Functional Morphology of the Sphyrnid Shark Cephalofoil: an Assessment of the Hydrodynamic Lift Hypothesis Using Computational Fluid Dynamics

The hammerhead sharks (Family: Sphyrnidae) are all characterized by a conspicuous lateral expansion and dorsoventral flattening of the head forming a structure known as a cephalofoil, however, there is substantial morphological variation within the clade. Many theories have been proposed regarding the functional aspects of this structure. One of these is that it may produce beneficial dynamic lift as the shark swims (in similar fashion to the cambered wings found on many modern-day aircraft). As sharks do not possess a swim bladder, part of their energy intake is expended on simply maintaining vertical station in the water column. If indeed the cephalofoil constitutes an anterior lift-generating feature as hypothesized, such energy expenditure could theoretically be reallocated. We digitized the head shaped of all eight living species of hammerhead shark, and performed a computational fluid dynamic (CFD) analysis to quantify the lift and drag forces associated with each of the various cephalofoil morphologies. For comparison, three carcharhinid species, the bull shark (C. leucas), the blacktip shark ( C. limbatus), and the leshark (N. brevirostris) were likewise analyzed. It was assumed that addition of a lifting structure to the morphology of the shark should have effected corresponding evolutionary changes in other lift-generating features. To test this hypothesis, morphometric data were gathered from numerous specimens and multiple regression coupled with an information-theoretic approach to model selection were used. The cephalofoil appears only to produce substantial lift forces at positive angles of incidence to the flow. These head morphologies, meanwhile, appear to be characterized by greater drag than their carcharhinid counterparts. Statistical analysis corroborates the current belief that hydrodynamic forces acting on the cephalofoil reduce stability during swimming. The ecophysiological impli