What's it all about? Shark

Ivy Susanti, The Jakarta Post, Jakarta

Sharks are fish without bones, so they are grouped together with rays, skates and ratfish in the cartilaginous fish (Elasmobranchii) subclass. They live in waters all over the world, in every ocean and even in some rivers and lakes. There are about 368 different species of sharks, which are divided into 30 families.

A shark's skeleton is made of cartilage, a tough fibrous substance that is softer than bone. Unlike bony fish, sharks do not have a swim bladder.

The oceanic whitetip shark (Carcharhinus longimanus), which measures up to 3.9m, is regarded as a dangerous species.

Size and shape

Sharks range widely in size -- some species are as small as a person's hand while others are larger than a bus. Full-grown sharks range in length from 18-centimeters (Spined Pygmy shark) up to 15m (Whale shark). Most sharks are intermediate in size and have human-like dimensions, ranging from 1.5 to 2.1m in length.

The largest living shark -- also dubbed the biggest fish -- is the whale shark (Rhincodon or Rhiniodon typus). Next largest is the basking shark (Cetorhinus maximus), which is about 12.3m long. Both are filter feeders and sieve enormous amounts of plankton through their gills as they swim.

Sharks have a variety of body shapes but most of them have streamlined, torpedo-shaped bodies that glide easily through the water. Bottom-dwelling species like the angel shark have flattened bodies that allow them to hide in the sand on the ocean bed. Some sharks have an elongated body shape (cookiecutter sharks and wobbegongs). Sawsharks have elongated snouts, thresher sharks have large elongated upper tail fins which are used to stun prey, and the hammerheads have extraordinarily wide heads. The goblin shark has a large, pointed protuberance on its head but its purpose is still unknown. (see anatomy chart).

Teeth

Sharks may have as many as 3,000 teeth at any one time. Most sharks do not chew their food but gulp it down whole in large pieces. The teeth are arranged in rows. When one is damaged or lost, it is soon replaced by another. Most sharks have about five rows of teeth with the front set the largest and the one that does most of the work.

Habitat

Pelagic sharks live in the open ocean and include the great white shark and the basking shark. Benthic sharks are those that live on the ocean floor, and count among their number the zebra horn shark, the wobbegongs and the angelshark.

The other species live in relatively warm waters (hammerheads, bull sharks and tiger sharks). The thresher, mako, basking and blue shark live in temperate waters. Others, like the dogfish, Greenland and goblin live in cold waters.

Sharks can migrate distances exceeding 1,600 km. The blue shark (Prionace glauca) can migrate some 2,000 to 3,000km in its seasonal journey from the U.S. state of New York to Brazil.

How to avoid shark attack

If you want to reduce the risk of becoming a victim of shark attack, bear in mind the following rules from senior biologist in ichthyology (the study of fish) at the Florida Museum of Natural History, University of Florida, George Burgess, in his book The Kids' How to Do (Almost) Everything Guide:

1. Always swim in a group. Sharks most often attack lone individuals. 2. Don't wander too far from shore. By doing so, you isolate yourself and place you away from assistance. 3. Avoid the water at night, dawn or dusk. Many sharks are most active at these times and are better able to find you than you are to see them. 4. Don't enter the water if bleeding. Sharks can smell and taste blood, and trace it back to its source. 5. Don't wear shiny jewelry. The reflected light looks like shining fish scales. 6. Don't go into waters containing sewage. Sewage attracts bait fish which in turn attract sharks. 7. Avoid waters being fished and those with lots of bait fish. Diving seabirds are good indicators of such activities. 8. Don't enter the water if sharks are present. Leave immediately if sharks are seen. 9. Avoid an uneven tan and brightly colored clothing. Sharks see contrast particularly well, so use extra caution when waters are cloudy. 10. Don't splash a lot. Also, keep pets out of the water. Erratic movements can attract sharks. 11. Use care near sandbars or steep drop-offs. These are favorite hangouts for sharks. 12. Don't relax just because porpoises are nearby. Sightings of porpoises do not indicate the absence of sharks. Both often eat the same foods. 13. Don't try to touch a shark if you see one! 14. If attacked by a shark, the general rule is "Do whatever it takes to get away!" Some people have successfully chosen to be aggressive, others passive. Some yelled underwater, others blew bubbles. Burgess says he would go down fighting.

Types of Shark Attack

Burgess, also the Director of the International Shark Attack File, says that sharks occasionally attack humans, but not for food. They sometimes grab people by mistake or in a bid to protect their territory.

He says there are two kinds of shark attack: provoked and unprovoked. The yearly average of unprovoked shark attacks on humans is 75, resulting in about 10 deaths. But these worldwide numbers are considered small given the millions of humans that enter the water.

* Provoked attacks happen when you touch sharks. They often involve unhooking sharks or removing them from fishing nets. However, there have been a number of incidents involving divers who were attacked after grabbing or feeding sharks while underwater.

* Unprovoked attacks happen when sharks make the first contact. They can take three forms:

Hit and run attacks that take place near beaches where sharks try to survive by capturing fish. In pounding surf, strong currents and murky water, a shark may mistake the movements of humans, usually at the surface, for those of their normal food, fish. The shark makes one grab, lets go, and immediately leaves the area. Legs or feet are often bitten; injuries are usually minor and deaths rarely occur.

Sneak attacks take place in deeper waters. The victim doesn't see the shark before the attack. The result can be serious injury or death, especially if the shark continues attacking.

Bump and bite attacks happen when the shark circles and actually bumps the victim with its head or body before biting. As in the sneak attack, the shark may attack repeatedly and cause serious injury or death.

Sources: www.sharkattacks.com, www.enchantedlearning.com, www.greatwhite.org, www.megmawl.com, The Florida Museum of Natural History.