Meeting Abstract
Hammerhead sharks are known for their unique, dorso-ventrally compressed head called a cephalofoil. Due to its aerodynamic shape and anterior location, previous researchers hypothesized that the cephalofoil acts as lifting body, increasing the maneuverability of hammerhead sharks. Observational studies also show that scalloped hammerheads are actively moving their head approximately 10 degrees in volitional swimming, suggesting a degree of muscular control over the cephalofoil. Previously, it has been shown that scalloped hammerheads have significantly more axial muscles in the anterior trunk region than compared to the pointy nosed Galapagos reef shark. This allowed for a greater range of dorso-ventral movement of the cephalofoil. Here, we examine the hypaxial and epaxial muscles of three species of hammerhead shark (Sphyrnidae) in comparison to three species of pointy nosed sharks (Carcharhinidae). We hypothesized that hammerhead sharks would have more axial musculature in the anterior trunk region and that hypaxial muscles would originate more anteriorly in the hammerheads. We found significantly more axial muscle in the two more ancestral hammerhead species (scalloped and great), although the most derived bonnethead shark did not fall into this pattern. However, hypaxial musculature was present at the first gill slit in all hammerhead species and absent in the pointy nosed sharks. We predict that the farther anterior presence and greater mass of hypaxial and epaxial muscles in the anterior trunk region of hammerhead sharks correlates to the increased range of motion of the cephalofoil and, in turn, greater maneuverability of these sharks.
