Shark

I

Introduction

Shark, any of several fast-swimming, generally large fish known for their many sharp teeth, distinctive dorsal fin, and skill in locating underwater prey. Like other fishes, sharks are generally cold-blooded and breathe with gills. But unlike the more common bony fishes, which have skeletons made of rigid bone, sharks—and their relatives skates, rays, and chimaeras—have flexible internal skeletons made of cartilage.

Among the most ancient vertebrates in the sea, ancestors of modern sharks originated almost 400 million years ago. Today sharks live in essentially the same way they did more than 200 million years ago, before the rise of the dinosaurs. Sharks range in size from the dwarf dogfish, less than 20 cm (8 in) in length, to the massive whale shark, which reaches lengths of more than 15 m (50 ft). Most sharks inhabit tropic or temperate marine waters, but some species have been found in polar seas. The bull shark leaves ocean waters to enter freshwater rivers and lakes, including the Zambezi River in southeastern Africa, the Mississippi River in the United States, and Lake Nicaragua in southwestern Nicaragua. Depending on the species, sharks inhabit either shallow coastal waters or the open ocean. Some species, such as the sixgill shark, live at depths of more than 1,800 m (6,000 ft).

No matter where they live, sharks play a crucial role in the ecosystems to which they belong. As highly efficient predators, sharks keep ecosystem populations in check by hunting and killing other animals, particularly those that are weak or diseased. Studies indicate that many shark species face the risk of extinction, mainly due to overfishing. As top predators, sharks perform a crucial role in maintaining the delicate balance within marine ecosystems. Without sharks, the behavior and increased numbers of some prey species may cause damage to the surrounding ecosystem. For example, the population increase of some prey species may cause populations of other species to decline.

II

Physical Characteristics of Sharks

Sharks have streamlined bodies that permit short bursts of movement through the water at high speeds. Their long bodies taper into a blunt or slightly pointed snout on one end and a powerful tail fin on the other. This shape reduces drag in the water, enabling sharks to glide gracefully, expending minimal energy as they swim. Generally, sharks swim at speeds less than 5 km/h (3 mph), but many can reach speeds of more than 20 km/h (12 mph) in short bursts. Mako sharks, swift, powerful, open-ocean sharks that reach lengths of up to 3.5 m (12 ft), are believed capable of speeds up to 48 km/h (30 mph).

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Although some sharks grow to more than 15 m (50 ft) in length, massive size in sharks is the exception rather than the rule. About half of all known species never grow to more than 1 m (3 ft) in length, and less than 20 percent reach lengths longer than 2 m (6 ft). Some of the best-known sharks, however, are the largest sharks. Nurse sharks, slow-moving sharks that live mostly in warm, shallow water, grow to more than 4 m (13 ft). Hammerheads, tropical water sharks known for their flattened, T-shaped heads, frequently exceed 3.5 m (12 ft), and great white sharks, perhaps the greatest predators of the sea, often reach lengths over 6 m (20 ft).

Hard, teethlike scales cover the shark’s body surface, giving its skin a sandpaper-like feel. These scales, called placoid scales or dermal denticles, do not get larger as the shark grows. Instead, the animal grows more scales to cover the growing body surface. Most sharks exhibit gray, blue, or brown coloring, shades that blend in well with their marine habitat. Sharks’ ventral sides, or undersides, are usually lighter in color than their dorsal, or top, sides. This countershading makes the animals more difficult to spot in the water. When viewed from above, their darker dorsal sides are difficult to distinguish from the ocean depths, and when viewed from below, their lighter ventral sides blend with the sunlit water above them.

In most species, the mouth and nostrils are on the ventral side, but in some species they are at the tip of the snout. Sharks are perhaps best known for their short, powerful, hinged jaws and multiple rows of sharp, triangular teeth, which they use to crush or tear pieces of flesh from their prey. Biologists believe that sharks’ formidable teeth are actually enlarged, modified placoid scales. A shark’s teeth are embedded in its gums, rather than firmly planted in the jaws, and sharks often lose teeth while killing and eating live prey. In most sharks, only the front two rows of teeth are upright and functional, but some species have as many as eight functional rows. Several rows of replacement teeth continually develop behind the teeth a shark actually uses. Developing teeth lie flat, just above the gums, and as teeth are lost, new ones move forward and upright to replace them. When a lemon shark loses a tooth, it takes only eight days for a new one to completely replace it. Some sharks may grow, lose, and replace as many as 30,000 teeth in their lifetime.

Just behind the head are five to seven gill slits, which the shark uses to filter oxygen from the water. Unlike most fishes, sharks do not have muscles that draw water across the gills. Although some sharks can pump water through their gills by opening and closing their mouths, most sharks rely on swimming movements to push water across their gills. Some sharks have spiracles, special gill slits just behind the eyes. Spiracles supply oxygen for respiration when a shark eats.

