The mantis shrimp, which isn't a shrimp, gets a lot of attention for its visual acuity and powerful punch. But there are actual shrimp that are equally impressive. There are various species of snapping shrimp that can close their claws fast enough that it produces a jet of water that undergoes cavitation, where the extreme turbulence creates pockets of low pressure where the water vaporizes. As these bubbles collapse, they reach temperatures above 5,000K and emit light along with a powerful snapping noise.
This, not surprisingly, can be valuable both on offense and defense, which is probably why there are over 500 known species of snapping shrimp. But how did this ability evolve in the first place? To find out, an international team of researchers obtained the claws of nearly 70 different species of snapping shrimp and subjected them to CT scans to identify their structure and musculature. After 3D printing models of what they found, they conclude that a surprisingly minor set of changes led to a big mechanical difference.
The study gets at a common conundrum in evolution. It's easy to see how generation after generation of small changes can refine a useful feature. It's harder to understand how a feature shows up in the first place, since any antecedents to the feature wouldn't be useful in the same way. So it is with snapping shrimp. Until the claws managed to produce powerful jets of water, it's not clear what could possibly be refined.