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Shark Biology:
Sharks arose about 350 million years ago and have remained virtually unchanged for the past 70 million years and still comprise a dominant group. Although they live in an environment that is fairly resistant to change and have not been majorly affected by humans until now, but to survive for this long on a geological time scale they must be doing something right. Their success is largely due to the original genetic traits they inherited from their more primitive ancestors. This gives sharks some amazing adaptations allowing them to occupy some varied ecological niches.
It is thought that sharks almost certainly evolved from placoderms, a group of primitive jawed fishes. It took a long series of successful and unsuccessful mutations with fin, jaw positions etc. to give us all the different designs of sharks around today. Most asked to draw a shark most people would draw a shape along the lines of the whaler shark family, tigers or a mackeral shark such as a porbeagle. However many people do not realize the sheer diversity in the shape of sharks, or that rays are really sharks.

Basic Shark Physiology:
• Introduction:
Don't worry this is going to be simple its written by a young woman not a biologist, meant to give people a basic idea. Any shark biologists out there want to write an article for inclusion, just send me a page written as a .txt file.
• Skeletal Support:
Sharks are cartilaginous fish unlike most fish they have no bones, this is not to say that they squidge around like jelly but instead of bone they have cartilage. This is a fibrous tissue that can actually be quite hard, your nose and ears are made of cartilage for example.
Because of this fact it is quite rare to fins fossils of sharks, cartilage decomposes, again this is why human skulls have no ears or nose present, however fossils can exist.
• Buoyancy:
As well as a lack of bones, sharks also lack what most bony fish have, a swim bladder. A bony fish's swimbladder is effectively his ballast tanks, they can vary the amount of gas in the swim bladder to rise up or down vertically in the water and remain stationary in mid water, just like a dirigible.
Sharks in contrast have no swim bladder, taking the analogy of a dirigible or zeppelin further a fish could be compared to a dirigible whereas a shark is more like an airplane. Sharks rely on lift generated by their large dorsal fins in the same way an airplane's wings provide lift, however this lift would not be enough on its own to support the shark with the size fins they have (unless moving faster), a lot of the sharks lift comes from its liver. Sharks have huge livers, these contain oil which is lighter than water and therefore floats, this makes sharks slightly negatively buoyant.
Contrary to popular belief sharks don't have to swim constantly or they die, however they do have to swim to avoid sinking to the bottom. This liver as opposed to a swim bladder means that sharks' bodies are incompressible allowing them to move between very deep water and the surface with ease. Those bony fish that live at high pressures usually die when brought to the surface, sharks don't. Little is known about deepwater sharks, however what is known is that they are abundant, the basic shark physiology is well suited to life at depth.
• Respiration:
Again unlike bonefish sharks do not have gill covers, most cannot pump water over their gills and must move to breathe. This is not strictly true as many sharks that were thought to need to move constantly have been observed lying motionless. Tiger sharks, to do this they can lie in a gentle current which passes oxygenated water over their gills for them. Many sharks and rays can, however pass water over their own gills, these are usually the bottom dwelling sharks such as Cat sharks, Dogfish, Port Jackson sharks etc.
• Reproduction:
Sharks have two methods of reproduction oviparous & viviparous, basically some sharks lay eggs, usually in an egg pod, known to kids from seeing mermaids' purses washed up on the beach. Sharks such as Port Jackson Sharks, Dogfish and several others lay eggs, these are attached to kelp fronds or other similar plants for camouflage and protection.
Most of the bigger sharks however (such as the Whaler family) give birth to live young, the young stay inside the female shark, when they are born they are fully developed and shoot away fast or risk becoming food. In their early years they are prey to many animals, only a few will survive. Sharks have a long gestation period and do not reproduce frequently. If we keep killing sharks faster than they can reproduce then there will soon be no sharks left.

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