The moray eel thrusting its head from a crevice, jaws agape, is an iconic underwater tableau, recognizable even to those unfortunate souls who care nothing for marine life.
At least in Malaysia, where I’ve done most of my diving, morays are common, but never does the thrill diminish. I normally try to avoid anthropomorphizing animals, but with morays I can’t help it: the grin-like curvature of the jaws, and the apparent intelligence with which they regard passing divers, suggests an eerie sentience.
As it turns out, morays may be among the world’s smartest fish, exhibiting the kind of complex behavior we usually associate with chimpanzees. At the very least, they display hunting adaptations that strike fear in the hearts of every reef fish.
First, the way morays swallow their prey is unique in the animal kingdom, and very different from the way other fishes go about ingesting their prey. From the University of Washington’s Biology of Fishes:
Generally speaking, the vast majority of living fishes are gape and suck feeders. They feed by opening the mouth while simultaneously lowering the mouth and expanding the sides of the mouth cavity. This great and sudden increase in volume creates negative pressure (i.e., a vacuum), which, in turn, results in a sudden rush of water into the mouth. When directed adequately by the predator, it’s this sudden rush of water that carries or pulls prey into the mouth to be swallowed.
That’s an ingenious system, and incidentally describes how I eat most baked goods, but morays can’t use it. Owing to their long, serpentine bodies, their heads are simply too narrow to create enough negative pressure to feed by the gape/suck method. They can’t expand their mouth wide enough to induce the vacuum.
But don’t feel too bad for morays, since they’ve evolved an incredible alternative to gaping and sucking: pharyngeal jaws. These are a second pair of jaws deep in the moray’s throat, operated on a kind of a spring. The moray grips its prey in its primary jaws; then, when the eel is ready to swallow, it launches the pharyngeal jaws up into its mouth at high velocity, uses them to grab the prey, and drags the hapless fish into the esophagus. If you’ve ever seen the Alien movies, this slow-motion clip will look familiar (courtesy of YouTube user sciencetranslator). The white protrusion at the end isn’t the tongue, but those second jaws:
Crazy, right? Pharyngeal jaws are just bonkers. A few other fish possess them, but those fish’s pharyngeal jaws are stationary in the back of the throat, and merely used to grind or crush food. Morays’ pharyngeal jaws not only leap forward, they’re studded with long, curved teeth. Contemplating the evolution of pharyngeal jaws is mind-blowing. (So, too, is the fact that moray anatomy wasn’t explicated until 2007.)
But that swallowing system isn’t the moray’s only amazing hunting adaptation. In fact, it might not even be the coolest.
Giant moray eels (Gymnothorax javanicus) in the Red Sea do something that, as far as humans know, no other fish does: they hunt cooperatively with another species.
In 2006, a team of marine biologists in the Red Sea observed a remarkable behavior: groupers, another big predatory fish, approaching giant morays in their crevices and making a strange, rapid head-shaking motion – a sort of summons:
…after which the morays would leave their holes and join the grouper in open water. Morays are nocturnal ambush feeders, so swimming freely in broad daylight is bizarre behavior for them – let alone in the company of another predator. (I’ve always found the sight of swimming morays, their exposed bodies wrinkled and writhing, a little illicit, like walking in on somebody changing.)
So: what’s the point of this partnership? The eel and grouper are hunting buddies. Groupers pursue reef fish in open water, while morays corner their prey in cracks and crevices. As the study’s authors put it,
The hunting strategies of the two predators are therefore complementary, and a coordinated hunt between individuals of the two species confronts prey with a multipredator attack that is difficult to avoid; prey are not safe in open water because of the grouper hunting strategy but cannot hide in crevices because of the moray’s mode of attack.
Sure enough, the study found that the groupers were substantially better at hunting when accompanied by the eels. (The eels also ate well with the groupers, but since it’s almost impossible to observe an ambushing moray in action, the authors couldn’t compare rates.) Additionally, the authors found that hungry groupers were far more likely than well-fed ones to make the head-shaking motions and ‘recruit’ the eels, supporting the notion of deliberate hunting coordination.
Before we hail the moray and grouper as a glorious example of bipartisanship, bear in mind that neither animal is sacrificing anything for the sake of the other. Both species are simply trying to maximize their own welfare; it just so happens that cooperation helps them do so. This sort of interspecies commensalism is almost undocumented in Kingdom Animalia. The study makes a fascinating point: the fact that groupers and eels swallow their prey whole, instead of gnawing, chewing, or tearing, may be critical to their cooperation. Perhaps, say, lions and leopards don’t hunt cooperatively because they’d wind up fighting over the carcasses.
I highly recommend that you read the discussion at the end of Interspecific Communicative and Coordinated Hunting between Groupers and Giant Moray Eels in the Red Sea. Suffice to say, it will make you reconsider the ostensible intelligence gap between primates and fish – either they’re smarter than we think they are, or our allegedly unique abilities to communicate and live mutualistically aren’t so unique. I’m sure that as our understanding of ecology deepens, we’ll find many more examples of creatures that rely on each other not only via unconscious food webs, but through deliberate cooperation.
Maybe I’m still wrong to anthropomorphize the moray, but I bet there’s more going on in that head than pharyngeal jaws.