Pharyngula

Jaws of the moray

Blogging on Peer-Reviewed Research

The vertebrate jaw is a product of evolution — we have a serially repeated array of pharyngeal structures as embryos (and fish retain them in all their bony glory as gill arches), and the anterior most arch is modified during our development to form the jaws. The fact that they’re serially repeated raises an interesting possibility: what if, instead of just the one developing into a jaw, others were transformed as well? You could have a whole series of jaws!

One animal has done exactly that. The moray eel has modified one of the more posterior pharyngeal arches into a second pair of jaws, with a set of muscles that can slide it forward to bite prey already held in the mouth.

i-bc339e0ce33b8633c43241c0cd94cad6-moray_xray.jpg
a, Posterior placement of the pharyngeal jaws in relation to the skull. The arrow points to the pharyngeal jaws. b, Pharyngeal jaws in their protracted position. The arrow points to the upper pharyngobranchial. Scale bar for a and b, 1 cm.

I know, everyone is thinking HR Giger and the Alien movies … and it is similar to that. These animals have toothed jaws in their throat, and a set of protrusive and retracting muscles to move them forward and back.

Why? In teleost fish, the jaws are wonderfully complex and malleable, and are impressive examples of morphological diversity. In most predatory fish, the common technique for capturing prey is suction feeding: opening the jaws, depressing the floor of the oral cavity, and flaring the opercula (gill flaps) creates a partial vacuum that sucks the prey into the mouth and holds it there while the teeth engage in their bloody work. This is a problem for eels, since they’re adapted for living in confined spaces and narrow crevices — a morphological feature that requires expanding the mouth and pharynx may not work well. And if they can’t use suction to draw food deeper into the throat, it might drift away each time they open their mouth to take another bite.

The moray has evolved a new eating strategy. Bite into the prey with the usual jaws, and then reach forward with the pharyngeal jaws and bite again. Open the main jaws and release, and the pharyngeal jaws pull the prey deeper into the mouth. The main jaws bite and hold again, and the pharyngeal jaws can reposition and bite more. The food is ratcheted deeper and deeper into the gullet by pairs of jaws taking turns to seize and tug.

The pharyngeal jaws can be seen in this quicktime movie. The eel bites on a bit of squid, and then about half way through the clip, you’ll see a disturbing, creepy something between its gaping jaws reach forward and bite into the meal. It is so cool, and so unexpected.

It’s a wonderfully, beautifully wicked mechanism.

i-7bec5bc7f2fc87d461eca815af33c37d-moray_jaws.gif
(click for larger image)

The left dentary has been removed in a?c, and the left maxilla has been removed in b and c. a, Pharyngeal jaw apparatus at rest. b, Pharyngeal jaw protracted: the levator internus (LI) and levator externus (LE) protract the upper jaw into the oral cavity, whereas the rectus communis (RC) protracts the lower jaw. During protraction, the upper pharyngobranchial is dorsally rotated by contraction of the LI and the obliqus dorsalis (OD). c, After prey contact, the adductor (AD) contracts to bring the upper and lower jaws together to deliver a second bite. The dorsal retractor (DR) and pharyngocleitheralis (PHC) retract the pharyngeal jaws back to their resting position behind the skull. Scale bar, 1 cm.

The closest resemblance to this mechanism elsewhere in vertebrates is snakes, which use gnathic transport in a similar way. They don’t have quite as elaborate a set of pharyngeal jaws, but they do use a pharyngeal ratchet to pull prey down into their throat.


Mehta RS, Wainwright PC (2007) Raptorial jaws in the throat help moray eels swallow large prey. Nature 449:79-82.

Comments

  1. #1 Brownian
    September 6, 2007

    Are ‘pharyngeal jaws’ what you used to scare off Pivar?

  2. #2 David Marjanovi?
    September 6, 2007

    The closest resemblance to this mechanism elsewhere in vertebrates is snakes, which use gnathic transport in a similar way. They don’t have quite as elaborate a set of pharyngeal jaws, but they do use a pharyngeal ratchet to pull prey down into their throat.

    Nooooo! No! No! No! No! Snakes use palate bones with teeth on them (the pterygoids in this case) for that purpose. The left and the right pterygoid move alternately. The whole affair is termed the pterygoid walk. The pterygoids stay in the skull throughout, completely unlike the pharyngeal jaws of the moray, and they are ordinary dermal bones that we have, too, not homologous to gill arches. There is nothing pharyngeal about them.

    You can see teeth on the palate of the moray in the drawings above. Having teeth on the palate is the normal condition for bony vertebrates; mammals and birds + crocodiles are the freaks for having lost all of them.

  3. #3 David Marjanovi?
    September 6, 2007

    The closest resemblance to this mechanism elsewhere in vertebrates is snakes, which use gnathic transport in a similar way. They don’t have quite as elaborate a set of pharyngeal jaws, but they do use a pharyngeal ratchet to pull prey down into their throat.

