All About Sharks
Selachimorpha) are fish with a full cartilaginous skeleton and a
streamlined body (versus fish with boney skeletons). They breathe with the
use of five to seven gill slits and have
replaceable teeth. They are some of the world's most misunderstood
predators, as they very rarely attack humans unless intimidated.
Sharks include species from the hand-sized pygmy shark, a deep sea
species of only 22 cm in length, to the whale shark, the largest fish in the
grows to a length of approximately 41 feet and which, like the great whales,
feeds only on plankton through filter feeding.
The sharks on
exhibit at the Indianapolis Zoo are dogsharks, a
relatively small species. The bull shark is the best
known of several species to swim in both salt and fresh water (it is found
in Lake Nicaragua, in Central America) and in deltas.
Photo by Kerrie Best
The skeleton of a shark is very different from that of bony fishes such
as cod; it is made from cartilage, which is very light and flexible,
although the cartilage in older sharks can sometimes be partly calcified,
making it harder and more bone-like. The shark's jaw is
thought to have evolved from the first gill arch. It is not attached to the
cranium and has extra mineral deposits to give it greater strength.
Like other fishes, sharks extract oxygen from seawater as it passes over
their gills. Some sharks have a modified slit called a spiracle located just
behind the eye, which is used in respiration. Due to their size and the
nature of their metabolism, many sharks have a higher demand for oxygen than
bony fishes and cannot rely on ambient water currents to provide enough oxygenated water. If these sharks were to stop swimming, the water
circulation would drop below the level necessary for respiration and the
animal would suffocate. Some sharks, such as
the nurse shark, can pump water over their gills as they rest.
Unlike bony fishes, sharks do not have
gas-filled swim bladders, but instead rely on a large liver filled with oil. The
liver may constitute up to 25% of their body mass and its
effectiveness is limited, so sharks employ dynamic lift to maintain depth
and sink when they stop swimming. Some sharks, if inverted, enter a natural
state of tonic immobility where they do not move - researchers use this condition for handling
The teeth of carnivorous sharks are not attached to the jaw, but embedded
in the flesh, and in many species are constantly replaced throughout the
shark's life; some sharks can lose 30,000 teeth in a lifetime. All sharks
have multiple rows of teeth along the edges of their upper and lower jaws.
New teeth grow continuously in a groove just inside the mouth and move
forward from inside the mouth on a "conveyor belt" formed by the skin in
which they are anchored. In some sharks rows of teeth are replaced every
8–10 days, while in other species they could last several months. The lower
teeth are primarily used for holding prey, while the upper ones are used for
cutting into it. The teeth range from thin, needle-like teeth for gripping
fish to large, flat teeth adapted for crushing shellfish.
The tails (caudal fins) of sharks vary
considerably between species and are adapted to the lifestyle of the shark.
The tail provides thrust, and so speed and acceleration are dependent on
tail shape. Different tail shapes have evolved in sharks adapted for
In general, sharks swim ("cruise") at an average speed of 5 mph, but when
feeding or attacking, the average shark can reach speeds upward of 12 mph.
The shortfin mako may range upwards of 31 mph and is considered to be the
fastest shark and one of the fastest fish. The great white shark is also
capable of considerable bursts of speed. These exceptions may be due to the
"warm-blooded" nature of these sharks' physiology.
Unlike bony fishes, sharks have a complex
skin made of flexible
collagenous fibers arranged around their bodies. This works as an outer skeleton, providing attachments for their swimming
muscles and thus saving energy. The skin also helps them reduce turbulence when swimming.
The fossil record of sharks extends
back over 450 million years - before land vertebrates existed and before
many plants had colonized the continents. The first sharks looked very
different from modern sharks. The majority of the modern sharks can be
traced back to around 100 million years ago.
Instead of bones, sharks have cartilagenous skeletons, with a bonelike layer
broken up into thousands of isolated apatite prisms. When a shark dies, the
decomposing skeleton breaks up and the apatite prisms scatter. Complete
shark skeletons are only preserved when rapid burial in bottom sediments
occurs. Mostly only the fossilized teeth of sharks are found, although
often in large numbers. As the teeth consist of mineral apatite
(calcium phosphate), they are easily fossilized.
