The most productive survey crew to work the Bahamas in years had no clipboards, no salaries and a mouthful of serrated teeth each.

Between 2016 and 2020, researchers fitted camera packages to seven tiger sharks on the Great and Little Bahama Banks, then left them to get on with their day. Together they filmed 20.6 kilometres of seafloor, one of them through a 360-degree camera that the team calls a first for any marine animal. That footage fed into a 2022 paper in Nature Communications led by Austin Gallagher of the research group Beneath The Waves.

Its headline number was 66,000 to 92,000 square kilometres of seagrass across the banks, the largest such system known on Earth. National Geographic likened the upper figure to Portugal. The study says the find adds 41 per cent to the world’s documented seagrass.

Before that, the record belonged to about 40,000 square kilometres behind the Great Barrier Reef, which only surfaced in a 2009 survey using towed cameras, despite lying in one of the planet’s most studied reef systems.

Why a meadow this big went unnoticed

How does something as big as a small country hide in some of the clearest water in the ocean? Mostly by being thin. Much of the Bahamian seagrass is sparse and low, its blades rising only a few centimetres from the sand and often partly buried in it, which from orbit reads as bare seafloor.

Earlier estimates for the banks varied roughly 30-fold, and the underwater checks behind them covered less than 5 per cent of the area. Those checks are called ground-truthing, which Beneath The Waves researcher Oliver Shipley described to the Naked Scientists as confirming that what the satellite reports is really there.

Divers doing that job cover a few hundred metres a day. The sharks managed an estimated 51.6 kilometres a day, at three times the typical depth of the human surveys.

Fitting a camera to a tiger shark

“Almost like a NASCAR pit crew.” That’s Shipley’s take on the tagging routine. A shark is caught on a baited line and held beside the boat, and a package carrying the camera, a satellite tag and a radio tag is fixed through its dorsal fin with biodegradable cable ties. Those ties meet at a link that dissolves in seawater, so after a set time the rig drops off, floats up and signals where to collect it.

Each deployment captured roughly one to two hours of video, with about 72 per cent of it spent over seagrass. More telling were two release sites where none of the satellite maps predicted any seagrass, yet the sharks cruised over it more than 70 per cent of the time. The authors took that as a sign every existing satellite-based estimate had been lowballing the banks.

Still, the footage covers only a sliver of the banks. It served as an independent cross-check on an estimate built from several satellite maps and 2,542 diver surveys. This is one study, and 92,000 square kilometres is its upper bound; the conservative figure is closer to 67,000.

What all that grass is storing

Seagrass draws carbon from the water and locks it into the sediment below, part of what scientists call blue carbon, meaning carbon held by ocean and coastal ecosystems. A first-pass estimate in the 2022 paper suggested the Bahamian meadows might hold a fifth to a quarter of all carbon buried in seagrass sediments worldwide, with more research needed.

A 2023 follow-up in Communications Earth & Environment cored ten meadows around the Exumas and put the stock at 420 to 590 million tonnes of organic carbon in the top metre of sediment. By its reckoning, yearly burial matches or exceeds the country’s entire greenhouse gas output. Square metre for square metre, Bahamian beds store less than the global average, so their clout comes from size.

Those cores also carried a warning: seagrass-derived carbon in the sediment has been falling since around 1980, which the authors link to tourism and boat traffic.

Why the sharks matter to the seagrass

It all started with a shark biologist noticing the scenery. Gallagher had been filming tiger sharks to understand their daily lives, and when the footage kept showing meadows, he contacted Carlos Duarte, a leading seagrass scientist at King Abdullah University of Science and Technology (KAUST). Gallagher later told National Geographic the meadows work like a savanna, with tiger sharks as the top predator.

That rank matters. Keeping grazers in check can stop meadows being eaten down, and both papers suggest the long-running protection of tiger sharks in the Bahamas, in place for more than a decade per a KAUST press release, has likely helped hold that balance.

The Nature Communications team also argues camera-carrying sharks remain one of the few practical ways to monitor an ecosystem this vast. Protections written for the sharks’ own sake have ended up guarding two things at once: the predators that keep the meadows in balance, and the most far-ranging data collectors on the banks, covering more ground in a day than a dive team manages in months.