Senses of the shark
Smell
The shape of the hammerhead shark's head may enhance olfaction by spacing the nostrils further apart.Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one part per million of blood in seawater. They are more attracted to the chemicals found in the guts of many species, and as a result often linger near or in sewage outfalls. Some species, such as nurse sharks, have external barbels that greatly increase their ability to sense prey.
Hearing
Although it is hard to test sharks' hearing, they may have a sharp sense of hearing and can possibly hear prey many miles away.[27] A small opening on each side of their heads (not to be confused with the spiracle) leads directly into the inner ear through a thin channel. The lateral line shows a similar arrangement, which is open to the environment via a series of openings called lateral line pores. This is a reminder of the common origin of these two vibration- and sound-detecting organs that are grouped together as the acoustico-lateralis system. In bony fish and tetrapods the external opening into the inner ear has been lost.
Speed
In general, sharks swim at an average speed of 8 kilometres per hour (5.0 mph) but when feeding or attacking, the average shark can reach speeds upwards of 19 kilometres per hour.The shortfin mako shark, the fastest shark and one of the fastest fish, can burst up to 50 kilometres per hour (31 mph). The great white shark is also capable of bursts of speed. These exceptions may be due to the warm-blooded, or homeothermic, nature of these sharks' physiology.
Sleep
Some sharks can lie on the bottom while actively pumping water over their gills, but their eyes remain open and actively follow divers. When a shark is resting, it does not use its nares, but rather its spiracles. If a shark tried to use its nares while resting on the ocean floor, it would be sucking up sand rather than water. Many scientists believe this is one of the reasons sharks have spiracles. The spiny dogfish's spinal cord rather than its brain, coordinates swimming, so it is possible for spiny dogfish to continue to swim while sleeping. It is also possible that sharks sleep in a manner similar to dolphins,[50] one cerebral hemisphere at a time, thus maintaining some consciousness and cerebral activity at all times.
Distribution and habitat
Sharks are found in all seas. They generally do not live in freshwater, with a few exceptions such as the bull shark and the river shark 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 at 3,700 metres (12,100 ft).
My reflection base on the researches:
Shark is a great animal,after done so many researches on sharks, I realised that sharks does not like what people though cruel and evil,in fact that sharks are a great and amazing animal.People fear sharks,however, people also hunt sharks because their delicious shark fin and shark oil.shark skin also are a kind of good material for making handbags and shoes.
in fact, now days the number of sharks are decearsing very fast.Every year more than 100 million shrks were killed by peoples.now days, prople hunting sharks not for protect thenselves but to hunt for themselves.so if we do not protect sharks from now, soon,shark will be another animal which disaper from our earth.
URL:http://en.wikipedia.org/wiki/Shark#Conservation
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