Hammerhead Shark (Sphyrna lewini): Guide to Characteristics and Behavior

The hammerhead sharks (Sphyrna lewini), also known as scalloped hammerhead sharks (in English: Hammerhead shark), constitute a group of sharks from the Sphyrnidae family, named after the peculiar shape of their heads, which are flattened and extend laterally in a shape similar to a "T" or "hammer". The eyes of these sharks are located at the ends of this T-shaped structure, with their small mouths centered and positioned underneath.

Most hammerhead shark species belong to the genus Sphyrna, while the winghead shark belongs to its own genus, Eusphyra. Various functions have been proposed for this cephalofoil structure, including sensory reception, maneuverability, and prey capture. This head shape provides them with enhanced binocular vision and depth perception.

Hammerhead sharks are globally distributed and prefer to inhabit warm waters along coasts and continental shelves. Unlike many other sharks, some species of this shark form large schools during the day and hunt solitarily at night.

Characteristics of the hammerhead shark (Sphyrna lewini)

Generally, Sphyrna lewini sharks have a light gray color with a greenish tint, and their bellies are white, which allows them to camouflage with the seabed when viewed from below, thus facilitating the stalking of their prey. Their heads have lateral projections that give them their characteristic hammer shape. Although this shape is generally similar, it varies slightly between species: for example, a distinctive "T" shape in the great hammerhead shark, a rounded head with a central notch in the scalloped hammerhead shark, and a rounded head without notches in the smooth hammerhead shark.

Hammerhead sharks have relatively small mouths compared to other shark species. Additionally, some species are known to form schools. At night, like many other sharks, they become solitary hunters. According to National Geographic, hammerhead sharks are found in warm and tropical waters, but during the summer, they participate in a massive migration in search of cooler waters.

Size, weight, and length

The known species of hammerhead sharks vary in size from 0.9 to 6.0 meters in length and can weigh between 3 and 580 kilograms. A specimen caught off the coast of Florida in 1906 weighed more than 680 kilograms. In comparison, the great hammerhead shark (Sphyrna mokarran) can reach up to 6.1 meters in length and weigh more than 600 kilograms, being the largest of the hammerhead sharks and notably more robust than other species of its genus.

Hammerhead shark skeleton

The hammerhead shark's skeleton, like that of other sharks, is primarily composed of cartilage rather than bone. This cartilage is lighter and more flexible than bone, providing advantages in terms of buoyancy and maneuverability in the water. Below are the characteristics and peculiarities of their cartilaginous bone structure, with simple comparisons to the giant Sphyrna mokarran, the great white shark, and the bull shark.

General characteristics

1. Cartilaginous Structure: cartilage is less dense than bone, which helps reduce the shark's weight and facilitates its movement in the water. This is a common characteristic among all sharks, including Sphyrna lewini, the great white shark, and the bull shark.
2. Hammer-shaped Head: the most distinctive feature of the hammerhead shark is its "T" or hammer-shaped head. This cephalofoil structure not only contains the eyes and nostrils at its ends but also houses a large number of electrical sensors called ampullae of Lorenzini. This unique design provides improved binocular vision and superior depth perception. In comparison, the giant hammerhead (Sphyrna mokarran) has a proportionally wider and more robust head than other species.
3. Vertebral Column: its vertebral column is designed to support rapid movements and sharp turns. Although all sharks have highly mineralized vertebrae to provide rigidity and protection, those of this species are specifically adapted to enhance maneuverability. In comparison, great white sharks and bull sharks have vertebrae that support more robust bodies and more powerful movements, suitable for their hunting habits.

Comparisons with Other Species and Classes of Sharks

1. Giant Hammerhead Shark: shares many structural features with the common hammerhead, but its skeleton is larger and more robust, adapted to support its greater size and weight. Its vertebrae are thicker, and its hammer-shaped head is wider, giving it an advantage in detecting and capturing prey.
2. Great White Shark (Carcharodon carcharias): the great white shark's skeleton is more massive and strong, designed to support its large size and powerful hunting method, which includes fast lunges and powerful bites. Unlike the common hammerhead, the great white shark has a more conventional head shape and a more streamlined body.
3. Bull Shark (Carcharhinus leucas): the bull shark has a strong and compact skeleton that allows it to move in both saltwater and freshwater. Its robust body and vertebral column are suited to support its aggressive behavior and powerful attacks. Compared to the hammerhead shark, the bull shark has a more compact structure and is less specialized in terms of sensory capabilities and maneuverability.

