A marvel of evolution: the dactyl club of stomatopods

June 16, 2012 • 9:51 am

Stomatopods, also known as “mantis shrimp,” are an order of marine crustaceans. They’re a nasty piece of work; as Wikipedia notes:

Called “sea locusts” by ancient Assyrians, “prawn killers” in Australia and now sometimes referred to as “thumb splitters” – because of the animal’s ability to inflict painful gashes if handled incautiously[4 – mantis shrimp sport powerful claws that they use to attack and kill prey by spearing, stunning or dismemberment. Although it happens rarely, some larger species of mantis shrimp are capable of breaking through aquarium glass with a single strike from this weapon. . .

Around 400 species of mantis shrimp have currently been described worldwide; all living species are in the suborder Unipeltata. They are commonly separated into two distinct groups determined by the manner of claws they possess:

  • Spearers are armed with spiny appendages topped with barbed tips, used to stab and snag prey.
  • Smashers, on the other hand, possess a much more developed club and a more rudimentary spear (which is nevertheless quite sharp and still used in fights between their own kind); the club is used to bludgeon and smash their meals apart. The inner aspect of the dactyl (the terminal portion of the appendage) can also possess a sharp edge, with which the animal can cut prey while it swims.

Both types strike by rapidly unfolding and swinging their raptorial claws at the prey, and are capable of inflicting serious damage on victims significantly greater in size than themselves. In smashers, these two weapons are employed with blinding quickness, with an acceleration of 10,400 g (102,000 m/s2 or 335,000 ft/s2) and speeds of 23 m/s from a standing start, about the acceleration of a .22 calibre bullet. Because they strike so rapidly, they generate cavitation bubbles between the appendage and the striking surface. The collapse of these cavitation bubbles produces measurable forces on their prey in addition to the instantaneous forces of 1,500 newtons that are caused by the impact of the appendage against the striking surface, which means that the prey is hit twice by a single strike; first by the claw and then by the collapsing cavitation bubbles that immediately follow. Even if the initial strike misses the prey, the resulting shock wave can be enough to kill or stun the prey.

The snap can also produce sonoluminescence from the collapsing bubble. This will produce a very small amount of light and high temperatures in the range of several thousand kelvins within the collapsing bubble, although both the light and high temperatures are too weak and short-lived to be detected without advanced scientific equipment. The light emission and temperature increase probably have no biological significance but are rather side-effects of the rapid snapping motion. Pistol shrimp produce this effect in a very similar manner.

Smashers use this ability to attack snails, crabs, molluscs and rock oysters; their blunt clubs enabling them to crack the shells of their prey into pieces. Spearers, on the other hand, prefer the meat of softer animals, like fish, which their barbed claws can more easily slice and snag.

Here’s a spearer:

Squilla mantis

And here’s a smasher.  Check out those second pair of appendages, known as “dactyl clubs”:

Odontodactylus scyllarus

Here’s a spearer in action:

I’m particularly interested in the smashers, since the way they get food is stunning. Here (via Faye Flam’s website, Planet of the Apes, which alerted me to this new research), is a video of a mantis shrimp busting open a clam.  If you’re not amazed at how evolution could produce such a weapon, you are jaded!

How can they do this repeatedly without damaging their “clubs”? Granted, they grow new ones each year when they molt, but they have to do these strikes thousands of times per year. A new paper in Science by James Weaver et al. (reference below, see also the Science perspective on it by K. Elizabeth Tanner, “Small but extremely tough“) did microstructural analysis of the club and found that it has several unique features to protect it.  The paper is extremely technical and difficult to read, so I’ll quickly summarize what they found.  The club consists of three sections:

  • The striking surface is made of hydroxyapatite, an extremely tough mineral made of calcium and phosphorus.  This is very rare in the exoskeletons of marine invertebrates, which are usually made of calcium carbonate.  Hydroxyapatite is a component of teeth and bones in vertebrates.
  • Behind the striking surface are layers of “chitosan,” a polysaccharide (sugarlike molecule). This not only helps deflect some of the striking energy back to the surface, but also prevents the inevitable cracks from growing (as Tanner says, “any crack is forced to continually change direcction, retarding crack growth.
  • Finally, the second layer is wrapped on the outside by a layer of chitin, which keeps prevents the club from disintegrating during its strikes

Not much more need be said except to marvel again at what natural selection can produce. The force of the animal’s blow is more than 1000 times its own weight; that’s the equivalent of a boxer landing a 100-ton punch!  Remember that all this evolved out of some simple, primal replicator through a blind and naturalistic process of gene sorting.

The research was partially funded by the U.S. Air Force, for it could have implications for designing not only aircraft frames but body armor for soldiers.

p.s. Be sure to check out Faye’s discussion of the physics of Ray Bradbury’s story “A sound of thunder.” You know the one: a hunter goes back to the past to kill a dinosaur that would already have been doomed, steps off the track, crushes a butterfly, and, in coming back to the present, finds that that one butterfly’s death dramatically changed the world.  The discussion of why time has a direction is nice, but what particularly struck me was Bradbury’s convergent discovery of LOLspeak.  When the hunter comes back to his present, he finds that one of the things that’s changed is language.  The sign that was in English before he went back to the past now reads:



Weaver, J. C., G. W. Milliron, A. Miserez, K. Evans-Lutterodt, S. Herrera, I. Gallana, W. J. Mershon, B. Swanson, P. Zavattieri, E. DiMasi, and D. Kisailus.  2012.  The stomatopod dactyl club: a formidable damage-tolerant biological hammer. Science 336:1275-1280.

