The peacock mantis shrimp strikes prey at 23 meters per second — fast enough that the water between appendage and target can't move aside smoothly. Pressure drops below the vapor threshold and the water itself ruptures, forming a cavitation bubble. When the bubble collapses microseconds later, it releases a shockwave, localized temperatures up to 50,000 Kelvin, and a flash of light called sonoluminescence. The collapse generates forces comparable to the initial strike. Every punch lands twice.
The first cavitation was an accident. Move an appendage fast enough through water and physics supplies the bubble whether you evolved it or not. But the current morphology is no accident. Smasher-type mantis shrimp have bulbous “heels” on their appendages that create optimal conditions for bubble formation. Spearer-type mantis shrimp — which stab rather than hammer — have sleek appendages that suppress cavitation entirely. The same physics is available to both. Only one lineage has evolved the shape to exploit it.
The boundary between side effect and weapon moved. What started as an incidental consequence of speed became a selectable trait once the organism's prey required forces exceeding what mechanical impact alone could deliver. The shrimp didn't invent cavitation. It adopted it — retrofitting morphology around a physical phenomenon that was already happening.
This is exaptation at its sharpest: not repurposing a structure for a new function, but evolving structure around a function that had no structure. The physics existed first. The anatomy grew to fit it. The weapon was there before anyone built a weapon.