Mechanism refers to the manner in which a behavior is produced and controlled at genetic, cellular, and organ levels. The mantis shrimp raptorial appendage primarily serves the function of capturing prey species. Speciation has led to diversification in the morphology and function of these specialized appendages. As a result, the specific anatomical mechanism by which mantis shrimp release their raptorial appendage and the function it serves is variable and dependent upon the hunting strategy of the organism in question. Nevertheless, several important aspects of the appendage’s morphology and mechanics are common throughout species.
Image is Figure 1 within Patek et al 2013 (Journal of Experimental Biology article)
The mantis shrimp raptorial appendage is capable of producing an impressive amount of force and is rapidly displaced once released. The mechanics of the appendage can be imagined as a simple spring (Patek et al 2009). In the same way that mechanical energy is stored as potential energy in the stretched or compressed coils of a spring, mechanical energy produced through muscle contraction is stored in the raptorial appendage during the period prior to release called loading. The loading of this anatomical spring serves the function of maximizing the amount of force, which can be released in a very short time interval, a phenomenon referred to as power amplification (Patek et al 2009). This is achieved through storage of mechanical potential energy in the elastic connective tissues of the appendage. Upon release, this energy is instantaneously released during the period referred to as unloading (Patek et al 2009).
Utilization of a Lever System
The work done by the mantis shrimp raptorial appendage is achieved through utilization of an anatomical lever system, which translates the mechanical energy produced from the power amplification system to accelerate the propodus and dactyl (the most distal segments of the raptorial appendage) toward potential prey items (Patek et al 2007). The lever-linkage system involves main four connections, the merus, the meral-V, the carpus, and the muscle linking the carpus to the merus each of which move around four pivot points. (See anatomical diagram) During unloading, the meral-V rotates toward the propodus around a pivot and moves a second pivot location distally. This rotates the carpus, which in turn rotates the propodus and dactyl outward (Anderson et al 2013). Because of the nature of levers, differences in the relative size and position of each of these linkages result in different mechanical characteristics of the raptorial appendage. Longer linkages allow for greater displacement of the appendage but decrease the mechanical advantage provided by the lever system resulting in lower maximum force outputs. Shorter linkages prevent the force generated to be displaced across large distances but maximize the mechanical advantage provided by the lever system and thus allow for maximum force production (Anderson et al 2013).
Image is Figure 2 of Anderson et al 2014 (Evolution article)
Large-Scale Differences Between Species Groups
Perhaps the most obvious differences between the striker and smasher appendages are changes size and dimension. Striker species have evolved long, thin appendages with small cross-sectional areas (a physiological dimension known to correlate with muscular force output. In comparison, smasher species have evolved shorter, wider appendages with larger relative cross-sectional areas (Blanco et al 2014). Increases in the cross-sectional area in the derived clade of smashers is among one of the many adaptions which allow greater force production through increase in the amount of muscle tissue dedicated to this specific function. Of course as discussed previously, changes in the dimensions of the appendage also have implications for mechanical aspects of the lever-linkage system (Anderson et al 2014). The longer merus, propodus and dactyl of striker species relative to smasher species allows greater displacement but at the cost of reductions in mechanical advantage leading to decreased maximum force outputs. Inversely, the smaller merus, propodus and dactyl of smasher species reduce possible displacement of the appendage but increase the mechanical advantage of the lever system allowing for greater force outputs. Thus striker species have appendages organized to maximize kinematic transmission rather than force transmission and the opposite is true of smasher species (Anderson et al 2014).
Image is Figure 6 of Anderson et al 2014 (Evolution article)
Small Scale Differences Between Species Groups
Research has demonstrated that smasher species have evolved longer average sarcomere lengths in the muscle fibers involved in movement of the raptorial appendage (Blanco et al 2014). Physiological studies have demonstrated that longer sarcomeres allow larger force outputs but at the cost of longer contraction rates. Thus in the appendage of smashers species, there is an optimization of force output at the cost of a considerable increase in contraction time. The inverse is true of striker species that have shorter sarcomeres on average thus producing less force but with higher contraction times. Such adaptation clearly serves to maximize the function of the raptorial appendage in the prey capturing system (Blanco et al 2014). Again we see that the smasher appendage is adapted to maximize force output at the cost of longer of a longer loading time and the opposite trend is true of striker species.
Differences in the mechanical characteristics of the raptorial appendage across mantis shrimp species are related to the function of the appendage in prey capturing systems. Smasher species produce considerably more force output than any other group of species but this requires a longer loading period in which energy is accumulated in the elastic tissues of the appendage and then released during unloading producing huge force outputs across a small distance. Inversely, striker species invest less time in the loading period resulting in less force per strike. However, less investment in the loading time allows striker species to load and release the spring mechanism of the appendage much faster compared to smasher species, allowing for much decreases in reaction time to stimuli which maybe a potential prey item. Smasher and striker species demonstrate adaptions at both cellular and tissue levels that maximize certain mechanical properties of the anatomical spring mechanism most relevant to their particular prey capturing system.