Amy Kenyon-Campbell. The Gale Encyclopedia of Science. Editor: K.Lee Lerner & Brenda Wilmoth Lerner. 4th edition, Volume 5, Gale, 2008.
The sharks are a group of more than 350 related species of cartilaginous fish, members of which are found in every ocean in the world. Far from their reputation as primitive monsters, the sharks are, in fact, some of the most fascinating, well-adapted marine organisms. Their many structural and functional adaptations, such as their advanced reproductive systems and complex sensory abilities, combine to make them very well suited to their environment.
Evolution and Classification
Sharks are often described as “primitive” animals, and little changed in millions of years of evolution. It is true that the first sharks evolved in the oceans more than 300 million years ago, in the Devonian era. However, the earliest species of sharks are all extinct. The species living in the oceans today evolved only 70-100 million years ago. The fact that the general body plan of the earliest sharks was so similar to that of living ones is a testimony to the suitability of their adaptation to the environment in which sharks still live.
Sharks and other modern fish are descended from primitive fish, called Placoderms, that were covered with bony, armor like plates. The descendants of the Placoderms lost the armor, but retained an internal skeleton. Most types of modern fish, such as trout, minnows, and tuna, have a bony skeleton. Sharks and their relatives, the skates and rays, are distinguished from other types of fish in that they have cartilage rather than bone as their skeletal material (cartilage is a translucent, flexible, but strong material that also makes up the ears and nose of mammals, including humans). Thus, the sharks are called the “cartilaginous fishes” (class Chondrichthyes).
Overview of Shark Groups
There are eight orders of living sharks.
The Squatiniformes order includes the angelsharks and sand devils. These sharks are flattened like rays and tend to live on the ocean bottom in water depths to 4,200 ft (1,300 m). They are found in most oceans, except the central Pacific and Indian Oceans and the polar areas. There are thirteen species, most of which are less than 60 in (1.5 m) long.
The Squaliformes order includes the dogfish sharks, bramble sharks, and roughsharks. This is a group of more than 90 species, 73 of which are dogfish sharks. Dogfish sharks generally have a cylindrical body and elongated snout. They are found in all oceans, usually in deep water. Their size ranges from the 10-in (25 cm) pygmy sharks to the 23-ft (7 m) sleeper sharks.
The Pristiophoriformes order consists of five species of sawsharks, with a long, flattened, saw-like snout. They are bottom-dwelling in temperate to tropical oceans, to depths of 3,000 ft (900 m). Adults are 3-5 ft (1-1.6 m) long.
The Hexanchiformes order consists of the frilled, cow, six-gill, and seven-gill sharks. There are five species, which are found in all oceans, mostly on continental shelves from 300-6,150 ft (90-1,875 m). The body length ranges from 77 in (195 cm) for frilled sharks to 16.5 ft (5 m) for a species of six-gill shark.
The Heterodontiformes order consists of eight species of bullhead sharks. They have a wide head, short snout, and flattened teeth for crushing hard prey. They are found in warm continental waters of the Indian and Pacific Oceans, to depths of 900 ft (275 m).
The Orectolobiformes order includes zebra sharks, nurse sharks, and whale sharks. This is a diverse group of 33 species, all found in warm water, mostly in the Indian Ocean and western Pacific. They may forage on the surface or at the bottom, mostly near shore to depths of about 330 ft (100 m). These sharks have two small projections called barbels under their snout, and most have a shortened, rounded nose and slender, elongated tail fins. Most species are 3-8 ft (1-3 m) long, but whale sharks may reach over 40 ft (12 m). Whale sharks are the largest fish in the world.
The Lamniformes order includes the sand tigers, basking sharks, megamouth sharks, mako sharks, and white sharks. There are 15 species, which are found in all but polar waters. The megamouth sharks were only discovered in 1982. The species in this order are found near shore or far from land, in shallow water and to depths of 3,900 ft (1,200 m). Most have a powerful, cylindrical body and elongated snout. Their length ranges from 3-19 ft (1-6 m), with basking sharks reaching over 33 ft (10 m).
The Carcharhiniformes order includes the cat-sharks, hammerhead sharks, and requiem sharks. The latter subgroup contains the blue, tiger, and bull sharks. The groundshark group consists of about 216 species found in all ocean habitats. It includes most of the species considered dangerous to humans.
