Sharks

FACT SHEET FROM THE EDUCATION DEPARTMENT

Sharks are some of the most mysteriousand misunderstood animals on Earth today.They are feared by most, loved by some and hunted by many people. Many movies and books have furthered myths about sharks. However, the more we study sharks, the more we learn the truth about these magnifi cent fi sh.

SHAPE, SIZE AND COLOR There are approximately 400 species of sharks, each with a unique shape and size. Many sharks have a sleek, streamlined body shape designed to help them swim quickly through the water to capture their prey. The fastest shark is the torpedo-shaped shortfi n mako, which can swim at 20 mph (32km/h). Sharks range in size from the 8 inch dwarf lanternfi sh (also known as the deepwater dogfi sh shark) to the 45 foot whale shark. The color of a shark’s body is designed to allow the shark to blend in, or camoufl age, with the ocean and surrounding environment. The top of a shark is typically a darker color that blends in with the dark water below when seen from above. Sharks have a light color underneath that blends in with the sunlight above when seen from below. This type of camoufl age is called countershading. Some sharks may have stripes or spots, become lighter over time or remain the same color their entire lives.

NO BONES ABOUT ITSharks and their relatives (including skates, rays, sawfi shes and guitarfi shes) belong to a group of animals called elasmobranchs, which includes fi sh with skeletons made of cartilage, not bone. Cartilage is a tough, fl exible material (connective tissue) that can be found in the human nose and ears. Sharks have fi ve to seven gill slits visible on the outside of their bodies. In order to breathe, water passes in through their mouth, across theirgills where oxygen is absorbed, and exits through their gill slits. Sharks do not sleep like humans do, but instead they have active and restful periods. Some species of sharks need to swim constantlyto allow oxygen-rich water to fl ow throughtheir gills. However, some sharks, like the nurse shark, can rest at the bottom because they have small holes called spiracles that force water across their gills in addition to taking in water through their mouth. Sharks also have an oily liver to help them stay afl oat. The skin of a shark is covered in tiny scales or skin teeth called dermal denticles.These skin teeth point toward a shark’s tail, so a shark feels smooth if touched from head to tail but feels like sandpaper if felt from tail to head. As a shark grows, more skin teeth are added to its body, which helps it to become a faster, more effi cient predator.

KEY TERMS

Ampullae of LorenziniSmall pores in a shark’s snout that detect electrical signals.

BycatchMarine animals caught accidentally by fi sherman who intended to catch another species.

CartilageHard, fl exible connective tissue that forms a shark’s skeleton.

CountershadingA type of coloration where an ani-mal’s top side is dark while the bottom is light. This coloring helps the animal to blend in with its surroundings.

Dermal DenticlesSmall tooth-like scales covering a shark’s skin.

ElasmobranchsSubclass of fi shes with skeletons made of cartilage and possessing fi ve or moregill slits: includes sharks, skates and rays.

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rksREADING LEVEL – MIDDLE

SHARK SENSESSharks have a number of specialized senses and adaptations, making them one of the most efficient predators on Earth. Depending on how far away their prey is located, a shark will utilize dif-ferent senses to detect it. A shark’s most acute sense is smell. When swimming, the shark moves its head from side to side in order to detect and follow the area of the most concentrated scent. A shark can smell blood or fish oil up to 1320 ft (400 m) away. Sharks hear underwater using internal ears on either side of their head. Sharks specialize in hearing low frequency sounds, like those made by injured or sick animals splashing in the water. Scientists have found that most sharks have ears that are capable of hearing low frequency sounds from 295 feet (90 m) away. How-ever, some sharks can hear these sounds from up to 800 feet (250 m) away. At 100 feet (30 m) away, the shark’s lateral line becomes effective. The lateral line is used to detect sound waves, vibrations and disturbances in the water. It is a complex network of holes that runs horizontally along the body. The holes are attached directly to nerves that stimulate a muscle response from the brain when vibrations

are detected. Sharks can typically see objects from about 50 feet (15 m) away. They see best at dawn and dusk by using a structure called a tapetum lucidum, which is a special layer of cells behind the retina in the shark’s eye. The tapetum lucidum acts like a mirror within a shark’s eye, allowing it to see 10 times better in dim light than humans. Sharks also have sen-sory organs called Ampullae of Lorenzini, which are most effective at about 6 feet (2 m) or less. They are fluid-filled pores sur-rounding the mouth and nose of sharks and rays.These pores detect weak electri-cal fields given off by animals as well as the Earth’s magnetic field. The Ampullae of Lorenzini are particularly useful in dark or murky waters when prey animals are hiding under the sediment.

