Respiration
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 an adequate supply of 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.
The process of ensuring an adequate flow of the Gills by forward movement is known as Ram
Ventilation. Some Sharks, such as the Nurse Shark, can pump Water over their Gills as they rest. The Respiration and Circulation
Process begins when Deoxygenated Blood travels to the Shark's Two-Chambered Heart.
Here the Blood is pumped to the Shark's 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 Ventricle of the Heart and the Cycle repeats. This way of Respiration
is highly inefficient and if the Shark were to stop moving Blood would not be able to move through the Body due to the lack
of strength of the Heart's Atrium.
Buoyancy
Unlike Bony Fishes, Sharks do not have Gas-Filled Swim Bladders, but instead rely on a large
Liver filled with Oil that contains Squalene.
The Liver may constitute up to 25% of their Body Mass for Buoyancy.
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-Researchers use this condition for
handling Sharks safely.
Osmoregulation
In contrast to Bony Fishes, Sharks do not drink Seawater; instead they retain high concentrations
of Waste Chemicals in their Body to change the Diffusion Gradient so that they can absorb Water directly from the Sea.
This adaptation prevents most Sharks from surviving in Fresh Water, and they are therefore
confined to a Marine Environment. A few exceptions to this Rule exist, such as the Bull Shark, which has developed a way to
change its Kidney function to excrete large amounts of Urea.
Teeth
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 liftetime. 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 "Conveyer 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.
Tails
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 different Environments.
The Tiger Shark's Tail has a large Upper Lobe which delivers the maximum amount of power for
Slow Cruising or Sudden Bursts of Speed. The Tiger Shark has a Varied Diet, and because of this it must be able to twist and
turn in the Water easily when hunting, whereas the Porbeagle, which hunts Schooling Fishes such as Mackerel and Herring, has
a large Lower Lobe to provide greater Speed to help it keep pace with its fast-swimming Prey.
It is also believed that Sharks use the Upper Lobe of their Tails to counter the lift generated
by their Pectoral Fins.
Some Tail Adaptations have purposes other than providing thrust. The Cookiecutter Shark has
a Tail with broad Lower and Upper Lobes of similar Shape which are luminescent and may help to lure Prey towards the Shark.
The Thresher Feeds on Fish and Squid, which is believed to herd, then stun with its powerful and elongated Upper Lobe.
Speed
In general, Sharks swim ("Cruise") at an average Speed of 5mph, but when Feeding or Attacking,
the average Shark can reach Speeds upwards of 12mph. The Shortfin Mako may range upwards of 31mph 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," or homeothermic, nature of these Sharks Physiology.
Dermal Dentricles
Unlike Bony Fishes, Sharks have a Complex Dermal Corset made of flexible Collagenous Fibers
arranged as a Helical Network surrounding their Body. This works as an Outer Skeleton providing attachment for their Swimming
Muscles and thus saving Energy. Their Dermal Teeth give them Hydrodynamic Advantages as they reduce turbulence when swimming.
Body Temperature
A few of the Larger Species, such as the Shortfin Mako, and the Great White, are mildly Homeothermic-able
to maintain their Body Temperature above the surrounding Water Temperature.
This is possible because of the presence of the Rete Mirabile, a Counter Current Exchange
Mechanism that reduces the loss of Body Heat.
Muscular Contraction also generates a mild amount of Body Heat. However, this differs significantly
from True Homeothermy, as found in Mammals and Birds, in which Heat is generated, maintained, and regulated by Metabolic Activity.
Etymology
Until the 16th Century, Sharks were known to Mariners as "Sea Dogs."
Accorfing to the OED the name "Shark" first came into use after Sir John Hawkins' Sailors
exhibited one in London in 1569 to refer to the Large Sharks of the Caribbean Sea and later as a general term for all Sharks.
the name may have been derived from the Mayan Word for Fish, XOC, prnounced "shock' or "shawk."
Evolution
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. Mostly
only the Fossilized Teeth of Sharks are found, although often in large numbers. In some cases pieces of the Internal Skeleton
or even complete Fossilized Sharks have been discovered.
Estimates suggest that over a span of a few years a Shark may grow Tens of Thousands of Teeth,
which explains the abundance of Fossils. As the Teeth consist of Mineral Apatite (Calcium Phosphate), they are easily Fossilized.
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.
Among the most Ancient and Promitive 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 1m long with stiff triangular Fins and slender Jaws. Its Teeth
had several Pointed Cusps which would have been worn down by use.
From the number of Teeth found in any one place it is most likely that Cladoselache did not
replace its Teeth as regularly as Modern Sharks.
Its Caudal Find had a siimilar shape to the Pelagic Makos and Great White Sharks. The Discovery
of Whole Fish found Tail first in their Stomachs suggest that they were fast swimmers with great agility.
From about 300 to 150 Million Years ago, most Fossil Sharks can be assigned to one of Two
Groups: One of thes, the Xenacanths, was almost exclusive to Freshwater Environments. By the time this Group became Extinct
(about 200 Million Years ago) they had achieved Worldwide distribution. The other Group, the Hybodonts, appreared 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 Megalodon, Carcharodon Megalodon, which like most Extinct Sharks, is only known from its Teeth.
