The scalloped hammerhead shark, Sphyrna lewini, is a marine creature recognized by its distinctive head shape.
Defining Features and Anatomy
The scalloped hammerhead shark is named for the unique “scalloped” indentations along the front edge of its flattened, hammer-shaped head, known as a cephalofoil. This specialized head structure offers several advantages, including enhanced vision, allowing the shark to see above and below itself simultaneously, and improved depth perception.
Scalloped hammerheads can reach lengths between 3.7 to 4.3 meters (approximately 12 to 14 feet) and weigh up to 335 pounds. Their bodies are greyish-brown or bronze on the dorsal side, fading to a white underside, with some individuals displaying black tips on their pectoral fins. They possess a tall, slightly hooked first dorsal fin, which is proportionately larger than their pectoral fins compared to many other shark species.
Global Habitats and Social Dynamics
Scalloped hammerhead sharks inhabit warm temperate and tropical seas across the globe, including the Atlantic, Indian, and Pacific Oceans. They are a coastal pelagic and semi-oceanic species, commonly found over continental shelves and in adjacent deep waters, ranging from surface waters down to depths of over 1,000 meters. These sharks exhibit daily habitat shifts, moving from offshore hunting grounds to island shelves, seamounts, and enclosed bays or estuaries at dawn, then returning to deeper offshore areas at dusk to forage.
They form large schools, sometimes comprising hundreds of individuals, particularly around seamounts and offshore islands during daylight hours. These aggregations are thought to consist primarily of females and may serve for social interaction, utilizing cleaning stations, or acting as staging locations for nocturnal foraging excursions into the surrounding pelagic environment. Satellite tagging has confirmed migratory connections between coastal nursery areas and these oceanic congregations, highlighting the need for protection of juvenile sharks in nurseries.
Diet and Hunting Strategies
Scalloped hammerhead sharks are generalist predators, consuming a diverse range of prey items. Their diet primarily consists of bony fish such as sardines, herring, and mackerel, as well as cephalopods like squid and octopus, and various crustaceans. They are also known to prey on other sharks and rays, even appearing unaffected by the venomous barbs of stingrays.
The distinctive cephalofoil plays a significant role in their hunting techniques. Scalloped hammerheads use their broad heads to pin down prey, such as stingrays, against the seabed. Their highly developed electroreceptors, distributed across the wide cephalofoil, allow them to efficiently sweep and detect the electrical fields generated by hidden or buried prey, providing a considerable advantage in locating food. Research suggests that scalloped hammerheads can temporarily close their gill slits during deep dives into colder waters to maintain body temperature, a strategy similar to some marine mammals, enabling them to effectively hunt deep-sea squid and other mesopelagic prey.
Conservation Challenges and Efforts
Scalloped hammerhead populations face considerable threats, primarily from widespread and intensive commercial fishing. They are often targeted directly for their fins, which are highly valued in the international shark fin trade, and are also frequently caught as bycatch in fisheries using longlines, trawls, purse seines, and gillnets. This species’ life history characteristics, including slow growth, late age at maturity (females at approximately 15 years, males at 10 years), and relatively low fecundity (producing litters of 12 to 41 pups), make them particularly susceptible to overexploitation.
The International Union for Conservation of Nature (IUCN) Red List classifies the scalloped hammerhead shark as Critically Endangered globally. Declines have been observed in various regions, with some populations, such as those in the Northwest and Western Central Atlantic, experiencing reductions of 83-85% between 1981 and 2005. In response to these declines, the scalloped hammerhead shark is listed under Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). This listing aims to regulate international trade in products from the species to ensure trade does not further threaten their survival. Certain distinct population segments of the scalloped hammerhead are also listed as endangered or threatened under the U.S. Endangered Species Act.