Thursday, April 14, 2016

SHADOWS IN THE SEA Sensory biology of sharks, skates, and rays The digestive system of Selachians is very primitive in structure; the flesh contains urea which gives it a distinctive odor and causes more rapid decomposition than in most Teleosts. The pectoral fins in many species are capable of little or no swimming movement; the breathing organs include not only gill slits but also spiracles on the sides or top of the head. The bodies of most sharks are shaped much like those of some OTHER PREY FISH Sharks come in many sizes. Ishmael, awed by the immensity of Moby Dick, rightfully called the whale "the mightiest animated mass that has survived the Flood." But the whale is a mammal, and the largest fish in the sea is a species of shark, the Whale shark {Rhincodon typus). The Whale shark's confirmed measurements are 45 feet in length and more than 13 tons in weight. Creditable reports have put its length at 60 feet and more. [Blue whales (Balaenoptera mus cuius) commonly grow to 90 feet, and have been known to reach 110 feet in length.] There are small sharks, too: some mature at less than 18 inches. One species, Squali- olus laticaudus, found at abyssal depths in the Pacific, retains a com- plete shark form but at full size is believed to be less than 3 inches long. Between the Whale shark and the tiny Squaliolus are sharks whose fame rests not on their size but rather on their versatility, feats, and repu- tation. Rightly or otherwise, this reputation is often bad, and the con- sensus of most seafarers, fishermen, and landsmen is that the best shark is a dead one. The notion that the shark deserves a hideous death seems to be uni- 6-FOOT MAN BASKING 12-FOOT AUTOMOBILE WHALE SHARK A 6-foot man is shown to scale with 6 of the largest sharks and the largest known ray, all drawn to reliably reported sizas. At left, top, is a Giant Devil ray ( breadth of 20 feet); at right, top, is a Thresher shark (20 feet, including tail) and, below it, a Hammerhead (15 feet). Four large sharks, from top to bottom, are a Great White ( 36 feet ) , Greenland ( 24 feet ) , Basking ( 40 feet ) , and Whale shark ( 45 feet ) . Courtesy, Scottie Allen 219 220 Shark and Company versal among sailors. Since the age of sail, seamen have usually caught sharks only to curse them and butcher them, though when shipwrecked they have been happy enough to eat them for survival on many occasions. More often they have hacked the shark into pieces, or chopped off its tail and hurled it back into the sea to be devoured by other sharks. In Panama, the natives have devised a fiendish death for captured sharks: crucifixion. They nail the shark's pectoral fins and tail to a board and then launch the board, sending the shark out to death under a glaring sun or into the jaws of other sharks attracted by the victim's bleeding and writhing. Native divers in the Red Sea share man's common terror of the shark, though they show it in another way. They give friendly names to the sharks as a means of placating the evil spirits lurking within them. Doctors J. T. Nichols and R. C. Murphy, the shark experts mentioned in Chapter 1, witnessed one attempt to kill an almost indestructible shark. They reported: "We have seen one hooked, shot full of lead from a re- peating rifle, then harpooned, hauled on deck, and disemboweled, yet it continued alive and alert for a long while, thrashing its tail and opening and shutting its weird, expressionless eyes by moving the whitish lower lids." And a "dead" shark is often very lively. One fisherman, for instance, had a hand bitten off by a disemboweled shark. A naval officer con- temptuously kicked a seemingly dead shark lying on deck; the shark's retaliation was immediate and massive— it tore off most of the calf of the officer's leg. The shark's hold on life is incredible. There is a reliable record of a shark that was cut open, gutted, and thrown back into the sea by a fisherman who then baited his hook with the shark's intestines— and caught the same shark again! The shark dies hard. Gavin Maxwell, writing in Harpoon at a Venture of an attempt to kill a gigantic harpooned Basking shark (Cetorhinus TnaxiTfTus)^ reports: He was ... a huge bull of unusually black coloring, and ... he was still moving, shuddering and undulating down his entire length, though he had been beached for two days ... At point-blank range I shot the shark between the eyes four times, so that the brain must have been completely obliterated. There was no visible effect; the movement of the body neither accelerated nor slowed. Then, to