THE OCEAN DEEP

The bull shark, Carcharhinus leucas, is the best known of several species that swim in both seawater and freshwater, as well as in deltas.[2]
SKELETON
The skeleton of a shark is very different from that of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made from cartilage, which is a flexible and dense connective tissue, but they are still considered bones. They function in the same way as human bones do. Like its relatives, rays and skates, the shark's jaw is not attached to the cranium. The jaw's surface, like its vertebrae and gill arches, is a skeletal element that needs extra support due to its heavier exposure to physical stress and its need for extra strength. It has therefore a layer of unique and tiny hexagonal plates called "tesserae", crystal blocks of calcium salts arranged as a mosaic.[3] This gives these areas much of the same strength found in real and much heavier bony tissue.
Generally there is only one layer of tesserae in sharks, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, have been found to be covered with two to three layers or more, depending on the body size. The jaws of a large white shark may even have up to five layers.
In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.
The fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers.
RESPIRATION
Like other fish, sharks extract oxygen from seawater as it passes over their gills. Shark gill slits are not covered like other fish, but are in a row behind its head. A modified slit called a spiracle is located just behind the eye; the spiracle assists the water intake during respiration and even plays a major role in bottom dwelling sharks, but is also reduced or missing in active pelagic sharks.[4] While moving, water passes through the mouth of the shark and over the gills — this process is known as "ram ventilation". While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small subset of shark species that spend their life constantly swimming, a behaviour common in pelagic sharks, have lost the ability to pump water through their gills. These species are obligate ram ventilators and would presumably asphyxiate if unable to stay in motion. (Obligate ram ventilation is also true of some pelagic bony fish species.)[5]
The respiration and circulation process begins when deoxygenated blood travels to the shark's two-chambered heart. Here the blood is pumped to the shark's gills via the ventral aorta artery where it branches off into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the dorsal aorta. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From there blood enters the ventricle of the heart and the cycle repeats.
HABITAT
Sharks are found all around the globe from the north to the south in all seas, they generally do not live in freshwater except for a few exceptions like the bull shark and the river sharks which can swim both in seawater and freshwater. Sharks are common down to depths of 2,000 metres (7,000 ft), and some live even deeper, but they are almost entirely absent below 3,000 metres (10,000 ft). The deepest confirmed report of a shark is a Portuguese dogfish that was found at 3,700 metres (12,000 ft).