Sharks have been swimming our oceans basically unchanged for millions of years and yet continue to be apex predators (at the top of the food chain). More than 400 different species of shark exist today, and they come in all different shapes and sizes. However, you might be surprised by the fact that more than half of shark species are less than three feet (1 meter) long. So how do sharks remain so successful?
Sharks are found all over the world and at a variety of depths. Although most sharks are marine, some 
sharks such as the bull shark (Carcharhinus leucas) can even live in freshwater! Some sharks such as the great white shark (Carharodon carcharias) are capable of swimming across entire oceans, whereas others live a relatively sedentary life, sticking to a small area.
Most fish have skeletons made of bone, just like mammals, reptiles, amphibians and birds. Sharks and rays, on the other hand, have skeletons made entirely of cartilage. Cartilage is the same flexible material that you have in your nose and ears. The advantage of cartilage is that it is light and flexible. This helps sharks to be swift and agile hunters.
Sharks also have a very unique skin. They don't have the large, prominent scales found in bony fish. Instead they are covered with smaller, tooth-like placoid scales called dermal denticles. These tough, protective denticles are aligned so that they channel water over the shark's body making them very fast.
Most bony fish have a special air bladder that helps them move up and down in the water column. Without this organ fish would sink to the bottom. Sharks do not have air bladders; they have a very large, oily liver that functions somewhat like an air bladder. However, this is not enough to keep them buoyant, so most sharks must keep swimming to keep from sinking. This is also important because most sharks also need to swim to breath. Unlike other fishes that can actively move water over their gills without swimming, sharks must swim to accomplish this.
One of the main reasons sharks are such effective predators is due to their amazing six senses. The shark's snout is definitely one of its most impressive (and prominent) features. As the shark swims, water enters through the two nasal passages and moves past folds of skin covered with sensory cells. Smell coming from the left of the shark will go through the left cavity before it goes through the right cavity. This is how a shark determines where a smell is coming from and heads in that direction. Fact: Sharks can smell 1 part per 25 million parts of water, smelling at a distance of hundreds of meters.
Sharks also have a very acute sense of hearing. Research suggests they can detect low pitch sounds well
below the range of human hearing.
In sharks, eyesight varies from species to species. Most sharks have a fairly wide field of view since their eyes are positioned on each side of the head. The most extreme example of this is the hammerhead, whose eyes are actually positioned on either side of their wide hammer-shaped heads.
Considered a sixth sense, sharks can sense weak electrical currents emitted by other animals with something called the Ampullae of Lorenzini. These are small clusters of electrically sensitive receptor cells positioned under the skin along the snout. These cells are connected to pores on the skin's surface via small jelly-filled tubes. This makes sharks very effective predators in low light conditions and in murky water.
Threats to Sharks
Sharks typically take a long time to reach reproductive maturity, mate rarely and have a relatively small number of babies (pups) at a time. Consequently, they don't replenish their population quickly. Sharks also have fairly long life spans - on average, sharks live 25 to 30 years, and some sharks can even live over 100 years old. A female shark will mate many times in her life. With this reproductive pattern, the death of every single shark obviously has a significant effect on the shark population.
One major obstacle to shark conservation efforts is our ignorance about sharks. We still don't fully understand their behavior, their breeding habits or their migration patterns. For most shark species we don't know their population size. This makes it very difficult to organize effective conservation methods. However, we can educate the public on the importance of sharks as apex predators, follow fishing regulations and practice catch and release techniques while we continue to learn about these amazing creatures.
Shark Facts
The spined pygmy shark only reaches 7 inches when fully grown and whale sharks can reach over 40 ft.
A great white shark can detect a single drop of blood in an Olympic-sized swimming pool.
Sharks may interpret sounds over many miles, listening specifically for distress sounds from wounded prey.
A shark's vision is seven times better than a human's and they are able to see as far as 60 feet under water.
The range of this electroreceptive capability seems to be fairly limited - a few feet in front of the shark's nose - but this is enough to seek out fish and other prey hiding on the ocean floor.
Shark Jaws
A unique characteristic of sharks is their teeth. Some bottom dwelling sharks have special grinding
teeth for crushing hard-shelled prey, but more active sharks have teeth suited primarily for cutting flesh. Sharks have many rows of teeth; up to eight rows at a time may actually be functional. The teeth are attached to the jaw by soft tissue, which causes them to continuously loose their teeth, up to 30,000 in a lifetime! This is crucial to the shark's effectiveness - worn or broken teeth are continually replaced by new, sharper teeth. This means sharks have a virtual conveyer belt of teeth that never run out.
Sharks have a very unique jaw structure, which makes their mouths effective weapons. The upper jaw rests below the snout but can be thrust forward and outward when the shark attacks its prey. This allows the shark to thrust its entire mouth forward to grab its prey, even though its mouth actually resides beneath its snout.
The combination of all these senses and adaptations makes the shark an unparalleled predator. The success of sharks is due largely to their unique characteristics and their ability to thrive in all waters.