make certain that the fish was dead, we cut off the entire forepart of the head with axes, but this, too, produced no change. Four days later, when we dragged the carcass off the beach, the body, now headless and disemboweled, was still twitching and jerking over its whole length. Yet in some ways, the shark is delicate. A relatively sHght injury to its gills, for instance, will usually cause a shark to bleed to death. If a shark is hoisted out of the sea by the tail, it has little chance of survival: Whence the Shadows? Ill the head-down suspension seems to have some effect on its nervous sys- tem. Some experts believe that the shark's primitive nervous system may be damaged by fright alone, a reaction animal behaviorists think they have detected in some mammals. A sports fisherman tells of catching a shark, removing its liver for chum, and then tossing the shark back into the sea as so much offal. The shark swam away, showing no apparent ill effects. A Dogfish (Mustelus canis) captured in Buzzards Bay, Massachusetts, had a large hole through the wall of its body. The wound had been plugged by a lobe of the liver which had simply grown into the hole! Stories are many of sharks' struggles against death and their apparent insensitivity to what in other creatures would be intense pain. But a headless, disemboweled shark writhing on a beach is not really strug- gling against death. Rather, its biologically simple body is throbbing with reflex actions. It is death that is doing the struggling, for snuffing out such a vibrant, basic form of life takes a long time. All evidence points to the behef that pain, as we know it, does not exist for Selachians— or fishes in general— or at least they have a very high pain-threshold. In man, the sensation of pain originates in certain nerve receptors that transmit impulses to the higher evolved nerve cen- ters of the brain. Presumably, the lower a creature on the evolutionary scale— and Selachians are well down it— the less developed is its sense of pain. The shark's tenacity of life begins at the moment of birth, when it emerges from its mother or its egg-case as a miniature replica of its el- ders: voraciously hungry, ceaselessly moving. Day-old pups, as shark young are called, have been seen going for baited hooks. Two of the au- thors have seen captured sharks give birth to pups that skittered across the deck of a boat, wriggled through the scuppers or leaped over the gun- wale and plunged into the sea— to begin a swim that would end only when they died. For, though sharks can rest on the bottom, they lack the swim bladders that give buoyancy to the Teleosts. This lack of a swim bladder (or, as it is sometimes called, air bladder) makes it impossible for the shark to maintain an equilibrium of depth. Its body is more dense than the water it displaces and will sink to the bottom unless sustained by constant motion. The shark, then, is con- stantly striving to keep itself from sinking. Only bv a continual un- dulation of its muscular tail and, to some extent, its fins, can the shark overcome the gravity that inexorably pulls it downward. Unlike the typical Teleost fishes which lie bloated in death on the surface of the sea, when the shark can swim no more its body settles to the oblivion of the deep. However, at least one species, the Sand Tiger shark {Carcharias 222 Shark and Co?npany tauriis), is said to have developed a kind of substitute for a swim bladder by swallowing air and keeping an "air pocket" in its stomach. Thus, its stomach is believed to act as a hydrostatic organ similar to the Teleost's swim bladder. In its lifelong swim, the shark does not sleep, at least as we humans know sleep. Sharks that spend their lives inshore seem to rest— or perhaps sleep— by swimming into shallow caverns, apparently alighting on rocky ledges, or seemingly resting on the bottom. Divers frequently are able to approach these "sleeping" sharks with ease. Sharks that spend their lives in the open ocean do not appear to rest, for, if they ceased moving, they would sink, often to abyssal depths. Of course, some sharks live in the great deeps permanently. The "sleep" of any shark, at any depth, however, is possibly only a physiological pause in its activity. The shark is a creature marvelously adapted to its environment. It achieved this harmony with the sea eons ago, and, from what we know of evolution, the shark's basic structure has remained virtually unchanged mainly because its prehistoric adaptation was so perfect, although much specialization has occurred among different