Among the most ancient and primitive sharks is Cladoselache, from about 370
million years ago, which has been found within the Paleozoic strata of Ohio,
Kentucky and Tennessee. At this point in the Earth's history these rocks
made up the soft sediment of the bottom of a large, shallow ocean that
stretched across much of North America. Cladoselache was only about
1 m long with stiff triangular fins and slender jaws. Its teeth had several
pointed cusps, which would have been worn down by use. The discovery of whole fish
found tail first in their stomachs suggest that they were fast swimmers with
From about 300 to 150 million years ago, most fossil sharks can be assigned
to one of two groups. One of these, the xenacanths, was almost exclusive to
freshwater environments. By the time this group became extinct (about 220
million years ago) they had achieved worldwide distribution. The other
group, the hybodonts, appeared about 320 million years ago and was mostly
found in the oceans, but also in freshwater.
Modern sharks began to appear about 100 million years ago. Fossil mackerel
shark teeth occurred in the Lower Cretaceous. The oldest white shark teeth
date from 60 to 65 million years ago, around the time of the extinction of
the dinosaurs. In early white shark evolution there are at least two
lineages: one with coarsely serrated teeth that probably gave rise to the
modern great white shark, and another with finely serrated teeth and a
tendency to attain gigantic proportions. This group includes the extinct
It is believed that the immense size of predatory sharks such as the great
white may have arisen from the extinction of the dinosaurs and the
diversification of mammals. It is known that at the same time these sharks
were evolving some early mammalian groups evolved into aquatic forms.
Certainly, wherever the teeth of large sharks have been found, there has
also been an abundance of marine mammal bones, including seals, porpoises
and whales. These bones frequently show signs of shark attack. There are
theories that suggest that large sharks evolved to better take advantage of
There are more than 360 described species of sharks,
which are related to rays and skates.
The sex of a shark can be easily
determined. The males have modified pelvic fins that have become a pair of
claspers. The name is somewhat misleading as they are not used to hold on to
the female, but fulfill the role of the mammalian penis.
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.
Sharks normally produce around a
dozen pups (blue sharks have been recorded as producing 135 and some species
produce as few as two). These pups are either protected by egg cases or born
live. No shark species are known to provide post-natal parental protection
for their young, but females have a hormone that is released into their
blood during the pupping season that apparently keeps them from feeding on
Sharks have keen olfactory senses,
with some species able to detect as little as one part per million of blood
in seawater. They are attracted to the chemicals found in the guts of many
species, and as a result often linger near or in sewage outfalls.
Sharks generally rely on their superior sense of smell to find prey, but at
closer range they also use the lateral lines running along their sides to
sense movement in the water, and also employ special sensory pores on their
heads to detect electrical fields created by prey and the ambient electric
fields of the ocean.
Shark eyes are similar to the eyes of other vertebrates, including
similar lenses, corneas and retinas, although their eyesight is well adapted
to the marine environment with the help of a tissue called tapetum lucidum.
This tissue is behind the retina and reflects light back to the retina,
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 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.
Sharks also have a sharp sense of hearing
and can hear prey many miles away. 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 has the greatest electricity sensitivity known in all
animals. This sense is used to find prey hidden in sand by detecting the
electric fields inadvertently produced by all fish. It is this sense that
sometimes confuses a shark into attacking a boat: when the metal interacts
with salt water, the electrochemical potentials generated by the rusting
metal are similar to the weak fields of prey, or in some cases, much
stronger than the prey's electrical fields: strong enough to attract sharks
from miles away. The oceanic currents moving in the magnetic field of the
Earth also generate electric fields that can be used by the sharks for
orientation and navigation.
This system is found in most fish, including sharks. It is used to detect
motion or vibrations in the water. The shark uses this to detect the
movements of other organisms, especially wounded fish. The shark can sense
frequencies in the range of 25 to 50 Hz.
view of the shark is of a solitary hunter, ranging the oceans in search of
food; this is only true for a few species, with most living far more
sedentary lives. Even solitary sharks meet for breeding or on 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.
Some sharks can be highly social, remaining in large schools, sometimes up
to over 100 individuals for scalloped hammerheads congregating around
seamounts and islands e.g. in the Sea of Cortez. Cross-species social
hierarchies exist with oceanic whitetip sharks dominating silky sharks of
comparable size when feeding.
When approached too closely some sharks will perform a threat display to
warn off the prospective predators. This usually consists of exaggerated
swimming movements, and can vary in intensity according to the level of
Despite the common myth that sharks are instinct-driven "eating
machines," recent studies have indicated that many species possess powerful
problem-solving skills, social complexity and curiosity. The
brain-mass-to-body-mass ratios of sharks are similar to those of mammals and
other higher vertebrate species.
