There are some shrimp species equipped with truly remarkable weaponry. The pistol shrimp’s massive claw closes with a powerful snap—louder than a gunshot, at 210 decibels—temporarily creating an air bubble with so much energy that it pops with an underwater shock wave and a flash of light and heat temporarily hotter than 9,000 degrees Fahrenheit. The shrimp use this impressive weapon to stun and kill prey.
How did such a tiny species evolve a claw that can create this so-called cavitation bubble? What machinery is required? An international team of researchers dug deep into the claw’s evolutionary history, and found the key is in the claw’s joints.
The issue with studying the evolution of this feature is that “either a claw closes fast enough to create a cavitation bubble or it does not,” according to the paper published last week in Current Biology. That means they researchers had to figure out how evolution’s slow changing of the claw’s appearance suddenly allowed the shrimp to create the bubble.
Through observation and 3D modelling, the scientists were able to construct the different phases of snapping claw evolution. It began with “pivot joints,” the simplest hinges that simply open and close the claws. Other shrimp soon evolved slip joints, where the moving part of the claw can both pivot and slide back and forth at its joint. Snapping shrimp have these kinds of joints on their non-snapping claw.
Eventually, some species of shrimp were able to latch these slip joints open and store energy in them before releasing them with a snap. Refinement led to snapping that could create cavitation bubbles. The fastest snappers lost the slip joint altogether and snapped with a cocking pivot joint. This occurred separately in two shrimp families.
But the researchers point out that the boundaries between these joints weren’t disjointed—slow, continuous evolutionary changes occurred that brought each increasingly complex claw its new function. And while work like this might requires lots of inference or some assumptions, the scientists think their methods could help how other new, strange functions occurred in other animals. How did geckos get their sticky pads, for example? Why can some termites spray ant repellent?