Sharks are commonly termed fish, even though they are only distantly related to the classical (bony) fish. What differentiates a shark from a bony fish? Sharks have a spinal column and are thus vertebrates. However, their skeleton is not made of bones, but of cartilage, and together with their nearest relatives the rays and the chimeras they form the class of cartilaginous fish.
Sharks have a highly streamlined body covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics. Over a period of more than 400 million years sharks have adapted themselves perfectly to life in the ocean and inhabit practically all ocean realms. Several shark species even live in sweet water. Whether they live on the ocean floor, swim freely in coastal regions, the open sea or the deep sea, sharks vary greatly in appearance. Their food spectrum ranges from plankton to mussels and molluscs, up to fish, birds and marine mammals.
Still, for marine life and of course for Sharks too, the oceans do not hold the same attraction everywhere. What counts are the largest food supply and the best environmental conditions. Such attractive productive regions include, for example, the relatively flat - only about 200 m deep - layers or shelf regions over the continental shelf. This is where the rivers deposit nutrients into the water which in turn feed the microorganisms which form the basis of the food pyramid. Most of the 500 known shark species live in these biologically highly productive continental shelf areas.
Sharks are in equilibrium with the populations of their prey and thus play a role in maintaining a stable marine ecosystem. In the oceans sharks are the most widely distributed hunters weighing more than 50 kg. They are called top predators because they are at the peak of the food pyramid. Some sharks are so-called super-predators which means they have no natural enemies except parasites and pathogens. Because they usually hunt older, weaker or sick prey, they also help maintain the health of the prey population. Healthy and strong individuals thus have a better chance of reproducing and passing on their genes.