Hammerhead shark

A scalloped hammerhead, one of eleven living representatives of the Sphyrnidae.

**Hammerhead sharks**, or the **Sphyrnidae**, are a family of sharks in the order Carcharhiniformes. Named for their characteristic, laterally flattened heads formed into what is known as a cephalofoil, most hammerheads belong to the genus Sphyrna, while one, the winghead shark, belongs to the genus Eusphyra, of which it is the sole member. Many not necessarily mutually exclusive functions have been proposed for the cephalofoils of these sharks, including sensory reception, manoeuvering, and prey manipulation. Hammerheads are found worldwide in warmer waters along coastlines and continental shelves. Unlike most sharks, several species of hammerheads usually swim in schools during the day, becoming solitary hunters at night. Some of these schools can be found near Malpelo Island in Colombia, Cocos Island off Costa Rica, and near Molokai in Hawaii. Additionally, large schools are also seen in the waters off southern and eastern Africa. Though many hammerheads are large and potentially dangerous, very few attacks are recorded for any species, save for the largest; the great hammerhead (*Sphyrna mokarran*), which has been known to attack humans on occasion. This family also contains the only known omnivorous shark; the bonnethead, which eats aquatic vegetation in addition to fish, mollusks and invertebrates.

## Description

Hammerheads range in size from 0.9 to 6.0 m (3.0 to 19.7 ft) in length and weigh from 3 to 580 kg (6.6 to 1,278.7 lb). They are usually light gray in color and have a greenish tint. Their bellies are white which allows them to blend into the ocean when viewed from the bottom and sneak up on their prey. Their heads have lateral projections which give them a hammer-like shape.

Hammerheads have disproportionately small mouths and seem to do a lot of hunting on or near the bottom. They are also known to form schools during the day, sometimes in groups of over 100 or more. In the evening, like other sharks, they become solitary hunters. *National Geographic* explains that hammerheads can be found in warm tropical waters, but during the summer, many species of hammerheads participate in a mass migration to search for cooler waters.

## Taxonomy and Evolution

Since sharks do not have mineralized bones and rarely fossilize, their teeth alone are commonly found as fossils. The hammerheads seem closely related to the carcharhinid sharks that evolved during the mid-Tertiary period. According to DNA studies, the ancestor of the hammerheads probably lived in the Miocene epoch about 20 million years ago.

Using mitochondrial DNA, a phylogenetic tree of the hammerhead sharks showed the winghead shark as its most basal member. As the winghead shark has proportionately the largest "hammer" of the hammerhead sharks, this suggests that the first ancestral hammerhead sharks also had large hammers.

### Cephalofoil

The theory has been advanced that the hammer-like shape of the head may have evolved (at least in part) to enhance the animal's vision. The positioning of the eyes, mounted on the sides of the shark's distinctive hammer head, gives the shark good 360° vision in the vertical plane, meaning they can see above and below them at all times. The shape of the head was previously thought to help the shark find food, aiding in close-quarters maneuverability and allowing sharp turning movement without losing stability. However, it has been found that the unusual structure of its vertebrae was instrumental in making the turns correctly, more often than the shape of its head, though it would also shift and provide lift. From what is known about the winghead shark, the shape of the hammer-head apparently has to do with an evolved sensory function. Like all sharks, hammerheads have electroreceptory sensory pores called ampullae of Lorenzini. By distributing the receptors over a wider area, like a larger radio antenna, hammerheads can sweep and scan for prey more effectively.

## Reproduction

Reproduction occurs only once a year for hammerhead sharks, and usually occurs with the male shark biting the female shark violently until she agrees to mate with him. Documentation of hammerhead courtship is rare and has only been caught on film only a scant few times. The hammerhead sharks exhibit a viviparous mode of reproduction with females giving birth to live young. Like other sharks, fertilization is internal, with the male transferring sperm to the female through one of two intromittent organs called claspers. The developing embryos are at first sustained by a yolk sac. When the supply of yolk is exhausted, the depleted yolk sac transforms into a structure analogous to a mammalian placenta (called a "yolk sac placenta" or "pseudoplacenta"), through which the mother delivers sustenance until birth. Once the baby sharks are born, they are not taken care of by the parents in any way. Usually, a litter cosists of 12 to 15 pups, except for the great hammerhead, which gives birth to litters of 20 to 40 pups. These baby sharks huddle together and swim toward warmer water until they are old enough and large enough to survive on their own.

In 2007, the bonnethead shark was found to be capable of asexual reproduction via automictic parthenogenesis, in which a female's ovum fuses with a polar body to form a zygote without the need for a male. This was the first shark species known to reproduce in this manner.

## Diet

Hammerhead sharks are known to eat a large range of prey including fish, squid, octopus, crustaceans, and other sharks. Stingrays are a particular favorite of many species. These sharks are often found swimming along the bottom of the ocean, stalking their prey. Their unique heads are used as a weapon when hunting down prey. The hammerhead shark uses its head to pin down stingrays and eats the ray when the ray is weakened and in shock. The great hammerhead, tending to be larger and more aggressive than most hammerheads, occasionally engages in cannibalism, eating other hammerhead sharks, including its own young. The bonnethead, unusually for hammerheads and for any shark in general, has been revealed to be omnivorous, with upwards of 54% of its diet comprising seagrass.

