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Sharks
Shark Taxonomy
Kingdom: Animalia, Phylum: Chordata, Class: Chondrichthyes, Subclass: Elasmobranchii,
Suberorder: Selachimorpha.
Since that time, sharks have diversified into 440 species, ranging in size from the small dwarf
lanternshark, Etmopterus perryi, a deep sea species of only 71/2 to 8 inches in length, to the
whale shark, Rhincodon typus, the largest fish, which reaches approximately 12 metres (39 ft)
and which feeds only on plankton, squid, and small fish through filter feeding. Sharks are found
in all seas and are common down to depths of 2,000 metres (6,600 ft).
The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. It
is currently thought that the sharks form a polyphyletic group: some sharks are more closely
related to rays than they are to some other sharks.
There are eight orders, listed below in roughly their evolutionary relationship from ancient to
modern:
 Hexanchiformes: Examples from this group include the cow sharks, frilled shark and even
a shark that resembles a marine snake.
 Squaliformes: This group includes the bramble sharks, dogfish and roughsharks, and
prickly shark.
 Pristiophoriformes: These are the sawsharks, with an elongated, toothed snout that they
use for slashing their prey.
 Squatiniformes: Also known as angel sharks, they are flattened sharks with a strong
resemblance to stingrays and skates.
 Heterodontiformes: They are generally referred to as the bullhead or horn sharks.
 Orectolobiformes: They are commonly referred to as the carpet sharks, including zebra
sharks, nurse sharks, wobbegongs and the whale shark.
 Carcharhiniformes (Carcharhinidae): Commonly known as groundsharks, the species
include the blue, tiger, bull, grey reef, blacktip reef, Caribbean reef, blacktail reef, whitetip
reef and oceanic whitetip sharks (collectively called the requiem sharks). They are
distinguished by a an elongated snout, a nictitating membrane which protects the
eyes during an attack and a heterocercal tail. Along with the houndsharks, catsharks
and hammerhead sharks.
 Lamniformes (Lamnidae): They are commonly known as the mackerel sharks. They
include the goblin shark, basking shark, megamouth shark, the thresher sharks, shortfin
and longfin mako sharks, and great white shark. They are distinguished by a large
mouth and jaws extending behind their eyes, two dorsal fins, five gill slits,and
eyes without nictitating membranes, and ovoviviparous reproduction. The
Lamniformes include the extinct megalodon, Carcharodon megalodon.
Life history - Shark lifespans vary by species. Most live 20 to 30 years. The spiny dogfish has
the longest lifespan at more than 100 years. Whale sharks (Rhincodon typus) may also live over
100 years.
Well-known species such as the great white and the hammerhead are apex predators at the
top of the underwater food chain. Their extraordinary skills as predators fascinate and
frighten humans, even as their survival is under serious threat from fishing and other human
activities.
*Rays are very similar to sharks. They belong to the same subclass, but rays have enlarged
pectoral fins (the side fins) and a well-developed rope-like tail. A major difference between
sharks and rays is apparent in their method of propulsion. While sharks swim mostly by the
action of their tail, rays swim by wave-like motions made with their pectoral fins, or “wings”.
Another difference between sharks and rays is that rays have mouths on the underside of their
body, because, unlike most sharks, rays tend to be bottom feeders.
*Skates are also close relatives of sharks and rays. They are similar in appearance to rays but
lack venomous tail spines and have a different mode of reproduction.
Skeleton - Shark skeletons are very different from those of bony fish and terrestrial
vertebrates. They lack true bones, instead they have cartilaginous skeletons:
Sharks, as well as rays and some other fishes, belong to the class Chondrichthyes, meaning
“cartilaginous fish.” Cartilage is a relatively soft, translucent, elastic tissue. In people, cartilage
is the soft, flexible material found inside the upper outer ear, in the nose, and in knee joints.