Five different kinds of rigid fins supported by cartilage aid sharks in swimming. Depending on the species, sharks may have one or two dorsal fins, which provide stabilization in the water. Sharks also possess two sets of paired fins located on their ventral surface. One set, the pectoral fins, is located just behind the head. Pectoral fins provide lift as sharks move up and down in the water column and stabilization when they swim at a constant depth. They are also used for steering and turning.

The second set of paired fins is the pelvic fins. Located toward the tail, these fins help the shark to swim in a level position. In males, modifications to the inner edge of each pelvic fin, called claspers, are used in mating. Some sharks also have an anal fin, a small ventral fin to the rear of the pelvic fins that provides further stability. The last fin is the tail fin, or caudal fin. Often crescent-shaped, the tail fin is the primary source of propulsion for sharks. The heavy tail and body muscles work together with the shark’s flexible cartilage skeleton to provide a powerful and smooth swimming stroke.

Not all sharks share this general shape and appearance. The angel shark has a flattened body, more like that of a skate, and the wobbegong shark is covered with fleshy flaps of skin that help to camouflage the animal. The hammerhead shark’s unusually shaped head provides lift in the water and enables it to make sharp, darting turns. Nurse sharks have unusually thick skin, which protects them from the sharp corals along the ocean bottom, where they seek protection.

A

Internal Organs

The shark’s digestive system is efficient, capable of digesting whole fishes and mammals, and even hard-shelled mollusks and crustaceans. Unlike most animals, which have long, coiled intestines, a shark’s intestine is short and straight. To increase intestinal surface area, shark intestines have a special feature called a spiral valve, a region of the intestine lined with membranous folds. The spiral valve functions to slow digestion and increase food absorption. The digestive tract empties into the cloaca, a rear chamber that also receives wastes from the urinary tract and eggs or sperm from the reproductive system. Wastes are eliminated from the body via the anus. A special salt-secreting gland, the rectal gland, helps eliminate excess salts absorbed from food and the marine environment.

Unlike their relatives the bony fish, sharks do not have a gas-filled swim bladder to help them float or change depth. Instead, sharks rely on their large, oil-filled livers for buoyancy. A shark’s liver is made of two large lobes that concentrate and store oils and fats, substances that are lighter than water. The liver can comprise as much as 25 percent of the shark’s total body weight and provides the shark with a small degree of buoyancy while swimming. But even with added help from the liver, a shark must swim continuously or it will sink to the bottom.

Sharks have two-chambered hearts that are relatively small compared to the rest of their bodies. Blood flows from the heart to the gills, where it collects oxygen from water and then distributes it to the other organs and tissues. The small heart produces weak blood pressure, and many sharks must swim continuously to create the muscular contractions needed to circulate blood throughout their bodies.

Most sharks are cold-blooded—that is, they do not generate heat by digesting food. Instead, the body temperature of most sharks matches the temperature of the water around them. There are, however, some notable exceptions. The body temperatures of fast-swimming sharks, such as blue sharks and makos, can rise almost 10 Celsius degrees (18 Fahrenheit degrees) above the temperature of the surrounding water. When swimming at great speeds, their muscles generate heat, which is transferred between adjacent arteries and veins.

B

Sensory Systems

Sharks’ well-developed sensory systems provide them with unmatched advantages when hunting or feeding. Almost one-third of their brain is devoted to the sense of smell. A shark’s sense of smell is so powerful that it can detect odors in the water hundreds of meters from their source. Sharks can detect as little as one part per million of substances in the water, such as blood, body fluids, and chemical substances produced by animals under stress. Some sharks can detect as few as ten drops of liquid tuna in the volume of water it takes to fill an average swimming pool.

Sharks’ eyes detect very small movements and they can see in low-light conditions, making them effective hunters in dark waters. Like cats and other nocturnal hunters, sharks possess a reflective layer in the back of their eyes, called the tapetum lucidum, which magnifies low levels of light. In clear water, sharks see their prey when it is about 20 to 30 m (70 to 100 ft) away. Sharks’ eyes also contain cells that detect color, and behavioral studies suggest that sharks can see colors as well as black, white, and shades of gray. These studies also revealed that shiny objects and bright colors, such as yellow and orange, may attract sharks.

Sharks use an additional sensory system, called the lateral line, to detect vibrations in the water caused by sounds or animal movements. The lateral line consists of a narrow strip of sensory cells running along the sides of the body and into the head. This sensory system is especially sensitive to sounds in the low-frequency ranges, such as those emitted by struggling wounded fish or other animals.

Additionally, sharks sense the weak electrical currents associated with the functioning of nerves and muscles in living animals. The shark’s electrosensors, called the ampullae of Lorenzini, are distributed in clusters over its head. This electroreception system is effective only over distances of less than 1 m (3 ft). It may aid sharks in the final stages of feeding or attack. Biologists also agree that this system may somehow enable sharks to detect the weak electromagnetic fields of Earth, aiding them in migration.

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