The mantis shrimp, which is not a shrimp, gets a lot of attention for its visual acuity and powerful punch. But there are real shrimp that are equally impressive. There are several species of bite shrimp that can close their claws fast enough to produce a jet of water that undergoes cavitation, with the extreme turbulence creating pockets of low pressure where the water evaporates. When these bubbles collapse, they reach temperatures above 5,000 K and emit light along with a powerful clicking sound.
This, unsurprisingly, can be valuable both on offense and defense, which is probably why there are over 500 known species of bite 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 biting 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 small set of changes made a big mechanical difference.
The study addresses a common evolutionary conundrum. It’s easy to see how generation after generation of small changes can refine a useful feature. It’s more difficult to understand how a function appears in the first place because any antecedents of the function wouldn’t be useful in the same way. It’s the same with snapping shrimp. Until the claws managed to produce powerful jets of water, it’s not clear what could possibly be refined.
But an evolutionary analysis proved to be the key to understanding things. After the researchers took CT scans that showed the shell and musculature of the shrimp’s claws in their native conformation, they were able to map out certain features that showed up in closely related species.
The analysis showed that the ancestor of these shrimp likely had a fairly mundane claw that bent on a spindle. If the muscles attached to one side of the spindle contracted, the claw would open; contraction of those stuck on the other side would close it. Perfectly functional, but incapable of these powerful snaps.
But the evolutionary analysis also showed that descendants replaced this simple spindle with a system that had never been recognized before. They call it a “slip joint” and it’s a slightly different way of opening and closing the claw. Instead of turning the base of one side of the claw, the entire base can shift its location slightly. This shift takes it to one side or the other of a small ridge on the shell at the base of the claw. Slide one side of the rim and the combination of internal forces and the shape of the shell will twist the tang to close. However, if the muscles move it to the other side of the comb, the forceps is open in the most favorable configuration.
The slip connection in itself has no obvious advantages and some shrimp with this version of the claw are unable to generate a powerful click. But it allows for a slightly different configuration of the muscles internally, which allows the authors to reverse torque. In this configuration, there are muscles that, when the claw is open, can contract and generate a force that holds the claw open. But when the forceps shifts slightly towards closure (driven by other muscles), it is enough to convert the same contraction of the same muscles into a force that prompts it to close. In other words, by shifting the location of the forceps, the contractile torque of these muscles is reversed.
This is the configuration that can finally generate a snap (although not all types with this configuration snap). Amazingly, once this muscle configuration evolved, a few species switched back to using pivot joints, but were able to maintain torque reversal and a powerful click.
Without the evolutionary analysis, it would have been extremely difficult to figure out this path. But that creates a simple picture. Small changes can shift the shrimp between one of two claw configurations, neither of which seems to offer any particular benefits in and of itself. But one of those configurations allows for the evolution of a new muscle system that has a number of important energetic advantages, in that the same muscles can contribute to both the opening and closing of the claw. These energies can then be refined so that the claw is able to generate the shrimp’s signature snap.
And it really is a signature. The authors tried different configurations to correlate with the speed of the snap, but were not entirely successful. As they write, “we are unsure about the differences between species because most of the closures exceeded our frame rate for filming.”
Current Biology2017. DOI: 10.116/j.cub.2017.11.044 (About DOIs).