Shark skeletons are made entirely of calcified cartilage. Physically, cartilage serves as a skeletal
support structure that is gristly, relatively rigid, and resistant to forces. Cartilage is also less
dense than bone, making sharks’ skeletons lighter in weight, therefore helping them
to stay buoyant in the water. Sharks have no rib cage and therefore on land a shark's own
weight can literally crush it.
Jaw - Like its relatives, rays and skates, the shark's jaw is not attached to the cranium. The
jaw's surface, like the shark's vertebrae and gill arches, needs extra support due to its heavier
exposure to physical stress and its need for strength. In certain locations where strength is
particularly important - such as the jaws and parts of the backbone - shark cartilage is fortified
with tiny, hexagonal crystals of calcium salts called "tesserae" (after the little tiles that
compose mosaics). Tesserae give shark cartilage much of the strength of bone without
most of the accompanying weight.
Generally there is only one layer of tesserae in sharks, but the jaws of large specimens, such as
the bull shark, tiger shark, and the great white shark, have two to three layers or more,
depending on body size. The jaws of a large white shark may have up to five layers. In the
rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.
Teeth - The teeth of sharks are embedded in the gums rather than directly fixed to the
jaw, and are constantly replaced throughout the shark's life. Multiple rows of
replacement teeth are grown in a groove on the inside of the jaw and moved forward
in a "conveyor belt"; some sharks lose 30,000 or more teeth in their lifetime. The rate
of tooth replacement varies from once every 8–10 days to several months. In most
species teeth are replaced one at a time, while in the cookiecutter sharks the entire row of teeth
is replaced simultaneously.
The shape of a shark's tooth depends on its diet: those that feed on mollusks and crustaceans
have dense flattened teeth for crushing, those that feed on fish have needle-like teeth for
gripping, and those that feed on larger prey such as mammals have pointed lower teeth for
gripping and triangular upper teeth with serrated edges for cutting. The teeth of planktonfeeders such as the basking shark are greatly reduced and non-functional.
*Shark skin feels like sandpaper:
Shark skin feels rough because it is made up of tiny teeth-like structures called placoid scales,
also known as dermal denticles. These scales point towards the tail and help to reduce friction
from surrounding water when the shark swims. Because of this, if someone rubbed the skin
from the head towards the tail, it would feel smooth, but in the opposite direction the skin feels
very rough, like sandpaper.
Unlike bony fish, sharks have a complex dermal corset made of flexible collagenous fibers and
arranged as a helical network surrounding their body. This works as an outer skeleton, providing
attachment for their swimming muscles and thus saving energy. In the past, sharkskin has been
used as sandpaper. Their dermal teeth give them hydrodynamic advantages as they reduce
turbulence when swimming.
*Extra Note: The silky shark acquired its name because it has very small scales, giving it a
smooth, “silky” touch.
Fins - *Sharks have five different types of fins:
The fin skeletons are elongated and supported with soft and unsegmented rays named
ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers.
Sharks can only drift away from objects directly in front of them because their fins do not allow
them to swim backwards.
Pectoral (2): These fins are large and project from both sides of the shark near its front end.
The paired pectoral fins are used primarily for lift as the shark swims.
Pelvic (2): Two similar but much smaller fins are located behind the pectoral fins and are used
for stabilization while the shark swims.
Dorsal (1 or 2): Sharks may have one or two dorsal fins. Both rise vertically from the top of
the shark. The first dorsal fin is the fin that most people picture when they think of sharks: it
is the fin that commonly appears skimming along the water's surface. The other dorsal fin is
called the second dorsal fin and it is usually much smaller than the first dorsal fin. It is located
near the tail of the shark. The dorsal fins are used for stability.
Anal (1 or 0): Many sharks have an anal fin, which functions primarily for stability. It is
located on the bottom side of the shark between the pelvic and caudal fins.
Caudal (1): The caudal fin is also called the tail fin. This fin provides thrust for moving the
shark through the water as it swims.
