Functional Morphology of Elasmobranch Jaws: Testing the Strain Rate of the Hyomandibulae Cartilage

Abstract

This project explored and analyzed the strain and structure of elasmobranch jaws, particularly the hyoid arch. The cartilaginous element tested was the hyomandibula in four individuals each of the angel shark, Squatina squatina, spiny dogfish, Squalus acanthias, and sandbar shark, Carcharhinus plumbeus. We put them through trials that tested for strain and we performed geometric morphometric analyses on the results. ! Chondrichthyans have a modified gill arch called the hyoid arch that they use during prey capture and mastication. In all our tested shark species, the cartilaginous hyoid arch expands the buccal cavity. This expansion functions to create negative pressure in the mouth to draw in prey for suction feeders, and allows a wide buccal area to take in as much prey as possible for bite feeders. The hyoid arch in elasmobranchs consists of the ceratohyal, basihyal, and hyomandibula cartilages. The hyomandibula is significant since it connects the ceratohyal to the chondrocranium, connecting functions of the upper and lower jaws. ! Teleosts and elasmobranchs evolved different jaw structures, especially in the hyoid region: teleosts have craniums ossified with dermal bone, with the hyoid arch incorporated into the suspensorium and opercular series. The hyoid expands laterally in teleosts, as shown in previous research with Amia calva and Micropterus salmoides (Wilga 2008), while it compresses in elasmobranchs, except in mako and sandbar sharks (Wilga 2008). Lamniform and carcharhiniform sharks have long, posteriorly oriented hyomandibulae to make room for a wide buccal cavity. Previous research done on buccal pressure shows that lamniform and carcharhiniform sharks do not rely much on suction feeding, but rather on bite feeding (Wilga 2008). Suction feeders, in particular the whitespotted bamboo shark, laterally compress their buccal cavity to generate negative pressure to draw the prey in (Wilga & Sanford 2008). Even the very large megamouth shark uses negative pressure by pulling its basihyal cartilage posteriorly and ventrally rotating its hyoid arch to expand its buccal cavity (Tomita et al 2011). ! All chondrichthyan skeletons are cartilaginous, yet many readily feed on bony teleosts. Some, like the Horned shark, Heterodontus francisci, are durophagous, even with cartilaginous jaws (Huber et al. 2005). Since cartilage is more malleable than bone, the question is brought up as to how chondrichthyans can feed on durable, hard prey with cartilaginous jaws. Hence, our project focuses on the strain levels of the hyoid arch elements in sharks and rays. We tested the hyoid arch since it swings the lower jaw ventrally from the chondrocranium to expand the buccal cavity and experiences much of the pressure from mastication. ! We also examined cartilage mineralization. One suggestion in literature was that the mineralization of cartilage makes it stiffer and therefore more resistant to strain (Porter et al. 2007). Mineralization occurs in chondrichthyan jaws as prismatic or globular calcification in the form of blocks called tesserae. They form around the perichondrium, the outer layer of the extracellular matrix (Dean and Summers 2006). Since the calcified tesserae add another layer to the cartilage, one of our hypotheses was that cartilage elements with more mineralization would be more resistant to strain than those that had a thinner layer of mineralization. ! Our second hypothesis was that shorter, more square-like hyomandibulae would deform less than longer, thinner hyomandibulae. We obtained data on hyomandibulae shape by measuring their length and cross-sectional areas, and used length data to test this hypothesis. We predicted that long, thin specimens had more area to deform than short, squat specimens, leading to a higher Poisson’s ratio of strain. Our third hypothesis was that mineralized area and hyomandibula cross-sectional area were correlated, since a thicker layer of mineralization would add to the extracellular matrix area of the cartilage. Significant correlations would suggest that larger hyomandibulae are more resistant to strain than smaller ones.