32 thoughts on “A marvel of evolution: the dactyl club of stomatopods

    1. There’s actually a famous science fiction story by Fred Saberhagen, called “Smasher,” in which the (regular-sized) mantis shrimp are the heroes.

  1. The striking surface is made of hydroxyapatite, an extremely tough mineral made of calcium and phosphorus.

    While technically true, I can’t help but think that saying this compound is “made of” phosphorus is misleading. Does it really scare the average reader to see the word “phospate” used? It certainly gives a better impression of what sort of compound we’re talking about.

  2. The force of the animal’s blow is more than 1000 times its own weight; that’s the equivalent of a boxer landing a 100-ton punch!

    No it isn’t. Basic biophysics tells us that comparisons across radically different size scales are meaningless because of different surface/mass ratios.

    It’s like saying a flea the size of an elephant could jump over the Astrodome. Well, no, it couldn’t, because it would be too heavy even to support its own weight against gravity, never mind jumping. That’s why elephants aren’t built like fleas, or boxers like shrimp. Different size scales call for different body plans.

    That said, the clam-smashing trick is pretty impressive.

    1. But the comparison is intended to bring out the different scaling, isn’t it?

      Or should we shrink our everyday scales down and compare to isolate the difference better?

    2. Gregory, if there’s no “boxer’s punch” of any force that would be the equivalent, what analogy would you suggest to help people understand how hard in proportion to its size the shrimp is punching from a resting position? It’s not just analogies across scale, but all analogies, that are inaccurate in certain respects. Analogies are meant to help us appreciate an aspect of something unfamiliar, and need only be accurate as to that adpect. The point of the flea-elephant analogy is not to argue that fleas could physically jump over the astrodome if they were as big as an elephant, and not even first-graders draw that conclusion when they hear it. The point is only to illustrate how far fleas jump in relation to their size. And it does that job quite nicely. So, NOT meaningless. Your point that different surface/mass ratios enable different feats is important and interesting, and science teachers might well take the opportunity, when making analogies across different size scales, to teach that lesson too. But it’s a separate point.

      1. “And it does that job quite nicely.”

        I don’t think it does. If you want an idea of what a jumping flea is like on flea scale, picture a space-suited astronaut jumping a mile in slow motion on the asteroid Ceres. That, I think, is a better analogy than elephants jumping over the Astrodome on Earth, and (in my opinion) gives a much better feel for what’s really going on, which is small forces acting against negligible weight.

        So no, I don’t think it’s a separate point, because pretty much anything animals do on very small scales — jumping, flying, walking on walls, or whatever — works in a radically different way than similar-looking actions at much larger scales, by virtue of cube/square scaling laws. You really can’t begin to understand the micro-world without taking those scaling differences into account. So in my opinion these facile analogies that ignore such differences don’t aid understanding; they hinder it.

        1. You objected that the analogy– if a flea was the size of an elephant it could jump over the aerodrome — was “meaningless” because a flea couldn’t “really” jump over the aerodrome at that size due to its altered surface/mass ratio. But the analogy wasn’t intended to suggest a flea could “really” jump over the aerodrome at that size, but only to convey a more intuitive sense of how far a flea can jump in relation to its size. So now you obect that the experience of being a jumping flea is more like the experience of an astronaut jumping a mile on Ceres. But again, the point of the flea-elephant analogy isn’t to convey what it’s like for a flea to jump, it’s to convey a sense of how far it can jump relative to its size. You ignored my refutation of your original criticism and simply raised a different critici, one which showed the same lack of understanding. Analogies help us understand unfamiliar things by comparing them to more familiar things. The point of an analogy to an elephant and an aerodrome on earth is that it gives us a more familiar frame of reference for understanding how far a flea can jumpt relative to its size. The image of an astronaut making a mile-long jump on an asteroid invites us to compare one unfamiliar thing with another. Your comparisons are interesting and fun, but off point. You are barking up the wrong tree

          1. The point I’m trying to make is that the ratio of jump distance to body length, or punching force to body weight, is simply not an interesting metric, precisely because it’s not invariant with respect to scale. So analogies that extrapolate such meaningless metrics to human scale do not in fact help us understand anything worth knowing. Garbage in, garbage out.

            And by the way, I think you’re wrong about whether first-graders will conclude that elephant-sized fleas could actually jump over the Astrodome. (I’d bet good money we could find some Hollywood scriptwriters who believe it.) If that’s what Teacher tells them, and if nobody bothers to explain scaling laws to them, how are they to learn any different? So in my opinion teachers should stop saying things like that.

