History

Evolution of the species

Sharks are 450 million years old and have been on this planet longer than almost any other animal. They have lived through every major mass extinction event and have survived long past many of their competitors. With over 3,000 species spanning nearly half a billion years, sharks are one of the most evolutionarily successful species to ever live.

Hammerhead Shark probably appeared in Earth's oceans about 20 million years ago and were as large as some contemporary hammerhead sharks. To date, there are over 529 different species of sharks registered and over 10 species of Hammerhead Sharks have been recognized.

Species Description

Sphyrna mokarran is the scientific name of the Great Hammerhead Shark. The etymology of Sphyrna is Greek: sphyrna means "hammer", mokarran is an arabic name meaning "great".

It is Wilhelm Peter Eduard Simon Rüppell (1794-1884), a German naturalist explorer, that first described the species in 1837. He led two major expeditions to northeast Africa and helped create the Senckenburg Natural History Society in 1817.

During his second expedition, from 1831 to 1834, near Massaoua (Eritrea) on the Red Sea, he observed a stranded hammerhead shark on a strip of sand while chasing a prey. Eduard Rüppell noticed that it was different from the first four species inventoried by the French zoologist Achille Valenciennes and gave it the Arabic name of mokarran. It was a 2.92 m male which was released alive. In his description taken from his book Neue Wirbeltiere zu der Fauna von Abessinien gehörig of 1840 (page 66) Eduard Rüppell writes:

"This hammerhead shark that I found in the Red Sea does not match any of the four species that Mr. Valenciennes described in his ninth volume of the Memoirs of the Museum. The shape of its head is so different ...".

He then described the differences between his mokarran and the Zygaena malleus.

In the same year, Eduard Rüppell described and drew four other sharks: Rhizoprionodon acutus (Milk Shark), Triaenodon obesus (Whitetip Reef Shark), Carcharhinus albimarginatus (Silvertip Shark) and Negaprion acutidens (Lemon Shark).

Biology

Physical Characteristics

The Great Hammerhead Shark is the largest of all hammerhead shark species : the maximum total size ever recorded is reported as 610 cm, though 350 to 400 cm is more common for a mature adult. Like most sharks, female Great Hammerheads tend to grow larger than males. The weight ranges between 200 to 500kg. The heaviest individual ever recorded weighed up to a massive 580kg.

It belongs to the Sphyrnidae family. They get their name from their laterally expanded, dorsal–ventrally compressed head, referred to as the cephalofoil. For S.mokarran, it is wider than any other hammerhead shark with a straight and smooth front edge making it more distinguishable from its peers.

Great Hammerhead Sharks have a set of well-evolved fins that allow the massive fish to navigate swiftly in the ocean waters. The long-sickle-shaped dorsal fin and a pair of large pectoral fins provide stability in water while the elongated tail fin acts as a propeller, capable of pushing the shark at speeds of up to 40km/h. It has a relatively large second dorsal fin and anal fins. The pelvic fins is also easily identifiable due to its concave rear margin.

More than any other shark, the Great Hammerhead is a master of senses. It uses over seven sensory systems (with touch and taste) to survive and wander the oceanic waters.

• The sense of vision is very fine-tuned: each eye is located at the end of the cephalofoil, allowing the shark a wide angle vision of its surroundings.
• The sense of hearing also proves to be essential, as sound travels faster in water than in air. Great Hammerheads can hear its potential prey from a distance of many kilometers.
• Its sense of smell is extremely acute. Because its nostrils are on the edge of the cephalofoil, it can sample a large amount of water to then follow the scent of blood in the water column.
• The lateral line system is unique to aquatic vertebrates. It consists in canals filled with fluid and hairs stimulated by water movement or pressure from a single direction. It allows the shark to determine the direction and rate of water movement.
• The cephalofoil concentrates Ampullae of Lorenzini, tubular structures located under and parallel to the skin on the ventral surface and posterior to the eyes. The Great Hammerhead has a larger amout as its snout is wider than any other sharks. Thousands of those receptors help the animal locate preys by detecting a difference in voltage due to the electric field generated by their activity.

