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
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
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.
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.
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.”
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.
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
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.
what makes the shark such a fearsome predator?
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.
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.
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.
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.
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.
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
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.
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.
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!
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.
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.
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.
Sharks possess one sense that humans do not: the ability to
detect electrical fields.
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.
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.
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.
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.
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.
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