Stress, Strain, and Deformation of Chondrichthyan Hyomandibulae During Feeding
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The powerful and aggressive feeding habits of various shark species are clear indications that individuals are capable of applying incredible force to their prey. Sharks have been known to crush hard bone and shell (Wroe et al., 2008), and are notorious for their ability to attack and crush large prey. As cartilaginous fishes lacking any hard bone, we sought to study how the jaws of these chondrichthyans are able to withstand such intense pressure while continuing to remain functional and supportive. Of the various cartilages of the jaw, we identified the hyomandibular cartilage of the hyoid arch as our primary focus. The hyoid arch is a modified gill arch that acts as a hinge between the upper and lower jaws (Maisey, 1980). This cartilage is particularly significant as it helps suspend the jaw and allows the other elements of the jaw to function properly and effectively. Differences in pressure of the buccal cavity and pressure produced by the jaw during feeding vary in different types of chondrichthyans. Chondrichthyans feed either by suction feeding, bite feeding, or a mix of both, and jaw structure is crucial to determining which behaviors are implemented (Tomtia et al., 2011). The arrangement of the hyoid arch and the surface area of the cartilages that make up the hyoid arch can help determine what type of feeding a shark uses. For example, by calculating which cartilages have the largest surface area, the relative stiffness of the jaw can be estimated, therefore indicating the most probable type of feeding (Wilga 2008). Cartilage stiffness was tested to determine whether stronger ceratohyal cartilage was present in sharks that suction feed. The stronger the cartilages were, the more force they could withstand during the high pressures that occur during suction (Tomita et al., 2011). The hyoid area, the area made by the opening of the hyoid arch, can provide further insight into the determination of shark feeding (Wilga, 2008). There are five main shapes and orientations of the hyoid in chondrichthyans. They include a posteriorly oriented hyoid, an anteriorly oriented hyoid, and a short, laterally oriented hyoid. Particular positioning and shape of this feature directly affects the size of the jaw opening and buccal cavity. Posteriorly oriented hyoids allow for a very large and wide bite, and are seen in bite feeders. Short, lateral hyoid positions are well suited for cutting back and forth, but do not allow for as large of a bite. This small area creates a better setting for suction feeding and helps create stronger negative pressure inside the buccal cavity to draw in prey. Anteriorly oriented hyoids swing the jaw open much wider so the most amount of water and prey possible can flow into the buccal cavity. This is more often seen in ram and filter feeders (Wilga, 2008). Difference in hyoid orientation is an important distinguishing feature to recognize and can help connect jaw mechanics to ecological scenarios.