species. A tough skin plated with row upon row of teeth; three great muscles flexing nearly the length of each side of its body; a strong, gristly, resilient skeleton— these form the dwelling place of what might be said to be the essence of the shark. In addition, there is a tiny brain and a nervous system perfectly attuned to the animal's activity in its environ- ment. The silhouette of a typical shark is unmistakable. Unlike the mouth of the typical Teleost, the mouth of most sharks is curved and lies on the under side of its head. Its tail, or caudal, fin is almost always asym- metrical, with the upper lobe usually the far longer one. Its fins are flipper-like and differ from the Teleost's fins, which are held rigid by a network of rays or spines. Sharks cannot move their side fins freely to swim, as Teleost fishes can. A shark's fin arrangement is also distinctive. The pectoral fins are generally larger than those of the Teleost. The ventral, or pelvic, fins have, in the male, appendages called "claspers," which are intromittent or sexual organs. Aft of the ventral fins, between the vent and the tail, is the anal fin. The caudal itself sweeps upward, forming the two lobes, the upper of which may have a notch, whose purpose is not known. And jutting from the back of most sharks is the familiar dorsal fin that, when seen, is the warning banner of a shark's presence. The skeleton of the shark is formed of cartilage, but in some species so much calcium is deposited in the cartilage that it is almost as rigid as bone. Never, however, is true bone developed. This lack of bone does not mean a lack of skeleton; the familiar structural framework of the Whence the Shadows? Ill fish is there, at least at first glance. But, demonstrating in still another way its tendency to remain basically simple, the shark has a skeleton that differs considerably from that of the bony fish. Without going into anatomical detail, it may be said that the Teleost's skull is a far more complex bony structure than the Selachian's cartilaginous skull. / ' The skin of fish, like the skin of man and other vertebrates, consists of an epidermis, an outer layer of cells, which is continually wearing away and being replaced, and the dermis, an inner layer of more com- plex cells which include the pigment cells that determine color. Gen- erally, the skin of fish is covered with scales, and most fish scales are of two types: cycloid scales, found in such fish as carp and herring, and ctenoid scales, which have minute spine-like projections at their exposed edges (a black bass has ctenoid scales). Sharks have a third type of scales— placoid. And these scales are really dermal teeth, set in the shark's hide. Of all the many oddities of the shark, this is one of the most difficult to grasp, perhaps because it is so uncomplicated. These scales, called dermal denticles, are truly teeth. Each denticle in the shark's hide has the two attributes of a tooth: its surface is covered by dentine, and it has a central pulp canal containing a nerve and blood vessels. In some species, these denticles are visible to the naked eye; in other species, they are microscopic. But, no matter the size, they are teeth. The denticles give the tough hides of most sharks a sandpaper-like roughness that can scratch or even tear a swimmer's flesh. This abrasive hide, called shagreen, can smooth down the hardest woods and, in fact, was once used for that purpose by cabinetmakers, as has been mentioned. Denticles are anchored in the skin of the shark much as collar but tons are held in a shirt. The sub-surface base of the denticle is larger than the opening through which the visible portion projects. The denti- cles project backward, which is very obvious if the skin is stroked from the tail toward the head. In some species, such as the Nurse shark (Ginglymostoma cirratum), the denticles are so large and so closely spaced that it is difficult to drive a harpoon into the hide. Other species produce scattered patches of denticles. The variety of denticle forms is nearly as great as the variety of shark species. Denticles are blunt, scalloped, spade-shaped, thorn-like, geometric, and even heart-shaped. By a growth process called hypertrophy, certain denticles develop independently of others and become comparatively gigantic structures with no apparent relationship to the smaller and microscopic denticles. The possession of denticles is one of the many characteristics shared by sharks, skates, rays, and the links between them.



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