Peacock mantis shrimp are champion underwater boxers, wielding a pair of specialised club-shaped limbs to smash snail shells into smithereens. They are also surprisingly quick on the draw, and it turns out that this confers a rather unexpected advantage. Sheila Patek and Roy Caldwell at the University of California, Berkeley, have discovered that each strike of these powerful biological hammers rains two successive blows on a shrimp's hapless prey. They reveal that the limb's impact causes the first blow, but the second is due to the implosion of vapour bubbles next to the prey surface, which form as a result of the astonishing speed of a shrimp's strike(p. 3655).
In the late 1960s, a series of classic papers by Malcolm Burrows described the click mechanism that makes mantis shrimp the fastest strikers in the animal kingdom. A latch holds the animal's hammer in place while limb muscles contract, and when the latch is suddenly freed, the resulting movement is much faster than the original muscle contraction. Recently, Patek and her colleagues discovered that a specialized spring stores the elastic energy that drives the limb's explosive smashing action. Patek and Caldwell are fascinated by this biomechanical marvel, but it wasn't until they got a chance to try out new imaging technology that they realised just how spectacular a shrimp's strike really is.
Studying mantis shrimp strike forces is challenging. `What's great about these animals is that they're very aggressive; they'll hit anything', Patek enthuses. But she was taken aback by the sheer force of the animals' blows;when she placed a force sensor in front of the feisty creatures to measure their strike forces, the shrimps promptly exceeded the sensor's capacity. `It was challenging to design waterproof sensors with sufficient capacity', Patek recalls. When she finally did record some strikes, she was astonished to find that the tiny shrimps can generate impact forces of thousands of times their body weight.
Taking a closer look at the data, Patek was puzzled to find that each strike produces two force peaks, about half a millisecond apart. The first peak could be explained by the limb's physical blow. But what was causing the second force peak? To answer this, Patek had to face a more pressing problem;most existing cameras simply aren't fast enough to capture the shrimps'pounding action on a microsecond timescale. So she was thrilled when a newly released high-speed video camera was capable of capturing a staggering 100,000 frames per second. More importantly, the mantis shrimp consistently struck the force sensor within the small spatial area resolvable at these frame rates.
Realising that this was her chance to see what causes the second force peak, Patek set to work. Synchronizing the high-speed camera with a force sensor, she smeared shrimp paste on the sensor to entice the aggressive animals to strike. Watching the strike footage, Patek and Caldwell were amazed to see what causes the second force peak. A shrimp's hammer moves so fast that it creates an area of low pressure as it rebounds from the force sensor,causing vapour bubbles to form. The pair saw that the second force peak coincides with the implosion of these cavitation bubbles. Incredibly, on average the cavitation force exerted by the collapse of these bubbles was half of the limb's impact force, but in some cases the cavitation force was far greater than the limb's impact force! Patek and Caldwell conclude that cavitation is a force to be reckoned with, and helps a shrimp demolish its prey. But exactly how a shrimp's limbs emerge unscathed from the continuous battering by cavitation forces is a mystery.