Do Moray Eels Hunt With Groupers? Teamwork on the Reef

How the partnership was discovered

The partnership was first studied in detail by Redouan Bshary and colleagues, who observed it on coral reefs in the Red Sea and published their findings in PLoS Biology in 2006. The two partners were the roving coral grouper (Plectropomus pessuliferus), a fast, spotted predator of open water above the reef, and the giant moray (Gymnothorax javanicus), the heaviest moray in the world.

The researchers showed four things. The fish did not meet by chance: groupers sought out morays. Groupers used signals to start joint hunts and to lead morays to hiding prey. Signaling depended on how hungry the grouper was. And both species benefited from hunting together.

The head-shake invitation

A typical joint hunt starts with the grouper. It swims to a crevice where a moray is resting and shakes its head rapidly from side to side, just in front of the eel. Often the moray then leaves its shelter, and the two predators move off across the reef together, the eel weaving through holes while the grouper swims above or beside it.

Groupers did not shake their heads at morays at random. In the Red Sea study, the signal was linked to hunger: well-fed groupers signaled less than hungry ones. That suggests the grouper is not simply reacting to the eel but signaling when it actually needs help.

Pointing at hidden prey

The second signal is even more remarkable. When a prey fish escapes into a crevice that the grouper cannot reach, the grouper may do a headstand over the hiding place, pointing its head down at the spot. A moray, or another partner, then comes over and goes in after the prey.

In 2013, Alexander Vail, Andrea Manica and Redouan Bshary described this headstand in the roving coral grouper and in the coral trout (Plectropomus leopardus) of the Great Barrier Reef in Nature Communications. The fish used it with several partners: giant morays, humphead (Napoleon) wrasses and even octopuses. The researchers argued that the headstand is a referential gesture, a signal that draws a partner’s attention to an object, much like pointing. It is aimed at the prey’s location, has no effect on its own, is directed at a potential partner, gets a voluntary response and shows signs of being intentional. Before this, such gestures had been attributed outside humans only to great apes and ravens.

Why both partners win

The partnership works because the two hunters have opposite strengths. A grouper is fast in open water but cannot follow prey into narrow holes. A moray, with its long, slim body, can search deep inside the reef, but is not built for a chase in open water. A fish escaping one partner is driven toward the other: prey fleeing the moray leaves the reef and meets the grouper, and prey hiding from the grouper is found by the moray.

There is another reason the teamwork lasts. Whichever partner catches a fish swallows it whole immediately, so there is no carcass to fight over. Bshary and his colleagues suggested that this is why such partnerships are rare among predators: in most combinations, one partner could steal or monopolize the kill, and cooperation would break down.

Coral trout pick the right partner

In a 2014 study published in Current Biology, Vail, Manica and Bshary tested coral trout in experiments modeled on classic tests of cooperation in chimpanzees. The trout used a model moray as a potential partner. They recruited the “moray” more often when prey was hidden in a place the trout could not reach on its own, the situation in which help is really needed. They also quickly learned to choose the more effective of two partners.

Chimpanzees had been the only animals other than humans known to show both abilities, knowing when to collaborate and with whom. The trout performed comparably. The authors concluded that these skills may depend more on an animal’s ecological needs than on brain size or close relationship to humans.

Other hunting partnerships on the reef

Morays have other partners. In Brazil, snappers have been seen recruiting green morays while foraging among mangroves, and the green moray of the western Atlantic is a close relative of the giant moray.

Octopuses also hunt in mixed groups. A 2024 study of the day octopus (Octopus cyanea) in the Red Sea found that goatfish, groupers and other fish join octopuses on hunts. The fish tended to lead the search, while the octopus decided whether and when the group moved, and an octopus sometimes “punched” a fish partner that was taking advantage of it. The intelligence of these animals is explored in How smart are octopuses?

What it says about fish intelligence

Fish are often thought of as simple animals, but the grouper and moray partnership shows communication, timing and partner choice in creatures with small brains. Fish do not need a large brain to cooperate in complex ways when their way of life rewards it.

Many questions remain, such as how young groupers learn the signals and how widespread similar partnerships are. For other animals with impressive mental skills, see smartest animals, and for the moray’s own hunting equipment, see Why do moray eels have two sets of jaws?