Habitat and Distribution

The Great Hammerhead Shark is widely distributed in coastal and pelagic tropical waters. It inhabits deep waters, to depths of 300 m, shallow lagoons and coral reefs and and is considered an upper-trophic level consumer. Due to its known long-range movements it is likely an important mobile link species between ecosystems playing a major part in the functioning, structure, and stability of these systems.

The great hammerhead’s range typically spreads across the globe. They can be observed in ocean waters anywhere between latitudes of 40 N and 37 S. In the Pacific Ocean, Great Hammerheads are found from Southern California and Baja California to Peru and from Ryukyu islands, Japan, China to Australia in the South. The species is found dwelling in continental shelves throughout the Indian ocean. In the Atlantic ocean it occurs from Senegal to Morocco and the Mediterranean Sea in the east, to Uruguay till North Carolina, in the west including the Caribbean Sea and the Gulf of Mexico.

In French Polynesia, the Great Hammerhead Shark has only been observed in some atolls of the Tuamotu and occasionally in Tahiti:

They are known to migrate long distances (1200 km in 62 days) but little is known about their migratory patterns. Bridging large travelling distances, they can swim on one of its sides. This swimming method is called rolled swimming, where the shark rolls to its side and continues swimming forward. This posture reduces drag and conserves energy while the large dorsal fin is used to obtain lift. Great Hammerheads may use the rolled swimming method very often in order to keep their energy consumption rate under control.

Reproduction and life cycle

Males mature at about 225 to 270 cm, whereas females mature at about 210 to 300 cm.

S. mokarran is viviparous, meaning they give birth to fully grown pups like mammals. Litter size ranges from 6 to 42 pups after 11 months' gestation. The size at birth is between 50 to 70 cm with females breeding once every two years. Pups are born in late spring to summer in the Northern Hemisphere and between December and January off Australia. There is no parental involvement after birth.

To date, no Great Hammerhead nurseries have been discovered in French Polynesia, although the presence of large females at high intensity period in Rangiroa matches the calving period established in Australia.

Behaviour and Diet

It is a solitary species. Very little is known about their social behaviour in key steps of their life circle (migration, reproduction, growth). This wide range of distribution provides a variety of potential prey for the Great Hammerheads. They consume sting rays, crabs, squids, small bony fishes like sardines, catfish, croackers and box fish and are highly opportunistic animals. They have also been spotted feeding on smaller sharks such as Grey Reef Sharks (Carcharhinus amblyrhynchos) and Blacktip Sharks (Carcharhinus limbatus), especially in the Tuamotu.

Conservation status

The Great Hammerhead Shark Sphyrna mokarran, is an understudied shark species mainly due to its cryptic and migratory nature. However, several studies suggest that the species is globally subject to a variety of anthropogenic stressors.

Its proven vulnerability to human activities and the alarming evidence of the global decline of the Great Hammerhead populations, has led to its official listing on the IUCN Red List of Threatened Species. Since 2018, the species has been assessed as 'Critically Endangered'.

The Great Hammerhead Shark is captured on a global scale. It is both a targeted and a by-catch (i.e., non-targeted) species in both inshore and offshore fisheries, by industrial and artisanal fisheries. Shark fin trade is a major factor in the decline of shark populations worldwide. Due to their large size and high cartilage fiber content, common to all hammerhead sharks, Great Hammerhead fins are highly valued on shark fin markets, contributing significantly to the decline of the species. This high mortality rate, combined with the late sexual maturity of the species and the fact that females breed only once every two years, increases the risk of extinction of the species which has no time to regenerate.

The Great Hammerhead Shark population has declined significantly in the Indian Ocean, the Mediterranean Sea and the Atlantic Ocean. However, there is a lack of data available for the Pacific region. It is estimated that the global population of Great Hammerhead Sharks has declined by more than 80% in 70 years. Although data are still globally lacking, uncertainty remains about current levels of exploitation that are potentially similar to those of the scalloped hammerhead shark Sphyrna lewini, also assessed as globally 'Critically Endangered'. For these reasons, the Great Hammerhead Shark is, to date, also considered as 'Critically Endangered' (A2bd).