Meeting Abstract
The muscles of the adductor mandibulae complex (AMC) in sharks are complexly subdivided and diverse in architecture. A possible benefit of the subdivisions is observed when manipulating the jaws in fresh specimens. As the jaws are moved closed, the angles of the muscle divisions reconfigure, and may affect the force output to the jaws during feeding. Here we investigate of the anatomy and mechanics of the AMC divisions during jaw closure in sharks that employ suction and bite feeding mechanisms. The goal of the study is to determine if the reconfiguration of the muscle divisions result in a mechanical benefit related to feeding mechanism. Mechanical models of the AMC in suction and bite feeders show that the AMC is capable of transferring a stable level of force to the jaw tips from 0 to 100 percent jaw closure using two disparate methods, which appear to have evolved with the constraints associated with the different feeding mechanisms. In suction feeders, stable force transfer is a consequence of a reduced gape, which does not significantly alter the insertion angles of the posterior-more adductors, and large processes on the upper jaw that restrict posterior excursion of the anterior-more adductors. These muscles occlude much of the lateral gape, are geared low-to-high and have moment arms that do not change with jaw closure. In bite feeders, stable force transfer is accomplished by changes in the insertion angle of the adductors, some switch from large to small moment arms during jaw closure, while others switch from small to large. As the jaws close, total force transfer to the jaws remains the same as divisions increase and decrease force transfer capabilities. These muscle divisions also allow for a wide gape without a reduction of bite force that is useful in capturing and processing large prey.
