Sharks continue to maintain their reputation as the most feared and impressive fish occupying the oceans of the world. These marine hunters belong to a super-order called Selachimorpha, which is made up of approximately 440 different species. These diverse species can be found in waters all over the planet and are believed to have existed even before the time that dinosaurs roamed the earth, millions of years ago. Some modern scientists share the commonly held belief that sharks have actually been in existence for some 420 million years. As one of the world's oldest residents, sharks are certainly all the more fascinating to us, holding some intriguing secrets about the prehistoric world that humans are yet to discover.Under the umbrella of sharks as a species, there are more than 360 different kinds of these underwater hunters.

The different shark species within these orders are extremely diverse in terms of their physical characteristics, distribution, diet and habits. The smallest known shark species is the Lantern Shark, which reaches an average length of only 17 centimetres (or 6.7 inches). The largest is the Whale Shark, which measures an awesome 12 metres (or over 39 feet).

The Great White Shark averages an impressive 6.6 to 7 metres (21.6 to 23 feet) and the grizzly Ragged Tooth Shark comes in at about 3.6 metres, or just under 12 feet. With these mammoth hunters gliding through the waters, it is no wonder that sharks have earned themselves a fearsome reputation.

Different shark species tend to prefer different waters (in terms of the prey found therein and the temperatures of the ocean) but, generally, sharks are found all over the world. They may live anywhere between relatively warm, shallow depths and about 2 000 metres (or over 6 500 feet) from the surface, where the waters are dark and icy. Some sharks are even found skating along the ocean floor, camouflaged to blend in with the sand on which they lie in wait for their prey. There are no known sharks found at 3 000 metres or 10 000 feet below the water's surface.

There are a few rare species that have been designed to live in fresh water (such as rivers and lakes), while the vast majority are ocean dwellers. Generally, sharks prefer warmer waters, as they are exothermic creatures (broadly referred to as being cold-blooded, although this is not strictly the case) and rely on the water around them to regulate their body temperature. There are certain species that migrate, while others remain within a defined area for their entire life.

The body of a shark is streamlined to allow these experienced hunters to glide through the water, unhindered by the resistance that the water (which is denser than air) offers. Sharks that skate along the sandy floor of the ocean are generally flattened, allowing them to slide along unnoticed and without unnecessary bulk (since they do not need to exert the same powerful force that is required by sharks that swim against the density of the water).

Unlike most other fish, which have both bones and cartilage making up their skeletons, sharks have only a cartilage skeleton. This increases their flexibility and decreases their weight to allow for even faster movement through the water, which is essential for their hunting and general travel.

Unlike most other fish, which have both bones and cartilage making up their skeletons, sharks have only a cartilage skeleton. This increases their flexibility and decreases their weight to allow for even faster movement through the water, which is essential for their hunting and general travel.

Sharks have several rows of teeth. As the front row are used to their capacity, they fall out and are replaced by the strong, beable and brand new row of razor-sharp teeth behind them. These rows of teeth are used only for capturing their prey and tearing chunks of flesh off of the victim. Unlike humans, sharks do not chew their food, but gulp it down as solid hunks of meat.

All sharks are carnivores, meaning that they survive exclusively off a diet of meat and do not eat vegetation in any form. However, although restricted to meat, their diet remains varied. Some species, including the mammoth Whale Shark, swim through the waters with their mouths open, allowing plankton and other small creatures to enter through huge filters.

To get an adequate volume of food, this requires these types of feeders to swim like this for most of their lives. The bottom-feeders that skate along the floor of the ocean crush crabs, clams and similar animals that live at this depth and eat these at leisure, while the more ferocious species (such as the Great White and Mako sharks) actively hunt fish, squid, dolphins, seals and turtles. Sharks thrive off prey that has a high body fat content.

Some sharks are known for the feeding frenzies that sometimes take place among them. These frenzies occur when a group of sharks come across a large school of fish or a similar assembly of potential prey. They begin to attack the prey from all angles in a crazed, uncontrolled manner. This results in the injury and even death of many fellow sharks, as they snap their powerful jaws without discrimination between prey and fellow hunters. The resulting injuries are sometimes fatal to fellow sharks and those that survive frequently bear massive scars from their encounters.

As a species, sharks are believed to be relatively intelligent and teachable. They can learn to adopt patterns of behaviour once they are familiar with these and have been shown to recognise certain people and objects time and again. It has not, as yet, been easy to gauge exactly how much sharks are able to learn, since they are elusive, and sometimes vicious, animals.

Testing is not easily conducted so much is left to coincidental findings or rare experiences and experiments.

Sharks, like many other animals today, are often victims of hunting at the hands of human beings. In some eastern countries, shark fins are considered to be a delicacy and their cartilage is believed to have medicinal value. This has resulted in the brutal slaying of millions of sharks every year. In order to save the population numbers of these invaluable creatures, large-scale initiatives are required. As their focus, these initiatives have to educate local communities regarding the value of sharks and their role in the ecosystem, and implement legislation to protect the animals and punish the hunters. Often, it is because locals and tourists alike are not aware of the biological implications of upsetting an ecosystem that they carry on supporting such industries.