There are hundreds of species of sharks, ranging in size from the dwarf lantern shark (~20 cm or 8 inches) to the whale shark (up to 20 m or 65 feet), inhabiting all oceans. Contrary to popular belief, sharks are not indiscriminate killing machines; whale sharks  and bonnethead sharks are omnivores, eating plankton and seagrass respectively.

While each shark is unique, they share some common features. All sharks have a skeleton made of cartilage, not bones. Cartilage is lighter than bone, which lets them preserve energy to stay afloat and swim farther. Their body is covered in tiny scales with a tooth-like structure called dermal denticles, which overlap one another along their body, facing tail ward. A shark that is rubbed head-to-tail feels smooth, but if it is rubbed tail-to-head it feels like sandpaper. These denticles provide protection and make the shark more hydrodynamic, reducing friction in the water. 

Many sharks have countershading, a colouration that makes them less visible to predators and prey, specifically having a light underside. Unlike many bony fish, sharks lack a swim bladder, a gas-filled chamber that allows fish to  regulate buoyancy. Instead, sharks have large livers filled with low-density oils that  help with their buoyancy so they can swim up and down in the water column with ease. Deep sea sharks are often targeted for their livers, as about 40-70% of its mass is oil which contains squalene,  a compound with a large commercial demand. 

Behaviour and Senses 

Sharks have an extremely impressive sensory system. Sharks can detect chemical signals from distant sources and the direction they originated from.

Sharks also have specialized eyes and  depend on this sense. Their vision is well adapted to their marine environment, given that light levels are so variable . Their eyes have similarities to those of both humans and cats, but they don’t blink.

Sharks are very sensitive to sound, having tiny openings on their heads leading to internal ears. They pick up low frequencies from potential prey.

Their sense of touch is slightly different, as sharks use a line that runs along each side of their body called a lateral line. The lateral line has tubes  that allow water to flow through the skin help sharks detect changes in moving water, vibration, and pressure, along with sound. As a result, sharks can detect prey even in total darkness. 

Sharks have a weak sense of taste. Their taste buds are in their mouth and throat rather than on a tongue, as sharks have a small piece of cartilage that lacks tastebuds instead. Sharks use their sense of taste to try their prey before committing to consuming them, which helps in avoiding prey that might be dangerous. 

In addition to the standard five senses, sharks have adapted a sixth sense called electroreception. It is used to detect the weak electric fields that all living animals generate, and helps them find prey that is undetectable with their other senses.