Wednesday, October 18, 2017

float like a butterfly, sting like shrimp

The peacock mantis shrimp (Odontodactylus scyllarus), is a species of crustacean that is unique in having the world’s fastest moving appendage. It uses its club-like front appendage to bash or stab its prey with explosive speed, resulting in a powerful punch that effectively kills its prey. It is so powerful in fact, that mantis shrimps have been known to break the 1/4 in. thick glass that is used for aquariums. When measured by Sheila Patek, a researcher from UC Berkeley, found the mantis shrimp to punch at 52 mph (or 23 m/s) with an acceleration of 104,000 m/s^2, over a time of 0.0027 s (Patek et al., 2004). Using the F=ma equation, and the fact that it weighs 0.015 kg, the force at which the mantis shrimp strikes is 1560 N, about the same force as a .22 rifle. Patel estimates that the amount of power needed for a strike of this force is 4.7x10^5 Watts per Kg (Patek et al., 2004), which, using the equation P=W/t and t=0.0027 s, gives 1,269 Joules of work. The mantis shrimp is capable of producing this much energy due to the energy storing abilities of its limb structure. Instead of simply pulling back with its lateral extensor muscle and swinging forward, like a human would punch, the mantis shrimp uses the combination of a latch mechanism and a spring mechanism. The latch mechanism is a series of latches within the mantis shrimp’s appendage that locks the limb into place until the muscles are fully contracted (Burrows and Hoyle, 1972).This latching of the limb allows a greater buildup of muscular potential energy, PEmuscle, leading to a greater kinetic energy, and force, upon release. However, this mechanism alone does not produce enough potential energy to make such a fast strike possible, since muscle fibers can store only a limited amount of energy (Alexander and Bennet-Clark, 1977). In order to have enough energy for its strike, the mantis shrimp also uses a spring mechanism. The spring mechanism is composed of a saddle-shaped spring that compresses when the mantis shrimp pulls its limb back for the strike. (Figure 1). This compression causes a massive increase in the spring’s potential energy, resulting in the necessary amount of kinetic energy needed for the strike when the limb is released. Overall, the work-energy equation for the peacock mantis shrimp’s strike is Wnet=PEmuscle + 1/2Kx^2.

Figure 1: The saddle-shaped spring of the peacock mantis shrimp compressed (above)
and released (below) (Patel et al., 2004).

Patek, S. N., Korff, W. L., & Caldwell, R. L. Nature 428, 819-820 (2004).| doi:10.1038/428819a
Burrows, M. & Hoyle, G. J. Exp. Zool. 179, 379−394 (1972).

Alexander, R. M. & Bennet-Clark, H. C. Nature 265, 114−117 (1977).

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