Groupers hunt for small fish in open water, but sometimes they will beckon to a moray eel or a humphead wrasse to go hunting with them. Morays and wrasses will grab fish out of crevices and places a grouper can't reach -and if it flushes more of them out, the grouper can catch them. But what is weird is how the grouper recruits an eel, as observed by Alexander Vail from the University of Cambridge.
The groupers always summoned the wrasses and morays with a vigorous shimmy, but they also used a second, much rarer signal—a headstand, combined with head-shaking. Vail thinks it was a signal, one that said: “The prey’s in here, guys!”
When doing their headstands, the groupers always swam over the location of hidden prey that they had failed to catch. They only used the move when a moray or wrasse was nearby, continued to do so until one arrived, and stopped as soon as one did.
Most morays and all wrasses headed towards the grouper’s location when they saw the signal, causing the prey to break their cover. (The fact that the prey didn’t abandon their hiding spots beforehand shows that the headstand itself isn’t a hunting tactic.) And when the morays ignored the headstand, the groupers actually swum after their partner and either performed their “recruitment shimmy” or forcibly tried to push the eels in the right direction.
The researchers were impressed, but they caution that such cooperative behavior doesn't necessarily mean these fish have high intelligence. See videos of the tag teams in action at Not Exactly Rocket Science. Link