University of California .::. Department of Integrative Biology .::. Berkeley, CA 94720-3140 U.S.A.

 

Mantis shrimp mechanics

This page describes some recent discoveries about the mantis shrimp's strike. A series of video clips and pictures at the bottom of this page illustrate some of incredible phenomena surrounding the mantis shrimp's strike.

Mantis shrimp (Stomatopoda) use a specialized pair of forelimbs, the raptorial appendages, to capture prey. Peacock mantis shrimp (Odontodactylus scyllarus) smash open snails and consume the contents as their primary source of food. High speed video images, recorded at 5000 frames per second, revealed that peacock mantis shrimp forelimbs reach maximum speeds from 12-23 m/s (in water!). This may be the fastest feeding strike produced by an animal appendage.

While recording these images, we noticed cavitation bubbles forming between the limb and the snail. As a result of the limb’s extraordinary speed, the water cavitates (vaporizes) when the limb strikes the prey. Cavitation is a destructive phenomenon; when these vapor bubbles collapse, they essentially cause a small implosion in the water which produces heat, light and sound. For example, rapidly rotating boat propellers are often badly damaged by cavitation, to the point of developing holes in the metal.

Such extreme speeds in water require substantial energy storage and release. Energetic calculations show that these movements cannot be controlled by muscle contractions alone. In other words, the mantis shrimp needs a potent power amplification system in its limb. Earlier studies showed that mantis shrimp have latches which hold the limb in place until the animal is ready to strike. We added to this model through the discovery that mantis shrimp have a stiff, saddle-shaped spring located on the upper surface of their feeding limb. This saddle shape, also known as a hyperbolic paraboloid or anticlastic surface, is well known to engineers and architects as a surface that uniformly distributes loads and requires few materials. The newly discovered spring allows the animal to store additional energy to generate such extreme speeds once the latches are released.

IMAGES AND VIDEO CLIPS:

Here a peacock mantis shrimp smashes a snail (regular speed video). Filmed at 5000 frames per second, and played back at around 30 frames per second (slowed down 333x), the formation of a cavitation bubble can be seen between the limb and the striking surface. Filmed at 20000 frames per second, and played back at 30 frames per second, cavitation is visible as the limb strikes a snail.

A peacock mantis shrimp (Odontodactylus scyllarus) strikes a force sensor. Filmed at 100,000 frames per second in color, this movie clip shows the mantis shrimp's dactyl heel striking a force sensor. The formation and collapse of cavitation bubbles are visible between the sensor's surface and the mantis shrimp's appendage.
Synchronous high-speed video and force output of a peacock mantis shrimp's strike.Two force peaks are generated during the strike of a single raptorial appendage. This movie clip shows simultaneous high-speed video images and force sensor output, both sampled at 100,000 samples per second. The first force peak is caused by the impact of the mantis shrimp's appendage against the force sensor. The second force peak is caused by the collapse of a cavitation bubble. Following the primary cavitation bubble collapse, the rebound phase of cavitation is visible in the form of a light cloud of cavitation bubbles, which eventually collapse with lower total forces than the first primary bubble collapse.

In the image below, cavitation is visible between the limb and a snail (indicated by black arrow).

Scyllarus Cavitation


The image below shows a a saddle-shaped surface, otherwise known as a hyperbolic paraboloid.

 

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