Tails - Sharks have very distinctive tails. The tails (caudal fins) of sharks vary considerably
between species and are adapted to the lifestyle of the shark. Different tail shapes have evolved
in sharks adapted for different environments. Sharks possess a heterocercal caudal fin in
which the dorsal portion is usually noticeably larger than the ventral portion. This is
due to the fact that the shark's vertebral column extends into that dorsal portion, allowing for a
greater surface area for muscle attachment which would then be used for more efficient
locomotion among the negatively buoyant cartilaginous fishes. This is in contrast to most bony
fishes, which possess a homocercal caudal fin.
Physiology: Buoyancy - Unlike bony fish, sharks do not have gas-filled swim bladders for
buoyancy. Instead, sharks rely on a large liver, filled with oil that contains squalene and the fact
that cartilage is about half as dense as bone. The liver constitutes up to 30% of their body mass.
The liver's effectiveness is limited, so sharks employ dynamic lift, to maintain depth and then
sink when they stop swimming. Dynamic Lift is done using their pectoral fins in a manner similar
to the use of wings by aeroplanes and birds. As these fish swim, their pectoral fins are
positioned to create lift which allows the fish to maintain a certain depth. Sand tiger sharks are
also known to store air in their stomachs, using the stomach as a swim bladder. Most sharks
need to constantly swim in order to breathe and cannot sleep very long, if at all, or they will
sink. However certain shark species, like the nurse shark, are capable of pumping water across
their gills, allowing them to rest on the ocean bottom.
Some sharks, if inverted (turning them upside down) or stroked on the nose, enter a natural
state of tonic immobility (paralysis). Researchers can use this condition to handle sharks safely.
The shark remains in this state of paralysis for an average of fifteen minutes before it recovers.
Scientists have exploited this phenomenon to study shark behavior. When testing sharks under
this "tonic" state, scientists will put a chemical plume in the water awakening the shark.
Respiration - Sharks have 5-7 pairs of gill slits located on the sides of their heads. Unlike
bony fish, they do not have gill covers. Water must continually flow across these slits in order for
the shark to breathe. This can be accomplished by the shark's swimming, by it standing still in a
current, or by it fanning water across the gills with its fins (this is done by the nurse shark). A
modified slit called a spiracle lies just behind the eye; the spiracle assists water intake during
respiration and plays a major role in bottom dwelling sharks. Spiracles are reduced or missing in
active pelagic sharks. While the shark is moving, water passes through the mouth and over the
gills in a process known as "ram ventilation". While at rest, most sharks pump water over
their gills to ensure a constant supply of oxygenated water. A small number of species have lost
the ability to pump water through their gills and must swim without rest. These species are
obligate ram ventilators (Great Whites and Hammerheads) and would presumably asphyxiate
if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species.
The respiration and circulation process begins when deoxygenated blood travels to the shark's
two-chambered heart. Here the shark pumps blood to its gills via the ventral aorta artery where
it branches off into afferent brachial arteries. Reoxygenation takes place in the gills and the
reoxygenated blood flows into the efferent brachial arteries, which come together to form the
dorsal aorta. The blood flows from the dorsal aorta throughout the body. The deoxygenated
blood from the body then flows through the posterior cardinal veins and enters the posterior
cardinal sinuses. From there blood enters the heart ventricle and the cycle repeats.
Thermoregulation - *Most sharks are cold-blooded or more precisely ectothermic
(poikilothermic), meaning that their internal body temperature matches that of their ambient
environment.
*Some sharks have warm-blooded capabilities:
There are five species of sharks that have been documented as having some warm blooded
characteristics.
All five of these species belong to the family Lamnidae (also called mackerel sharks), and
include: the white, shortfin mako, longfin mako, porbeagle, and salmon sharks.
Members of the Lamnidae family are endothermic (homeothermic) and maintain a higher body
temperature than the surrounding water. In these sharks, a strip of aerobic red muscle located
near the center of the body generates the heat, which the body retains via a countercurrent
exchange mechanism called the rete mirabile ("miraculous net") which is a highly-developed
network of blood vessels. The common thresher shark has a similar mechanism for maintaining
an elevated body temperature, which is thought to have evolved independently.
