By Cara Hodgson
Sharks are one of the increasingly rare organisms seen on coral reefs. They have been eliminated from many reefs due to demand for their fins to make shark fin soup, a Chinese delicacy. In 2011, there were some big “wins” for sharks with shark finning and trading banned in several areas. Because shark sightings are now so rare just about everywhere, Reef Checkers are asked to record any sharks during their dives – even of those observed off of the transects.
One type of shark has always held a fascination as a kind of prehistoric-looking oddity that one might think was dreamed up by a Hollywood horror filmmaker – not a result of millions of years of evolution. This is the hammerhead shark. A related shark is the bonnethead – with a head shaped more like a shovel.
Looking at the wide separation between the hammerhead’s eyes, and the flat surface of the head, one wonders about the evolutionary advantage of this design?
Several hypotheses explaining the evolution of the hammerhead shark’s head – called a “cephalofoil” – have been proposed.
One is Stephen M. Kajiura’s enhanced electrosensory hypothesis. All sharks have special gel-filled pits on their lower jaw and around their “face” that are used to detect electromagnetic radiation, such as the nerve impulses in the muscle of a fish hiding in the sand. A preferred prey item for hammerheads is the stingray – often resting buried under sand. Hammerheads have more electrosensory pores (called Ampullae of Lorenzini) than other sharks because they are spread over the wider cephalofoil of the hammerhead. Kajiura hypothesizes that the wider, flatter head allows hammerheads to have electroreceptor pores more spaced out so that the sharks can search and forage a larger area – sort of like a wide beam flash light.
A second hypothesis is that since the hammerhead’s eyes are positioned at the ends of the cephalofoil, some researchers think that this helps them to see better than other sharks. This could be because of a wider overlap in binocular vision.
The third hypothesis proposed is the ability of the cephalofoil to improve the shark’s movement in the water by providing hydrodynamic lift. Most sharks need to swim continuously to pass water over their gills. The additional lift provided by the cephalofoil may reduce the effort needed for hammerheads to swim. Also, hammerheads have more muscles around their head and vertebral column which results in greater flexibility and greater ability to move their heads. Other sharks do not have musculature to allow them to depress their heads.
This extreme movement is needed for sharks that feed on benthic prey since the shark needs to make a rapid turn away from the ocean floor after attacking its prey. If the turn is not made sharply enough, the shark could end up running into the ocean floor. In addition, since it is dangerous to capture stingrays, hammerheads have developed a way to hold the stingrays down with their cephalofoils until they are traumatized and immobilized, so that they can feed on it without being impaled by the stingray’s tail spines.
The cephalofoil also seems to provide a similar function to the pectoral fin on other sharks since the hammerheads have a much smaller pectoral fin than other species. Because the cephalofoil is larger than a pectoral fin and is further from the center of gravity on the shark, it therefore is better for movement and provides better hydrodynamic lift than a fin centered in the middle of the shark’s body.
Regardless of the ultimate cause or causes of the evolution of the hammerhead shark’s head, the hammerhead has adapted to become an aggressive and efficient predator while remaining one of the most identifiable creatures of the sea.