    Nooooo! No! No! No! No! Snakes use palate bones with teeth on them (the pterygoids in this case) for that purpose. The left and the right pterygoid move alternately. The whole affair is termed the pterygoid walk. The pterygoids stay in the skull throughout, completely unlike the pharyngeal jaws of the moray, and they are ordinary dermal bones that we have, too, not homologous to gill arches. There is nothing pharyngeal about them.

    You can see teeth on the palate of the moray in the drawings above. Having teeth on the palate is the normal condition for bony vertebrates; mammals and birds + crocodiles are the freaks for having lost all of them.

  4. #4 David Marjanovi?
    September 6, 2007

    Pharyngeal Jaws are common in fish

    Yes, the cichlids are famous for them (having differently shaped teeth in the normal and the pharyngeal jaws gives them a lot of possibilities for easy adaptation to weird ecological niches, without having to evolve a mammal-style differentiated dentition in one jaw). Pharyngeal jaws that are so freakishly and stunningly mobile are apparently unique, however.

  5. #5 David Marjanovi?
    September 6, 2007

    Pharyngeal Jaws are common in fish

    Yes, the cichlids are famous for them (having differently shaped teeth in the normal and the pharyngeal jaws gives them a lot of possibilities for easy adaptation to weird ecological niches, without having to evolve a mammal-style differentiated dentition in one jaw). Pharyngeal jaws that are so freakishly and stunningly mobile are apparently unique, however.

  6. #6 Ichthyic
    September 6, 2007

    I’m surprised neither PZ nor the NPR report went into the other common feeding technique used by morays:

    when faced with a prey item that is simply too large to swallow whole, they will tie themselves in a knot, run the knot up their bodies until it reaches their head, and rip off a big hunk of whatever they have their teeth sunk into as they pull their head through the knot.

    I used to demonstrate this nightly to visitors at the Catalina Marine Science Center (Big Fisherman Cove, near the isthmus).

    I would take a large, frozen, king mackeral or small bonita, drill a hole through the skull, and tie a rope through it.

    then I would take the onlookers down to the rocks at the base of the pier, and dangle the frozen fish in the water between the rocks.

    sure enough, within a couple of minutes, one of the large (up to 5 feet or so) morays would inevitably bite onto the frozen fish. Whereupon, I would actually pull the moray all the way out of the water, still attached, and it would tie itself in a knot and end up removing an actual large hunk of frozen fish before dropping back into the water.

    now, don’t let it be said that they are vicious beasties that attack divers, because they are normally quite shy and very rarely attack divers, even when provoked. However, it was always instructive to divers who would not afterwards be so incautious as to stick their hands in unexplored holes in the rocks.

    BTW, anybody who ever wants the demonstration for themselves, if you have a boat and plan to head for Catalina in the future, I’d be happy to oblige.

  7. #7 Ichthyic
    September 6, 2007

    Please come join me for a dive on my local reef and I’m sure I can change your minds.

    where’s your local reef, Fernando?

  8. #8 Fernando Magyar
    September 6, 2007

    Ichthyic, over the years I’ve spent a lot of time in the water and have seen more than my fair share of morays on the reef but I’ve yet to see one tie itself into a knot. I was planning on diving this Saturday but the seas here are going to be a bit rough so I won’t be able to test this on the weekend but you can be sure that I will be on the lookout for an opportunity to do so in the very near future. Don’t worry I will be very very careful. I don’t type too well with my toes.

  9. #9 Ichthyic
    September 6, 2007

    heh, I have a buddy in Sarasota.

    It’s not hard to get a moray to do this:

    -get a fish that’s far too large for them to swallow

    -go to a reef area where you know one or more hang out

    -use the fish to lure one out of it’s hole

    -let it get a good grip on the fish, but don’t let it retreat into it’s hole.

    almost every time, if it doesn’t let go (rare), it will indeed tie itself in a knot.

    heck, I’ve been watching morays do this since I was 15, though I must admit, I’m not sure if I recall having experimented with tropical morays to see if they do the same thing as the temperate ones. No reason to doubt that they do, however.

  10. #10 Ichthyic
    September 6, 2007

    … be happy to show you anytime you want to do a dive in the Southern California area.

    oh, btw, do you photograph any of your underwater forays?

    website?

  11. #11 Ichthyic
    September 6, 2007

    I figured there had to be some vids out there of tropical morays doing the “knot”, and of course, there are:

    http://www.youtube.com/watch?v=oSE3gbAKslU

    in a tank, but it’s a tropical snowflake moray.

    I had forgotten I used to have a tank with one who did this as well when I was about 12.

  12. #12 Skemono
    September 7, 2007

    Um. This is kinda disturbing. I looked at the video Ichthyic provided, and at the end, the other videos that it provides are almost all about Ann Coulter. Seriously, here’s a list of the ones I got:
    “Zebra Moray Mating Dance”
    “Ann Coulter, Canada, and Vietnam”
    “Carlson and Coulter take on Canada”
    “Ann Coulter Crazy on the Today Show”
    “Adam Corolla hangs up on Ann Coulter”
    “Late Show – Clinton’s reply to Coulter”
    “Ann Coulter gets owned”
    “O’Neal in Beijing”

    Does YouTube look at the last page you visited and pick out words or something?

