The mantis shrimp is an aggressive marine crustacean that uses specialized forelimbs (called raptorial appendages) to capture its prey. Mantis shrimp that are “smashers” use a hammer-like strike to destroy the shells of snails and other mollusks, exposing the soft body of the animal so that it can be eaten. The mantis shrimp’s strike can even smash aquarium glass. It does this with a tough bulbous heel on the raptorial appendages, which function in both feeding and protection. The raptorial appendage, like most of the mantis shrimp’s body, is composed of tough exoskeletal material. It is divided into four segments: the merus (closest to the body) houses the major muscle groups. Next is the carpus, propodus, and then the dactyl, which differs in shape depending on the species of mantis shrimp. “Smashers” bear the hard heel on their dactyls. While there are many different species of mantis shrimp, the raptorial appendages use the same principle to generate rapid and forceful movement. This principle is called power amplification.
Power amplification systems amplify the mechanical power generated by relatively slow muscle contractions by separating muscle contraction and movement into two sequential steps: the load phase and the release phase.
In the load phase for the mantis shrimp raptorial appendage, flexor muscles in the merus contract to engage small hardened parts of the merus against other parts of the exoskeleton, which function like a latch to keep the whole appendage in place and prevent movement. At the same time, extensor muscles in the merus contract and bend other exoskeletal parts of the merus (the saddle and ventral bars), which store energy like a compressed spring. These flexor and extensor muscles are antagonistic, meaning that they produce opposite movements if they contract individually (your biceps, which flexes the arm, and triceps, which extends the arm, are a pair of antagonistic muscles); however, contracting at the same time enables the large extensor muscle to contract slowly while the appendage is flexed and “latched.” Instead of moving the appendage, the extensor muscle’s slow contraction stores energy as elastic potential energy, essentially loading a spring while it prepares to strike.
When the mantis shrimp is ready to strike, the release phase begins as the flexor muscles relax to release the latch. The appendage’s saddle and ventral bars spring back to their original shape, releasing their stored elastic energy and causing the dactyl segment to rotate forward at speeds up to 45 miles/hour! Because the appendage motion in the release phase takes place over only milliseconds, the mantis shrimp greatly increases the power of its strike.
There’s more to this story, though: check out this related strategy describing how the mantis shrimp’s extremely fast appendages produce a cavitation bubble that creates even more force.
This summary was contributed by Allie Miller.
“All animals face an overriding constraint on their ability to produce fast movements – muscles contract slowly and over small distances. Repeatedly over evolutionary history, animals have overcome this limitation through the use of power amplification mechanisms. These mechanisms decrease the duration of movement and thereby increase speed and acceleration (Alexander, 1983; Alexander and Bennet-Clark, 1977; Gronenberg, 1996a).” (Patek et al. 2007:3677)
“Mantis shrimp, like all crustaceans, control movement with antagonistic pairs of muscles that alternately abduct and adduct their appendages. However, in the load phase of a power-amplified strike, mantis shrimp simultaneously activate the antagonistic muscles connecting the carpus and merus segments in the raptorial appendage as they prepare for a high-powered strike (Fig. 1). Specifically, they contract large, slow extensor muscles in the merus while contracted flexor muscles in the merus brace a pair of sclerites to prevent movement (Burrows, 1969; Burrows and Hoyle, 1972; McNeill et al., 1972). When the extensor muscles have fully contracted and the animal is ready to strike, the flexor muscles turn off, releasing the sclerites, and the appendage rapidly rotates outward toward its target (Burrows, 1969; Burrows and Hoyle, 1972; McNeill et al., 1972).” (Patek et al. 2007:3678)
“One hypothesized elastic storage structure, the saddle, only contributed approximately 11% of the total measured force, thus suggesting that primary site of elastic energy storage is in the mineralized ventral bars found in the merus segment of the raptorial appendages.” (Zack et al. 2009:4002)
“Skeletal structures can channel work into elastic materials; when these structures are allowed to relax to their resting state, energy is released over a much shorter time scale than the underlying muscle contraction, thereby resulting in power amplification…The use of elastic structures to amplify the power output of skeletal muscle is fundamental to rapid accelerations in animals.” (Zack et al. 2009:4002)
“Two key structures have been identified as probable energy storage structures – the meral-V and saddle…A “ventral bar” of exoskeleton that extended from the meral-V to the ventral surface of the merus in the peacock mantis shrimp…and acts as part of a four-bar linkage system to couple stored elastic energy to the rapid rotation of the carpus.” (Zack et al. 2009:4003)