Coral reefs can communicate with fish, and many of them are crying for help

If you go snorkeling off the coast of Fiji in the south Pacific, you’ll run into gorgeous coral reefs full of color, sound and texture. There are big boulder-like corals, branching spiky corals, fish darting around every bend and the crackling sound of reef creatures at work.

“There’s a lot of space for things to hide in and there’s just a huge amount of biodiversity there,” says Danielle Dixson, a coral reef ecologist at Georgia Tech. 

But as Dixson watches a video from the reef, it cuts to another patch of coral that’s eerily quiet, with none of the trademark pops and crackles. A field of sand and coral rubble is broken up by brown algae waving in the current.

“[It’s] extremely degraded,” Dixson says. “There’s hardly any coral. A lot of the fish just aren’t there.”

The two places look worlds apart, but they’re actually right next to each other — two halves of the same ecosystem. And here’s the difference: That bustling, vibrant side is part of a protected area where fishing is banned. On the other side, fishing is allowed.

Like so many corals around the world, the health of this reef is closely tied to the fish that live on and in it. Normally, there are fish munching on the algae on the degraded part of the reef, keeping it in check. When fishing shifts that balance, the algae can take over and destroy the corals. That means less habitat for the remaining fish, which leads to more algae, and then even less coral and even fewer fish — the vicious cycle goes on and on.

Georgia Tech ecologist Mark Hay calls it part of “the coral reef death spiral.”

“Everything that’s going on,” Hay says, “global warming, overfishing, pollution — means there’s less coral, more algae and more contact between corals and algae.”

And Hay says that contact can start to degrade a reef incredibly quickly — within just two days. “Those corals … start bleaching and tissues start dying where they’re in contact,” he says. “And then on a couple of those corals, that bleaching just spreads [to] the rest of the coral.”

But the researchers have found an interesting twist to the story: In a healthier ecosystem, when the algae starts to grow, corals can communicate the threat to a group of tiny saviors — little fish called gobies.

“The coral releases some kind of chemical that the goby responds to within about five minutes,” Dixson says. It’s a sort of chemical distress call, a smell that wafts out through the water to draw the gobies toward expanding algae.

Dixson has found that smells in general are a big part of life on these reefs. Among other things, some fish born away from the reefs use smell to find corals to settle on as adults.

“When you’re in the open ocean, there’s actually very little to orient yourself to,” Dixson says, “it’s just vast blue water. And if coral reef fish are able to find the smell of coral in the ocean, they follow that home like a yellow brick road.”

Unfortunately, it also works the other way around. Fish are lured by the smells of healthy reefs, but they’re repelled by the smells of unhealthy ones. And that can help lock in that “death spiral.”

The results of this research are published in the latest issue of the journal Science. The details may be new, but the general principle — how life tends to converse — definitely isn’t. “Most organisms don’t have eyes, don’t have ears,” Hay says, “so they decide whether to eat something next to them — or run from it or mate with it — based totally on chemistry alone. And that’s a world that we’re pretty much blind to.”

But we’re slowly starting to see — or smell — that world. And this new understanding of how corals communicate with other species may do more than just turn up another reason that reefs are in trouble: The Georgia Tech ecologists say what they’ve learned might make it possible to manipulate smells to help revive degraded reefs around the world.

Soon, we humans might be able to respond to coral reefs’ distress calls too.

Ari’s story comes to us from the series Small Matters, produced by the Center for Chemical Evolution, and supported by NSF and NASA.