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You’re enjoying a calm dive on a Red Sea reef, sunlight streaming through clear water, illuminating colorful corals and reef fish. As you drift over a coral outcropping, you notice a grouper hovering in front of a crevice. From the shadows, a moray eel peers out. For a moment, nothing happens. Then, just inches from the moray’s face, the grouper begins rapidly shaking its head—once, twice, six times a second. The eel watches the grouper’s behavior for a beat before it glides into the open. The fish pair up and swim off along the reef, the grouper taking the lead.
You’ve just witnessed one fish ask another for help.
Behavioral ecologist Redouan Bshary first saw this in 1998, while following individual groupers at Ras Mohammed National Park in Egypt’s Red Sea.
“All of a sudden, a grouper approached a moray and shook its head,” he recalls. “I initially thought they would start fighting.” Instead, the moray emerged from its crevice and the two swam off, side by side. “It was immediately obvious they had started hunting together.”
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In 2006, Bshary and his colleagues at Switzerland’s University of Neuchâtel published findings from more than 400 hours of observation across 14 groupers, documenting the remarkable behavior.
The partnership makes sense once you understand how each predator hunts alone. Groupers are fast, open-water ambush hunters but helpless when prey darts into a crevice they can’t reach. Moray eels thread through those same crevices to corner hidden prey, forcing it to flee into open water.
“If grouper and moray coordinate in time and space,” Bshary says, “there is no safe place for prey outside or inside.”
Together they create what the study describes as “a multipredator attack that is difficult to avoid.”
What makes this more than coincidence is that grouper actively recruit resting morays. They have two signals for doing so. The first—shaking their head about four times per second—invites a nearby moray to join a hunt. The second is used when prey has already escaped into a crevice. The grouper tips into a near-vertical position above the hiding spot and makes wide, slow head movements, about once per second, to draw the moray in.
“It is more like a hound pointing to a fox or badger in a burrow,” Bshary says.
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On multiple occasions, this second signal attracted not just morays, but Napoleon wrasses too. Crucially, any grouper will signal to any moray in its home range, and all morays understand it.
The numbers reflect how well it works. Groupers hunting with morays caught nearly five times more prey per hour than groupers hunting alone, while morays—never observed catching prey on their own during daylight—caught prey regularly when paired with a grouper. In more than 30 hours of observed joint hunting, neither species showed aggression after a successful catch. Each swallowed its prey whole and moved on, which the researchers suggest may be exactly why the partnership could evolve at all. No carcass means nothing to fight over.
Since the study’s publication, Bshary has received reports of similar behavior at other reef sites, and a Japanese film team has recently captured footage. The behavior may be more widespread than a single study in a single park suggests.
But what does it reveal about fish? “Fish can be more strategic than they usually get credit for,” Bshary says. “The signals by the groupers have all the hallmarks of intentionality, and intentionality is usually considered an advanced cognitive tool.”
On your next dive, watch for a moray moving across the reef—then look for who might be following. Slow down. Pay attention. The reef is communicating. You just have to open your mind to it.
