The Scalloped Hammerhead, Sphyrna lewini, is one of the most recognizable and widely distributed shark species in the world’s oceans. It is defined by a unique, laterally expanded head structure. This shark inhabits warm temperate and tropical waters globally, making it a familiar sight across multiple coastlines and open ocean environments. Its anatomy and behavior reveal a sophisticated predator adapted to active migration and specialized hunting.
Defining the Scalloped Hammerhead
The physical structure of the Scalloped Hammerhead is distinguished primarily by its flattened, wing-like head, known as the cephalofoil. The species earns its common name from the specific shape of the cephalofoil’s anterior margin, which features a prominent central indentation and smaller notches on either side. The eyes and nostrils are positioned at the tips of the cephalofoil extensions, providing a wide-set sensory platform.
The Scalloped Hammerhead is a moderately large species with a slender, streamlined body. It is typically brownish-gray or bronze on the dorsal side, fading to pale white underneath for countershading. Adults can reach a maximum length of up to 4.3 meters (14 feet), with females generally growing larger than males. This species possesses a tall, distinctively hooked first dorsal fin and is estimated to live for over 30 years in the wild.
Global Range and Unique Schooling Behavior
The Scalloped Hammerhead exhibits a circumglobal distribution, thriving in coastal warm temperate and tropical seas across the Atlantic, Pacific, and Indian Oceans. Classified as coastal pelagic, it is found near continental and insular shelves, as well as venturing into adjacent deep waters. Juveniles typically inhabit shallow, inshore areas, utilizing bays and estuaries as protective nursery grounds. Adults undertake seasonal migrations between offshore feeding grounds and near-shore breeding areas.
This shark is known for its tendency to form large schools, a social behavior rarely observed among large shark species. These aggregations can involve hundreds or even thousands of individuals. Schooling is predominantly observed during daylight hours, often centered around offshore structures like seamounts and oceanic islands. This daytime grouping may serve social functions, such as orientation, mating, or protection, before the sharks disperse at dusk to hunt individually in deeper waters.
Hunting Strategy and Diet
The distinct shape of the cephalofoil is a highly specialized sensory tool that dictates the shark’s hunting strategy. Like all sharks, it possesses electroreceptors called the Ampullae of Lorenzini, which detect the faint bioelectric fields generated by living creatures. The wide lateral expansion of the cephalofoil spreads these electroreceptors across a greater surface area than in other shark species. This distribution maximizes its sensory range, allowing the shark to sweep the ocean floor like a metal detector.
This enhanced electroreception is effective for locating prey hidden beneath the sand, such as stingrays, which are a major component of its diet. Once a ray’s electrical signature is detected, the shark uses its head to pin the prey against the seabed. The diet is diverse, consisting mainly of bony fish (sardines and mackerel), cephalopods (squid and octopus), and smaller elasmobranchs (sharks and rays). This active foraging often occurs at night when the sharks leave their daytime schools to pursue prey in deeper, offshore waters.
Conservation Status and Threats
Despite its wide distribution, the Scalloped Hammerhead population has faced a significant decline, leading to its classification as Critically Endangered by the International Union for Conservation of Nature (IUCN). Its highly migratory nature and tendency to form large, predictable schools make it extremely vulnerable to fishing pressure. When a fishing vessel encounters these dense aggregations, large numbers of individuals can be harvested quickly.
The primary driver of this population collapse is the intense demand for its fins in the global shark fin trade. The species is also frequently caught unintentionally as bycatch in commercial fisheries using longlines and trawls. International efforts to mitigate this decline include listing the Scalloped Hammerhead under Appendix II of CITES (Convention on International Trade in Endangered Species). This mandates that international trade must be regulated to prevent overexploitation.