Arthropod evolution is amongst some of the most dynamic processes in biological history, with the exoskeleton constantly developing in entirely unique directions to serve the survivability of the specific host animal. The Pistol Shrimp exhibits one of the most creative evolutionary characteristics: the development of a primary claw with a unique geometry that utilizes the laws of fluid mechanics to develop a shockwave-inducing weapon. Unlike many other arthropods, who have developed exoskeletal properties for defensive purposes, the Pistol Shrimp is a highly aggressive animal that uses its appendage to hunt prey. Analysis of the biomechanical design of this creature will not only provide greater insight into the capacity of natural evolution but will also better inform the design of industrial machinery that functions very similarly to the Pistol Shrimp claw.
One study created an artificial model of the claw to better illustrate the mechanism, displayed in the image above. The claw is composed of three primary components known as the “dactyl”, the “plunger”, and the “socket”, represented by their respective letters above. The dactyl, the large, curved top portion at the front of the claw, opens to allow water to fill into the socket in the claw’s interior. The plunger protrudes from the bottom of the dactyl and is inserted into the socket as the dactyl closes with high speed and force. The insertion of the plunger forces the water out of the socket through a small opening in the front of the claw, causing the water to eject from the claw at a jet velocity of about 25 meters per second. According to Bernoulli’s Law, a fluid traveling at high speeds will experience a static pressure drop proportional to said speed. In this instance, the water ejected from the Pistol Shrimp will experience a pressure drop of about 3(105) Pascals. When a fluid’s static pressure drops below its vapor pressure, bubbles of vapor form and then immediately collapse in a phenomenon known as “cavitation”. The collapsing of these vapor bubbles generates an incredibly strong shockwave that can incapacitate or kill prey.
Cavitation occurs incidentally in both other organic life functionality and in industrial machinery. Equipment that involves high fluid velocity suffers from shockwaves that lead to serious wear of materials over time. In other aquatic instances, cavitation prevents certain animals from swimming at high speeds due to the pain caused by developing and collapsing vapor bubbles in the surrounding water. A separate arthropod, the Mantis Shrimp, is an aggressive hunter whose habits incidentally create cavitation bubbles that damage its own exoskeleton over time. While in almost all other instances cavitation is a negative by-product of other functions, the Pistol Shrimp uses this principle to its advantage. In fact, the Pistol Shrimp utilizes cavitation so effectively, it raises the question as to how this previously deemed negative phenomenon can benefit future engineering. Several attempts have been made to replicate the pistol shrimp claw mechanics to achieve the same nucleation of cavitation bubbles. However, a claw closing time of 600 microseconds and a rotational speed of over 1100 rotations per second, according to one particular study, is incredibly difficult to reproduce. The findings made are intended to be implemented into designs for fuel injection systems, ship propellers, and even medical treatments.
Title image by Arthur Anker/FLICKR