, many of which grow in swamps or along streams. Starting one hundred million years ago, three or four independent groups took to the seas. What we call by a single name, the seagrasses, are, in fact, several different clans that each took to the oceans.

Seagrasses embody adaptability and resilience. This is hope, in biological form.