Structural and Functional Adaptations
Sharks are generally fusiform in body shape, with a narrow snout, wider body, and a tapering tail. Sharks have one or two fins on their dorsal surface (back), a pair of pectoral fins, a pair of pelvic fins, usually a single anal fin on the ventral surface (belly), and a caudal (tail) fin. Usually the upper lobe of the caudal fin is larger than the lower lobe. The pelvic fins of male sharks have a projection called a clasper, which is used in sexual reproduction.
Locomotion and Buoyancy
Sharks swim by moving their caudal fin from side to side in a sweeping motion, which propels them forward through the water. The large upper lobe of the caudal fin of most sharks provides most of the forward thrust. Sharks, like makos, which sometimes need to swim at high speed, also have a well-developed lower caudal fin lobe for greater thrust. As a shark moves through the water, it angles the pectoral fins to change direction.
Sharks are slightly heavier than water, so they naturally tend to sink. Buoyancy or lift is provided in two ways. First, sharks store large quantities of oil in their liver. Because oil is less dense than water, storing this oil decreases the overall density of the shark, and increases its buoyancy. Second, as a shark swims, its pectoral fins provide lift, in much the same way the wings of an airplane does. If a shark stops swimming it will sink, but its stored oil and relatively light skeleton help it to float and decreases the amount of energy that must be expended on swimming.
Sharks are “cold-blooded” (or poikilothermic) animals, meaning their body temperature is the same as that of the water in which they live. The term cold-blooded is misleading, however, because sharks living in warm water are “warm-blooded” in actual temperature.
Some fast-swimming sharks in the Mackerel shark order (for example the mako and white sharks) can actually raise their core body temperature somewhat above that of their surroundings. In these sharks, heat generated as they swim is conserved by a special vascular network surrounding the muscles. This network helps to conserve heat in the body core, rather than allowing it to dissipate into the cooler water. Just as chemical reactions in a laboratory proceed faster when heat is applied, so too do metabolic reactions at higher temperatures. With their higher core body temperature, these species are able to be more active and efficient predators than most other sharks and bony fish.
Sharks use their gills to absorb oxygen from the water. Most sharks have five gill slits on each side of their body, behind the mouth and above the pectoral fins. Water enters the mouth of the shark, enters a canal between the mouth and the gills (the orobranchial cavity), and then passes back to the outside through the gill openings. As the water passes over the gills, oxygen is absorbed into the blood across the thin skin of the gill surface, and carbon dioxide moves into the water.
Water can flow across the gills by two mechanisms. First, as the shark is swimming it may hold its mouth open, allowing water to flow in and then out through the gill slits as the fish moves forward. Some sharks, however, can get enough oxygen when they are not swimming by gulping water into their mouth, then forcing the water out through the gills with muscular contractions of the orobranchial cavity. It is not true that all sharks must always keep swimming to breathe.
Water and Salt Balance
Fish living in the ocean are in danger of dehydrating because water moves out of their body into their salty environment through the process of osmosis. Basically, this occurs because the saltconcentration in the ocean is much higher than that in the blood of fish. In part, sharks solve their dehydration problem by having a relatively high internal concentration of salts and other molecules. In addition to the salts naturally present, sharks have additional solutes (i.e., dissolved substances) in their blood, so the total osmotic activity of dissolved substances is similar to that in seawater. They maintain their blood at this concentration by excreting the excess salt they ingest in their diet. A special gland near the end of the intestine, called the rectal gland, absorbs extra salt from the blood and passes it into the intestine to be excreted. These two adaptations function together to ensure that sharks do not dehydrate.
Sharks have the same five senses of sight, hearing, smell, taste, and touch that humans have. Moreover, some of these senses are more acute in sharks. Sharks also have an additional sense; they can detect weak electric fields in the water.
Sharks are known to possess a complex visual system, and can even see color. A problem for sharks is that, if they are in deep or murky water, the light level is very low. Several features of the shark eye make it well-suited to vision in dim light. Unlike most fish, sharks have a pupil that can adjust to the amount of light in the environment. Also, shark eyes have high numbers of the structures that actually detect light (the rods), so that even in low light an image is formed. Finally, sharks have a special reflective membrane (the tapetum lucidum) at the back of the eye, which enhances their vision in low light even further. Cats have a similar membrane in their eyes, which is why their eyes seem to reflect light in the dark. The membrane, for both cats and sharks, helps them see in dim light.
