Sneaky Shrimp Smash and See Sircularly

Yes, I am aware that it is spelled with a `c'. As Josh has already alluded to, I have had this draft sitting around for a while now...

Mantis shrimp are about as extreme as a small crustacean can get! Not only are they beautifully coloured, they can generate shock waves, smash glass and see circularly polarised light. They even feature in a trading card game. Needless to say they have also been the subject of many great articles.

The punch of the mantis shrimp is so forceful that it is capable of generating cavitation. Plenty of people talk about it but no-one really seems to explain how it is capable of punching so quickly, especially underwater! I did track down one guy who does discuss this, and the paper that it's all drawn from. Essentially, the mantis shrimp is able to get enough energy into the punch by pre-tensioning a resilient elastic mechanism within its arm. This means that it doesn't have to be able to generate a huge amount of power: it can tension the elastic relatively slowly, then release the whole lot of energy very quickly (this is the idea behind the flywheel in more mundane mechanical systems).

Their claws can accelerate at 100 thousand metres per square second and reach a top speed comparable to your car on the highway: the forces involved range into the thousands of newtons, which isn't bad considering they reach only a few dozen centimetres length (around 25cm). The rapid motion of the mantis shrimp's arms means that the shear rate at their surface is huge: so huge that the pressure difference between fluid streams around the claw (definitely high Re) are sufficient to cause cavitation.

Cavitation is the formation of a gas bubble inside a body of liquid as a result of rapid motions. It can be heard when you do donuts in your boat (not that I've done this.... *ahem*) and can cause quite a bit of damage: the rapid collapse of cavitation bubbles generates shockwaves that can cause pressures of up to 9MPa (with a bubble only 2.5mm across!). The collapse of these bubbles could be considered to be a great example of an irreversible adiabatic process... the compression of the gas raises its temperature to around 5000K, allowing it to briefly fluoresce, in a process collectively referred to as sonoluminescence.

The net effect is that a prey item gets smashed with a knobbly calcified arm, then battered by broadband sound at 240dB, then eaten! This seems a little excessive for catching and eating snails... hardly the most nimble prey.

In addition to their deadly cutlery, mantis shrimp are able to distinguish between circular and linear polarisations of light. They do this by passing light through a cell which acts like a broadband quarter waveplate, taking circularly polarised light in and giving linear polarisations out. As it happens, this waveplate is `better' than our own manufactured waveplates, in the sense that it outperforms any human-manufactured material over the visible spectrum (waveplates are generally manufactured with a specific wavelength or very narrow range of wavelengths in mind, so don't work well at other frequencies). Even more interesting, the cell itself is also a photoreceptor! This is just the beginning when it comes to mantis shrimp vision... but I'll leave that to those better qualified.

By the way, the actual shrimp is fluorescent too!

Enjoy!