Hammerhead shark
The hammerhead sharks are a group of sharks that form the family Sphyrnidae, so named for
the unusual and distinctive structure of their heads, which are flattened and laterally extended
into a "hammer" shape called a cephalofoil. Most hammerhead species are placed in the
genus Sphyrna, while the winghead shark is placed in its own genus, Eusphyra. Many different,
but not necessarily mutually exclusive, functions have been postulated for the cephalofoil,
including sensory reception, manoeuvering, and prey manipulation. The cephalofoil gives the
shark superior binocular vision and depth perception.[2]
Hammerheads are found worldwide in warmer waters along coastlines and continental shelves.
Unlike most sharks, some hammerhead species usually swim in schools during the day,
becoming solitary hunters at night. Some of these schools can be found near Malpelo
Island in Colombia, the Galápagos Islands in Ecuador, Cocos Island off Costa Rica,
near Molokai in Hawaii, and off southern and eastern Africa.[citation needed]
Description[edit]
The known species range from 0.9 to 6.0 m (2 ft 11 in to 19 ft 8 in) in length and weigh from 3 to
580 kg (6.6 to 1,278.7 lb).[3][4] They are usually light gray and have a greenish tint. Their bellies
are white, which allows them to blend into the background when viewed from below, and sneak
up on their prey.[5] Their heads have lateral projections that give them a hammer-like shape.
While overall similar, this shape differs somewhat between species; e.g., a distinct T-shape in
the great hammerhead, a rounded head with a central notch in the scalloped hammerhead, and
an unnotched rounded head in the smooth hammerhead.[6]
Hammerheads have disproportionately small mouths compared to other shark species. They are
also known to form schools during the day, sometimes in groups over 100. 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, they participate in a mass migration to
search for cooler waters
.
Taxonomy and evolution[edit]
Since sharks do not have mineralized bones and rarely fossilize, only their teeth 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.[8]
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.[9] Fossils show that hammerheads might have evolved earlier during
the Paleocene.[1]
Cephalofoil[edit]
The hammer-like shape of the head may have evolved at least in part to enhance the animal's
vision.[10] The positioning of the eyes, mounted on the sides of the shark's distinctive hammer
head, allows 360° of vision in the vertical plane, meaning the animals can see above and below
them at all times.[11][12] They also have an increased binocular vision and depth of visual field as a
result of the cephalofoil.[2] 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. The unusual structure of its vertebrae, though, has been found to be 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 hammerhead
apparently has to do with an evolved sensory function. Like all sharks, hammerheads
have electroreceptory sensory pores called ampullae of Lorenzini. The pores on the shark's head
lead to sensory tubes, which detect electric fields generated by other living creatures.[13] By
distributing the receptors over a wider area, like a larger radio antenna, hammerheads can
sweep for prey more effectively.[14]
Reproduction[edit]
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.[15] 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 consists 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.[15]
Diet[edit]
Hammerhead sharks eat a large range of prey such as fish (including other
sharks), squid, octopus, and crustaceans. Stingrays are a particular favorite. 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 weak and in shock.[15] 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.[17] In addition to the
typical animal prey, bonnetheads have been found to feed on seagrass, which sometimes makes
up as much as half their stomach contents. They may swallow it unintentionally, but they are able
to partially digest it. This is the only known case of a potentially omnivorous species of shark.[18]
Relationship with humans[edit]
According to the International Shark Attack File, humans have been subjects of 17 documented,
unprovoked attacks by hammerhead sharks within the genus Sphyrna since 1580 AD. No human
fatalities have been recorded.[28]
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. The young swim mostly in shallow waters along shores all over the world 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.[29] This practice, known as finning, is lethal to the shark.
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, but others 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. 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 their swimming by is a sign that the gods are watching over
the families, and the oceans are clean and balanced.[30]
