And now come the fish. In an article published today in PLoS Biology, Redouan Bshary from the University of Neuchâtel and colleagues describe the astonishing discovery of coordinated hunting between groupers (Plectropomus pessuliferus) and giant moray eels (Gymnothorax javanicus) in the Red Sea. These two species make a perfectly complementary pair. The moray eel can enter crevices in the coral reef, whereas the grouper hunts in open waters around the reef. Prey can escape from the grouper by hiding in a crevice and from the moray eel by leaving the reef, but prey has nowhere to go if hunted by a combination of these two predators. The article offers a description and accompanying videos, such as the one showing a grouper and eel swimming side by side as if they are good friends on a stroll. It also offers quantification, which is truly hard to achieve in the field, of the tendencies involved in this mutually beneficial arrangement. The investigators were able to demonstrate that the two predators seek each other's company, spending more time together than expected by chance. They also found that groupers actively recruit moray eels through a curious head shake made close to the moray eel's head to which the eel responds by leaving its crevice an d joining the grouper. Groupers showed such recruitment more often when hungry.
The observed role division comes "naturally" to two predators with different hunting specializations, and is therefore far simpler to achieve than for members of the same species. Also, recruitment is quite common in the animal kingdom—for example, primates have specialized signals to solicit each other's support in fights. What is truly spectacular about this study is that the entire interaction pattern—two actors who seemingly know what they are going to do and how this will benefit them—is not one we usually associate with fish. This is probably because we tend to develop cognitively demanding accounts for our own behavior and believe that absent the same cognition, the behavior simply cannot take place. It is very well possible, however, that our accounts overestimate the amount of intelligence that goes into complex behavior. Moreover, we have a tendency to underestimate the intelligence of animals at lower rungs of the evolutionary ladder. In fact, it is the ladder idea itself that is wrong. The best way to approach animal intelligence is from an evolutionary and ecological perspective focused on the tasks that each species faces in nature. In this regard, these two reef predators show us that if it comes to survival, highly intelligent solutions are within the reach of animals as different from us as fish. (Watch a grouper signal to a giant moray eel resting in a cave by shaking its head in front of the moray in this video.)