Spectrum; 23; Myths and Man-Eaters

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That was a young lemon shark. His main diet is fish, but in a pinch, he and others of his species will eat practically anything, including man. Sharks have been of absorbing interest to semen and seaside dwellers in general for untold generations. Their staring eyes and savage teeth have inspired legend and myth since man first discovered their existence along some prehistoric sea coast. We're at the University of Miami Institute of Marine Sciences on Virginia Key.

I'm David Proud, Science Editor for National Educational Television. As a matter of fact, sharks predate men by tens of millions of years. Some experts have traced the origins of the shark back to the Devonian period about 275 million years ago. And yet, less is known about the behavior of this living fossil than any other major group of fishes. One reason for this is that, difficulty in studying the animal, a shark in captivity isn't the same shark found in the open sea. Another problem is simply the vast scope of the study. There are nearly 300 known species of shark. In every ocean in the world, they are found in the shallowest and deepest of waters. There are cold water sharks and warm water sharks. Although the majority seem to prefer the temperate and tropical seas. They range in size from the 60-foot long whale shark to a small but still carnivorous ten inch shark found out the coast of Cuba. This amazing range of size, behavior and activity has given birth to a massive popular misinformation about the shark.

A worldwide folklore based on centuries of fear and understandably hasty observation. It wasn't until 1958 when the United States Navy convened a meeting of shark experts in New Orleans that the first definitive studies of shark physiology and behavior began. We're visiting with one of those experts today, Dr. Warren J. Wisby of the University of Miami. Dr. Wisby's work is focused on the vision and hearing in the shark. Both of these areas are of vital importance in determining how the shark locates food over long distances and on a more pragmatic level what stimulus causes a shark to attack. And now let's go aboard the research vessel, Arius and meet Dr. Wisby. Hello, Dr. Wisby. Welcome aboard. Thank you. Have a seat. Thank you.

I'd like to talk to you about sharks. Fine. One of my favorite subjects. Dr. Tommy, what is it about sharks that make it unique for many other kinds of fish? Well, they differ from bony fishes primarily in that they do not have bones in their skeleton. They have cartilage in their skeleton. They have bony structures, very tiny ones in their skin, and their teeth are bony in structure, but this is the only place the body would occur. Their teeth also are unique in that as they wear out and get old, they drop out, and the new one comes into place, and the animal always has a new shark set of dentures. Also, I guess the fact that they don't have swimbladders, whereas most bony fishes have swimbladders. That would indicate that they have to keep moving. Right. They must swim in order to stay afloat when they stop swimming they sink. Tommy, are sharks naturally vicious? No, I think that this is an emotion, and we probably shouldn't describe emotions to sharks.

They are very efficient eating machines. They're well equipped to capture their prey and to eat it, and benefit from it, but probably not vicious. I understand that they're very hard to kill. Yes, they're extremely tenacious of life, if you catch one, for example. It is possible to cut it open, remove its liver, bait a hook with the liver, throw the carcass of the shark overboard, and soon thereafter catch the same shark on his own liver. On the other hand, in captivity, we have a great deal of difficulty keeping them alive, and they seem there to be among the more delicate of our animals. I think we probably don't yet know just how to treat them in captivity. So many stories about sharks are in circulation, one of which I think a favorite is that you can always spot when a shark is in the area by seeing its fin cut through the water. I think this is not very dependable. Most sharks do not swim at the surface. You would never see their fin and should not depend on seeing it,

in order to know whether there was a shark there or not. What about the idea that sharks always only exist in warm waters? No, sharks are found in northern waters as well as tropical waters, but the majority of species are found in tropical and temperate waters. There's another fairly widespread belief that the shark can only strike if it turns over on its belly. No, this is not true. Most sharks never do it. It would be unwise to depend on that pause if a shark were to approach. Dr. Peoples' attitudes about sharks are quite violent. They test them and won't go near them or in the case of skin divers, many skin divers, and many people who deal with the sea, they're almost unconcerned. Is this dichotomy in viewpoints? Have you found this to be true? Yes. In this country here, for example, some people fear to venture into the water while other people swim a little foolheartedly.

I think with sharks almost routinely. There are other cultures, for example, in the South Sea. There is a culture where the people believe that their ancestors made a pact with sharks. If they do not harm sharks, sharks will not harm them. And they routinely get into the ocean where they fish and spearfishes using coconut husks for flotation and apparently ordinarily do so with impunity. It does work. At least it seems to. Tell me, is there anything in all that you can say good about sharks? Do they serve men in any way? Well, they serve the same purpose in their environment as sharks, wolves, tigers, lions, that sort of thing. Do on land. They eat dead, injured, dying animals, and are therefore sometimes called scavengers. Most of the animals they eat are alive. In addition to this, their skin makes very, very useful leather products. Their fins are eaten for soup. The flesh is edible.

