The Biology of Sharks
The shark is one of the most feared predators in the ocean. There are very few marine predators that are as dangerous to human beings as some of the large predatory sharks such as the Great White Shark, the Tiger Shark and the Bull Shark. However, many species of shark are harmless to humans and shark attacks on humans are statically rare.
Movies like ‘Jaws’ (1975) have portrayed sharks as blood thirsty man-eaters, which has further increased the public awareness of this potential threat and shark attacks, when they do happen, creates great news so this keeps the man-eater image of the shark firmly rooted in the mind of the public.
In truth sharks have more to worry about from their greatest predator, man; than humans have to worry about shark attacks. More people are killed each year by deer, dogs, and pigs than are killed by sharks. The risk of dying from a lightning strike or from drowning is far greater than the risk of dying from a shark attack. Of the more than 300 existing species of sharks, many are incapable of harming humans; however, the populations of many species of shark are declining rapidly due to human fishing.
Observing these magnificent creatures in the wild is a fascinating experience. Here with the use of videos and photograph we will try to give you some sense of this experience so that you can also learn to enjoy and respect sharks.
Now lets first look at the biology of the shark to see if we can better understand these wonderful creatures.
The key to understanding, interpreting and even predicting shark behavior is to first understand how sharks perceive their world. This is largely determined by the wide array of senses that sharks have at their disposal.
In addition to the five senses that human beings have, sharks have several other senses that they routinely use to perceive the world around them.
The sense of hearing is well developed in most sharks and is the furthest reaching sense of the shark. Sharks hear sounds underwater in the range between approximately 10 – 1500 Hz. These low-frequency sound waves can travel over a mile in the ocean waters enabling sharks to hear the sounds created by the erratic movements of an injured or dying fish from long distances.
The acuteness of the shark’s sense of smell is legendary. Studies have shown that sharks are able to detect blood at one part per million. That’s about the same as mixing two teaspoons of blood in an average sized swimming pool. This strong sense of smell is due to the shark’s highly sensitive olfactory system. On either side of the shark’s snout there are paired nostrils which are used solely for smelling. These nostrils lead to extensive olfactory epitheliums that are loaded with chemoreceptors. These chemoreceptors detect molecules within the water that places near them. The olfactory bulb of the shark’s brain is also enlarged, further enhancing the shark’s sense of smell. Sharks mainly use their acute sense of smell to find prey. However, male sharks can also use this sense to detect pheromones produced by female sharks, thus helping them to locate potential mates in the vast oceans.
Like most fish, the shark also has a lateral line on each side of its body. The lateral line is a system of water filled pores and connected canals that has pressure-sensitive haircells to detect changes in water pressure. This information is used by the shark to sense potential prey and predators. The lateral line system, however, is only effective for relatively close distances ranging up to several hundred feet.
The shark also has small pores on its snout and lower jaw that lead to the Ampullae of Lorenzini. These are small sacs filled with mucus that are sensitive to small electric currents. The Ampullae of Lorenzini help the shark detect magnetic fields, including the weak magnetic fields produced by prey. The Ampullae of Lorenzini also allow the shark to use the magnetic field of the earth for orientation.
Although less important that the other senses, discussed above, vision is still used by most sharks. The eyes of the shark are about ten times better adapted to low light conditions than human eyes. Some species of shark also have color vision, although their photo-receptors tend to be more sensitive to blue-green wavelengths than those of human eyes. Many species of shark have a third eyelid called the nictitating membrane that covers the shark’s eye when the shark is attacking prey. Other species simply roll their eyeballs under the tissue surrounding the eye to prevent possible injury when they are attacking prey.
The shark also has a good sense of touch and a good sense of taste, although these senses are probably less important to the shark than those discussed above.
Other Interesting Facts About Sharks
One extraordinary adaptation of the shark is its ability to freely move up or down in the water column.
Unlike bony fishes, which have swim bladders to maintain buoyancy, sharks are able to move freely between varying depths in the water. Swim bladders are sacs of gas, so when moving between different depths fish with swim bladders can run the risk of exploding or imploding because of the changes in pressure. However, because sharks do not have swim bladders, they must keep swimming to avoid sinking to the bottom.
This does not mean that all sharks must always keep swimming. Some species of sharks actual rest on the bottom. These sharks are able to pass water through their respiratory system by a pumping motion of their pharynx so that they can continue to breathe while resting. Other shark species do not have this ability and they must swim constantly in order to keep water flowing through their respiratory system.
Although most sharks are predators, and/or scavengers, the two largest species of shark, the whale shark and the basking shark are plankton feeders, like Baleen Whales.
Sixty Sharks 