Sharks are a group of elasmobranch fish, which constitute a vital sector of marine and estuarine nekton and are of great commercial importance all over the globe, apart from being a major component in marine food web.
Sharks are crucial to marine ecosystems. They maintain a balance in populations of prey species and keep the ocean healthy by removing ill or diseased animals.
The various direct and indirect products obtained from sharks are today used in food, tourism and pharmaceutical industries.
The main unique physical attributes used to identify sharks are a skeleton made from cartilage, exposed gill slits and a large, oil filled liver to aid buoyancy.
Since sharks lack a swim bladder, they have a natural tendency to sink. Their lightweight skeleton combined with a liver full of oil helps them overcome this tendency.
A few species of sharks, like the bull shark, are able to adapt to fresh water enough that they can swim up rivers. Bull sharks have been found in the Mississippi River as far upstream as Illinois. One bull shark was reportedly found in the Amazon River, about 2600 miles (4200 km) from where the river meets the sea.
Shark