The teeth are sold in all these souvenir stores. They have many uses to make. There is a place in the ecology in the shark. Tell me, is there a shark that we could look at, doctor, and take advantage of your expertise and have you give us an anatomy lesson? Yes, we caught one yesterday and I'd like to show it to you. Thank you. Let's go right over this way. Dr. Wisby, what type of shark is this? This is a female of a great hammerhead. We have at least three species of hammerheads in these water. This one happens to get bigger than any of the others. It looks pretty big now. What's its full growth? This is about a 10-foot specimen and they will get 18 feet. Tell me, doctor, are these common in Florida waters? One of our more common sharks, yes. How fast do they go? I would guess their ordinary cruising speed would be from three to four knots and they could undoubtedly go four or five times that fast in a burst.

The skin of this shark is smooth one way and very, very sharp like sandpaper the other. What does a shark have embedded in their skin? Small bony structures, very hard called dendicov and correspond really to the scales of bony fishes. Could you describe the basic structure of the shark? Yes, I think so. It's obvious why this animal is called a hammerhead shark. The head is misshapen and formed into a hammer. Most sharks come to a point right here. On the ends of the hammer, we have two primary sensory structures, the eyes with the nicotating membrane and the olfactory aperture, which is here, there's one on the other side, and also on the other side, underside of the head, we can see the very large mouth with the many rows of teeth. What is the nicotating membrane for, doctor? The nicotating membrane, it is thought, operates as our eyelid does that is for protection of the eye.

It's rather hard, almost bony structure. What about the gills? Sharks have gill slits. This species happens to have five, and they are the excurrent canals, exhalant canals for the water. The water goes in through the mouth, out through the gill slits, passing over the gills, and the animal derives oxygen from that and is charged as CO2. That's different from a bony fish gills, isn't it? Yes, a bony fish would have an operculum here, a gill structure, and with only one open, through which the water would issue. What about all these fins? What purpose do they serve, doctor? The pectoral fins primarily serve the same function as the under part of the head, that is, as the animal swims, it holds it up in the water. The shark has no swim bladder, and would sink if it were to stop swimming. The other fins are primarily for maneuverability, except for the very large tail fin, which is used in forward propulsion by a sinuous movement of the animal's body that would drive it through the water.

Well, you've mentioned everything except the shark's hearing structure, doctor, and I know that you're a special interest. What about that? Yes, sharks have internal ears corresponding to the internal ears of some of the higher animals, with no external opening, however. But of more importance, I think, is the fact that they have down each side of their body, a lateral line system consisting of small nerve endings, and we think that by means of these nerve endings that the sharks detect underwater sound and vibrations. You're testing that now, are you not? Yes. As a matter of fact, we've made some films of our research on vision and on hearing, which you might like to see now. Why don't we look at them now? Fine. Dave, this is a film that was made by some of our staff members. The diver in the water there is going to take that hydrophone down to the bottom, and accompanied by a skin diver who will spear a fish. We're going to record the sounds made by fishes after their speared. The sounds are low-frequency bursts, and so low that humans can almost not hear them.

At a later stage of the experiments, we recorded also the sounds of people swimming, normally, regularly, people struggling in the water, people swimming in the scuba gear. And we find that the low frequency portions of struggling sounds of people sound like the low frequency portions of struggling sounds of groupers and other fishes. Now, we've taken the grouper sounds out into the open ocean, played them back into the water, and we find that we can almost without fail attract sharks from as far away as 300 feet. So, the sharks have a directional homing sense, Doctor? Yes, as a matter of fact, it's a very precise one. They may make an error initially of about five degrees, but this error they can easily correct as they swim along. We have an airplane in the air incidentally where we watch the sharks, and we position the boat from the airplane. We've not played these sounds to many, many sharks,

and almost invariably, as I said, they come into the low frequency sounds, produced by struggling fishes, but not to the other kinds of sounds that we play high frequency sounds, high frequency pulse sounds. What's this? And this is a view inside of the laboratory. We keep the sharks in that large tank you saw there. Now, we're looking at what we call our sound tunnel. This is essentially a rigid walled piece of plastic in the far one, and glass in the near one. We have a sound source at one end. You see it suspended in the air above the box. That sound source is a J9 hydrophone furnished by the US Navy. J9 transducer rather, and it is then lowered, clamped over the end of the tube so that we have a watertight seal in effect between the transducer and the tube.

