Cooperative hunting in groupers and moray eels

May 28, 2015 • 11:43 am

Here’s a short video from Nature that isn’t based on a single research paper, but on the continuing work of Redouan Bshary, a biologist in Switzerland who studies interspecific communication and behavior in fish.  The article itself, a summary of Bshary’s work written by Alison Abbott, is called “Animal behaviour: Inside the cunning, caring and greedy minds of fish“, and deals largely with the relationship between cleaner wrasses and their “cleanees.”

It turns out that this relationship is far more complex than the simple symbiotic relationship I’ve always taught (“cleaneee gets dead skin and parasites removed, cleaner gets a meal”). There is cheating by the cleaners, punishment by those fish who get a bum job of cleaning, and all kinds of complex social dynamics.

The video accompanying the article, however, deals with a different interspecies relationship—that between groupers and moray eels who seem to hunt “cooperatively”:

Now from the video alone, it looks as if the grouper is getting all the benefits, using the moray eel to flush prey for him (note that this also involves the moray leaving the safety of its den in the reefs). The experiments shown at the end demonstrate learning ability in groupers, but one question is missing in all this: What does the moray get out of it?  This behavior would not evolve (or persist, if it’s not genetically based) if both species didn’t derive some benefit from it. My immediate reaction would be that the moray gets some noms, too, but this isn’t mentioned in the video, which seems to show that all the benefits devolve to the grouper.  A better video would explain the benefits to both partners.

One alternative, which I find unpalatable and not biologically sound, is that the groupers are enslaving the morays as sort of a hunting d*g, to flush prey for them.  In this scenario the moray gets nothing. However, in nature we don’t see this kind of slavery very often, though it’s the norm for humans and their d*gs.

h/t: John

41 thoughts on “Cooperative hunting in groupers and moray eels

  1. Hey – dogs get noms from humans for the stuff they do. Question seems to me, do humans get rewards for the stuff they do for cats?

  2. We don’t know if the eels are obtaining food hidden in the coral when it runs from the grouper. Maybe the grouper does not get all the food.

    1. Groupers seem to recruit the morays using an invitation to hunt display or by pointing to the prey location. If both grouper and moray eat the same kinds of prey, but morays catch their prey it tight spaces and groupers in open water, both might benefit provided prey flee groupers by seeking crevices and into open water when pursued by morays. Thus, a grouper-moray mutualism may be mediated by adaptive escape behavior where prey find themselves trapped, if I may mix metaphors, between a hard place and the deep blue sea. Morays should not (evolve to) enter the game if there is nothing in it for them.

      1. Fascinating. I had the same question. Based on your comment, it might be that each species benefits about equally. The pray fish can be either in the hole or in the blue. Not many other places to go.

        1. But those fish are already either in holes or in the blue. What advantage is it for the eel to leave safety to share in hunting? Could the presence of the grouper keep other predators away?

      2. That’s my belief too. That the prey is more vulnerable to the eel when it’s on the run looking to hide, into the waiting eel’s reach.

  3. The human d*g relationship clearly also has benefits for the d*g. They get housed and fed, in return for being humiliated. I suspect that the population density of domestic canines is greater than one would expect for a mammal of that size so it seems to work well on a genetic level too. Of course neutering and spaying might cut down on reproductive success…..

    Didn’t see the grouper make the moray sit up and beg for a treat. However as noted in the video the presence of the grouper in the open water might inhibit the prey from moving out from the coral thus benefiting the eel. That at least is a plausible mutual benefit.

      1. You are probably aware that there is increasing “evidence” ( anecdotal, mostly, sofar), discussion about the health negatives of canine spaying and neutering? Hormones are not only advantageous to health but in some cases necessary. The current proclivity in shelters of very early neutering is raising concerns. Many dog breeders are urging people to refrain from neutering/spaying until a dog is at least a year and indefinitely when possible. Of course canine over population is a huge problem – there may be alternatives to early spaying/neutering that involve retaining hormones. I can supply some papers to any interested.

    1. I suspect that the population density of domestic canines is greater than one would expect for a mammal of that size so it seems to work well on a genetic level too.

      Oh absolutely. # of Gray wolves in US: 3,600. Number of domestic d*gs in US: 83 million.

