Part of a lecture about sharks — what they are, how their bodies

Section 4. You will hear part of a lecture about sharks — what they are, how their bodies work, and the trouble they are in. First, you have some time to look at questions 31 to 36. Now listen carefully and answer questions 31 to 36. WOMAN: Good morning. Today's subject is sharks, animals that have been swimming in our oceans for a truly staggering length of time, some four hundred million years, which means sharks are older than trees, and older even than the dinosaurs. Let me start by placing them. Sharks are a kind of fish, but they differ from ordinary fish in one fundamental way: an ordinary fish has a skeleton of bone, whereas a shark's skeleton is made entirely of cartilage, the same light, flexible material that stiffens your own ears and nose. This makes a shark lighter in the water and helps keep it afloat. WOMAN: Their senses are extraordinary. A shark's sense of smell is famously acute, it can detect a single drop of blood diluted in a vast volume of water. But the sense that fascinates biologists most is one we don't share at all: sharks can sense the faint electricity given off by the muscles of other animals, through tiny pores in the skin around the snout. In murky water, or even when prey is buried in the sand, a shark can find it by this electrical sense alone. WOMAN: Now, their teeth. A shark's teeth are not fixed for life as ours are, they sit in rows, and when a front tooth breaks or falls out, a new one simply rotates forward from the row behind to replace it, like a conveyor belt. A single shark may get through many thousands of teeth in a lifetime, which is why fossil shark teeth are so commonly found. Before you hear the rest of the lecture, you have some time to look at questions 37 to 40. WOMAN: Let me correct a popular myth. Films have given sharks a reputation as monsters that hunt people, but this is badly wrong. Attacks on humans are extremely rare, and of the more than five hundred species of shark, the great majority are harmless to us, indeed many are small, and the two largest of all, the whale shark and the basking shark, eat nothing but tiny plankton, filtered from the water as they swim. WOMAN: I want to end on a serious note, because the real story is the reverse of the myth. It is not sharks that threaten us, but we who threaten them. Vast numbers are killed each year, many caught only for their fins, and because sharks grow slowly and produce very few young, their populations recover extremely slowly once reduced. A great many species are now endangered. And this matters for the whole ocean, because as top predators sharks keep the populations below them in balance, remove the sharks, and the entire food web can unravel. Protecting them, then, is not sentiment, it is about the health of the sea itself. That's where we'll stop. That is the end of Section 4. You now have some time to check your answers.