It's through that that the sounds are placed. Right. The shark is placed in the middle of the tube, as you will see. This is about the only precarious part of the experiment that we have. These sharks are young and small, but they are fast and snapped viciously. This little animal will now be brought to an operating table, where an electrode will be inserted near his heart. Our procedure is essentially a Pavlovian conditioning procedure, and we use as the conditioned response the slowing down of the heart rate. The shark's heart normally beats at about one time per second. When he's very excited or very frightened, his heart skips a beat or two. The electrode is carefully placed so it doesn't injure the animal's heart and the sharks live rather well for a number of days with that insertion in place.

Now the next step will be to place him within that little loose site too, that you see there. We don't like to have him swimming around inside the tunnel because we must know exactly what the sound level is at the place where the shark is. So he's put down inside the tube. This is the heart electrode being attached to the recorder. And the other electrode has to do with the electric shock that we can administer in order to condition his heart rate. Now this shark is conditioned to when it hears a certain sound and he's shocked at the same time his heart rate slows down. Right, and then eventually when he hears the sound alone, his heart rate will slow down. And in effect he will be saying, I heard it. In this way we can manage to find out what their thresholds are

against various levels of background noise and also what the range of hearing is as far as frequency is concerned. Strangely enough we find that they hear best in the area where humans hardly hear at all. Probably they don't hear anything above four or five hundred cycles and hear best down around ten or twelve cycles. Very low indeed. This animal's heart is beating about the normal rate about one time per one and two tenth seconds or something like that. Set up the sound gear and project the picture of the sound as received by the hydrophone at the shark. And we compare this then with the sound as produced by the transducer. We insist that the images be identical and then we know that there's no distortion taking place. Now if you watch the little needle up in the upper right hand corner it will indicate when the sound is being played.

This frequency will be so low that we cannot hear it. There goes the sound as you can see. Now if you watch that strip of paper as it comes out from under the roller you'll see that that heartbeat is much longer than those that preceded it. Almost twice as long. Right and the following two actually are longer than normal. Now that means the shark has heard. He heard that sound right. Now the normal procedure would be that we would begin to attenuate the sound until we reached the point where he did not hear it. Then we would know this was his threshold. That's another one there. There's an excellent response. That's longer than normal as a matter of fact. It's almost three times as long as a normal response. We have here the eye of a shark. We're working also on color vision in sharks. We have here the eye of a shark which has been dark adapted. You can see that it's pupil is decreasing in size as we watch it.

We have been able to find cones in the retinas of all the kinds of sharks that we've examined and we therefore feel that we have circumstantial evidence to indicate that they can see color. Since this is the mechanism used by most animals in seeing color. What we must do of course then is to ask the animal whether he sees this image or this light as a color. Another addition to it. Exactly the same way we did before. Except that well here light levels are so low that we have to use a war surplus sniper scope so that we can watch them when the light levels get so low that we can no longer see. Their eyes are very well adapted for seeing in very very dim surroundings. Much less light is needed by sharks than is needed by humans. Now we have the animal in the apparatus and he's being shown a colored light and he's being conditioned to respond to it.

Is he receiving electric shock now doctor? No this shark is this is the blue light we're watching now and he has been conditioned not to respond to a blue light. Now we will watch the process a little farther and see that the same animal will respond to a red light and our experimental setup is such that it is not possible for the animal to react because of intensity. Did you see him wink his eye that time? Yes I did. That's the response he says I see it I see it. That's the same membrane that you showed on the hammerhead. Right that's the dictating membrane. Well after seeing the film's doctor was be the major impression that I get is that the shark's vision and hearing are ideally adapted to the murky refractive environment in which it lives but an environment that's also a fine sound conductor. That's right as matter of fact those two senses combined with their extremely keen sense of smell make them what they are a very efficient eating machine.

The functional animal. Exactly. Well another piece of functional equipment in the shark that we've spoken about before is his teeth and you seem to have quite a specimen over here. This is the jaw of a rather large shark. As you can see there are many many rows of teeth in the jaw. Not all of them are exposed however at the same time. The membrane comes up and covers the teeth up to about here. Now if for example this one were to fall out this one would move into position and all those behind it would shift so that they're ready to come up. You'd always have a brand new shark set on the outside. And this sulfur-newing teeth continue until the shark dies? That's right throughout its life. There seems to be a difference here doctor between the teeth and the lower jaw which are very straight and pointed and the teeth in the upper jaw which lean over somewhat and seem to have a serrated cutting edge on it. It's quite sharp. That's exactly right.