Two tiny pores on the top of the sharks head lead to their inner ears. The inner ear contains organs for detecting sound waves in the water, as well as three special canals that help the animal orient in the water. The sound receptors are very sensitive, especially to irregular and low-frequency (20-300 Hz) sounds. These are the types of noises a wounded prey animal would be likely to make. The distance at which a shark can hear a sound depends on the intensity of the sound at its source: a vigorous disturbance or a loud underwater noise will produce sound waves that travel further in the water than those produced by a smaller disturbance.
A shark’s nostrils are two pores on the front of its snout. As the shark swims forward, water passes through the nostrils and chemicals in the water are detected as odors. The nose is used only for detecting odors, not for breathing. Some sharks can detect as little as five drops of fish extract in a swimming pool of water. Sharks can easily use their sense of smell to detect and home in on prey, by swimming in the direction of the increasing scent.
Evidence suggests that sharks can taste their food, and that they have preferred prey. Small taste buds line the mouth and throat of sharks, and they seem to reject foods based on their taste. Some scientists argue that the reason most shark attacks on humans involve only one bite is that the animal realizes, after biting, that the person does not taste the same as the expected prey.
Sharks have two types of touch sense. One is the ability to sense when an object touches their body. The second is the ability to detect an object by the movements of the water it causes. This is similar to how you might detect where a fan is located in a room, because you can feel the movement of the air on your skin. Sharks and other fish have a specialized, very sensitive receptor system for detecting these types of water movements. This sensory system involves a series of tiny, shallow canals and pits running beneath the surface of the skin, known as the lateral line and the pit organs. The movement of water against the canals and pits is detected in receptor organs, and this information is used to “visualize” the presence of nearby organisms and objects.
All organisms in sea water generate a weak electric field around them, like an invisible halo. Small pits in the skin of sharks end in receptors that can detect extremely low-voltage electric fields in the water. Sharks use this sense to locate their prey at close range. Some sharks can even find their prey under sand and mud.
Feeding and Diet
All sharks are carnivorous, meaning that they only eat other animals. The range of prey eaten by sharks is extremely broad, from snails to sea urchins, crabs, fish, rays, other sharks, seals, and birds. Some sharks eat carrion (animals that are already dead), but most only eat live prey. Sharks eat relatively little for their size, compared to mammals, because they do not use energy to maintain a high body temperature. Sharks eat the equivalent of 1-10% of their body weight per week, usually in one or two meals. Between meals they digest their food, and they do not eat again until they have finished digesting their previous meal.
Sharks that eat prey with hard shells, such as bullhead sharks, have flat crushing teeth. Bullheads eat a variety of prey, including barnacles, crabs, sea stars, and snails, which they crush with their rear teeth. The two largest sharks, whale sharks and basking sharks, eat nothing larger than 1-2 in (2-5 cm) long. These whales filter their tiny prey (called krill) from the water using their gills as giant strainers. The whales swim through the water with their mouth open, and small crustaceans in the water get caught in meshlike extensions of the gills. Once caught, the krill are fun-neled back to the whale’s throat and swallowed.
Species such as white sharks, makos, tiger sharks, and hammerheads attack and eat large fish, other sharks, and marine mammals such as seals. The feeding biology of the white shark has been well studied. This shark often approaches its prey from below and behind, so it is less visible to its victim. It approaches slowly to within a few meters, then rushes the final distance. If the prey is too large to be taken in one bite, the shark will bite hard once, and then retreat as the prey bleeds. When the prey is weakened, the shark again approaches for the kill.
Reproduction and Growth
Sharks have fascinating reproductive systems, with some advanced features for such an ancient group of organisms. Unlike bony fish, sharks have internal fertilization. The male shark uses projections from his pectoral fins, called claspers, to anchor himself to the female. He then transfers packets of sperm into the female’s urogenital opening, using pulses of water. The sperm fertilize the eggs inside the female, but what happens next to the developing embryo depends on the species.
Some species of sharks lay eggs with the developing embryo covered by a tough, protective case. This is known as oviparous reproduction. The embryos of these sharks are well supplied with nutritious yolk, unlike the tiny eggs of most bony fish. After some time, the egg hatches and a young shark emerges. Bullhead sharks, whale sharks, and zebra sharks are examples of oviparous species.
