Benji Jones is a senior environmental reporter at Vox, covering biodiversity loss and climate change. Before joining Vox, he was a senior energy reporter at Insider. Benji previously worked as a wildlife researcher.

In recent decades, warming seas, diseases, and other threats have wiped out half of the world’s corals and 90 percent of those in Florida. And this past summer, the problem accelerated. A devastating heat wave struck the Caribbean, pushing the reef in the Florida Keys — the largest in the continental US — closer to the brink of collapse.

The decline of coral reefs is an enormous problem for wildlife and human communities. Reefs not only provide habitat for as much as a quarter of all marine life, including commercial fish, but they also help safeguard coastal communities during severe storms. Simply put, we need coral reefs.

Coral reefs, meanwhile, need crabs.

Lucky for them, help is on the way. Scientists are in the process of building a crab army — hundreds of thousands of crustaceans strong — that they’ll unleash on Florida’s reefs, giving this ailing ecosystem a tool to fight back.

Crabs to the rescue

If you find crustaceans icky, Jason Spadaro’s lab is not a place you want to visit. Housed in a large, hurricane-proof building on Summerland Key in the Florida Keys, it’s full of tanks that are full of crabs — dozens of them. Some are the size of fingernails; others are as large as dinner plates. They all look a bit like rocks.

Spadaro, a crab enthusiast and marine ecologist at the Mote Marine Laboratory and Aquarium, is heading up an ambitious plan: to breed a quarter of a million Caribbean king crabs each year. It’s not about cultivating seafood, though these crabs are indeed delicious, Spadaro said. It’s about helping coral reefs survive.

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The key is in the crabs’ diets: These critters consume enormous quantities of seaweed, also known as macro algae. Algae has been choking reefs throughout the world and especially in Florida, making it hard for them to grow and recover from damaging events like marine heat waves.

Algae harms coral. Crabs eat algae.

Algae is one of the few winners in a world dominated by humans. It thrives on our waste, such as sewage and runoff from farmland, which is full of nitrogen and phosphorous — nutrients that algae need to grow.

As pollution runs into the ocean, algae booms.

Meanwhile, animals that eat algae have declined precipitously in recent decades. In the 1980s, an unknown pathogen wiped out longspined sea urchins in the Caribbean. These marine invertebrates — which take the shape of an overfilled pin cushion — eat loads of algae. Similarly, overfishing and the loss of various ecosystems has caused declines in algae-eating fish, such as parrotfish.

Like a fertilized pasture with no cows, a field of algae on a reef with no herbivores grows unencumbered. In the last decade, the extent of algae on reefs globally increased by roughly 20 percent, turning them from brilliant fields of color to monochrome patches of green.

This is a serious problem for coral.

When a thick layer of seaweed covers the reef, it’s hard for baby corals — which spend their early days as larvae swimming in the ocean — to find a spot on the seafloor and start a colony. This seaweed not only takes up floor space but it can also limit the amount of sunlight that reaches the bottom (coral needs light to grow) and produce chemicals that dissuade corals from settling. Abundant algae also competes with adult colonies for space, crowding them out.

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