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Sharks: Banishing the myths about Nature’s perfect predators
Visitors can get close to several shark species that call the
Tennessee Aquarium’s new Ocean Journey building home

CHATTANOOGA, Tenn. (Feb. 21, 2005) – Amid the fantastic coral formations and colorful fish in the Tennessee Aquarium’s Secret Reef, a dark shape looms. As it glides closer, the torpedo-like body, tall dorsal fin and tooth-filled mouth are undeniably that of a shark.

Called the “perfect predator” by scientists, sharks inspire a strange combination of fear and fascination. Many find it difficult to observe them without imagining the two-note musical theme from the movie, “Jaws.” However, sharks are not the indiscriminate killing machines portrayed in popular culture, but are fascinating animals that are a vital part of the ocean environment.

Movies, books and television often describe sharks as eternally hungry, ready and willing to devour any living creature in their path. The reality is that shark attacks against humans are uncommon, and fatal attacks are very rare.

"There are a lot of myths and misconceptions about sharks and shark bites," said Rob Mottice, Tennessee Aquarium coordinator of fish culture and acquisition. “Sharks should be respected, not feared. Sharks do much less harm to people than people do to sharks. Tens of thousands of sharks are killed each year. People are more likely to be killed by lightning than by a shark.”

With thousands of sharks being killed on a daily basis, the survival of many species is now threatened, said Mottice. Although sharks are not considered endangered by the U.S. government, some regions have put in place protections for dwindling species.

“The Secret Reef and Shark Island give us the opportunity to bring people face-to-face with sharks in a meaningful way,” said Jackson Andrews, Aquarium director of operations and husbandry. “By allowing people to form a connection with these fascinating animals, we hope they will come away with a new understanding of and appreciation for sharks and the role they play in the environment.”

A variety of sharks can be found in the Tennessee Aquarium’s Ocean Journey building. From the beautifully patterned epaulette and bamboo sharks in Shark Island to the sleek and toothy sand tiger and sandbar sharks seen cruising in the Secret Reef exhibit, the shark residents of Ocean Journey are very different in appearance. However, like all other sharks, they have no bones in their bodies. Instead, shark skeletons are made entirely of cartilage – the same semi-rigid material that makes up the tip of the human nose and ear.

But what makes the shark such a fearsome predator?

The most obvious answer is the enormous teeth and powerful jaws clearly visible in many shark species. Both the sand tiger and sandbar sharks found in the Secret Reef exhibit have large mouths with several rows of visible teeth.

Sharks are veritable tooth factories. They lose teeth throughout their lives – some during feeding, others simply fall out. Replacement teeth line the jaw of a shark and move forward to take the place of missing teeth. The replacement rate for teeth in the front of the shark’s mouth can be as often as every two weeks. Aquarium staff members frequently clean lost teeth out of the tanks that house the sand tiger sharks.

The shape of a shark’s teeth depends on its diet, and some species possess more than one type of tooth. Some shark experts can look at a shark tooth and determine which species the tooth came from. Sand tiger shark teeth are thin and needle-like with a wicked backward curve that helps the shark grab and hold food. Sandbar sharks have teeth with serrated edges for cutting. Other sharks have short, blunt teeth that are used to crush invertebrates like lobsters or crabs.

Although the meat-eating habits of many toothy sharks are legendary, other sharks feed by filtering plankton from the water using gill rakers – special comb-like bristles located in their throats. Some of the world’s largest sharks use this technique, including the whale shark.

Whale sharks are the largest sharks in existence today and can reach lengths of 60 feet. However, as a filter feeder, the whale shark poses no threat to humans. The largest shark ever found was the megalodon shark (Carcharodon megalodon) – the original great white shark. Megalodon was a huge shark with a body that may have reached lengths of 80 feet and large teeth that stood more than six inches high. The megalodon is extinct today, but its relative, the great white shark (Carcharodon carcharias) is well known today. The modern great white shark rarely exceeds 20 feet in length, but is still one of the largest predatory animals in the world.

However, even the largest shark with the sharpest of teeth is ineffective if it cannot find prey. Sharks’ ability to find food in the vastness of the ocean is nothing short of remarkable. To aid them in their pursuit of prey, sharks have six highly developed senses.

Sight
Most sharks have excellent vision – an important sense in the low-light, murky conditions of the open ocean. Sharks’ eyes have low-light photoreceptors that make them about 10 times more sensitive to light than human eyes. Experiments have shown that sharks can see some color and that they often gravitate to bright and shiny objects.