## Species

Ten living species of hammerhead are known within two genera (two species of which are currently new to science, with another five extinct species known from fossils. Species denoted with a "†" are classified as extinct.

* Genus Eusphyra
	+ *Eusphyra blochii* (winghead shark)
* Genus Sphyrna
	+ *Sphyrna corona* (scalloped bonnethead)
	+ *Sphyrna couardi* (whitefin hammerhead)
	+ *Sphyrna gilberti* (Carolina hammerhead)
	+ *Sphyrna lewini* (scalloped hammerhead)
	+ *Sphyrna media* (scoophead)
	+ *Sphyrna mokarran* (great hammerhead)
	+ *Sphyrna tiburo* (bonnethead)
	+ *Sphyrna tudes* (smalleye hammerhead)
	+ *Sphyrna zygaena* (smooth hammerhead)
	+ † *Sphyrna arambourgi*
	+ † *Sphyrna gibbesii*
	+ † *Sphyrna integra*
	+ † *Sphyrna laevissima*
	+ † *Sphyrna magna*

## Relationship with Humans

Of the ten known species of hammerhead, only three are known to be particularly dangerous to humans: the scalloped, great, and smooth hammerheads. As of 2013, 33 attacks have been recorded, with no fatalities.

The great and the scalloped hammerheads are listed on the World Conservation Union's (IUCN) 2008 Red List as endangered, whereas the smalleye hammerhead is listed as vulnerable. The status given to these sharks is as a result of overfishing and demand for their fins, an expensive delicacy. Among others, scientists expressed their concern about the plight of the scalloped hammerhead at the American Association for the Advancement of Science annual meeting in Boston, Massachusetts. The young swim mostly in shallow waters along shorelines worldwide to avoid predators.

Shark fins are prized as a delicacy in certain countries in Asia (such as China), and overfishing is putting many hammerhead sharks at risk of extinction. Fishermen who harvest the animals typically cut off the fins and toss the remainder of the fish, which is often still alive, back into the sea. This practice, known as finning, is lethal to the shark. Hammerheads are themselves among the most common victims of such practices.

In Native Hawaiian culture, sharks are considered to be gods of the sea, protectors of humans, and cleaners of excessive ocean life. Some of these sharks are believed to be family members who died and have been reincarnated into shark form. However, some sharks are considered man-eaters, also known as *niuhi*. These sharks include great white sharks, tiger sharks, and bull sharks. The hammerhead shark, also known as *mano kihikihi*, is not considered a man-eater or *niuhi*; it is considered to be one of the most respected sharks of the ocean, an *aumakua.* A similarform of respect is shown for the tiger shark. Many Hawaiian families believe that they have an *aumakua* watching over them and protecting them from the *niuhi*. The hammerhead shark is thought to be the birth animal of some children. Hawaiian children who are born with the hammerhead shark as an animal sign are believed to be warriors and are meant to sail the oceans. Hammerhead sharks rarely pass through the waters of Maui, but many Maui natives believe that hammerhead sharks swimming by is a sign that the gods are watching over the families, and the oceans are clean and balanced.

### Hammerheads in Captivity

Great hammerheads have been kept in captivity at several facilities, including Atlantis Paradise Island Resort (Bahamas), Adventure Aquarium (New Jersey), Georgia Aquarium (Atlanta), Mote Marine Laboratory (Florida), and the shark reef exhibit at Mandalay Bay (Las Vegas). Aquarist Dean Trinh has successfully raised scalloped hammerhead pups, which he has claimed were collected by hand from an environment in which they might otherwise face the prospect of starvation. Other aquarists have noted the importance of providing adequate room for hammerheads to swim constantly and for the provision of mechanical filtration systems to maintain suitable water quality for shark survival. Bonnetheads, the smallest species, fare exceedingly well in captivity due to their familiarity with professional public aquarists and their small size. In 2015, the proprietors of the nightclub Atlantis in Adelaide, South Australia, announced plans to keep two hammerhead sharks in a 3-metre (10 ft) diameter cylindrical tank on their premises. The proposal prompted animal-rights activists to start a petition calling for the abandonment of the idea. The petition attracted 35,000 signatures in two weeks.

## Conservation

In March 2013, three endangered commercially valuable sharks, the hammerheads, the oceanic whitetip, and porbeagle, were added to Appendix II of CITES, bringing shark fishing and commerce of these species under licensing and regulation.

## In Popular Culture

Hammerheads are among the most recognized of sharks due to their heads. They are often featured in numerous documentaries on sharks, including episodes of Shark Week. Several films that feature hammerheads include Shark Tale, Shark Night 3D, Finding Nemo, Flipper, the Pirates of the Caribbean trilogy, and Hammerhead: Shark Frenzy. They are often depicted as symbols in public media, and in innumerable books regarding sharks and the sea.