Osmoconformers vs. Osmoregulators - The blood of the shark is usually isotonic to its
watery home. This means that there is an equal concentration of solutes within their body as
there are in the ocean in which they live. So, they maintain osmotic balance with the seawater.
These types of animals are also known as osmoconformers. This is the opposite of
osmoregulators, which maintain a different osmotic balance to that of their environment. Marine
animals are usually osmoconformers, while freshwater species are generally osmoregulators.
Senses: *Sharks have six keen senses to detect prey:
Sharks use the senses of smell, taste, vision, hearing, touch through the lateral line
system and skin, and a sixth sense us humans do not have, electroreception for capturing
prey.
Smell - The shape of the hammerhead shark's head may enhance olfaction by spacing the
nostrils further apart. A shark has two noses, on each side of its head. These noses also have
two holes. Through the one hole the water gets in and through the other it gets out. The water
floats permanently through the nose, so the shark can recognize every change. A shark’s nose is
extremely good. For example, the great white shark can smell one single drop of blood in a full
bathtub of water.
If a shark isn’t sure its prey tastes good or not he uses its fourth sense, taste. Instead of a
tongue the shark uses its whole body to test the taste of its prey. The cells of taste are spread
out over the whole skin. Touching the shark can prove if the prey is good or not.
Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony
fish) between the anterior and posterior nasal openings, with some species able to detect as
little as one part per million of blood in seawater. They are more attracted to the chemicals
found in the guts of many species, and as a result often linger near or in sewage outfalls. Some
species, such as nurse sharks, have external barbels that greatly increase their ability to sense
prey.
Sight - Shark eyes are similar to the eyes of other vertebrates, including similar lenses, corneas
and retinas, though their eyesight is well adapted to the marine environment with the help of a
tissue called tapetum lucidum. This means that sharks can contract and dilate their pupils, like
humans, something no teleost fish can do. This tissue is behind the retina and reflects light back
to it, thereby increasing visibility in the dark waters. The effectiveness of the tissue varies, with
some sharks having stronger nocturnal adaptations.
Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To
protect their eyes some species have nictitating membranes. This membrane covers the eyes
during predation, and when the shark is being attacked.
However, some species, including the great white shark (Carcharodon carcharias), do not have
this membrane, but instead roll their eyes backwards to protect them when striking prey. The
importance of sight in shark hunting behavior is debated. Some believe that electro- and
chemoreception are more significant, while others point to the nictating membrane as evidence
that sight is important. Presumably, the shark would not protect its eyes were they unimportant.
The use of sight probably varies with species and water conditions. In effect the shark's field of
vision can swap between monocular and stereoscopic at any time.
The hammerhead’s eyes and nostrils are spread far apart. This may help the shark search
laterally, or horizontally, for food. It is thought that the head structure may also give the shark
some electrosensory advantages with an increased surface area for electroreceptors (we’ll get to
those later…just remember that this shark has a larger head with more room for things called
electroreceptors).
Hearing *Sharks have an excellent sense of hearing:
Sharks have better hearing than humans.
*Sharks have ears located inside their heads, rather than external ears like humans:
Sharks can hear best at frequencies below 1,000 Hertz, which is the range of most natural
sounds. Humans generally hear sound waves whose frequencies are between 20 and 20,000 Hz.
This keen sense allows sharks to locate potential prey that are swimming and splashing in the
water.
