Most sharks are predatory, though the largest kinds feed on plankton, and some can grow to a large size.
EXISTENCE:
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.
FEATURES:
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.
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 deep water sharks, however what is known is that they are abundant, the basic shark physiology is well suited to life at depth.
Sharks can be found all over the world, in both cold and warm waters. Some, like the blue shark, spend most of their time roaming the open ocean, while others, like the bull shark inhabit warmer, murky coastal waters.
FEED:
With a variety of species and sizes, sharks eat a variety of prey. Huge whale sharks eat tiny plankton, while the relatively smaller white shark eats toothed whales, pinnipeds and even sea turtles.
FEAR OF SHARKS:
Not all sharks attack humans, and the risk of shark attack, relative to other dangers, is relatively slim. But some species do attack, or interact with, humans more than others. The International Shark Attack File maintains a list of attacking species of shark, along with whether the attacks were provoked or unprovoked, fatal or non-fatal.
While shark attacks are a scary prospect, sharks have much more to fear from humans than we do of them in the grand scheme of things. Some estimate that up to 73 million sharks are killed each year just for their fins. Other threats to sharks include intentional harvesting for sport or for their meat or skin, and being caught as bycatch in fishing gear.
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