Peculiarities

• Sensory Adaptation: the head design allows for a wider distribution of electrical sensors, giving it an advantage in detecting prey buried in the seabed. This feature is less pronounced in the giant hammerhead but remains a key advantage.
• Flexibility and Lightness: the cartilaginous structure of shark skeletons provides great flexibility and reduces weight, enhancing their hunting ability and energy efficiency.

Classification of Sphyrna lewini

• Kingdom: Animalia
• Phylum: Chordata
• Class: Chondrichthyes (cartilaginous fish)
• Order: Carcharhiniformes
• Family: Sphyrnidae
• Genus: Sphyrna
• Species: Sphyrna lewini

Taxonomy of Sphyrna lewini or Hammerhead Shark

Hammerhead sharks, like other sharks, do not have mineralized bones, which means they rarely fossilize, and generally, their teeth are found as fossils. Their closest relatives are requiem sharks (Carcharhinidae). DNA and fossil studies indicate that the ancestor of the hammerhead shark probably lived in the early Miocene, about 20 million years ago.

Using mitochondrial DNA, a phylogenetic tree of hammerhead sharks has been constructed, showing that the winghead shark is its most basal member. Since this shark has proportionally the largest "hammer," it is suggested that ancestral hammerhead sharks also possessed large cephalofoil structures.

Cephalofoil

The hammer shape of the heads of these sharks may have evolved, at least in part, to enhance their vision. The position of the eyes, located on the sides of the hammer-shaped head, allows for 360° vision in the vertical plane, meaning they can see both above and below them at all times. Additionally, this configuration provides greater binocular vision and depth of field. While it was initially thought that the head shape helped in food search, facilitating maneuverability and allowing sharp turns without losing stability, it has been discovered that the structure of their vertebrae is key to making these turns correctly. The hammer-shaped head also distributes the electroreceptive sensory receptors, known as ampullae of Lorenzini, over a wider area, allowing them to detect electric fields generated by other organisms more effectively.

How Does the Hammerhead Shark Reproduce?

Reproduction in hammerhead sharks occurs once a year, usually when the male bites the female until she accepts mating. These sharks have a viviparous mode of reproduction, where females give birth to live young. Fertilization is internal, and the male transfers sperm to the female through one of his claspers. The embryos initially develop sustained by a yolk sac. When this is exhausted, the yolk sac transforms into a structure similar to a mammalian placenta, through which the mother provides nutrients until birth. The pups, born in litters of 12 to 15 individuals (20 to 40 in the case of the great hammerhead), group together and swim towards warmer waters to grow until they reach a suitable size to survive on their own. In 2007, it was discovered that the bonnethead shark can reproduce asexually through automictic parthenogenesis, where a female's egg fuses with a polar body to form a zygote without the need for a male.

Eggs of Sphyrna lewini

Hammerhead sharks, like many other sharks, have a viviparous reproductive system, meaning they do not lay external eggs; instead, the young develop internally in the mother. Despite this, it is interesting to know what the process would be like if they laid eggs and some characteristics of their reproduction.

In oviparous shark species, the eggs are usually large and protected by a leathery capsule. These capsules, often known as "mermaid's purses," are designed to protect the developing embryo from predators and adverse environmental conditions. The eggs have adhesive filaments that allow them to attach to stable substrates on the seabed, preventing them from being swept away by currents.

In the case of hammerhead sharks, although they do not lay eggs, the embryos go through an initial stage similar to an egg inside the mother, where they are contained in a yolk sac. This sac provides essential nutrients during the early stages of development. As the embryo grows, it consumes the nutrients from the yolk sac, which eventually transforms into a structure analogous to the placenta in mammals, allowing direct nutrient exchange between the mother and the embryo.

The process of development within the mother's body offers several advantages over egg-laying. These include continuous protection of the embryo from predators and a stable environment for development. At birth, the young are better prepared to face the challenges of the marine environment.

Once the young are born, generally in litters of 12 to 15 individuals, or even up to 40 in the case of the great hammerhead shark, they are completely independent and receive little or no parental care. These young group together and seek warmer, shallower waters where they have a better chance of finding food and shelter while they grow and develop until they reach a size that allows them to survive in the open ocean.

What does the hammerhead shark eat? Its diet

Hammerhead sharks have a varied diet that includes fish (including other sharks), squid, octopus, and crustaceans. Rays are a favorite prey, thanks to the location of their crescent-shaped mouths, which facilitates hunting rays and flatfish on the ocean floor. They use their heads as tools to immobilize and stun their prey before eating them. The great hammerhead shark, which is larger and more aggressive, sometimes practices cannibalism, even consuming its own young. Additionally, bonnethead sharks have been found to ingest seagrasses, which they can partially digest, classifying them as potentially omnivorous, similar to whale sharks, which have also been recently discovered to be omnivorous.

