A lionfish is a swimming paradox. It is painted in fierce hues of orange and white but it has an almost melancholy expression. It has fearsome venomous spines sticking out of its back, but elegant fan-shaped fins protruding from its side. And although it floats with an indolent air, it is actually an active and skilled predator—one that hunts in teams using surprisingly sophisticated tactics.
Oona Lönnstedt has spent hundreds of hours watching lionfish, both in the wild and in her lab at James Cook University. During night dives, Lönnstedt often saw teams of two to four lionfish positioning themselves around schools of smaller fish and using their fan-like pectoral fins to corral their prey “like fishermen with their nets”. The hunters then take turns to dart into the school of prey, picking them off one at a time.
That’s how the hunts end. But Lönnstedt was more interested in how they start.
She noticed that they always begin when one lionfish swims up to another, points downwards, flares its pectoral fins, and quickly undulates its tail. After a few seconds of this, it slowly waves one pectoral fin, then the other. The partner almost always responds by undulating its own fins, and the pair moves off in search of victims.
Despite her extensive fieldwork, Lönnstedt only ever saw lionfish make this display before they started hunting together. It looked like a message with a clear meaning: “Let’s hunt.”
Lönnstedt tested her interpretation in an experiment. She would place one zebra lionfish (Dendrochirus zebra) in a maze-like tank. Six tasty cardinalfish sat behind a transparent barrier at one end of the tank. A second lionfish was hidden from view by some opaque dividers at the other end.
After Lönnstedt added the cardinalfish, the first lionfish spent more time next to the prey compartment. But it also repeatedly left these tempting morsels, and swam to the other end of the maze to hang out in front of the second lionfish. Once there, it often made the distinctive fin displays.
These fin displays only happened when the tank contained both prey and another predator to recruit. The second predator didn’t have to be a lionfish of the same species—there are at least a dozen—as long as it was a lionfish. If the potential partner was a grouper, the first lionfish wasn’t interested.
Once the partners were united, Lönnstedt removed the partitions and let them do their thing. Working as a team, the duo captured almost twice as many fish than each of them would normally manage alone.
“There has been anecdotal evidence of cooperative hunting in lionfish since the late 1980s,” says Lönnstedt. “Ours is the first study that empirically demonstrates that it occurs. These are highly complex animals with advanced social behaviours, and they are ridiculously good at catching prey.”
“I find the paper amazing,” says Redouan Bshary from the University of Neuchâtel, who studies fish behaviour. “Fish social behaviour is much more complex than previously assumed. Moving away from a stimulus of major interest—prey—in order to actively recruit a partner that is initially out of sight suggests planning and awareness of objects that [they can’t see].”
Last year, Bshary showed that coral groupers use special gestures to recruit giant moray eels. The two fish hunt cooperatively, with the sinuous moray chasing prey that hides in cracks and crevices and the fast grouper catching any that flee into open water.
But in these pursuits, the two partners are merely hunting next to each other and relying on their complementary abilities. The lionfish are doing something more impressive: they’re working together to corral their prey and taking turns to go in for the kill.
Now, Lönnstedt wants to find out more about how they coordinate their movements. Why does the fish that initiated the hunt always go first? And why do they take turns—is it a cooperative strategy, or does each fish simply need time to recover from its assault? And finally, does the flared fin display that launches the hunt actually convey any information? Is it just a “Come hunt with me” signal, or does it convey information about the location of food, like the famous waggle dance of bees?
PS: Lionfish are known for their painful venom, which Lönnstedt has experienced first-hand. “It’s excruciatingly painful,” she says. She now wears special gloves to stop the spines from piercing through and if that fails, hot water can dull the pain a bit. “If you’re out somewhere on a boat, it’s always good to bring a thermos of boiling water as an extra precaution.”
Reference: Lönnstedt, Ferrari and Chivers. 2014. Lionfish predators use flared fin displays to initiate cooperative hunting. Biology Letters http://dx.doi.org/10.1098/rsbl.2014.0281