Grouper and Giant Moray are Unlikely Hunting Buddies, by Don Orth

A grouper swims into the face of a Giant Moray Eel and says, “Let’s hunt together.”  The Giant Moray says “Ok, why not?”  And such begins a cooperative hunting trip among two unlikely hunting buddies.  It's not a riddle. 

A fish must eat or it will not survive long, especially in the warm ocean waters.  We often think of a fish by its feeding strategy.  For example, the Pickerel and Barracuda are camouflaged sit-and-wait predators, the Goosefish also sits and waits, but it has a lure to attract prey, sturgeons are benthic foragers, and Menhaden, Sardines, and Whale Sharks are filter feeders.  The morphology of each fish is specialized to facilitate its dominant feeding strategies. But the Grouper and the Moray Eel often hunt together to increase prey captures.  This cooperative hunting behavior in two unrelated carnivores was first observed by Bshary and associates in the Red Sea.  It may be the first reported occurrence of cooperative hunting by unrelated species.

The Roving Coralgrouper, Plectropomus pessuliferus Fowler, 1904, is a large grouper (Serranidae, Epinephelinae, Epinephelini) that roams on the reefs in the Indo-Pacific Ocean and reaches 1.2 m.  The groupers are a highly photogenic marine fishes.  All members of this genus have the same dorsal fin formula “VIII, 11” and three antrorse spines on the lower margin of the preopercle.  All seven species of Plectropomus possess the beautiful pattern of blue spots on the head, body, and median fins (see photo).  Roving Coralgroupers usually live and hunt alone.  Only during the breeding season, do they join other Roving Coralgroupers in spawning aggregations. This grouper is most likely a protogynous hermaphrodite, like other close relatives. The Roving Coralgrouper is a diurnal predator with a large mouth. These large-bodied rover predators use their burst speed and vacuum action of the large buccal cavity to capture prey in open water.  Consequently, its prefers to eat other fishes, typically damselfishes (Pomacentridae), wrasses(Labridae), and anything else that moves too slow.

Photo of  Roving Coralgrouper Plectropomus pessuliferu.  Photo by Pere Rubio

The Giant Moray Gymnothorax javanicus (Muraenidae) is the largest of the moray eels, reaching up to 3 meters and up to 30 kg.  They are widespread in the Indo-pacific region as well as other shallow warm oceans.  Adults have black specks, which graduate into leopard-skin spots behind the head, with a black area surrounding the gill opening (see photo).   Giant Morays rest in crevices during the day and, unlike the Roving Coralgrouper, hunt nocturnally.  Because of their body form, the moray can access prey that are hiding in reef crevices.  Their wide jaws have a unique set of secondary jaws, the pharyngeal jaws; these jaws assist with holding prey.  Fish, cephalopods, molluscs, and crustaceans often fall prey to these double-jawed morays.   The Giant Moray also displays long breeding migrations, transparent leptocephalus larvae, and hermaphroditism, but it’s the cooperative hunting with the Roving Coralgrouper that illustrates this animal’s incredible intelligence. 

Giant Moray Eel Gymnothorax javanicus.  Photo by Andrew Bruckner

The fascinating study by Redouan Bshary and associates, of University of Neuchâtel, Switzerland, described cooperative hunting behavior of the Roving Coralgrouper and the Giant Moray Eel.   By cooperation we mean that the two species communicate regarding their needs and cooperate to help each other capture more prey.  Further, true cooperation occurs only when individuals play different roles during a hunt. How do these two species communicate?    If the grouper is hungry, s/he will approach with head shaking (3-6 shakes/second) directly in front of the Giant Moray’s head.  This gesture has been referred to as the “shimmy signal.”  The spiny dorsal fin is always depressed. Apparently, this signal means “let’s go hunting together.”   The moray understands the signal and the two fish swim off together (58% of the observations).  The grouper does the head shake behavior while performing a headstand over the hiding place of its prey.   This signals to the moray to investigate the crevice, which they do.  Because the moray flushes the prey, the groupers caught almost five times as many prey items per unit time than when morays were absent.  Sometimes they observed moray eels catch prey when hunting with a grouper, but the two species never displayed any aggression when hunting. 

The Red Sea investigators documented many interactions between the two fish species; 35% of these were 2 minutes or longer (up to 44 minutes).   Clearly, this was not a random occurrence.  The Giant Moray stayed within 1-2 grouper body lengths during the interaction.      Both hunting buddies increased their feeding success when hunting cooperatively.   No cheating is possible because both of these fishes swallow their prey whole, leaving no trailing parts to “fight” over.

Other SCUBA divers have subsequently videotaped this cooperative hunting behavior. The behaviors of these hunting partners were videotaped by Bshary and his coauthors. See here.  However, others have also witnessed and videotaped the behavior, including FrederikW, Karel Mestdagh, and David Whitehead101. 

The grouper and moray are two unlikely hunting buddies that participate in true cooperative behavior, in which both partners benefit from the association.  The head-shaking and head-down behavior of the Roving Coralgrouper bears all the characteristics of an intentional “referential gesture.”   It communicates the location of the hidden prey to the hunting partner.  The joint hunting expeditions are more successful because the adaptations of the two fish are complementary.  The intentionality of a gesture is a characteristic of communication among primates and has been seldom investigated in fishes (Vail et al. 2013).    However, this behavior suggests that cognitive processes may underly the gesture.  We have no simple way to ask sophisticated questions on cognition in wild, free-living fishes.

References

Bshary, R., A. Hohner, K. Ait-el-Duoudi, and H. Fricke. 2006. Interspecific communicative and coordinated hunting between groupers and Giant Moray Eels in the Red Sea. PLoS Biology 4(12): e431.    DOI: 10.1371/journal.pbio.0040431

Vail, A.L., A. Manica, and R. Bshary. 2013. Referential gestures in fish collaborative hunting. Nature Communications  4:1765  DOI: 10.1038/ncomms2781