Female sharks of most species are ovoviviparous live-bearers, which means they retain their eggs inside the body until the young hatch, which are then born “alive.” This method provides the young with protection from predators during their earliest developmental stages. Examples of ovoviviparous sharks are dogfish sharks, angelsharks, and tiger sharks. Some species of sharks have a modification of this type of reproduction. In the white and mako sharks, the embryos hatch inside the mother at age three months, but then stay in the mother for some additional time, obtaining nourishment by eating nutrient-rich, unfertilized eggs the mother produces for them. A further bizarre twist occurs in the sand tiger shark, in which the earliest embryo to hatch in each uterus eats its siblings, so only two offspring are born (one from each uterus).
The most advanced form of shark reproduction occurs in the hammerheads and requiem sharks (except the tiger shark). In these sharks, early in embryonic development a connection (placenta) is created between the embryo and the mother. The embryo obtains nutrients through the placenta for the remainder of its growth, before being born alive. This type of development is called viviparity, and it is similar to the development process of mammals.
Compared to most bony fish, sharks reproduce and grow relatively slowly. Bony fish tend to lay thousands or more tiny eggs, most of which are scattered into the environment and die. Sharks have relatively few (zero to around 100) offspring each year, and the mother invests much energy in each to increase the chance that it will survive. Some female sharks put so much energy into a litter that they must take two years to recover their strength before breeding again. Although young sharks are born relatively large and able to take care of themselves, they grow slowly, sometimes only a few centimeters a year. It may take 15-20 years for an individual to reach sexual maturity. Such low reproductive rates and slow growth combine to make sharks highly vulnerable to overfishing.
Historically, sharks have been fished for their meat and for liver oil, which was the best source of vitamin A until the 1940s. Shark fin soup is a traditional Asian delicacy and shark meat has recently gained popularity; these are greatly increasing the killing of sharks in marine fisheries. In addition to their food value, many sharks are caught and killed for sport by individuals and in specific shark-catching competitions. Often, sharks are unintentionally caught in nets and lines set for other species. Modern methods used by many commercial fishing fleets involve either baited long-lines stretching for miles, or long drift-nets that entangle and kill anything in their path. Sharks caught by these methods are often either dumped, or are finned (the fins are removed for shark fin soup) and thrown back to die. In the 1980s, 50% of sharks caught recreationally and 90% of sharks caught commercially were discarded back to the ocean dead.
Since the mid-1960s, scientists studying sharks have warned that indiscriminate and wholesale slaughter of these animals was driving their populations to a dangerously low level. Many people, with visions of sharks as monsters, had little interest in saving them. Some sharks do attack humans. However, the risks are very small: a person’s chance of being killed by lightning is 30 times greater than that of dying in a shark attack. Each year, humans kill more than one million sharks for every human bitten by a shark.
It is now quite clear that the fishing mortality described above is having a severely negative effect on shark populations. Sharks have relatively low reproductive and growth rates, and they are being fished much faster than they can replace themselves. Scientists have determined the maximum number of sharks that can be caught each year to maintain the population. In the 1980s, the actual number of sharks killed in areas of the North Atlantic Ocean exceeded that target by 35– 70%. Without rapid changes in this wasteful overfish-ing, many shark species will become endangered.
There are numerous reasons to conserve shark populations, in addition to the fact that they are beautiful animals about which there remains much to learn. Perhaps most importantly, sharks are important predators in marine habitats. Removing them will affect the populations of their prey, which would
have impacts on all other species living in the ecosystem. On a different note, scientists have recently discovered a chemical in shark blood called squalamine, which functions as an antibiotic. Further tests on this chemical and others from sharks may produce chemicals toxic to cancer cells. If sharks become endangered, it will not be possible to harvest these medically useful chemicals.
The U.S. Department of Commerce has established guidelines for and restrictions on shark fishing based on the acceptable maximum catch estimated by researchers. The guidelines limit the recreational and commercial catch of sharks, prohibit finning, and reduce the numbers of shark fishing tournaments. In Australia, species such as the great white shark have been declared endangered, and are now protected from indiscriminate killing. With wider enforcement, guidelines such as these may mean that sharks live to enjoy another 350 million years roaming the world’s oceans.