  13. #13 Ichthyic
    September 7, 2007

    Does YouTube look at the last page you visited and pick out words or something?

    interesting question. It might track cookie data on your comp.

    try disabling cookies in your browser, delete your internet cache and recent history files, and then go back there and see if the same pattern emerges, or whether you get something entirely different.

    make sure you have cookies turned OFF.

    … and if you thought it was checking the last page you visited…

    what were you doing perusing Ann Coulter’s stuff???

    HUH?

    :P

    the only value in her materials is in using as an emetic, IMO.

  14. #14 Ichthyic
    September 7, 2007

    hmm, tried it myself.

    nope. seems the coulter links are pre-programmed for some reason. my guess would be a tag system someone is abusing.

  15. #15 Carlie
    September 7, 2007

    Ichthyic – am I the only one who was screaming “Use tweezers, for god’s sake!” when watching that video? Sure, it was a little eel, but the fingers seemed a tempting target.

  16. #16 David Marjanovi?
    September 7, 2007

    I think they knew about the extra jaws, they just had no idea how mobile they were.

    That’s what I think. The pharyngeal jaws themselves are not presented as a new discovery.

    (BTW, I didn’t write only cichlids had them. I wrote they were famous for theirs.)

  17. #17 David Marjanovi?
    September 7, 2007

    I think they knew about the extra jaws, they just had no idea how mobile they were.

    That’s what I think. The pharyngeal jaws themselves are not presented as a new discovery.

    (BTW, I didn’t write only cichlids had them. I wrote they were famous for theirs.)

  18. #18 Sven DiMilo
    September 7, 2007

    David Marjanovi? is (as always) correct above. (Hi David–have “known” you for years via lurking the Dinosaur listserve archives).

    Lago: Anyone with a vertebrate bio course in their background knows about pharyngeal jaws; it’s the protraction that’s new and (oh so) cool…I agree though that PZ’s wording implies otherwise.

    Ichthyic: thanks! I knew hagfish were self-knotters but did not know about morays. (My one trip to Catalina was a highlight of my life (many reasons)).

    John Scanlon: Could you briefly summarize what you think are the best bits of evidence for a “marine” origin of snakes? As opposed, I mean, to a) aquatic (freshwater) or b) fossorial?
    thanks

  19. #19 Ichthyic
    September 7, 2007

    Sure, it was a little eel, but the fingers seemed a tempting target.

    yup. learned that one the hard way myself, doing exactly what you see in the video.

    which brings up another amazing thing about morays-

    they’re tremendously hardy buggers, and can even stand being flung across a living room by a surprised 12 year old.

    suffice it to say I did indeed use tweezers after that.

  20. #20 Ichthyic
    September 7, 2007

    The morays have a good slander case, then.

    well, better than Pivar did, anyway.

    :p

  21. #21 David Marjanovi?
    September 8, 2007

    In the absence of the expert, let me try… It looks like the snakes’ closest relatives are a bunch of Cretaceous marine “lizards”, including the great and mighty mosasaurs. Furthermore, the burrowing snakes are not the deepest branch on the snake family tree, which is instead where we find large marine Cretaceous animals that retained small hindlegs.

    Today’s sea snakes are something else, however — a branch within the elapids, the group that the cobra and the venomous snakes of Australia belong to.

  22. #22 David Marjanovi?
    September 8, 2007

    In the absence of the expert, let me try… It looks like the snakes’ closest relatives are a bunch of Cretaceous marine “lizards”, including the great and mighty mosasaurs. Furthermore, the burrowing snakes are not the deepest branch on the snake family tree, which is instead where we find large marine Cretaceous animals that retained small hindlegs.

    Today’s sea snakes are something else, however — a branch within the elapids, the group that the cobra and the venomous snakes of Australia belong to.

  23. #23 David Marjanovi?
    September 9, 2007

    On a related note, the Mosasaurs had pterygoid teeth.

    That’s normal.

  24. #24 David Marjanovi?
    September 9, 2007

    On a related note, the Mosasaurs had pterygoid teeth.

    That’s normal.

  25. #25 Sven DiMilo
    September 11, 2007

    …and one reason it’s interesting is that it reminds me of Dawkins’s essay The Giant Tortoise’s Tale, in which he points out that terrestrial tortoises have aquatic ancestors, which had terrestrial ancestors, which had (as did we all back then) aquatic ancestors.
    If the snakes really have a marine origin then terrestrial snakes too have aquatic–>terrestrial–>aquatic ancestors, and modern seasnakes go one better: aquatic (the seasnakes)–>terrestrial (their elapid ancestors)–>aquatic (the putative marine protosnakes)–>terrestrial (lizard ancestors)–>aquatic (proto-tetrapods).
    Very cool!

  26. #26 Daniel B
    August 16, 2008

    How the hell did that evolve? Some things seem easier to lose. but dear god.. that’s wax on wax

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