Hydrodynamic properties and ecomorphology of the hammerhead shark (Family Sphyrnidae) cephalofoil

One of the hypotheses explaining the unique cephalofoil of hammerhead sharks (Family Sphyrnidae) suggests that the cephalofoil influences hydrodynamic efficiency by generating lift during swimming. Non-sphyrnid sharks generate lift using the ventral surface and pectoral fins. Cephalofoil and pectoral fin measurements were made on 196 sphyrnid sharks. Allometric relationships with body size were calculated. The results indicate that hammerhead pectoral fins are relatively smaller than those of other sharks. The morphometric data suggest that the hammerheads have smaller pectoral fin aspect ratios relative to other sharks. The sum of pectoral fin and cephalofoil surface areas does not vary significantly among sphyrnid species and therefore the cephalofoil may represent an alternative lifting surface.; To investigate this, models of eight sphyrnid species were subjected to varying flow conditions. Results indicate that the cephalofoil does indeed behave as an aerofoil. Aspect ratios, which are correlated positively with aerofoil lift/drag ratios, increase with increasing average adult size. Cephalofoil shape was compared using geometric morphometrics. Landmark data were obtained for 155 specimens. Principal warp analysis, multivariate analysis of variance, and canonical variates analysis were used to compare species. Static allometry was indicated for three of the seven species investigated. Uniform shape transformations generate most of the shape differences among the species. Non-uniform transformations discriminate between genera. The hydrodynamic implications of these shape variations is most apparent in viewing the aspect ratios of the species. The uniform components depict an evolutionary trend toward higher aspect ratios, a trend toward greater angularity in Sphyrna, and a greater degree of compression and shearing in Eusphyra. It thus appears that the shape of the head, body size and ecology have a functional relationship.; Phylogenetic analyses suggest two alternative scenarios regarding evolution of the cephalofoil. These two hypotheses are in topological conflict with one another. One suggests that the hydrodynamic cephalofoil is derived and may be an exaptation, while the other suggests that it is primitive and has been diminishing with diversification of the clade. Additional phylogenetic information is required to resolve this conflict.