The appendage of the mantis shrimp strikes with a tremendous amount of force enhanced by cavitation bubbles

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The mantis shrimp is a marine crustacean distinguished by its ability to deliver high-velocity powerful strikes that can break mollusk shells and even aquaria glass. It does this with its raptorial appendages--forelegs specialized for protection and feeding. There are many different species of mantis shrimp, and the morphology of its raptorial appendage classifies the mantis shrimp as either a spearer, smasher, or undifferentiated. While the rapid movement and initial force of the striking mechanism is due to power amplification, smasher and undifferentiated species can add extra force to their strike by producing cavitation bubbles.

The raptorial appendage is divided into four segments: the merus (closest to the body), carpus, propodus, and dactyl. The shape of the dactyl differentiates the shrimp as a spearer, smasher, or undifferentiated species. Spearing appendages are long, pointed segments that slice through the water and soft prey. Smasher and undifferentiated appendages, on the other hand, have blunt, bulbous “heels” on the dactyl used to deliver forceful blows in close range. This rapidly accelerating heel is what creates a cavitation bubble.

Cavitation bubbles are created when an object moves through water at very high speeds and creates extreme velocity gradients in the flowing water. Under the right conditions, this produces a cavity or bubble in which the pressure is so low that the water vaporizes. When the bubbles collapse under higher surrounding pressures, they emit energy in the form of sound, light, and heat waves. The whole process occurs in a matter of milliseconds. Cavitation is a common phenomenon with boat propellers, which create a continuous stream of bubbles that collapse and erode the propeller metal over time.

In the case of smasher and undifferentiated appendages in mantis shrimp, their anatomy and striking speeds create the perfect conditions for a single cavitation bubble. When the bubble bursts, its strength is almost equal to that of the strike itself. A single cavitation bubble is proportional to the firing of a 22-caliber bullet. It’s the one-two punch of the deep sea. The spearer appendage does not create a cavitation bubble because of its sleek design.

Check out this video that shows the formation of a cavitation bubble, as well as this TED talk in which Dr. Sheila Patek describes the discovery of cavitation bubbles in mantis shrimp strikes.

This summary was contributed by Allie Miller.

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References

“Mantis shrimp are renowned for their unusual method of breaking shells with brief, powerful strikes of their raptorial appendages. Due to the extreme speeds of these strikes underwater, cavitation occurs between their appendages and hard-shelled prey…[E]ach strike generates two brief, high-amplitude force peaks, typically 390–480 μs apart…[I]t is evident that the first force peak is caused by the limb's impact and the second force peak is due to the collapse of cavitation bubbles...Despite their small size, [Odontodactylusscyllarus can generate impact forces thousands of times their body weight. Furthermore, on average, cavitation peak forces are 50% of the limb's impact force, although cavitation forces may exceed the limb impact forces by up to 280%. The rapid succession of high peak forces used by mantis shrimp suggests that mantis shrimp use a potent combination of cavitation forces and extraordinarily high impact forces to fracture shells” (Patek and Caldwell 2005:3655)
 
“Skeletal structures can channel work into elastic materials; when these structures are allowed to relax to their resting state, energy is released over a much shorter time scale than the underlying muscle contraction, thereby resulting in power amplification...The use of elastic structures to amplify the power output of skeletal muscle is fundamental to rapid accelerations in animals.”  (Zack et al. 2009:4002) 

Journal article
Elastic energy storage in the mantis shrimp's fast predatory strikeJournal of Experimental BiologyNovember 28, 2009
T. I. Zack, T. Claverie, S. N. Patek

Journal article
Extreme impact and cavitation forces of a biological hammer: strike forces of the peacock mantis shrimp Odontodactylus scyllarusJournal of Experimental BiologySeptember 16, 2005
S. N. Patek

Journal article
How Snapping Shrimp Snap: Through Cavitating BubblesScienceJuly 27, 2002
M. Versluis

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Living System/s

Organism
Peacock Mantis ShrimpOdontodactylusSpecies

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