by Matthew Norton
Mantis shrimp are remarkable creatures for a number of reasons. They can see ultraviolet light, something that even human eyes cannot do, and for crustaceans they can be unusually social with their fellow mantis shrimps (within the same species). However it is the modification of their limbs into powerful clubs or sharp spears, depending on the species, which draws the most attention.
‘Puncher’ species use their club shaped limbs to strike, with extreme force and speed, the shells of prey that are slow moving, if not fixed in place. For example in the peacock mantis shrimp, Odontodactylus scyllarus, the impact force when the club meets it target ranges from 400 to 1500N at maximum speeds of 31-51 miles per hour. These strikes also generate vapor bubbles (cavitation bubbles) which, when they collapse, release energy in the form of shockwaves and sudden changes in temperature that are damaging to any solid surface. Even when the club fails to impact with its target the shockwave from the cavitation bubbles can be enough to stun, and even kill the prey of mantis shrimp.
‘Spearer’ species often lie in wait in their burrows and launch their spears to pierce the bodies of their prey, which are softer, but faster than the prey that puncher species typically attack. However these strikes appear slower, for example the maximum strike speeds of Lysiosquillina maculata and Alachosquilla vicina are 5.1 and 13 miles per hour respectively. This may simply be because these speeds may be sufficient to catch prey with their spears, whereas punchers may need the faster strikes to generate the force required to damage the shells of their prey.
These limbs function with such deadly speed and power from the build-up and rapid release of energy, from elastic strain. They achieve this by a latch system which only releases when the muscles and tendons are fully contracted and by a number of power amplification mechanisms. These include a specialised spring for storing additional elastic energy and a leverage system to minimise the loss of that energy from rotation at the limb joints. The limb itself could be thought of as merely as the system that integrates all these components.
However, given that there are so many species of mantis shrimp, possibly around 400 species, the mechanics of their modified limbs could vary greatly, with varying speed and power.
From a human perspective
The powerful strikes of puncher mantis shrimp, the larger species at least, has proven to be a problem for those who try to keep them in aquariums as there have been instances where they have inflicted damage onto aquarium glass. However, for these clubs to be effective weapons they also have to be able to withstand the power of their own strikes with little damage, which in itself is a remarkable feat.
The secret to how this is achieved is in the microstructure of the shell around these clubs which is designed to minimise the spread of cracks, which would cause it to fall apart. The spiral arrangement of mineral chitin fibres forces the cracks to constantly shift in different directions to progress through the microstructure. Also the helical arrangements are positioned perpendicular to the impact surface, minimising the surface area of each helix hit by the impact, and the chitin concentration is especially high concentration at the surface, both of which minimise the cracks that generated by the impact.
Given what this structure can withstand it is hardly surprising that it has been used to design materials used in sports equipment, lightweight body armour and airplane frames, all of which need to withstand impacts with considerable force. This is one of many examples of how materials built by marine animals can inspire new designs for synthetic materials that better fulfil our needs in a modern world.
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