4．
Sharks have lived in the oceans for over 450 million years, long before dinosaurs appeared. There are now about 360
of sharks, whose size, behavior, and other characteristics differ widely.
Every year, we catch and kill over 100 million sharks,
for food and for their fins. Dried shark fins are used to make shark fin soup, which sells for as much as $50 a bowl in fine Hong Kong
. Other sharks are killed for sport and out of fear. Sharks are vulnerable(脆弱的) to overfishing because it
most species 10 to 15 years to begin reproducing and they produce only a few offspring.
by movies and popular novels, most people see sharks as people-eating monsters. This is far from the
. Every year, a few types of shark injure about 100 people worldwide and kill about 25. Most
are by great white sharks, which often feed on sea lions and other marine mammals. They sometimes mistake human swimmers for their normal prey,
if they are wearing black wet suits.
If you are a typical ocean-goer, your
of being killed by an unprovoked （无缘无故的）attack by a shark are about 1 in 100 million. You are more
to be killed by a pig than a shark and thousands of times more likely to get killed when you drive a car.
Sharks help
human lives. In addition to providing people with food, they are helping us learn how to
cancer, bacteria, and viruses. Sharks are very healthy and have aging processes similar to ours. Their highly effective immune system
wounds to heal quickly without becoming infected, and their blood is being studied in connection with AIDS research.
Sharks are among the few animals in the world that almost
get cancer and eye cataracts. Understanding why can help us improve human
. Chemicals extracted from shark cartilage have killed cancerous tumors in laboratory animals.