How can a mantis-shrimp strike create a collapsing vapor bubble?

A peacock mantis shrimp can strike hard prey so quickly that the surrounding water participates in the blow. Experiments have recorded two brief force peaks: one from direct impact of the appendage and another associated with the collapse of a nearby cavitation bubble.

Muscles first load elastic structures in the striking appendage while a latch-like arrangement restrains motion. Releasing the restraint allows stored elastic energy to accelerate the appendage. This spring-assisted release explains how the strike can deliver energy more rapidly than muscle shortening alone would allow.

As the appendage moves rapidly through water, pressure in part of the surrounding flow can drop below the pressure at which liquid water remains stable at that temperature. Some liquid becomes vapor, creating a cavity. This is cavitation: the water need not be heated to its ordinary boiling temperature, and the cavity is not a bubble blown from the animal’s mouth.

When the cavity encounters higher surrounding pressure, it can collapse rapidly. That collapse produces a short pressure disturbance and can contribute an additional force on a nearby target. High-speed imaging combined with force and sound measurements links the second peak to this event. The relative contribution varies between strikes and target arrangements; neither the bubble’s size nor a simple drawing determines how much damage occurs. The mechanism joins biological energy storage with fluid pressure and a liquid-to-vapor transition.