A new description of muscle architecture in the adductor mandibulae complex of spiny dogfish, Squalus acanthias.

The Spiny Dogfish (Squalus acanthias) has been a model organism for teaching since the late 1800s. This fish is in just about every textbook and lab exercise that focuses on comparative vertebrate anatomy. Visualization of the musculoskeletal system of S. acanthias is relatively consistent in the literature with one acceptation, the adductor mandibulae complex (AMC). The AMC is a group of muscles that close and protrude the jaws during feeding. Graphic and pictorial visuals, and general descriptions of the AMC differ from literature source to literature source. Here we reexamine the architecture of the AMC divisions in fresh and preserved specimens of S. acanthias through dissection, recording our findings through photo documentation. The main goals are to provide a new and accurate description and illustration of the AMC for universal use in comparative anatomy educational literature, and discuss the function implications of the muscle architecture. Our results reveal the presence of a previously overlooked division of the AMC. This division extends from the caudolateral surface to the upper jaw superior to the jaw joint. The fibers of the muscle converge onto a broad tendon that extends rostrodorsally to insert onto the postorbital process of the cranium. The muscle tendon lies directly under the skin, making it immensely easy to cut if off if one isn’t dissecting with this muscle in mind. Furthermore, the mechanical position of the muscle suggests that it would function in upper jaw protrusion, altering the description of the feeding mechanism in this well studied species.