Author: Gill, 1872
Field Marks:
The hammer or mallet-shaped lateral expansions of the heads of these sharks are unique.
Diagnostic Features:
Head with laterally expanded blades, shaped like a double-bitted axe or mallet in profile; eyes circular or nearly so; nictitating eyelids internal; spiracles absent; anterior nasal flaps short and triangular, not barbel-like; internarial width usually about 7 to 14 times the nostril width (but only 1.1 to 1.3 times it in Eusphyra, which has tremendously expanded nostrils); labial furrows vestigial or absent; teeth small to moderately large, more or less bladelike, with acute and narrow to moderately broad cusps, no lateral cusplets, and with basal ledges and grooves strong to absent; teeth weakly differentiated in upper and lower jaws; tooth rows 24 to 37/25 to 37. Precaudal pits present. First dorsal fin moderate-sized to very large but not keel-like, much shorter than caudal fin; first dorsal base ahead of pelvic bases, varying from equidistant between pectoral and pelvic bases to closer to pectoral bases; midpoint of first dorsal base always in front of pelvic origins; second dorsal fin much smaller than first; pectoral fins with radials extending into distal web of fins. Ventral caudal lobe strong, undulations or ripples present in dorsal caudal margin. Neurocranium without primary supraorbital crests, but with tips of preorbital and postorbital processes fused to form unique secondary supraorbital crests; vertebral centra with strong, wedge-shaped intermedial calcifications. Valvular intestine with a scroll valve. Colour light grey or brownish above, white below, no colour pattern. Development viviparous.
Habitat, Distribution and Biology:
The hammerheads are a small but common family of wide-ranging warm-temperate and tropical sharks found in continental and insular waters on or adjacent to their shelves but with none being truely oceanic. Depths range from the surface, surfline and intertidal region down to at least 275 m depth. Hammerheads are very active swimmers, ranging from the surface to the bottom, and occur in all warm seas. Several species occur in schools, sometimes with hundreds of individuals. Some of the larger species seem to find fish baits on longlines quicker than other sharks, and expire more swiftly than most other species after being caught. The unique flattened and laterally expanded prebranchial head of hammerheads has been interpreted by some shark biologists and functional morphologists as a bowplane primarily serving to increase manouvering capabilities in these sharks, but sensory physiologists and shark behaviourists are more impressed by the increased sensory capacity afforded by their expanded heads, like the sensor plate of a metal detector. The more wide-spaced eyes may enhance binocular vision anteriorly, the expanded nasal capsules allow larger nasal organs and perhaps a more acute and more directional olfactory sense, and the increased head area allows more extensive lateral line canals and Ampullae of Lorenzini and possibly more capable pressure and electromagnetic senses. In the genus Sphyrna the sequentially increased lateral expansion of the head among species may indicate enhanced manouverability as well as sensory enhancement in the more broad-headed species; however, head expansion is carried beyond what seems optimal for increasing manouverability in the bizarre winghead shark (Eusphyra blochii) but certainly may indicate a more capable sensorium in this shark. Hammerheads are versatile feeders that take a wide variety of bony fishes, elasmobranchs, cephalopods, crustaceans, and other prey; some habitually feed on other elasmobranchs, and one species (S. tiburo) has enlarged, almost Heterodontus-like posterior teeth as an adaptation to crushing invertebrate prey. Some of the larger species have been involved in attacks on people, but recent studies on a few species of these sharks show that they are not particularly aggressive in unbaited situations.
Interest to Fisheries:
The larger species of Sphyrna are important elements of tropical inshore and offshore fisheries. The small species of Sphyrna and the small Eusphyra blochii figure in local fisheries where they occur.
Remarks:
The present classification of the family follows Gilbert (1967, 1967a) and Compagno (1979).