    1. Try floating with your head down, pointed to a crevice. Maybe an eel will chase some little fish out to you. 😉

  4. Why wouldn’t the eel take a nice appetizer out of the back of the grouper when it comes to invite for a hunt? The narrator also says that hunting success of both fish increases. Is the eel even hunting? Could the vision of the grouper above hold other fish in place making them easier to pick off? Interesting stuff.

    1. Ann– The moray IS the eel — you’re thinking of the grouper. And, yes, part of this “understanding” appears to be the grouper “flying cover” so the eel is free to forage outside its burrow.

      Looks like an air-ground operation to me.

  5. I recently observed what looked like cooperative hunting between diving birds and a sea lion. To me it seemd a clear case of opportunistic behavior from smart animals that have learned a thing or two about how other animals may behave. This is a tad long, so please forgive.

    I saw a small flock of circling diving birds (mostly gulls but there were clearly some diving ducks in the mix) wheeling over a small patch of water in Elliot Bay in Seattle. Not unusual in as much as the birds had clearly spotted a school of bait fish near the surface.

    About a quarter mile away was a large male sea lion floating at the surface and basking in the sun (yes, the sun. In Seattle. At least I think that’s what it was. It’s what people outside the Pac NW call that big yellow thing in the sky, anyway).

    Manwhile the birds seemed frustrated. It looked like the fish were to deep for the birds to get to because very few made dives, but they were all very agitated. Several birds peeled off and flew over to the sea lion. I figured it must have been fishing and the birds figured they could get some scraps from him.

    However, the birds started to harrass the sea lion, diving at him and making a raucous noise sort of waking him from his torpor. I’d never seen that before. They kept bugging him until he dove. When he resurfaced they again harrassed him. It seemed like they were steering him, like a Border Collie and a stray sheep, towards the school of fish.

    As the sea lion got closer it became obvious that had HE become aware of the school of fish because he started heading fast, straight for the wheeling birds and the fish below. I watched as the sea lion repeatedly chased the school of fish into a small shallow, false bay against a breakwater where, cornered, the fish could not escape him. This brought the fish near the surface and the whole flock of birds began diving from the air and the water.

    In this case both birds and sea lion ate, so I can see where this benefits both. But the sea lion was not successful on every run; he would surface often without a fish.

    Perhaps the grouper was just being a smart opportunist and has learned how to manipulate the eel’s behavior. It looked to me like the grouper was alerting the eel to the presence of prey much as the birds were alerting the sea lion. I would bet that at least some of the time it is the eel who is the successful hunter of the two.

    1. It’s what people outside the Pac NW call that big yellow thing in the sky, anyway).

      Other than around sunset and sunrise when the sun can be orange, most of the time the sun is actually white.

      ( i’ll be out in Seattle in less than two weeks to visit my newly born granddaughter )

      1. Color is hard, especially of light sources rather than reflective objects. A lot of perception depends on context and surroundings. And, for something as bright as the Sun, which overpowers the human optical system (potentially to the point of permanent blindness), it’s even harder still.

        A good way to get an idea of just how hairy the problem is would be to start with this Wikipedia article:

        Especially note the chromaticity plot — the image at the bottom of the page on the right.

        Also good is this page:

        and especially the animated plot of spectral power distributions.

        In short…it is not an error to describe the Sun as being yellow, but it is incomplete. In particular, the Sun and the sky lie along a certain blue / yellow color axis, and any representation that faithfully puts the sky on the blue side of that axis in roughly its proper spot is going to have the Sun appearing at least slightly yellow in comparison. Even, it should be noted, if the rendition uses paper white for the Sun.

        Now…here’s something to really bake your noodle.

        Put a styrofoam cup upside down in the sunlight. (Even better, use a styrofoam sphere instead.) Position yourself so you’re at right angles to the Sun, so you see half of the foam lit by the Sun and the other half in its own shadow.

        Take a picture.

        On your computer, compare the colors of the sunlit half and the shadowed half.

        For the most dramatic results, do the experiment at dawn or dusk, but also compare with the results at noon.

        For bonus points, do all this with a camera that can shoot RAW photos, and set the white balance off the various parts of the foam.

