Sphyrnidae, also known as hammerhead sharks, hammer sharks, or bonnetheads, are a family of elasmobranchs in the order Carcharhiniformes. Their heads extend sideways with eyes on each end. Most species belong to the genus Sphyrna, while the winghead shark is classified under the genus Eusphyra. The cephalofoil, the hammer-shaped structure of their heads, serves several functions including sensory reception, maneuverability, and prey capture. This shape provides superior binocular vision and better depth perception.
The hammerhead shark is found mainly in tropical and subtropical coastal areas worldwide, along coastlines and continental shelves. Although generally solitary, some species form schools during the day and hunt alone at night.
These sharks feed on various invertebrates, bony fish, and cartilaginous fish. Larger specimens may also consume other sharks, including smaller individuals of their own species (cannibalism). The sensory organs in the cephalofoil allow them to detect prey mainly on the seabed, such as rays. All these sharks are viviparous, developing a placenta to nourish their young. While the larger species are potentially dangerous, accidents with humans are rare. Some species are commercially fished for their fins, and the IUCN classifies some of these species as “vulnerable” or “endangered” due to overfishing.
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Characteristics, Traits, and Adaptations
Morphology and Size
Hammerhead sharks have a hydrodynamic body. The great hammerhead (Sphyrna mokarran) is the largest known, reaching up to 6.10 meters in length and weighing nearly half a ton. In contrast, the scalloped bonnethead is the smallest species, reaching only 92 centimeters.
Coloration Pattern
The coloration is usually brown to light gray with a greenish hue and a white belly, helping them camouflage on the seabed while stalking their prey. Some species have irregular spot patterns or dark edges on their fins. The small-eye hammerhead has a special coloration ranging from golden yellow to orange due to its carotenoid-rich diet.
Cephalofoil
The head's widening, characteristic of the order Sphyrnidae, is notable. In species of the genus Sphyrna, the head represents between 17% and 33% of the total body length, while in the winghead shark (Eusphyra blochii) it can reach between 40% and 50%.
Senses and Vision
The eyes are located at the ends of the cephalofoil, providing binocular vision and a 360-degree field of vision in the vertical plane. This allows them to see both above and below at all times. The head shape also helps in prey detection and allows sharp turns without losing stability.
Additional Physical Features
The nostrils have short, lobed flaps, with a considerable distance between them. The mouth, located below the cephalofoil, is parabolic. The teeth are sharp and feature a single central cusp. Hammerhead sharks have five gill slits and a reduced spiracle. The first dorsal fin is large and sickle-shaped, while the second dorsal fin and the anal fin are smaller. The caudal fin is asymmetrical, with a large upper lobe and a smaller but well-developed lower lobe.
Skeletal Structure
The neurocranium lacks primary supraorbital ridges, replaced by secondary supraorbital ridges unique among sharks. The vertebrae have strong wedge-shaped calcifications.
Additional Senses
Besides the usual senses, hammerhead sharks detect frequency waves generated by movements in the water and electric fields produced by other fish, allowing them to find prey buried in the sand.
They can form groups of over a hundred individuals during the day and become solitary hunters at night. Hammerhead shark fossils are rare due to the lack of mineralized bones, with teeth being the most common remains.
Habitat and Distribution
Hammerhead sharks inhabit marine and coastal waters and occasionally brackish waters. They are present in all oceans, especially in warm ones. The head structure, with widely separated eyes and nostrils, likely enhances their sensory capabilities.
Feeding and Diet
They feed on various bony fish, elasmobranchs, cephalopods, and crustaceans. Young are born fully developed, and some large hammerhead sharks are known for their aggressive diet, including squids, octopuses, other hammerhead sharks, and even cannibalism.
Geographical Distribution
The small-eye hammerhead shark is found along the western Atlantic coast and primarily lives in shallow, muddy, and turbid waters. Hammerhead sharks are distributed globally, especially in the tropical and subtropical coastal areas of the oceans. Some species, such as the great hammerhead and the scalloped hammerhead, inhabit warm regions worldwide. The smooth hammerhead is also found in temperate areas and migrates to colder regions near the poles during the summer. Smaller species have more limited distribution areas; for example, the winghead shark is found in the Red Sea and the Asian coasts of the Indian Ocean up to northern Australia, while the common hammerhead is located along the Atlantic coasts of Europe and northwest Africa. The scoophead is found along the tropical coasts of North and South America, and the scalloped bonnethead has the smallest distribution, present from the Gulf of California to Peru.
Hammerhead sharks primarily inhabit coastal areas, the continental shelf, and around archipelagos, and are rarely found in deeper regions. The scalloped hammerhead can be found at depths of over 270 meters, while the great hammerhead rarely lives below 80 meters, and the smooth hammerhead usually inhabits less than 20 meters deep. Smaller species and juveniles of larger species prefer shallow waters, with the small-eye hammerhead adapted to turbid and muddy bays and estuaries, resulting in the regression of their eyes.
Interaction with Humans
Generally, hammerhead sharks do not typically attack humans, preferring smaller prey, and divers can observe them safely if they do not chase them. However, they can be dangerous if there are injured animals in the water, as their sense of smell is highly developed. The more aggressive species are the great hammerhead (Sphyrna mokarran) and the smooth hammerhead (Sphyrna zygaena).
Many of these species are endangered due to hunting and fishing for their fins, used in the famous "shark fin soup," considered an aphrodisiac. Conservation campaigns have been initiated in Costa Rica, Colombia, and Ecuador, and diving with these animals is promoted.
Use in Commercial Fishing
Hammerhead sharks, especially the larger species, are fished both commercially and for sport. They are caught in coastal and offshore waters using longlines, bottom nets, and trawl nets. Their fins are highly sought after in Asia for shark fin soup; the body, often still alive, is returned to the sea after the fins are cut off, where it dies. However, the meat, skin, and liver oil are also used, and the remains are employed in fishmeal production.
Additionally, in regions where Sphyrnidae sharks are not specifically hunted, they are frequently caught accidentally due to their association with commercial species. Concrete figures on catches or population sizes are unavailable due to a lack of precise data on deep-sea fishing. Hammerhead sharks are also caught in nets set up to protect beaches, especially along the coasts of Australia and South Africa.
Classification and Categories of Sphyrnidae
Hammerhead sharks include two genera and nine species:
- Eusphyra
- Eusphyra blochii - winghead shark
- Sphyrna
- Sphyrna corona - scalloped bonnethead
- Sphyrna couardi - whitefin hammerhead
- Sphyrna lewini - common hammerhead
- Sphyrna media - scoophead
- Sphyrna mokarran - great hammerhead
- Sphyrna tiburo - bonnethead
- Sphyrna tudes - small-eye hammerhead
- Sphyrna zygaena - smooth hammerhead
Hammerhead Sharks You Should Know
