Hammerhead Sharks


Misunderstood Monsters

Believe it or not, sharks are an advanced evolutionary species. Why wouldn't they be - they've been around for 25 million years. Hammerheads display characteristics that aside from mammals, few other animals exhibit. These characteristics include internal fertilization, live birth, large brains and a high degree of learning ability.

Sharks do not normally bite humans. Sharks will bite in self defense, although they must be provoked (for example - pulling their tale) to attack. Hammerheads, when left alone to swim the oceans, will avoid humans whenever they can.

What is a shark? A shark is a fish with a skeleton made almost entirely of soft, flexible cartilage where as you and I are made of hard bones. Since a shark is not made of bone at all, minerals deposit into their teeth and backbones making them as strong and as hard as bone.

Hammerhead sharks are classified into the order of Carcharhiniformes. That means that they are ground sharks, they have an anal fin and two dorsal fins without spines, five gill slits, their mouth is behind their eyes and they have a spiral intestinal valve. The hammerheads spiral intestinal valve is like a spiral staircase. Food passes through this valve extremely slowly. This slow rate means that hammerheads can not feed often, contrary to popular belief, and they have a very slow growth rate. Hammerheads five gills allow the shark to breathe by extracting oxygen from the water and then pushing it over the gill slits.

Hammerheads do not spend all day swimming around. If they did, they would use up a lot of their energy (imagine if we always had to keep walking - even in our sleep!). Instead they have a liver that is filled with oil which is lighter than water. This gives most sharks enough buoyancy to seem like their floating in the water.

The hammerhead, along with all other sharks, have evolved a large range of senses. Light in the ocean tends to lean toward the blue-green end of the spectrum. This is the range of colors hammerheads are most sensitive to, although they can see red light. Hammerhead sharks are also sensitive to low light because the ocean floor has little if any light. Hammerheads vision is also very sharp although testing has shown that they might be far sighted (they can not see up close).

Another superior sense hammerheads have is their hearing. Hammerheads can hear sounds in the entire range we can. But their specialty is low frequency vibrations like those made but a wounded fish. A hammerheads ear also contains canals used for balance and motion detection.

Hammerhead sharks are swimming noses. They have the ability to detect chemicals, like blood, in solution at concentrations as low as one part per million. Hammerheads swing their head from side to side like a metal detector to sample the water with their noses. Hammerheads will also swim in a zigzag pattern for miles tracking the scent of wounded prey, much in the same way a dog tracks a wild animal.

Also located in their noses are sensors called ampullae of Lorenzini that let the hammerhead detect very weak electrical fields, chemical and thermal changes in the water. They can detect an electrical field 0.01 microvolts per cm which is equivalent to a AA battery with terminals almost a mile apart. Hammerheads use this ability to detect prey buried in the ocean out of sight. Why? Every animal, even fish, have a heartbeat that produces a very weak electrical field. But the hammerhead can detect this electrical field and zoom in on otherwise hidden prey.

Another unusual thing hammerheads do is form schools. These schools can contain hundreds of individuals, with the largest known schools containing as many as 500. The reason why hammerheads school and other sharks do not is unknown. Hammerheads only school during the day. They break up at night to do their feeding. Because the schools contain mainly small to medium sized hammerheads, it is believed that they school to reduce the risk of predation from larger sharks. It is also believed that an order of dominance exists in the schools based on age, size and sex.

Hammerheads also have a method of communication. Scientists know of 9 different communications. One such communiation is when a large female hammerhead in the center of a school shakes her head violently back and forth. This motion sends out pulses in the water that smaller females respond to by swimming to the outside of the school. Scientists believe large females do this for mating. When the smaller females are forced to the outer edges of the school, the large female becomes the center of attention for males.

The strange T-shaped head design, which contains their eyes and nostrils, occurs in all 9 species of the hammerheads. There are several different theories on the odd shape of the hammerheads head. Some scientists say their head helps them in diving and gives them extra lift. One theory says that because hammerheads favorite meal is stingray, the odd shape of their head positions their eyes further apart, away from the stingrays painful tailbarb. Another theory says that because their eyes and nostrils are separated further than any other shark, they have an increased sight and smell ability. Yet more scientists reason that the malformed head might scare away predators.

Since the hammerhead have evolved for a long time, they use internal reproduction whereas most ocean dwelling animals reproduce externally. While a male mates with a female, the male bites her body and fins, possibly in an attempt to hold the female long enough for mating to occur. Hammerheads have live births with a litter size of generally 15-30. After the hammerhead gives birth, the baby hammerheads are on their own with no help from their mother.

Hammerhead sharks are distributed in tropical and warm temperate oceans, both coastal and insular.

A favorite meal of the hammerheads is the stingray. Their diet may also include fish, shrimp, squid, crabs, snails, sharks smaller than them and lobster.

The hammerhead is brown to gray on top with a lighter underbelly.

Large hammerheads are harder to study than small hammerheads. Large hammerheads are very fragile and do not survive well in captivity. For that reason, much of what we know about hammerheads is based solely on the smaller hammerheads.

All 9 species of the hammerhead belong in the family Sphyrnidae and the genus Sphyrna. The 9 species of hammerheads are scientifically known as S. zygaena (smooth hammerhead), S. lewini (scalloped hammerhead), S. mokarran (great hammerhead), S. couardi, S. tiburo (which consists of S. tiburo tiburo and S. tiburo vespertina and are also known as bonnetheads or shovelheads), S. media, S. corona, S. blochii, and S. tudes.

SCIENTIFIC NAME COMMON NAME
Sphyrna zygaena Smooth Hammerhead
Sphyrna lewini Scalloped Hammerhead
Sphyrna mokarran Great Hammerhead
Sphyrna tiburo Bonnethead or Shovelhead
Sphyrna media Scoophead shark
Sphyrna corona Scalloped Bonnethead
Sphyrna blochii Winghead shark
Sphyrna tudes Smalleye Hammerhead
Sphyrna couardi Whitefin Hammerhead

Sphyrna lewini or the scalloped hammerhead grows to be about 11 feet. They are gray on top and paler on their underbelly. Scalloped hammerheads live in tropical waters of both the Atlantic and Pacific. They can be found both inshore and offshore. The scalloped hammerhead has smooth-edged teeth with an indentation on the midline of their head.

The great hammerhead, S. mokarran, can grow to be as long as 19 feet in length although they are normally about 15 feet in length. Great hammerheads are an olive-brown color with a lighter underbelly. They live mostly offshore in the tropical and subtropical waters of the Atlantic and Pacific. The great hammerhead has a head which is almost entirely rectangular. The great hammerhead, as the name implies, is the largest of the hammerhead sharks.

Sphyrna zygaena, the smooth hammerhead, grows to be around 13 feet long. They tend to be mostly gray on top and lighter below. The smooth hammerhead can be found both inshore and offshore in the Pacific and Atlantic Oceans.

Bonnetheads, Sphyrna tiburo, also called shovelheads, are small hammerhead sharks. They are further divided into two groups: Sphyrna tiburo tiburo from the western Atlantic and Sphyrna tiburo vespertina from the eastern Pacific. Bonnetheads typically grow to be about 3.5 feet long. Bonnetheads are grayish-brown in color with light underbellies. They are normally found in shallow subtropical and tropical waters. Bonnetheads have round, shovel-shaped heads. They are normally found in small schools of about 20.

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