A small opening on each side of their heads (not to be confused with the spiracle) leads directly
into the inner ear through a thin channel. The shark inner ear is a fluid-filled structure
consisting of a cartilaginous sac to which are attached three D-shaped cartilage tubes. These
fluid-filled tubes are set at right angles to one another and are lined with hair cells.The lateral
line shows a similar arrangement, which is open to the environment via a series of openings
called lateral line pores. The receptors in the lateral line are neuromasts, each of which is
composed of a group of hair cells. The hairs are surrounded by a protruding jelly-like cupula,
typically 1/10 to 1/5 mm long. The hair cells and cupolas of the neuromasts are usually at the
bottom of a visible pit or groove in the fish. The hair cells in the lateral line are similar to
the hair cells inside the vertebrate inner ear, suggesting that the lateral line and the
inner ear share a common origin.
This is a reminder of the common origin of these two vibration- and sound-detecting organs that
are grouped together as the acoustico-lateralis system. In bony fish and tetrapods the external
opening into the inner ear has been lost.
Lateral line - This system is found in most fish, including sharks. It detects motion or vibrations
in water. The shark uses its lateral line to detect the movements of other organisms, especially
wounded fish. The shark can sense frequencies in the range of 25 to 50 Hz.
Electroreception - The Ampullae of Lorenzini are the electroreceptor organs, and they number
in the hundreds to thousands. Sharks use the Ampullae of Lorenzini to detect the
electromagnetic fields that all living things produce. This helps sharks (mostly the hammerhead)
find prey. The shark has the greatest electrical sensitivity of any animal. Sharks find prey hidden
in sand by detecting the electric fields they produce. Ocean currents moving in the magnetic
field of the Earth also generate electric fields that sharks can use for orientation and possibly
navigation.
Reproduction - *Males have claspers and females do not:
Only male sharks have paired external organs called claspers. Claspers are modifications of the
pelvic fin located on the inner side of the pelvic fins. These organs are used during reproduction.
All sharks have internal fertilization (meaning that sperm is deposited within the female shark).
Mating has only been observed in a few species of sharks. During mating, the male bites the
female’s pectoral fin and rolls her over. The male inserts one clasper into the female’s cloaca to
deposit sperm.
Mating has rarely been observed in sharks. The smaller catsharks often mate with the male
curling around the female. In less flexible species the two sharks swim parallel to each other
while the male inserts a clasper into the female's oviduct. Females in many of the larger species
have bite marks that appear to be a result of a male grasping them to maintain position during
mating. The bite marks may also come from courtship behavior: the male may bite the female
to show his interest. In some species, females have evolved thicker skin to withstand these
bites.
Unlike most bony fishes, sharks are K-selected reproducers, meaning that they produce a small
number of well-developed young as opposed to a large number of poorly-developed young.
Fecundity (reproductive potential) in sharks ranges from 2 to over 100 young per reproductive
cycle.
Sharks display three ways to bear their young, varying by species, Oviparity, Viviparity
and Ovoviviparity.
*Oviparous (egg laying species):
Oviparous species lay eggs that develop and hatch outside the mother’s body.
In most of these species, the developing embryo is protected by an egg case with the
consistency of leather. Sometimes these cases are corkscrewed into crevices for protection. The
mermaid's purse, which can wash up on shore, is an empty egg case..
*Egg cases of many sharks and skates are referred to as “mermaid’s purses”:
This name is given to the egg cases of skates and many sharks.
*Each mermaid’s purse contains one fertilized egg from which a young shark or skate
later emerges:
Shark species that utilize this mode of reproduction include the swell shark, chain dogfish, and
angel shark.
Extra Note: The hooks and screws on the side of a mermaid’s purse are called “tendrils” and
they are used to latch on to foliage, rocks, or coral on the bottom of the ocean floor to be
protected from currents or other creatures looking for a snack.
Ovoviviparity - Aplacental viviparity: Eggs hatch inside the female, no placenta to nourish the
pups. Most sharks are ovoviviparous, this means that the egg's yolk and fluids secreted by
glands in the walls of the oviduct nourishes the embryos. The eggs hatch within the oviduct, and
the young continue to be nourished by the remnants of the yolk and the oviduct's fluids. As in
viviparity, the young are born alive and fully functional.
