Classification
Kingdom Animalia
Plylum Chordata SubPhylum Vertebrata
Class Chrondrichthyes SubClass Elasmobranchii
Order Lamniformes
Family Lamnidae
Genus Carchardon
Species carcharias
People’s Fear of Sharks
Many people have a big fear of sharks, especially Great Whites. A lot of this comes from movies like Jaws - which is based on a number of attacks that really happened in New Jersey in 1916. Some people think that there are tons of sharks waiting to eat them if they venture out into the ocean, which is simply not true. Sharks rarely ever eat humans and are solitary animals. They either travel alone or in groups of two.
This image is from www.eng.utoledo.edu/~lcendere/gw/pics.htm
Shark attacks are quite rare, in the US there are two to three non-fatal attacks on swimmers, surfers, and divers per year. By the number of reported incidents, Dogs kill more people each year than Great Whites have killed in the last 100 years. There are lots of divers who swim with sharks and are not attacked. In fact, there are places where you can be guided on a dive with sharks after just one dive lesson. They get used to people and get quite tame. Sharks are misunderstood animals.
Physical Appearance
Great White Sharks are a very large species of shark. They are streamlined swimmers, and have a torpedo-shaped body with a pointed snout.
They have about 3000 teeth, arranged in several rows. The first two rows of teeth are used for grabbing and cutting prey, while the teeth in the last rows rotate into place when front teeth are broken, worn down, or fall out. The teeth are triangularly shaped with serrations on the edges.
The back of the shark is a dull grey colour and the underside is coloured white. They have three main fins: the dorsal (on back) and two pectoral fins (on the sides). The tail is crescent shaped. There are five gill slits on Great White Sharks.
Most of the largest sharks are found in South Australia. The largest one ever recorded was 6.4m (21ft) long, and weighed 3312kg! The maximum length able to grow is thought to be 25ft. Though some people claim to have seen sharks as long as 31ft. The smallest Great White shark caught was 47inches long, but was newly born.
An encounter with a Great White
In the magazine, "Sharks and Divers #1" (Sept. 1997) Jeremiah Sullivan had some great experiences swimming with Great Whites off South Australia. There are some amazing pictures, one of him holding the tail of a 16ft shark and him being towed around.
This is an exerpt of what he said:
"Soon, I was so close to him I could stroke his torso as he swam by – he seemed quite unconcerned. On his next pass I tested him further by hooking my right hand around the upper lobe of this tail. When his momentum jerked me into his slipstream, he didn’t so much as flinch…the only acknowledgement of my presence was a slight but very deliberate glance over his right side as he towed me out to sea…I turned and began swimming with all my strength towards the cage…I remember cursing myself for not calculating this risk a little more carefully…I turned towards the big boy…everything was quiet…He cruised up, looked at me sideways and appeared to say, ‘Oh, Jeremiah, it’s you!’"
Five more times that afternoon shark and man interacted. There were three Great Whites at the site that afternoon. The sharks had every chance to attack, but didn’t.
Food
Sharks are important predators in the marine ecosystem. Their favorite prey are seals and sea lions. As juveniles they eat fish, and rays. When they become full grown they eat marine animals such as whales, seals, dolphins, large tuna fish, sea otters, and dead animals that they find floating on the surface. In order to catch its food a shark will go along the sea bottom and look for shapes at the surface. If it sees something similar to the shape of a seal they charge full speed. They ram the prey and give it a first bite all in one motion, which stuns and injures the prey. It then disappears and allows the prey to bleed to death. When it’s certain the prey’s dead it begins to feed. Sharks don’t chew their food, they just rip it into mouth-sized pieces and swallow it whole. A big meal can last a shark up to two months
Habitat
Great White Sharks live in all coastal temperate waters, and have been known to occasionally make dives into the deep water of open oceans. They can be found in water as shallow as three feet deep, and as deep as 1280metres. They can be found on the following coaslines: California to Alaska, the east coast of the USA, most of the Gulf coast, Hawaii, most of South America, South Africa, Australia (except the north coast), New Zealand, Mediterranean Sea, West Africa to Scandinavia, Japan, and the eastern coastline of China to Russia.
