The peculiar heads of hammerhead sharks are called "cephalofoils," a term that reflects an interpretation of the head as a hydrodynamic structure that helps to direct the shark's head upward while it is swimming or to make the shark more

Question

maneuverable. An alternative hypothesis is that the head increases the shark's chemosensitivity or electrosensitivity. Proponents of the two interpretations have used anatomical, behavioral, and evolutionary information to support their views and to discredit the alternative hypothesis. Hydrodynamic Function • The cross section of the cephalofoil is shaped like a wing, and the angle of its trailing edge can be altered in a manner reminiscent of changing the angle of the flaps on the trailing surface of an airplane's wing to control the amount of lift. • The sizes of the two anterior planning surfaces, the cephalofoil and the pectoral fins, are inversely related. That is, species with wide cephalofoils have relatively small pectoral fins, and vice versa. As a result of that relationship, the total surface area of those surfaces is fairly constant. • Hammerheads are among the least buoyant species of sharks. Sensory Function • Hammerheads swing the head from side to side when they are searching for prey buried in the sediment. This is the same way a person searches for buried objects with a metal detector. • The ampullae of Lorenzini are distributed over the entire ventral surface of the hammer. Additional Observations • The least-derived extant species of hammerhead, the winghead shark (Eusphyra blochii), has the broadest hammer; its width is nearly 50 percent of the total length of the shark. • The size of the hammer in more derived species of hammerheads does not change unidirectionally according to current phylogenetic interpretations of the lineage (that is, there appears to be no tendency for the size of the hammer to have increased or decreased during the evolution of the extant hammerheads). • The nostrils of the winghead shark are near the midline of the hammer, but they lie toward the ends of the hammer in more derived species. • The nostrils of the scalloped hammerhead (Sphyrna lewini), which is a well-studied species, lie near the ends of the hammer and collect water via a prenarial groove that probably increases the volume of water flowing across the olfactory epithelium. • The surface area of the olfactory epithelium of the scalloped hammerhead is no larger than that of non-hammerhead sharks. It appears that observations have taken us as far as they can and it's time to try a different approach to testing hypotheses about the functional significance of the hammerhead morphology. a. What experimental tests can you propose to evaluate the two hypotheses? (Don't worry too much about how you would carry out a manipulation—if a test involves putting sharks into a flow tank, for example, assume that you have access to a tank and sharks of the appropriate species and sizes.) b. Is there another possibility that you miss if you consider the hydrodynamic and sensory functions to be alternative hypotheses?

Answer 1

a. Obviously there is no single answer to this question; its point is to encourage students to move from observation to manipulations that compare the responses of hammerhead sharks with different cephalofoil morphologies to each other and to non-hammerhead sharks. Here are some examples:
Chemosensation
• Test the sensitivity of the olfactory system to odors and directionality by measuring the minimum concentrations of odor molecules that sharks can detect, the latency of response, and the ability of sharks to home in on the source of an odor.
• Test the effect of partially or completely blocking one or both nares on the sensitivity, latency, and directionality of a shark's response to odors.
Electrosensation
• Carry out the same kinds of manipulations using synthetic electrical stimuli and occluding some or all of the ampullae with an insulating paste.
Hydrodynamic
• Compare the ability of hammerheads and non-hammerheads to navigate a three-dimensional maze, preferably one that requires abrupt changes of direction.
• Modify the size and shape of the cephalofoil by gluing on plastic extensions, and evaluate the speed and precision of a hammerhead's ability to change its direction of movement or to maintain itself in the water column. The latter test might be made more challenging by adding weights to make the shark negatively buoyant.
b. The hydrodynamic and sensory hypotheses are not mutually exclusive; both could be correct, and the morphology and behavior of hammerhead sharks could represent a series of compromises between the optimal morphologies for the two functions.