Some species practice oophagy, where the first embryos to hatch eat the remaining eggs in the
oviduct. This practice is believed to be present in all lamniforme sharks, while the developing
pups of the grey nurse shark take this a stage further and consume other developing embryos
(intrauterine cannibalism). The survival strategy for the species that are ovoviviparous is that
the young are able to grow to a comparatively large size before birth.
The whale shark is now considered to be ovoviviparous after long having been classified as
oviparous. Extrauterine whale shark eggs are now thought to have been aborted. Most
ovoviviparous sharks give birth in sheltered areas, including bays, river mouths and shallow
reefs. They choose such areas for protection from predators (mainly other sharks) and the
abundance of food. Dogfish have the longest known gestation period of any shark, at 18 to 24
months. Basking sharks and frilled sharks appear to have even longer gestation periods, but
accurate data are lacking.
Viviparity - *Viviparous (live-bearing species): Placental viviparity: give birth to live
sharks, placenta nourishes pups.
Finally some sharks maintain a placental link to the developing young, this method is called
viviparity. This is more analogous to mammalian gestation than that of other fishes. The young
are born alive and fully functional. Hammerheads, the requiem sharks (such as the bull and blue
sharks), and smoothhounds are viviparous.
Behavior - The classic view describes a solitary hunter, ranging the oceans in search of food.
However, this applies to only a few species, with most living far more sedentary, benthic lives.
Even solitary sharks meet for breeding or at rich hunting grounds, which may lead them to cover
thousands of miles in a year. Migration patterns in sharks may be even more complex than in
birds, with many sharks covering entire ocean basins. However, shark behavior has only begun
to be formally studied, so there is much more to learn.
Sharks can be highly social, remaining in large schools; sometimes over 100 scalloped
hammerheads congregate around seamounts and islands, e.g., in the Gulf of California. Crossspecies social hierarchies exist with oceanic whitetip sharks dominating silky sharks of
comparable size during feeding.
When approached too closely some sharks perform a threat display to warn off prospective
predators. This usually consists of exaggerated swimming movements, and can vary in intensity
according to the threat level.
Feeding - Most sharks are carnivorous. Some species, including tiger sharks, eat just about
anything. The vast majority seek particular prey, and rarely vary their diet.
Whale, basking and megamouth sharks use filter feeding. These three species evolved plankton
feeding independently and use different strategies. Whale sharks use suction to take in plankton
and small fishes. Basking sharks are ram-feeders, swimming through plankton blooms with their
mouth wide open. It normally does this at the surface of the water, and also has the habit of
sunning itself at the surface which is why it is named the “basking shark.” Food is strained
from the water by gill rakers located in the gill slits. The 1000-1300 gill rakers can strain
up to 2000 tons of water per hour. These sharks feed along areas that contain high densities of
large zooplankton (i.e., small crustaceans, invertebrate larvae, and fish eggs and larvae).
Megamouth sharks make suction feeding more efficient, using luminescent tissue inside the
mouth to attract prey in the deep ocean. This type of feeding requires gill rakers, long slender
filaments that form a very efficient sieve, analogous to the baleen plates of the great whales.
The shark traps the plankton in these filaments and swallows from time to time in huge
mouthfuls. Teeth in these species are comparatively small because they are not needed for
feeding.
Other highly specialized feeders include cookiecutter sharks, which feed on flesh sliced out of
other larger fish and marine mammals. The teeth in these sharks are enormous compared to
their size. The lower jaw’s teeth are particularly sharp. Although they have never been observed
feeding they are believed to latch onto their prey and use their thick lips to make a seal, twisting
their bodies to rip off flesh.
Some seabed dwelling species are highly effective ambush predators. Angel sharks and
wobbegongs use camouflage to lie in wait and suck prey into their mouths. Many benthic sharks
feed solely on crustaceans which they crush with their flat molariform teeth.
