Hammerheads are some of the most distinctive and well known of the more than 400 species of sharks. This family includes the massive great hammerhead, which may reach 20 feet in length; scalloped hammerheads that sometimes mass together in the hundreds; diminutive three-foot bonnetheads; and several others--eight species in total. They are most famous for their physical appearance.
The family's defining characteristic is the oddly-shaped head, resembling either a hammer or a flattened spade, depending on species, with eyes at either end of the stalks and nostrils on their leading edge. The form seems to confer greater maneuverability and possibly enhanced sensory perception (see "Other Sensory Features"), and also may be used to brace a favorite food, rays, against the seafloor.
Like most sharks, hammerheads have five pairs of gill slits. As the shark swims, water passes through the gills, which extract oxygen.
Hammerheads typically have tall, scythe-shaped dorsal fins, located at the midpoint of the back. Their pectoral fins stabilize the front of the body. Small fins in the shark's posterior (a pair of pelvic fins, an anal fin, and a second dorsal fin smaller than the main one) and the powerful caudal fin of the tail, which has a much larger upper lobe than lower, complete the array.
Bonnethead sharks, the smallest of the hammerheads, have robust dentition for crushing crustaceans. Larger species, however, have sharper, planed teeth for dispatching fish (including those rays) and squid.
Other Sensory Features
As in other sharks, hammerheads have a "lateral line" running from their heads along their sides. This network of hair-cell structures (called neuromasts) help detect movement. In another component of the hammerhead's impressive electrosensory arsenal, pores in the shark's head, called the ampullae of Lorenzini, detect magnetic fields in nearby living creatures.