Hammerhead sharks (Final draft). C. vg. (http://pages.eidosnet.co.uk/~imageworld/index.htm?greathammer.htm&1) A great hammerhead shark Introduction: This essay was written to convince people that hammerhead sharks belong in our oceans today. Sharks have been roaming them over 350 million years, 100 million years before the age of the dinosaurs. We should not even think to rid them of the undersea worlds we have today. Hammerheads are some of a few sharks that are starting to die out What did they do to deserve that? I find hammerhead sharks fascinating because of the unique shape of their head. How do they use it? Does it give them an advantage over other sharks? Read on to find out. The hammerhead shark There are nine different species of hammerhead sharks. They are the only species of shark with the hammer shaped head, hence their name. The specific shape of their heads allows them to have more in depth vision and to navigate using magnetic fields. Hammerhead sharks have been around for 25 million years, and are well adapted to prove it: They have large brains and a high degree of leaning ability, both characteristics of which are more commonly found among mammals.
Hammerhead sharks are members of the Sphyrnidea family, and so far, there are nine species in this group. Size Hammerhead sharks range in size from three to over 20 feet, and their weight can be up to 1,000 pounds. Hammerheads are a grey-brown color on top, and lighter on their stomachs. This coloration scheme is popular in most predatory animals due to camouflage: light-colored stomachs match the light sky above, and when prey look down, their backs match the dark depths below, helping them to stay invisible until the moment they attack. Senses All sharks have what people describe as a sixth sense. This sense is the ability to sense electric fields. When a fish, for example, just moves its gills, or moves it s fin, it is changing the electric fields in the water surrounding it, which is what the sharks can detect, because of gel-filled canals called the ampullae of Lorenzini in the shark s head. Hammerheads can sense electric signals as low as half a billionth of a volt. These canals are also heat sensory, meaning they can sense the heat every animal gives off, by changing them into electric signals: kind of like our eyes, which perceive everything upside-down until the occipital lobe in our brain flips the image around. They also have an excellent sense of smell. The fact that their nostrils are so far apart is thought by some scientists to be the reason they have a better sense of smell than most sharks. Habitat Hammerhead sharks are found all over the world, usually in warm temperate waters, on continental shelves where the water less than 65 feet (20 meters) deep. Hammerheads are rarely found in tropical waters. In the summer months, these sharks migrate to cooler waters, and are often found near the surface when they swim in open the oceans. Prey Their prey includes fish, cephalopods, other sharks and stingrays, which seem to be their favorite. Hammerheads are apparently immune to their stings; they use their head to pin down the ray while taking bites. Hammerheads are often stabbed in the throat
by these animals, which is another reason scientists think the eyes are positioned so far apart. These sharks have also been known to be cannibalistic. Hammerheads have extremely slow growth rates, contrary to the usual belief, because they have a spiral intestinal valve. This valve is like a spiral staircase, and food cannot pass through it very quickly, meaning hammerhead sharks cannot feed very often. Traveling Hammerheads travel in groups called schools of up to 500 sharks. The thought reason for these schools is thought to be for protection. There is believed to be a dominance order in these groups based on age, size and gender. There is a visible difference between male and female hammerhead sharks: females tend to be larger, as is the case in most shark species. When traveling in these shoals, large female hammerhead sharks position themselves at the center, while the males, and smaller females stay swimming on the outer edges. This pattern is for mating, which is explained in the next section. Mating habits Since the female sharks in this species outnumber the males 6 to 1, the males always have a chance to mate with a healthy strong female, which they know will be at the center of the shoal. In the evening, the sharks then swim off in pairs to mate, although sometimes a whole pack of males can be going after one female. The male wraps himself around the female, and holds on to her with his teeth, from which females have visible scars. This is why females in most shark species have skin, three times thicker than male sharks. Reproduction The eggs are fertilized inside the female and the pups are then born live, something that is unusual for sharks: Other species of shark are born in purse shaped egg cases called mermaid purses, where the embryos live in for a number of weeks off the egg yolk before breaking out. These egg cases got their name from people who found them washed up on shore, thinking these purses belonged to mermaids. Female hammerheads are usually pregnant from 10-12 months, before they swim off to shallow waters to give birth to live pups.
These pups are born ready to fend for themselves against hungry predators: This includes their own mother, who will eat them if they are not quick enough to leave. One female can give birth to 15-30 pups at one time. That was what scientists thought, but a catch on May 23, 2006 of a pregnant female off Boca Grande in Florida made them think otherwise: a female weighing 1,280 pounds, a worldrecord, was found to be carrying 55 pups. Hammerhead sharks are slow to reach sexual maturity, and so even though they can have a large amount of pups at one time, not all of them will survive until they are able to mate, since hammerheads have many predators when they are young. Hammerheads often die of starvation when they are that age as well. Human threats Hammerhead shark numbers are dwindling today, and some species of hammerhead sharks are facing the threat of extinction. Populations have declined 89 percent since 1986. The main threats to hammerheads are humans. Only small infant sharks have natural predators, other sharks being one of them. Humans have been hunting sharks for a very long time, sometimes for understandable reasons such as food, but other times not. Examples include hunting sharks for sport, fear, and most of all greed: shark fisheries, that usually only hunt sharks for their fins, dry the fins and sell them for a fin-soup, which is considered a delicacy in some countries and can cost up to $100 a bowl. Of each shark the shark fisheries catch, they only use the sharks fin, which they cut off mercilessly and throw the rest of the shark back into the sea, sometimes alive, where it has to endure a slow and agonizing death. At first, it was thought sharks never got cancer, and that their liver oil would be valuable to us as a cure for cancer. This turned out to be a hoax, since sharks do in fact get cancer, especially cartilage cancer. But until that was discovered many sharks were killed: a large shark cartilage factory in Costa Rica, which was owned by Americans killed more than 111,000 sharks per year, and an estimated 200,000 sharks per month in American waters, for the cancer curing oil they contained. It is estimated that humans kill 26 to 73 million sharks only for their fins every year. Among these are many of the larger species of hammerheads.
Changing human attitudes Some people think that fish, which includes sharks, do not feel pain: but many citizens, scientists and more, believe all animals do, and therefore we cannot allow this shark-slaughter to go on. Humans pose more of a threat to sharks then vise versa, and have more of a chance of dying from a bee sting, lightning strike or a snake bit than from a shark attack. When we enter the water, we should be more afraid of drowning then being attacked by a shark. It is extremely important that everybody is aware of the importance sharks play in the undersea ecosystems. Without sharks, the whole food chain could go out of whack: there would be an overpopulation of their prey, causing all sorts of abnormalities, such as not enough plant life, or other prey that the shark-prey usually feed on to name one example. Millions of years of evolution have made the shark an extremely well adapted animal with a very important role at the top of the food chain. Sharks also help clean up the ocean: they are the animals that keep the population healthy by eating dead or sick animals. Conclusion Hammerheads, as are all sharks, play an important role in undersea life. It is essential that we understand the importance of sharks in the ocean ecosystems, and stop pushing them toward extinction. Without sharks in our oceans, undersea worlds would just not be complete.