Other sharks feed on squid or fishes, which they swallow whole. The viper dogfish has teeth it
can point outwards to strike and capture prey that it then swallows intact. The great white and
other large predators can either swallow small prey whole or take huge bites out of large
animals. Thresher sharks use their long tails to stun shoaling fishes, and sawsharks either stir
prey from the seabed or slash at swimming prey with their tooth-studded rostra.
Many sharks, including the whitetip reef shark are cooperative feeders and hunt in packs to herd
and capture elusive prey. These social sharks are often migratory, travelling huge distances
around ocean basins in large schools. These migrations may be partly necessary to find new
food sources.
Speed - In general, sharks swim ("cruise") at an average speed of 8 kilometres per hour
(5.0 mph) but when feeding or attacking, the average shark can reach speeds upwards of
19 kilometres per hour (12 mph).
*The fastest shark is the shortfin mako:
The shortfin mako shark, the fastest shark and one of the fastest fish, can burst up to
50 kilometres per hour (31 mph). And has an average adult size of 10 ft. (The great white shark
is also capable of bursts of speed. These exceptions may be due to the warm-blooded, or
homeothermic, nature of these sharks' physiology.)
The shortfin mako body is conic-cylindrical and extremely hydrodynamic, meaning that its body
shape is helpful for swimming at fast speeds underwater. Because it can swim so fast, this
shark feeds on very fast fish such as tuna and swordfish.
*Other interesting information about the shortfin mako:
The shortfin mako’s color is brilliant metallic blue on its top and white on bottom. Countershading helps to camouflage the shark. From above, this shark would look like the deep-blue
ocean and from below it may look more like the lighter colored surface of the water.
The shortfin mako has a very wide distribution range and has the unique capability of raising its
body temperature above the surrounding water temperature. Therefore, it is considered to have
some warm-blooded characteristics.
*The largest shark, and also the largest fish in the ocean is the whale shark, growing
up to 60 feet (18 m) in length:
The whale shark inhabits warm, tropical open seas. Whale sharks are grayish, bluish or brownish
on top, with an upper surface pattern of creamy white spots between pale stripes. The belly is
white. The function of the distinctive pattern of body marks is unknown.
The whale shark is generally considered harmless.
Extra Note: Whale sharks have been observed “bumping” fishing boats at the surface of the
water, possibly after being provoked. In reality, these sharks are at more risk of being hit by
boats than we are of being hurt by them!
*The whale shark feeds on plankton: It feeds actively by opening its mouth widely and
sucking in water. As the water that has been sucked in is pushed back out of its gills, plankton
becomes trapped against its gill rakers that line its gills.
Extra Note: the smallest shark is a deepwater dogfish shark, found in the Caribbean Sea. Its
mature length is under eight inches long!
Sleep - Sharks do not sleep like humans: Sharks have active and restful periods:
Scientists once believed that sharks could not sleep for more than a few minutes at a time since
their breathing requires movement to force oxygen-rich water across their gills. While some
species of sharks do need to swim constantly, this is not true for all sharks. Some sharks, such
as the nurse shark, have tiny structures called spiracles that allow water across their gills when
resting, they can lie on the bottom while actively pumping water over their gills, but their eyes
remain open and actively follow divers. When a shark is resting, it does not use its nares, but
rather its spiracles. If a shark tried to use its nares while resting on the ocean floor, it would be
sucking up sand rather than water. Many scientists believe this is one of the reasons sharks
have spiracles. The spiny dogfish's spinal cord rather than its brain, coordinates swimming, so it
is possible for spiny dogfish to continue to swim while sleeping. It is also possible that sharks
sleep in a manner similar to dolphins, one cerebral hemisphere at a time, thus maintaining some
consciousness and cerebral activity at all times.
Distribution and habitat - Sharks are found in all seas. They generally do not live in
freshwater, with a few exceptions such as the bull shark and the river shark which can swim
both in seawater and freshwater. Sharks are common down to depths of 2,000 metres
(7,000 ft), and some live even deeper, but they are almost entirely absent below 3,000 metres
(10,000 ft). The deepest confirmed report of a shark is a Portuguese dogfish at 3,700 metres
(12,100 ft).