Where does the hammerhead shark live? Habitats and global distribution

The hammerhead shark Sphyrna lewini is a coastal-pelagic and semi-oceanic species that inhabits continental and insular shelves as well as adjacent deep waters. This species frequently approaches the coast and can enter closed bays and estuaries. During the summer, large groups of young individuals migrate toward the poles in certain areas. Adults are solitary or form pairs or small groups, while juveniles often form large schools.

In the Atlantic, it can be found from Nova Scotia to the Virgin Islands and from Brazil to southern Argentina in the west, and from the British Isles to Ivory Coast, including the Mediterranean Sea, in the east. In the Indian Ocean, it inhabits the waters of South Africa, India, and Sri Lanka. In the western Pacific, it ranges from the Gulf of Tonkin to southern Japan and Siberia, as well as the coasts of Australia and New Zealand. In the central and eastern Pacific, it is found in the Hawaiian Islands, California, Panama, the Galapagos Islands, Ecuador, and Chile.

Why the hammerhead shark is endangered

Studies indicate that hammerhead shark populations in the Gulf of Mexico and the Caribbean Sea have declined by approximately 80% in the past two decades. Globally, it is considered endangered, both on the IUCN (International Union for Conservation of Nature) Red List of Threatened Species and in the Venezuelan Red Book of Wildlife.

The governments of Costa Rica and Colombia have denounced shark finning for many years. In the Cocos Islands in Costa Rica and Malpelo in Colombia, it is common to see large schools of sharks, promoting diving with these animals. Both countries have worked together on the conservation of hammerhead sharks.

In Costa Rica, research conducted by the non-governmental organization Misión Tiburón, using conventional and acoustic tagging methods, discovered that adult hammerhead sharks migrate from the pelagic waters surrounding Cocos Island to the mangroves in Golfo Dulce, a tropical fjord on Costa Rica's Pacific coast. Here, females give birth to live young, and juveniles remain in the shallow roots of the mangroves for about three years. After this time, they migrate back to Cocos Island to feed in pelagic waters. In May 2018, thanks to the efforts of Misión Tiburón, the Costa Rican government declared the wetlands in Golfo Dulce a sanctuary for the hammerhead shark, creating 4,000 hectares of "no-take" zone, the first shark sanctuary in Costa Rica.

In Colombia, a notable case of finning involved four Costa Ricans fishing for sharks on Malpelo Island. They had finned nearly 2,000 sharks before being caught and sentenced to 10 years in prison. Subsequently, Costa Rica banned shark killing in the country and the presence of boats on Malpelo Island. Shark finning remains a persistent problem in both countries, aggravated by illegal fishing in their waters by large Asian vessels, which also buy fins from small fishermen and local traffickers. These fins are highly demanded in Asian countries due to the false belief that they improve male erectile function.

In Venezuela, the capture, transport, and trade of hammerhead sharks are prohibited in fishing operations conducted by national industrial vessels. Recently, it was included in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora due to the vulnerability of its current populations. Additionally, it is recommended to establish a minimum catch size, based on the size of sexual maturity, for artisanal fishing operations.

Threats

The main threats to hammerhead sharks include the overfishing of individuals that have not reached sexual maturity, the degradation of coastal areas (lagoons and estuaries) that are essential habitats for the species (nursery, breeding, and feeding areas), and the lack of fishing regulations controlling their exploitation by artisanal fleets.

Is the Hammerhead Shark Eaten? Importance in Fishing

The hammerhead shark is a species fished both commercially and by sport fishermen. It is easily accessible to both coastal fishermen and commercial offshore operations. The tendency of these sharks to form large schools facilitates their capture in significant quantities using longlines, bottom nets, and trawl nets. This ease of capture has led to the local extinction of populations in some areas. Although hammerhead shark meat is marketed, its fins and skins are particularly valuable. The fins of this species have represented up to 5% of the fin market in China, with estimates of up to 2.7 million sharks caught. The rest of the shark is used to produce vitamins and fishmeal. Hammerhead shark pups often inhabit shallow coastal nursery areas, making them especially vulnerable to fishing.

Is the Hammerhead Shark Dangerous? Aggressiveness Towards Humans

Hammerhead sharks are considered potentially dangerous to humans. According to the International Shark Attack File, there have been 17 unprovoked attacks by sharks of the Sphyrna genus, although none of them have been fatal. Scalloped hammerhead sharks have been observed to display threatening postures when divers approach, although at other times they do not exhibit aggressive behaviors.