Dr. Cheryl Wilga
geometric morphometrics; shape changes; hyomandibula; ceratohyal; basihyal; shark
The hyoid arch is critical in expanding the oral cavity during feeding and ventilation in fishes. The elements of the hyoid arch vary widely in morphology among fish taxa. The hyoid arch is medial to the jaws and is made up of paired hyomandibular cartilages (HY) dorsally, paired ceratohyal cartilages (CH) ventral to the HY, and a single, medial basihyal cartilage (BH) that interconnects the two sides. As the jaws open, the CH-BH complex is depressed expanding the floor of the oral cavity. The coracohyoideus muscle pulls the BH ventroposteriorly, which in turn swings the CH ventroposteriorly. As the coracohyoideus muscle depresses the BH, the CH experiences substantial bending forces. Because of their critical role in the feeding mechanism, the various shapes of the hyomandibular cartilages among species are likely adapted for different feeding behaviors. The hyoid arch of suction feeders experiences larger forces during rapid hyoid depression and are expected to have robust hyoid elements to withstand muscular forces and strong intraoral negative pressure generated during suction. Bite feeders are expected to have relatively more gracile elements, since strong, rapid jaw adduction that does not involve the hyoid arch is more critical to prey capture. The goal of this project was to test for possible form-function relationships among four shark species with different feeding styles. Shape differences were analyzed using 2D geometric morphometrics in white-spotted bamboo sharks (Chiloscyllium plagiosum, suction feeder), sandbar sharks (Carcharhinus plumbeus, bite feeder), dusky smoothhound sharks (Mustelus canis, bite feeder), and spiny dogfish (Squalus acanthias, bite and suction feeder) to quantify the relationship between the morphology of the hyomandibular cartilage and the feeding mechanism. Landmarks and semilandmarks were digitized on images of the HY. Principal components analysis and canonical variate analysis were applied to distinguish the shapes of elements and to determine changes among different species. While dogfish, which uses both suction and bite to feed, have the most robust HY, bamboo sharks, which only use suction to feed, have the most gracile HY. Bamboo and smoothhound sharks are similar in having a process near the each end of the HY on the posterior/anterior side. The HY of sandbar sharks has a convex curve that bulges outward, while the HY of the other species are relatively straight. Thus, the morphology of the HY does not correspond to feeding style in these four species as expected, but rather may be associated with three-dimensional movements of the hyoid arch during feeding.