THE TOOTH OF THE MATTERDifferent species of sharks have different shapes and sizes of teeth that are spe-cialized for the kinds of food they eat. Notched teeth with serrated edges like those of the tiger shark are specialized for cutting and tearing into hard objects. These teeth make it possible for adult ti-ger sharks to eat through sea turtle shells.

Thin, long, pointy teeth allow the shortfin mako shark to grasp and hold onto prey. Great white sharks have narrow teeth in their bottom jaw used for piercing and holding prey, plus sharp, serrated, triangu-lar teeth in their top jaws for cutting. Port Jackson sharks have hard, flat teeth for crushing crabs, clams, oysters and other shellfish. Whale sharks have numerous tiny teeth—they filter-feed plankton and fish through screens on their internal gill slits. Some sharks, like the nurse shark, use barbels or whiskers to probe the bottom in search of food hidden in the sediment.

Sharks lose teeth constantly throughout their lives. When they eat animals with hard bones or shells, they lose teeth from their soft, cartilaginous jaw. Some sharks can lose up to 30,000 teeth in their life-time. The inside of a shark’s jaw has five to fifteen rows of teeth that usually lie flat until the tooth in front of it falls out. When a tooth is lost, another rotates for-ward to replace it within 24 to 48 hours. The process of tooth replacement in sharks is very similar to the movement of a con-veyor belt or the steps on an escalator.

FAST FACTS• The shortfi n mako is the fastest shark and can swim at 20 mph (32km/h).

• Sharks range in size from 8 inches to 45 feet in length.

• Sharks have a skeleton made of cartilage, not bone.

• Sharks can replace lost teeth in as little as 24 hours and may lose thousands of teeth in their lifetime.

KEY TERMS Lateral LineSense organ in sharks, running hori-zontally along the body, that can detect movement and vibrations.

Tapetum lucidumA special tissue layer in the eye that acts like a mirror bouncing light throughthe eye and improving a shark’s vision at night and in low light.

SHARK PUPSSharks reproduce by laying eggs, giving birth to live young or a combination of both. Some species of sharks lay eggs that have hooks to attach to underwater plants. Other shark species bear live young, whichare fully formed. The pups are not cared for by their parents and are ready to hunt for their own food as soon as they are born. The number of young varies by species. Sand tiger sharks have 1-2 pups while whale sharks can have up to 300 pups at one time.

SHARKS AND HUMANSFor years, a great deal of attention has been focused on the interaction between sharks and humans. Of the approximately 400 species of sharks, only 32 species haveever been documented in attacks on humans. According to the International Shark Attack File (ISAF), there are an average of 50-75 unprovoked shark attackseach year worldwide, and only about 5 result in a fatality. Scientists theorize that human attacks may happen as a result of territorial responses, inquisitive testing or mistaken identity.

Despite their reputation, most sharks are shy and harmless, avoiding people and other large animals whenever pos-sible. Only a dozen species of sharks are considered dangerous to humans. On the other hand, people kill millions of sharks every year by accident and intentionally. Many sharks die as accidental bycatch, caught by fi sherman who intended to catch another species. Sharks are also killed intentionally for their rough skin and sharp teeth to be used as sandpaper, leather and jewelry. Many sharks are also hunted for their liver oil, meat and fi ns to make shark fi n soup.

Shark populations are declining and, without protection, may not be able to bounce back from the pressures put on them by humans. Sharks are apex, or top, predators in the world’s oceans and play an essential role by feeding on sick and dying animals as well as larger animals that have few predators. Scientists who study sharks are working to fi nd out more about these amazing animals and how to better ensure their survival.

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