Sharks have even been known to engage in playful activities (a trait also
observed in cetaceans and primates). Porbeagle sharks have been seen
repeatedly rolling in kelp and have even been observed chasing an individual
trailing a piece behind them.
It is unclear how sharks sleep. Some sharks 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, they do not use their
nares, but rather their spiracles. If a shark tried to use their nares while
resting on the ocean floor, they 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 a spiny dogfish to continue to swim while
sleeping. It is also possible that a shark can sleep with only parts of its
brain in a manner similar to dolphins.
Contrary to popular belief, only a few sharks are dangerous to humans.
Out of more than 360 species, only four have been involved in a significant
number of fatal, unprovoked attacks on humans: the great white, tiger,
oceanic whitetip and bull sharks. These sharks, being large, powerful
predators, may sometimes attack and kill people, but all of these sharks
have been filmed in open water, without the use of a protective cage.
The perception of sharks as dangerous animals has been popularized by
publicity given to a few isolated unprovoked attacks, such as the Jersey
Shore Shark Attacks of 1916, and through popular fictional works about shark
attacks, such as the Jaws film series. The author of Jaws, Peter Benchley,
had in his later years attempted to dispel the image of sharks as man-eating
Habitat & Conservation
A December 10, 2006 report by the
Census of Marine Life group reveals that 70% of the world's oceans are
shark-free. They have discovered that although many sharks live up to depths
as low as 1,500 m, they fail to colonize deeper, putting them more easily
within reach of fisheries and thus endangered status.
The majority of shark fisheries around the globe have little monitoring
or management. With the rise in demand of shark products there is a greater
pressure on fisheries. Stocks decline and collapse because sharks are
long-lived apex predators with comparatively small populations, which makes
it difficult for them breed rapidly enough to maintain population levels.
Major declines in shark stocks have been recorded in recent years - some
species have been depleted by over 90% over the past 20-30 years with a
population decline of 70% not being unusual. Many governments and the UN
have acknowledged the need for shark fisheries management, but due to the
low economic value of shark fisheries, the small volumes of products
produced and the poor public image of sharks, little progress has been made.
Many other threats to sharks include habitat alteration, damage and loss
from coastal developments, pollution and the impact of fisheries on the
seabed and prey species.
Every year, somewhere between 26 to 73 million sharks are killed by
people in commercial and recreational fishing. In the past, sharks were
killed simply for the sport of landing a good fighting fish (such as the
shortfin mako sharks). Shark skin is covered with dermal denticles, which
are similar to tiny teeth, and was used for purposes similar to sandpaper.
Other sharks are hunted for food (Atlantic thresher, shortfin mako and
others), and some species for other products.
Sharks are a common seafood in many places around the world, including Japan
and Australia. In the Australian State of Victoria, shark is the most
commonly used fish in fish and chips.
Sharks are often killed for shark fin soup: the finning process involves
capture of a live shark, the removal of the fin with a hot metal blade, and
the release of the live animal back into the water. Sharks are also killed
for their meat. The meat of dogfishes, smoothhounds, catsharks, skates and
rays is in high demand by European consumers. The situation in Canada and
the United States is similar: the blue shark is sought as a sport fish while
the porbeagle, mako and spiny dogfish are part of the commercial fishery.
There have been cases where hundreds of de-finned sharks were swept up on
local beaches without any way to convey themselves back into the
sea. Conservationists have campaigned for changes in the law to make finning
illegal in the U.S.
Shark cartilage has been advocated as effective against cancer and for
treatment of osteoarthritis. However, a trial by Mayo Clinic found no effect
in advanced cancer patients.
Sharks generally reach sexual maturity slowly and produce very few offspring
in comparison to other fishes that are harvested. This has caused concern
among biologists regarding the increase in effort applied to catching sharks
over time, and many species are considered to be threatened.
Some organizations, such as the Shark Trust, campaign to limit shark
A popular myth is that sharks are immune to disease and cancer; however,
this is untrue. There are both diseases and parasites that affect sharks.
The evidence that sharks are at least resistant to cancer and disease is
mostly anecdotal and there have been few, if any, scientific or statistical
studies that have shown sharks to have heightened immunity to disease.