        And, as a “final exam,” repeat the experiment without the camera, only your eyes.

        …and I should add a trigger warning…you’ll never look at color the same way again…and this particular rabbit hole is unimaginably deep and twisty….



        1. But, as a prism shows, sunlight has all colors, which is the sedition of white light.

          white light

          apparently colorless light, for example ordinary daylight. It contains all the wavelengths of the visible spectrum at equal intensity.

          1. Ah…yes. Very common misconception, perpetuated by unfortunate oversimplifications for introductory texts.

            First, there is no real-world object that produces light of equal intensity at all wavelengths over the visible spectrum. Such a light source is sometimes an useful mathematical construct, and is known as Illuminant E…but it doesn’t actually exist.

            All real-world light sources emit more photons of some wavelengths compared to others. Lasers emit photons of a single wavelength, to the limits of Heisenberg uncertainty. Fluorescence also creates monochromatic single-wavelength light, but real-world fluorescent sources emit monochromatic light at multiple wavelengths, and practical fluorescent devices have phosphors that preferentially absorb those wavelengths and re-emit the energy over a broader spectrum. You could, however, go the other direction and filter out all but a single one of those wavelengths — and the filters can be relatively simple and imprecise as these things go, making that an economical but inflexible source of monochromatic illumination.

            And, on top of that, everything emits blackbody radiation according to its temperature. Things at room temperature don’t emit enough visible light for us to see, but anything with a four-digit temperature would be described as, “bright,” at least in some context.

            …but all that’s actually irrelevant to the question of what white actually is, as it turns out.

            There isn’t any such thing as white light. Not even in theory.

            What there are are reflective objects that uniformly reflect light regardless of wavelength, and that’s what we perceive as, “white.” (Or a neutral gray.) And there really are objects that’re close enough to perfect to fall within the margin of error of human visual systems, with PTFE (Teflon) thread seal tape being one of them. Tyvek is another, but typically not quite as reflective as PTFE (98.5% v99%+). Styrofoam is usually about 80% reflective, but again with a very flat spectral response — making it, in fact, a far superior white balance tool than any expensive gewgaw sold to photographers for the task.

            But what’s happening is that the white object appears as the same color as the light, and our visual systems, adapted to an environment with wildly variable light sources (noon Sun to horizon Sun to overcast to moonlight to fire to….), are quite adept at guesstimating the color of the illuminant and doing some very fancy math to figure out an object’s spectral reflective properties in comparison. If they match, you see the object as white or gray.

            Did I mention? It’s a rather deep rabbit hole….


            1. The roof of my motorhome is white. So are the sides except for the graphic designs. My boat is white or whatever that colorless color is, duh.

    2. Very cool story. Here on the coast of South Carolina I’ve see hunting interactions between sea-birds and porpoises. Usually it looks like the sea birds taking advantage of the porpoises herding of the fish, but it’s probably more complicated than that.

  6. That’s neat. I’ve seen hundreds of Grouper in the wild and I’ve seen dozens of Moray eels, but I never knew of any kind of symbiosis between the two. A reef is like an under water metropolis.

  7. This was really interesting. It may be that the moray gets protection while it hunts for the both of them. So the partnership might be:
    a) The moray flushes out prey and it sometimes gets prey to eat.
    b) The grouper gets some prey to eat that get away from the moray.
    c) The moray has a big and alert fish hovering protectively over it while it is exposed, hunting over an extended range.

  8. My guess is that it’s cooperative hunting. The eyesight of the moray eel is not very good and they rely on smell. However, the grouper seems to be good at spotting and pointing at prey, and I’d think that any moray of worth its salt would make sure to get some whilst on the hunt.

    I wonder if there would be an added benefit to a moray who has a nest of babies somewhere. To go off on a hunt with a potential nest robber could be a good idea if they head out away from the nest. Could this be a good compromise on the part of the moray?

  9. As a fisherman, I always appreciate fishy posts. I’ve caught grouper in years past (they’re very tough fighters and tasty). However, I stay in my boat to avoid morays and other revenge seekers of the deep.

  10. Smart Groper, smart Moray, great post.
    Dumb human wants a groper to go fishing with.

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