Meeting Abstract

80.3  Friday, Jan. 7  Functional Morphology of the Feeding Apparatus in Hammerhead Sharks (Sphyrnidae): a Phylogenetic Perspective MARA, KR*; MOTTA, PJ; University of South Florida; University of South Florida kmara@mail.usf.edu

Because sphyrnid sharks possess cranial morphologies with extreme variation, they can be used to address questions about the evolution of cranial design and investigate the effects of changes in head morphology on feeding structures and ecologically relevant performance parameters such as bite force. Adult individuals of Eusphyra blochii, Sphyrna mokarran, S. lewini, S. tudes, S. tiburo, Carcharhinus acronotus, and Rhizoprionodon terraenovae were chosen to represent a continuum of head shape through phylogeny. The cross sectional areas of the four principal jaw adductors as well as the mechanical advantage of the jaws were used to estimate the theoretical maximum bite force. Additionally, the volume of each muscle along with the volume the jaws and hyoid arch were determined through CT scans. Both anterior (18.2 – 642.22 N) and posterior (71.08 – 1839.43 N) absolute bite force exceeded a full order of magnitude. Within sphyrnid sharks anterior (0.12 – 0.26) and posterior (0.76 – 1.01) mechanical advantage were relatively speed efficient and outgroup carcharhinids had slightly higher mechanical advantages. Size-removed bite force analysis within sphyrnids and relatives indicated that E. blochii, S. zygaena, and S. tiburo all produce less force than would be predicted based on their length. However, S. mokarran had relatively large bite force compared to other sphyrnids as did outgroup carcharhinids. Despite these differences, the feeding bauplan of hammerhead sharks has been conserved with few changes to the feeding apparatus and biomechanics despite differences in head shape, leading to the conclusion that differences in head morphology are most likely the result of selective pressures placed on the cranium by the sensory systems.