SHARKS DISLIKE THE YELLOW GEAR ----IT'S A SHARK TREND -------THEY DON'T SEE THE COLOR OF THE SURFER AND IS GEAR BECAUSE THEY ARE unable to discern colors because the retinas of the eyes of most species do not seem to have color-perceiving cones. OLD experiments conducted by Dr. Eugenie Clark indicated, however, that at least one shark was violently repelled by the color yellow. The experiments were performed at the Cape Haze Marine Laboratory at Sarasota, Florida. Dr. Clark was working with 8-foot Lemon sharks (Negaprion breviros- tris) enclosed in a pen next to a dock, trying to train them to push a "target" for food. One shark, trained to a white target, hungrily dashed toward it, as usual, one day. But Dr. Clark had substituted a yellow tar- get to test the shark's color perception. A few feet from the target. Dr. Clark reported, the shark whirled, did a back flip out of the water and then began going crazily around in circles. Transformed into what ap- peared to be a very neurotic shark, it refused to eat, and soon died. Did the mere sight of yellow do all this? Neither Dr. Clark nor any- one else knows. Certainly yellow isn't that repulsive to other sharks, for, during World War II, many yellow life-rafts were nudged and some- times attacked by sharks. Aristotle, a pioneer fish-watcher, said that fish could hear, "for they are observed to run away from any loud noises like the rowing of a gal- ley." There have been times when marine biologists were not as posi- tive as Aristotle that fish could hear, but in relatively recent times dis- coveries have been made which clearly demonstrate that fish can hear, and can discriminate pitch. Little, however, is known about the hearing of sharks in particular. There seems to be little doubt that Selachians can hear, or at least pick up vibrations accompanied by what humans sense as sound. Selachians respond to vibrations, such as the pulsations of a steamer's screws in the open sea, or the ringing of an underwater bell in a laboratory experimental tank. And they do appear to have ears- inside their heads. The question of how sharks can detect prey at considerable dis- tances has long fascinated both fishermen and marine biologists. Neither vision nor the sense of smell can explain some of the amazing prey- detection performances sharks have put on before observers' eyes. Al- though there is no doubt that the shark's super-sensitive olfactory system can detect minute quantities of blood whose odor is carried toward them by currents, the sense of smell alone cannot explain how sharks can track prey whose scent or blood is being carried away from the shark by cur- rents. Nor can vision alone be the sense sharks use to find prey that is behind obstructions, such as rocks. (Skin-divers have reported many such incidents.) Somehow, sound or vibration detection would seem to be the answer to these mysteries. Dr. Warren Wisby of the Institute of Marine Science at the University of Miami has been seeking the answer in a long-range Whence the Shadows? 231 study of the shark's sensory system. Wisby's subjects are Nurse sharks (Ginglymostoma cirratu?n), and his observations are carried on not in a tank— but in a drainpipe. The drainpipe, 16 feet long and 3 feet in diameter, was chosen so that distracting sounds and sights could be blocked out. One end of the pipe is buried in a box of water-soaked sand, which absorbs sound. The pipe rests horizontally on springs that further absorb sounds from the outside. When the shark is strapped on a kind of sled and suspended in the water-filled pipe, it is thus isolated from any stimuli except those which Wisby introduces. The shark is next conditioned to associate a sound with an electrical shock. When it detects a sound in its drainpipe prison, the shark's heart skips a beat— as it does when it gets an electrical shock. The telltale heart-skip, which proves that the shark hears a given sound, is regis- tered by a "lie detector." This is simply an electrode implanted near the shark's heart and connected to recording devices in the laboratory. From these recordings of shark reactions. Dr. Wisby believes, scientists may eventually be able to determine what types of sound attract— and repel— sharks. The sense of hearing alone does not fully explain the shark's de- tection of and reaction to low-frequency water vibrations— caused, for instance, by the struggles of a hooked fish. Certain fish, such as Croakers, make clearly audible sounds. But the struggles of a fish on a hook are not audible; they are vibrations undetectable by what we normally call hear- ing- Skin-divers, whose observations are adding vast lore to marine sci- ence, report that schools of fish do not always take Alight when sharks appear. Why are these fish apparently unconcerned about the presence of predatory sharks? One explanation, as yet unproved, is that they can somehow detect, possibly through varying vibration patterns, the difference between a "hunting" and a "non-hunting" shark

OR A PLANKTON EATER


AND A BEEFEATER SHARK