To protect the eyes during feeding, some sharks have a nictitating membrane, or extra eyelid. Great white sharks don’t have this eyelid. Instead, a great white shark rolls its eyes to the back of its head to protect itself while feeding.

Smell
Sharks’ nostrils are located on the lower part of their snouts. Because sharks use gills to breathe, the nostrils are used purely to smell. Smell is perhaps the keenest of a shark’s senses. Sharks can often detect the scent of prey from several hundred yards. Some sharks can smell components of blood and tissue at concentrations as small as one part per million. That’s the equivalent to as few as 10 drops of blood in the amount of water found in an average swimming pool!

Touch
Any movement by an animal through the water creates water movement that can be detected by a shark. Sharks have a special system called a lateral line that picks up this movement in the water. This narrow strip of sensory cells runs along the sides of the shark’s body and into the head. These sensory cells allow the sharks to detect the motions of sick or wounded prey when the animals are 3 to 10 feet away from the shark.

Hearing
Although sharks have no visible ears, they have sharp hearing and can detect sounds at a distance equal to the length of 10 football fields. Sharks can hear sounds as low as 10 hertz and are particularly sensitive to irregular sounds like those made by a wounded fish.

Taste
A shark’s mouth is filled with taste buds, and many researchers believe that sharks choose food items based on taste. Recent studies show that sharks seem to prefer prey with a high fat content. This makes good sense when you consider that sharks may go for long periods of time without eating. Some large sharks may go for months without eating. They need to make every bite count and fat delivers the greatest number of calories per bite.

Electroreception
Sharks possess one sense that humans do not: the ability to detect electrical fields.

All living things produce an electrical field created by muscle movement (including the beating of their hearts). Sharks have pores filled with a gelatin-like substance that are located in their snouts. These are called the Ampullae of Lorenzini. These receptors can help a shark find a fish hidden in the sand. The receptors also help the shark position its head and mouth when moving in for a final attack.

In addition to the variety of senses that have helped them to be top-tier predators, sharks have developed a variety of reproductive strategies that have allowed them to survive for millions of years. Most fish exhibit external fertilization, a reproductive strategy by which eggs and sperm are shed into the water. However, sharks rely on internal fertilization and several methods of hatching or birthing young, depending on the species.

Is that shark a male or a female?
The sex of sharks can be determined by the presence or absence of claspers. Claspers are the two finger-like, fleshy appendages found on the underside of a male shark at the base of its tail. Female sharks do not have claspers.

Some sharks are oviparous, meaning they lay eggs externally. Shark eggs are laid in tough, brown, leathery cases on the ocean floor, or the egg cases are attached to objects in the water. These eggs have enough yolk to nourish the embryos as they develop. When the eggs hatch, they are miniature versions of their parents. Other sharks exhibit an advanced mode of reproduction called viviparity. Viviparous reproduction in sharks means that the shark eggs are nourished internally through a connection to the mother instead of through a yolk sac. The eggs hatch in her uterus, and the young are born from the mother.

Perhaps the most common method of birthing young sharks is ovoviviparity. In this method of reproduction, the shark eggs hatch in the mother’s uterus before their development is complete. The embryos continue to mature inside the uterus without any connection to the mother. Instead, the young are nourished by the yolk sac. The young are born when development is complete.

Strange but true
Sand tiger sharks, which can be seen in the Secret Reef tank, exhibit a strange form of ovoviviparity. Female sand tiger sharks have two uteri. After fertilization, the largest embryo in each uteri are nourished by eating their smaller siblings, along with any additional eggs produced by the mother. This intrauterine cannibalism results in two large, healthy pups. The pups are usually 3 feet in length at birth. Although sand tiger sharks have the lowest reproduction rates among the sharks, their unusual way of hatching and birthing young ensures greater overall survival rates.

The Tennessee Aquarium inspires wonder and appreciation for the natural world. Admission is $17.95 per adult and $9.50 per child, ages 3-12. Each ticket purchased helps support Aquarium conservation programs. The IMAX® 3D Theater is next door to the Aquarium. Ticket prices are $7.95 per adult and $5.50 per child. Aquarium/IMAX combo tickets are $21.95 for adults and $12.50 for children. Advance tickets may be purchased online at www.tnaqua.org or by phone at 1-800-262-0695. The Aquarium, located on the banks of the Tennessee River in Chattanooga, is a non-profit organization. Open every day except Thanksgiving and Christmas, the Aquarium and IMAX are accessible to people with disabilities. Members enjoy unlimited visits and other benefits. Call 267-FISH to join.

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