A reproduction of its Jaws was based on some of the largest Teeth that Measured up to almost 7" long and suggested a Fish
that could grow to a length of 80 to 100 ft. The reconstruction was found to be inaccurate, and estimates revised downward
to around 43 to 52 ft.
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 Larger Prey.
Classification
Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class
Chonarichthyes. The Elasmobranchii also include Rays and Skates; the Chondrichthyes also include Chimaeras. It is currently
thought that the Sharks from a Polyphyletic Group: in particular, some Sharks are more closely related to Rays than they are
to some other Sharks. There are more than 360 described Species of Sharks. There are Eight Orders of Sharks, listed below
in roughly their Evolutionary Relationship from more Primitive to more Modern Species:
. Hexanchiformes: Examples from this Group include the Cow Sharks, Frilled Shark and even
a Shark that looks on first inspection to be 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 the Fish that they eat.
. 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: These are commomly referred to as the Ground Sharks and some of the Species
include the Blue, Tiger, Bull, Reef and Oceanic Whitetip Sharks (collectively called the Requiem Sharks) along with the Hound
Sharks, Catsharks and Hammerhead Sharks. They are distinguished by an Elongated Snout and a Nictitating Membrane which protects
the Eyes during an Attack.
. Lamniformes: They are commonly known as the Mackerel Sharks. They include the Gobin Shark,
Basking Shark, Megamouth Shark, the Thresher Sharks, Shortfin and Longfin Mako Sharks, and Great White Shark. They are distinguished
by their Large Jaws and Ovoviviporous Reproduction. The Lamniformes include the Extinct Megalodon, Carcharodon Megalodon.
Reproduction
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 positon 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 have a different Reproductive Strategy from most Fish. Instead of Producing huge numbers
of Eggs and Fry (99.9% of which never reach Sexual Maturity in Fishes that use this Strategy), 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. Now 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 their Young.
There are Three Ways in which Shark Pups are Born:
. Oviparity - Some Sharks lay Eggs. 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, found
washed-up on Beaches, is an empty Case. Oviparous Sharks include the Horn Shark, Catshark, Port Jackson Shark and Swellshark.
. Viviparity - These Sharks maintain a Placental Link to the developing Young, more anagomous Mammalian Gestation than
that of other Fishes. The Young are Born alive and fully functional.
Hammerheads, the Requiem Sharks (such as the Bull and Tiger Sharks), the Basking Shark and the smooth Dogfish fall into
this Category. Dogfish have the longest known Gestation Period of any Shark at 18 to 24 Months. Basking Sharks and Frilled
Sharks are likely to have even longer Gestation Periods, but accurate Data is lacking.
. Ovoviviparity - Most Sharks utilize this Method. The Young are nourished by the Yolk of their Egg and by Fluids secreted
by Glands in the Walls of the Oviduct. 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 Oophogy, 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 larger
size before being Born.
The Whale Shark is now considered to be in this Category after long having been Classified as Oviparous. Whale Shark
Eggs found 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 because of the protection from Predators (mainly other Sharks) and the abundance of Food.
Shark Senses
Sense of Smell
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.
Some Species such as NurseSharks, have External Barbels that greatly increase their ability to Sense Prey. The Short Duct
between the Anterior and Posterior Nasal Openings are not fused as in Bony Fishes.
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.
Sense of Sight
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.
Sense of Hearing
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 Lateral Line shows a similar
arrangement, as is open to the Environment via a Series of Openings called Lateral Line Pores.
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 Fishes and Tetrapods the External Opening into the Inner Ear has been lost.
Ampullae of Lorenzini
The Ampullae of Lorenzini are the Electroreceptor Organs of the Sharks and they vary in number from a Couple of Hundred
to thousands in an Individual. 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 inadvertantly 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
Electric Chemical 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.
Lateral Line
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.
Behavior
Studies on the Behavior of Sharks have only recently been taken, leading to little information on the Subject, although
this is changing.
the Classic 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, Benthic Lives. Even Solitary Sharks meet for Breeding or on Rich Hunting
Grounds, which may lead them to cover Thousands of miles 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 Threat.
Shark Intelligence
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.
In 1987, near Sitswinkle Bay, South Africa, a group of up to Seven Great White Sharks worked together to relocate the
Partially Beached Body of a Dead Whale to deeper Waters to Feed.
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 seen chasing an Individual trailing a Piece behind them.
Shark Sleep
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 thier Narestout 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.
Shark Attacks
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 Monsters.
In 2005, according to the International Shark Attack File, there were a total of 58 Unprovoked Attacks Recorded Worldwide,
of which Four were Fatal. In 2005 the International Shark Attack File (ISAF) took an Investigation on 105 Shark Attacks. Out
of those 105, 58 of the Attacks were unprovoked.
Habitat
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.
Conservation
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 to 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 SeaSpecies.
Shark Fishery
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 Dentricles, 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, in which Fillets are battered and deep-fried or crumbed
and grilled and served alongside Chips. When served in Fish and Chip Shops, it is called Flake.
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 Bl 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 Fishing. 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.