Reproduction
These sharks are ovoniparous, they give birth to 2-14 fully formed pups which are up to 1.5m (5ft) long. Fertilization of the eggs occurs in the female, and later the eggs actually hatch within the female. The young are nourished by eating unfertilized eggs and smaller siblings in the womb. There is no placenta with which they can get nourishment from their mother; they must fend for themselves. The female gives birth to live young, unlike many other sharks who lay eggs. The newborn gets no help from its mother. As soon as it’s born it swims away to begin living its life. A newborn is about 4ft long, and it grows 25cm (10inches) each year, reaching maturity at 10 years.
Senses
Sharks have some of the most highly developed senses of any creature. Their primary sense is the ability to smell. The nostrils can smell a drop a blood in 100 liters (25 gallons) of water. Their next important sense is the ability to detect electric charges. They can pick up electrical charges as small as 0.005 microvolts. The prey can be detected by the electrical field generated by a beating heart or gill action. Fish in hiding can also be detected this way.
Some females migrate to warmer water to give birth in the fall.
A Shark in Captivity
Great White Sharks aren’t usually kept in captivity. Most of the time, they are brought in because they are injured or sick, and they do not usually survive long. One healthy shark, nicknamed Sandy, was brought into captivity in 1980. She was a 7.5ft Great White and was brought to the Steinhart Aquarium in San Francisco, USA. The aquarium was donut-shaped, and visitors watched from a central viewing area. The tank was set up for fast swimming fish, with a current going around against which the fish swam. It was a good environment for a young shark. The caretakers said there were many ups and downs to having her stay there. She showed little interest in divers who went down into her tank.
They soon noticed that she often bumped her nose against the same metal seams in the tank that stuck out about 4inches. So to solve this they put a plastic covering over them to flatten out the seams. Then they noticed that she swam differently at one spot in the tank. At first nothing was detected, but then they realized that there was a tiny change in electrical activity, only 0.125 millivolts – most other sharks and fish wouldn’t have noticed it. They concluded that the only way to fix the problem was to drain the tank, and she would die if she kept bumping her head, so they let her go back into the wild.
The ocean sunfish (mola mola) is the largest bony fish in the world. It is a unique pelagic fish, and specimens of ocean sunfish have been observed up to 3.3 m (11 ft) in length and weighing up to 2,300 kg (5,100 lb).
Curiously, the Ocean Sunfish has a very small brain compared to its size: A 200 kg specimen may have a brain the size of a nut weighing only 4 g. It would explain their quiet behaviour and how often they get trapped into fisher nets.
According to the National Geographic if it lives to adulthood, a sunfish can gain over 60 million times its starting weight: "the equivalent of a healthy, bouncing human baby growing to a weight equal to six Titanics".
The images of scuba divers next to a giant sunfish gives us a better idea of their incredible size. Fortunately, ocean sunfishes float serenely in midwater.
Ocean sunfish is generally thought to be solitary fish, swimming freely in the ocean alone. However, certain sightings of molas in groups of more than ten have been reported. Frequently, giant sunfishes lie inert on the ocean surface, apparently sunning themselves.
Sometimes, ocean sunfishes are caught accidentally by fishers nets and a crane is needed to lift them on board.
Another threat to ocean sunfish are floating plastic bags which look like their natural prey, jellyfish. Sunfish can choke as they try to swallow the bags, or slowly starve as the plastic clogs their stomach.
Recently, a Gaza fisherman trapped a giant sunfish. He threw his nets into the water and was surprised when the fish got stuck in five of his nets. His catch measured 3.5 metres or 11 feet in length, 1 metre in width and weighed one tonne. Many fishers have lived the same experience, and it seems pretty impressive.
Some years ago, Australian Museum acquired such a large specimen that they had to use ropes to haul it up and get it inside the building through a window. As you can see, the size of other specimens is as big as a parking place.
On the other hand, giant sunfishes are an important menace for little ships. A collision at high speed can cause both injury to sailors and damage to yachts.
