Seagrasses, the pale green flowering plants that form meadows on the ocean floor, are home to all manner of life: turtles, fish, squid, seahorses, anemones, crab, dugongs. Yet for vast stretches of the marine world we still don’t know how much there is—and not just around far-flung atolls, but even along the coasts of some of the best-mapped countries on Earth.
Seagrass can stretch for hundreds or thousands of miles, and in areas with clear water that lets sunlight penetrate, may grow far deeper than humans in scuba gear can dive. So when Austin Gallagher, a marine scientist, and his team set out to see just how widespread these meadows were in the Bahamas, they sought help from some of the world’s best divers: tiger sharks.
By putting cameras on the deep-diving tropical predators, the group found to its amazement that the Bahamas is home to more seagrass than any other place on Earth—lots more.
Hours of video from the sharks revealed that seagrass covered at least 25,000 square miles, enough by itself to extend the world’s previously documented seagrass beds by 41 percent. In fact, the researchers suspect the meadows in that region are actually far larger—perhaps large enough to cover an area the size of Portugal, about 36,000 square miles, according to their study in Nature Communications.
Looking at the footage, “we could see these large expansive underwater meadows—all you could see was seagrass,” says Gallagher, who led the research and serves as CEO of the marine conservation science group Beneath the Waves. He was a National Geographic explorer in 2020-21.
Animals with cameras
Gallagher has studied marine life in the Bahamas for more than a decade and has been fascinated by the movement of highly mobile tiger sharks for even longer. He’d already been putting cameras on sharks “and having them reveal to us their daily lives,” he says.
Tiger sharks in the Bahamas form packs, like wolves in Yellowstone National Park. “These seagrass meadows are essentially the savanna, and that’s where all the big predators are,” Gallagher says. Nurse sharks, blacktips, and a few hammerheads are there, too, but tiger sharks “are the top, top, top dog.”
When Gallagher started noticing just how much seagrass the sharks swam through, he contacted one of the world’s leading seagrass experts, Carlos Duarte, at King Abdullah University of Science and Technology in Saudi Arabia. Intrigued, they embarked on a mapping exercise.
But using satellite data can be a misleading way to view underwater environments. It doesn’t tell the whole story. If grass is too deep or spotty or the water murky, it may not be visible from above. To understand just how vast seagrass is, it also needs to be confirmed from underwater.
Gallagher’s team conducted more than 2,500 individual human surveys, both with divers and with boats. They also equipped eight tiger sharks with satellite tags on their dorsal fins to help map locations the sharks traveled, and put cameras on seven others, including one virtual reality 360-degree camera, which logged long hours of footage over several years. By analyzing all of this information, they were able to show that previous accounts of the region’s seagrass had dramatically underestimated its extent.
“It really does underscore the incredible things that these animals can do,” he says.
The tiger shark work was not the first time marine animals have been used to help understand seagrasses. In 2015, cameras affixed to sea turtles revealed a massive decline in seagrass off western Australia following a heat wave. In 2018, researchers noticed sea turtles tagged with tracking devices were all converging on a spot in the Indian ocean. They followed the animals to the site and found a previously unknown deep seagrass bed in the Great Chagos Bank region of the ocean.
Unknown seagrass versus celebrated coral
That happened because seagrasses, while important, just haven’t received the level of attention of coral reefs even though the world’s 72 species of seagrasses are found across a wider range of the planet. “They haven’t been prioritized, says Benjamin Jones, chief conservation officer and co-founder of Project Seagrass, a United Kingdom-based seagrass conservation group. He estimates there are four or five times as many scientists globally studying corals as there are doing research on seagrass.
Read more about Project Seagrass here.
Seagrass “is the stuff you go past to get to a coral reef,” he says. One expert has referred to it as the “ugly duckling” of marine science. “The UK has a rich biological sciences history, but we know way more about kelp and seaweeds than we do about seagrasses,” Jones says.
Seagrass may be underappreciated, but it’s hard to overstate its importance. Not only does it house many species, but it also stores an immense amount of carbon, which helps battle climate change. In some places, it’s even more important to humans than coral reefs.
In a recent survey of a thousand residents from 147 coastal villages in Tanzania, Sri Lanka, Indonesia, and Cambodia, Jones found more people preferred fishing in seagrass than on coral reefs. While there are more fish and greater variety in coral reefs, for any given species, the fish living in protected seagrass are actually more nutritious for people.
As helpful as tiger sharks and sea turtles have been in documenting seagrass meadows, Project Seagrass also is enlisting humans to help improve our understanding. The nonprofit built a phone application, SeagrassSpotter, which can be used for crowd-sourcing newly discovered meadows that do not appear in any database. Already, citizen scientists using the app have helped map new beds around the Democratic Republic of Timor Leste.
“We’re really just beginning to realize how important seagrasses are,” Jones says. As such, it’s only growing more important to figure out how much of it exists, and where.
