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:
And here’s a smasher. Check out those second pair of appendages, known as “dactyl clubs”:
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:
SEFARIS TU ANY YEER EN THE PAST. YU NAIM THE ANIMALL. WEE TAEK YU THAIR.YU SHOOT ITT.
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.