    3. Thanks for pointing that out. Like you, I find that comparisons across size scales are typically wonky. For example, it’s been beaten into everyone’s head that insects are “incredibly powerful for their size” – yet they’re not. They’re no more or less powerful than their size and form dictate. We only label them super-strong because we measure their power against a target – “own body weight” – that happens to be ridiculously tiny at their scale, far more so than people realize.

      The mantis shrimp may very well have incredible punching power compared to other creatures its size, and that’s definitely worth the notice and the amazement! But as you rightly point out, the comparison with a human boxer is… well, what’s claimed may not be technically *wrong*, but I think it invites misunderstanding.

  3. Intersting to see an article on the evolutionary adaptations of mantis shrimp which doesn’t mention their incredible vision.

    1. That explains why I thought it looked for all the world as if the shrimp was visually inspecting the clam at various points. I thought I must be imagining it because shrimp brains must b too small. But a big brain isn’t needed if mantis shrimps have a specialized adaptation for visual inspection of prey, any more than ants need a big brain for calculating the complex algorithm that enables them, after wandering desultorily hither and thither away from their nest, to retun to it in a straight line. Thanks for mentioning it.

    1. It is clear that mantis shrimps are the apex of evolution. They’ve got the best eyes, the coolest weapons. Everything else was simply prelude to the appearance of the mantis shrimp.

  4. I work with venomous snakes, but those shrimp things are scary. I’ve also spent quite a lot of time checking out intertidal biodiversity and never encountered one, which is kind of disappointing. First time I met pistol shrimp was amazing (and the times I was slashed open by giant purple worms and had my hands pin-cushioned with sponge spicules, and so on), but these are something else.

    I understand that forces and such don’t scale linearly with size, as pointed out in comment 2, but many stomatopods are already very large, for arthropods. I’m sure the Science paper could lead to a serious bit of technological biomimicry, trying to copy the material properties while scaling up the musculoskeletal geometry. How about a new appendage for your Iron Man suit – or imagine having to keep clear of the Mantis Tanks that come to break up the next peaceful demo in your city…

  5. A thriller came out a few years ago about an island where life had evolved in isolation since the Cambrian. Every animal in the ecosystem was derived from mantis shrimp, from the doglike to the croclike. The scientists couldn’t step out of their armored lab without being torn to shreds in seconds. (It’s called Fragment by Warren Fahy if you have a spare afternoon).

  6. Reblogged this on thewordpressghost and commented:

    This is amazing. I missed how Wikipedia became a research tool.

    But, watching this thing strike was AMAZING.

    I just wish the second video had worked ….

    Kinda depressing when videos do not work.

    Don’t you think?


    1. What’s with reblogging? Do you add content, or do you always just reproduce it verbatim? Also, why do you think people will go to your website to read what they’ve already read here? Or is the link to let Jerry know that his posts are being used elsewhere?

      1. I’ve been wondering the same thing. I presume there’s some feature of WordPress that allows one to copy a post to one’s own blog and also post a “reblogged” comment on the originator’s site, all with one click.

        What I can’t figure out is why anybody would consider this desirable. Blogs are supposed to be for self-expression. What’s the point if you’re just going to fill it up with content copied from other sites? (And why would that be something to brag about?)

  7. I’ve often thought of getting a peacock mantis (Odontodactylus scyllarus) as a pet. Here in Cairns, Australia, demand is high due to the local University taking them for research, so a good size one was $80. I held off, but have no doubt the temptation will get the better of me. Watching them swim can be rather hypnotic.

  8. There’s another YouTube video of a supposedly “tame” peacock mantis shrimp that the guy hand-feeds with a piece of clam or something, and the shrimp very gently reaches forward and takes the “meat” from the guy’s hand. Very different from a third video, entitled something like, “my mantis shrimp and my roommate’s hand,” in which the roommate’s preferred fingers get promptly thwacked. But surely it can’t be possible to “tame” a shrimp? Maybe this is just their normal behavior when the proferred “meat” is closer to the shrimp than the person’s fingers, and has no shell that needs breaking. “Marvels of evolution” is a really interesting and entertaining genre: more, more!

    1. “Proferred,” not “preferred.” The stupid auto-spellcheck on the IPad auto-corrected proferred to preferred. Stupid IPad.

  9. In Planet Stories back about 1950, there was a short story, ‘Brooklyn Project”. A chronosphere is sent back in time three times. Each time the folks involved point out that nothing has changed. But it has and they simply are not aware. By the third time, they are blue with tentacles and exuding purple slime.

  10. Maybe I’m alone in this, but was anyone else reminded of the scene in Kill Bill where the heroine had to punch her way out of a coffin?

    1. Ha! No, I didn’t. But, she would have been out in no time if she could accelerate her arm as quickly as a mantis shrimp. Top speed is about 80 km/ hr, which is reached over a distance, at a guess, of about 2 – 3 cm.

    2. Kill Bill 2, actually. I get claustrophobic flashbacks even remembering that scene. Kill Bill 2 is second only to the first half of The Descent for most intense induced claustrophobia in any movie. (I could barely stay in my chair in The Descent.)

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