Reef alliances: goatfish hunt in packs, while groupers team up with moray eels

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In the Red Sea, a tiny fish has been cornered by a group of hunters working as a team. One of them chased it into a coral crevice, while the others circled around to block off any exists. With no escape, the predators – a group of yellow saddle goatfish – close in on their prey.

The goatfish are one of the few examples of fish that hunt in groups, and their strategy has just been described by Carine Strubin, Marc Steinegger and Redouan Bshary from the University of Neuchatel. Bshary has spent over a decade in the Red Sea, studying the local fish. “I spent a long time working on cleaner wrasses,” he says. “During that time, one happens to see a lot of things.”

The cooperative hunts of the goatfish were one of these incidental observations. After repeatedly snorkelling after the fish, Strubin gradually learned that they have distinctive spots on their bodies and blue lines around their eyes. Eventually, she could tell 17 different individuals from one another, and she learned that they form surprisingly stable social networks. Each fish would regularly keep in touch with the same companions, usually ones of similar size. They could belong to several different groups, but those groups would keep the same members over time.

The goatfish normally forage alone, using their whisker-like barbels to feel around for prey buried in sand. They only team up when they hunt among corals. The moment one fish shoots off towards a target, the others join in the hunt. Once the chaser drives its prey into coral crevices, the others act as blockers, swimming around to cut it off. Steinegger has started to test captive goatfish and he has duplicated the same coordinated hunts in the controlled conditions of a lab.

Many species of fish will attack the same prey, but very few truly work together during a hunt by coordinating their attacks. Until now, the most sophisticated example was an African elephantfish that flushes out its prey in tight formations, and communicates with one another using electric pulses. But the goatfish are more sophisticated still. They’re the only known species of fish where different individuals assume specific roles, not unlike hunting wolves, lions or chimps.

This is the second time that Bshary has observed Red Sea fish hunting in teams. In 2006, he described an equally remarkable alliance between two formidable predators: the giant moray eel and the roving coral grouper. Again, this discovery was the result of his work with the cleaner wrasse. “I followed groupers to see how they interact with several cleaners in a row, and I observed them signalling towards the morays,” he says.

Bshary saw that the groupers would visit the morays in their resting places and vigorously shake their heads. The signal is a call to arms, rousing the lazing morays to leave their crevices and swim off with the groupers. The groupers trigger the alliance if they’re hungry or frustrated. Bshary found that they were less likely to recruit morays if they’d just been fed, but more likely if they had just chased a prey fish into an inaccessible crevice.

The groupers lead the eels to a place where prey are hidden, and signal the right spot with more head-shaking; the morays investigate. The two species have complementary hunting skills. The groupers are open-water specialists, but the morays can probe in cracks and crevices. When both species hunt together, nowhere is safe. When the moray dives in, the fish has two options: stay and be eaten by the eel, or flee and be picked off by the grouper. Only one of the two predators will grab any individual prey, but both have a greater chance of eating if they work together. Certainly, the groupers are five times more successful at catching prey if their partners are around.

It’s not clear what either example says about how intelligent the fish are. “That’s the big question,” says Bshary. “One could turn it round and ask what the results tell us about the mental abilities of other collaboratively hunting species.” He notes that people have assumed that hunting in packs demands a higher intelligence, without actually working out why.

For example, the goatfishes’ tactics could emerge from just one simple rule: stay around the prey but keep as much distance from the other group members as possible. Strubin also points out that the chaser is the one that starts the pursuit, so it’s more likely to catch the prey in a straight chase. The blockers may be more likely to catch the fish if they circle around – a selfish strategy that looks a lot like cooperation.

The alliance between the groupers and morays is even less intellectually demanding. Both are simply acting in the way they normally would and by virtue of that, they end up playing different roles. Again, by behaving entirely selfishly, they both get more food. And again, a simple rule works: hunt next to the other species if they’re around and you’re hungry. The grouper’s signals might hint at something more complex, but Bshary points out that groupers hunt in the day and morays at night. The grouper would need to do something to rouse the eel’s attention.

With two examples in five years, it’s likely that there are more examples of reef team-ups left to discover, and that Bshary is the man to discover them. “Fish seem to be much cooler than people thought,” he says. “It’s a pleasure to describe collaborative hunting within and between fish species, because that is natural history with a strong element of surprise.”

Reference: Strübin, C., Steinegger, M., & Bshary, R. (2011). On Group Living and Collaborative Hunting in the Yellow Saddle Goatfish (Parupeneus cyclostomus)1 Ethology, 117 (11), 961-969 DOI: 10.1111/j.1439-0310.2011.01966.x

Bshary, R., Hohner, A., Ait-el-Djoudi, K., & Fricke, H. (2006). Interspecific Communicative and Coordinated Hunting between Groupers and Giant Moray Eels in the Red Sea PLoS Biology, 4 (12) DOI: 10.1371/journal.pbio.0040431

Image by Al Kok

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