It is thought that the teeth in the lower jaw are primarily for grasping, seizing and holding while the teeth in the upper jaw with the rapid sidewards motion of the head enable the shark to cut off a piece of flesh from its victim. What are these teeth over here doctor? They seem to be different from the ones that we're looking at. Yes they're all from different species and teeth are important to know about the different shapes of teeth since often teeth are embedded in the flesh of victims of shark attack and sometimes the shape of the wound can even tell you what sick-shaped tooth made that wound. For example here we have the tooth of a bull shark and a batiger shark. This is a mecho shark, a very fast fish eating shark. This is a great white shark. Probably the largest of our carnivorous sharks and certainly the one that earned for sharks their reputations as man eaters. What's the big one on the end doctor? We primarily have it here to indicate that although our great white shark is a very large shark that his ancestors were much larger this is a relatively small tooth of a prehistoric shark

probably very closely related to the great white. Does the possession of these highly specialized teeth make the sharks feeding habits any different? Yes they do as a matter of fact a shark is not required to swallow its victim whole. It can take off a piece of an animal much larger than itself or certainly too large to swallow a whole and this is what makes it primarily dangerous to man. Well since it does have all these cutting teeth and they do look very sharp what do you feel is the status of current repellents, the chemicals and dyes that are used to repel sharks? Actually none of them are 100% satisfactory. Many of them will turn a cruising animal but it's very hard to stop suddenly a highly motivated animal swimming at full speed towards the victim. In our work we don't depend on the chemical repellents or on things of that sort.

We use instead a bank stick and if you'd like to see how it functions we'd be happy to demonstrate. I would doctor. I handed it to Mr. Gruber a graduate student of mine. He's going to load it with a shotgun shell. Mr. Nelson is holding the shark. When the bank stick is thrust suddenly against the shark's body at discharges and as you will be able to see this animal expires. And now we have a very docile easily handled shark. But that's the only way that we know right now to really stop one. It seems to be the most effective way as far as we can tell. Well Dr. Most Skin Divers, thank you. Most Skin Divers and Bathers won't carry bank sticks with them when they're in the water. What should a swimmer do when he's in waters where sharks are present?

As a matter of fact the best thing to do is to swim in very clear water where you are easily visible to the shark and where the shark is very easily visible to you. And if you see a shark in the water, manage to remove yourself from the water but by swimming with slow, regular, strong strokes, not giving the impression that you are in distress. If possible, it would be better to swim underwater rather than on the surface for the same reason. Fish is in distress, most often float and flop at the surface. What about bright clothing, Doctor? What are the colors that we shark see, influence their feeding habits? Well, there is circumstantial evidence that certain bright colors may attract sharks. This still must be subjected to experimental evidence. But in the absence of that evidence, I would suggest rather dull, drab clothing. And if possible, a rather uniform color, such that it is not banded into distinct shapes. One of the symptoms of distress of a fish, certain nervous disorders, certain kinds of injuries,

produce a strongly banded fish. It would be well not to provide this kind of stimulus, I think. Well, Doctor, with the increase in shark population and the fact that there really are no chemical repellents that are considered generally worthwhile, is it safe to go in swimming in the ocean? Well, undoubtedly, danger from shark attack is far less than the danger from things that everybody does every day, such as crossing streets or taking a bath. These things are probably far more dangerous than going in swimming. If you had to sum up in a few words what shark researchers have found out about shark behavior and what there still is to find out about these animals, what would you say? Well, we have learned far less than we need to learn. There's more to be learned than we have even approached.

But the gratifying thing is that with every little bit of added information we get, the behavior of the animal becomes more and more predictable. And we hope eventually to make the shark as predictable in animal as those land animals that we know so well. Thank you very much, sir. You've been very informative. I enjoyed it. Sharks, stranger than any myth they inspire, sea hunters that have earned their reputation as man-eaters. But beyond that, an animal almost perfectly fitted for its main job in life, locating and killing enough food to stay alive in the sea. Plans are already underway to open up the continental shells for man. In the future, they could provide undersea living space and farming room for a considerable percentage of the world's population. If this and other dreams for utilizing the sea or ever to become reality, we must learn to cope with the dangers this new environment will offer. High up on this list is the shark. That's why scientists are busy untangling the fact from the fiction about sharks and soon they may provide the answers that will change the shark

from a sinister threat lurking beneath the ocean's surface to as Dr. Wisby refers to them just animals making a living. This is David Proudit for any tea at the University of Miami's Institute of Marine Science. This is NET, the National Educational Television Network.