*Some coastal shark species can survive in brackish estuaries with mixed fresh- and
saltwater:
Many juvenile sharks use these brackish areas as nursery grounds. There are only two known
shark species that are capable of surviving in freshwater for any length of time. Such sharks
have special physiological adaptations that allow them to thrive in freshwater. The bull shark
has been captured approximately 2,100 miles up the Amazon River, and about 1,700 miles up
the Mississippi River. The speartooth shark has been captured over 60 miles up the Adelaide
River in Australia. Though these two species of shark are capable of surviving in freshwater,
there are no populations living in completely landlocked freshwater lakes. They always have a
route that will connect them to the ocean.
There are also some species of rays that live in freshwater habitats. In Florida, the Atlantic
stingray is known to live in freshwater habitats connected to the sea. Some Atlantic stingray
populations live their entire lives in freshwater lakes. They represent the only North American
elasmobranch to permanently reside in freshwater.
Interaction with humans: Shark fishery - It is estimated that 100 million sharks are killed
by people every year, due to commercial and recreational fishing. Sharks are a common seafood
in many places, including Japan and Australia. In the Australian state of Victoria, shark is the
most commonly used fish in fish and chips, in which fillets are battered and deep-fried or
crumbed and grilled. In fish and chip shops, shark is called flake. In India, small sharks or baby
sharks (called sora in Tamil language, Telugu language) are sold in local markets. Since the flesh
is not developed, boiling breaks it into powder, which is then fried in oil and spices (called sora
puttu). The soft bones can be easily chewed. They are considered a delicacy in coastal Tamil
Nadu. Icelanders ferment Greenland sharks to produce hákarl, which is widely regarded as a
national dish.
Sharks are often killed for shark fin soup. Finning involves removing the fin with a hot metal
blade. Fishermen capture live sharks, fin them, and dump the finless animal back into the water.
The now immobile shark soon dies from suffocation or predators. Shark finning has become a
major trade within black markets all over the world. Shark fins sell for about $300/lb in 2009.
Poachers illegally fin millions of sharks a year. Few governments enforce laws that protect them.
Shark fin soup is a status symbol in Asian countries, and is considered healthy and full of
nutrients, with some even claiming fins prevent cancer and other ailments. No scientific proof
supports these claims; at least one study has shown shark cartilage of no value in cancer
treatment. The shark fin trade is a major problem and has gained international attention.
Conservationists have campaigned to make finning illegal in the U.S.
The shark species responsible for most unprovoked attacks on humans are the white shark, tiger
shark, and bull shark. However, all sharks, large and small, are predators capable of inflicting
wounds if provoked. They should all be treated with respect when encountered.
Unprovoked: incidents where an attack on a live human by a shark occurs in its natural habitat
without human provocation of the shark. (Incidents involving sharks and divers in public aquaria
or research holding-pens, shark-inflicted scavenge damage to already dead humans (most often
drowning victims), attacks on boats, and provoked incidents occurring in or out of the water are
not considered unprovoked attacks.)
"Provoked attacks" usually occur when a human initiates physical contact with a shark, e.g. a
diver bit after grabbing a shark, a fisher bit while removing a shark from a net, a person holding
a fish underwater.
*Sharks are especially vulnerable to overfishing:
Sharks have several biological characteristics that differ from those of most bony fishes, making
them more vulnerable to overfishing. These aspects of their life histories have served sharks
admirably for millions of years in their roles as apex predators, but now they have become
serious liabilities, or drawbacks, when faced with human predation. Sharks:
1) are slow growing, are slow to reach sexual maturity, have a long reproductive cycle, produce
low numbers of young, and have specific mating and nursing areas. (Each of these factors
contributes to the inability of most sharks to reproduce quickly in order to withstand
overfishing.)
2) are highly migratory creatures (fisherman can follow schools of migratory sharks)